Indexing articulation joint with sensors for establishing state of engagement and associated measurement device - Patents.com

Abstract

An apparatus comprising: a first relatively reorientable member, a second relatively reorientable member, and an indexing arrangement configured to provide a plurality of angularly indexed, lockable positions of the first and second members about a first axis, the indexing arrangement comprising: i) a series of features on the first member, the series of features extending annularly about the first axis, and ii) engagement features on the second member at at least three discrete, annularly spaced locations, the engagement features configured to interlock with a subset of the features on the first member when in a locked state, thereby providing a stable, repeatable relative rest position of the first and second members at each index position. The apparatus further comprises at least a first non-contact sensor mounted on the second member.

Description

[Technical Field]

[0001] The present invention relates to an indexing articulation joint, and in particular to an indexing articulation joint for use in high precision metrology apparatus. For example, in one specific embodiment, the present invention relates to an indexing articulation joint for an articulation head configured to support a measurement probe on a coordinate positioning apparatus such that the measurement probe can be positioned in a number of different rotational orientations. [Background technology]

[0002] As is well known in the field of coordinate positioning devices, and in particular in the field of coordinate measuring machines (CMMs), an articulating head (or rotary table) for a measurement probe (or object) comprises an articulatable member that facilitates reorientation of the mounted measurement probe (or object) about at least one axis of rotation. Typically, the articulating head provides two orthogonal axes of rotation, although fewer or more axes of rotation may be provided. Typically, the rotary table provides one axis of rotation.

[0003] Articulating heads with one axis of rotation have been described (see, for example, U.S. Patent No. 5,929,949), and articulating heads providing two orthogonal axes of rotation have been described (see, for example, U.S. Patent No. 5,929,949 and U.S. Patent No. 5,929,949). As described in these documents, it is also known to provide articulating heads with indexing arrangements that allow relatively rotatable parts of the articulating head to be locked at defined index positions. The indexing arrangements can be provided by providing two sets of intermeshing members, one for each of the relatively rotatable members. When engaged, the intermeshing members lock to provide relative rotation of the rotatable members. When the intermeshing members are disengaged, the rotatable members (and the measurement probe mounted thereon) are free to rotate relative to each other so that they can be repositioned to a new orientation (e.g., under the control of a potentiometer or encoder, as described in U.S. Patent No. 5,929,949) before being reengaged to lock the rotatable members (and the measurement probe mounted thereon) in the new orientation. Thus, measurement operations can be performed with the measurement probe held in a defined, known rotational orientation. Renishaw plc sells such an indexing joint under the product name PH10, the indexing arrangement comprising a series of balls in one of the rotatable members and three pairs of cylindrical "rollers" in the other member. A potentiometer mounted on the member having the three cylindrical rollers engages (via a series of gear members) with the member having the series of balls, using the output of the potentiometer when the members are unlocked so that the relative positions of the members can be monitored and controlled to ensure they are positioned in the desired relative orientation before being locked together. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 5,185,936 [Patent Document 2] European Patent No. 2889573 [Patent Document 3] International Patent Application Publication No. 2006 / 079794 [Patent Document 4] U.S. Patent No. 7,213,344 [Patent Document 5] U.S. Patent No. 7,263,780 [Patent Document 6] U.S. Patent No. 9,494,403 [Non-patent literature]

[0005] [Non-Patent Document 1] HJJBraddick, “Mechanical Design of Laboratory Apparatus”, Chapman and Hall, London, 1960, pages 11-30 Summary of the Invention [Means for solving the problem]

[0006] The present invention relates to an improved indexing articulation joint. In particular, the present invention has been found to be useful in facilitating high performance indexing articulation joints, for example, having a desired index position repeatability of 100 nm (nanometers) or less.

[0007] According to a first aspect of the present invention, there is provided an apparatus comprising a first relatively reorientable member, a second relatively reorientable member, and an indexing arrangement configured to provide a plurality of angularly indexed lockable positions of the first and second members about a first axis (of rotation), the indexing arrangement comprising: i) a series (e.g. a continuous series) of features on the first member, the series of features extending annularly about the first axis; and ii) engagement features on the second member at a plurality (e.g. at least three) discrete locations spaced annularly (about the first axis), the engagement features being configured to engage the first and second members when in a locked state. and an engagement feature configured to interlock with a subset of the features on the first and second members to thereby provide a stable (e.g., kinematically) repeatable relative rest position of the first and second members at each index position, the device further comprising at least a first non-contact sensor mounted to the second member (i.e., mounted so as to be rotatably fixed to the second member), the first non-contact sensor configured to sense an area on the first member and thereby provide a signal dependent on the spatial configuration of the first and second members when they are in their locked state.

[0008] As described in more detail below in connection with certain embodiments of the present invention, the inventors have identified that there can be significant advantages to locating a non-contact sensor (which measures the spatial configuration of a first member and a second member) on the second member as opposed to the first member. Specifically, the measurement of the spatial configuration of a first member and a second member can be less prone to error, or at least any error in said measurement can be more predictable and therefore more easily corrected.

[0009] The first relatively reorientable member and the second relatively reorientable member may be referred to as a pair of relatively reorientable members, eg, a first pair of relatively reorientable members.

[0010] At each index position, the material of at least those portions of the first and second members that mechanically couple the first non-contact sensor on the second member and the region on the first member, thereby defining their relative spatial configuration, can have a CTE of at least 2 ppm / ° C. Materials with CTEs less than 2 ppm / ° C. are known and can be used, but such materials may be expensive and / or may have other unsuitable properties (e.g., the material is too soft) that make them unsuitable for use in the desired application.

[0011] The first non-contact sensor may be positioned so that the radial line on which its sensing window is located is within ±15° of the radial line on which the first of the engagement features of the second member is located, preferably within ±10° of the radial line on which the first of the engagement features of the second member is located, more preferably within ±5° of the radial line on which the first of the engagement features of the second member is located, and particularly preferably within ±1° of the radial line on which the first of the engagement features of the second member is located.

[0012] The first non-contact sensor may be configured to measure the relative spatial configuration of the first member and the second member in only one dimension, or may be configured to measure the spatial configuration of the first member and the second member in at least two orthogonal dimensions, such as three orthogonal dimensions, for example. The first non-contact sensor may be configured to measure the relative height / separation of the first member and the second member (e.g., along the axis of rotation / first axis).

[0013] Optionally, the first non-contact sensor may be configured to measure the relative configuration (e.g., lateral position and / or rotational orientation) of the first member and the second member in a plane perpendicular to the axis of rotation (i.e., the first axis). For example, the first non-contact sensor may be configured to measure the relative lateral position of the first member and the second member (e.g., in at least one dimension perpendicular to the axis of rotation, e.g., in two orthogonal dimensions perpendicular to the axis of rotation). Specifically, for example, the first non-contact sensor may be configured to measure the relative rotational orientation of the first member and the second member about the first axis. Optionally, at least one first non-contact sensor may be configured to measure a combination of the above relative configurations.

[0014] As will be appreciated, the first non-contact sensor may be configured to provide a measurement of the relative configuration of the first body and the second body (in a plane perpendicular to the axis of rotation) with a resolution finer than an index increment of the indexing configuration, for example at least 5 times an index increment of the indexing configuration, optionally at least 10 times an index increment of the indexing configuration, for example at least 15 times an index increment of the indexing configuration. Preferably, the first non-contact sensor is capable of establishing the relative position of the first body and the second body to within 50 μm, for example within 10 μm, optionally within 1 μm, for example within 100 nm, optionally within 10 nm, for example within 2 nm.

[0015] The area of the first member sensed by the first non-contact sensor may comprise a scale member (e.g., a rotating scale member). Accordingly, the non-contact sensor and scale member may be generally referred to as an "encoder device" or a "position encoder device." In such cases, the non-contact sensor may be generally referred to as a "readhead." The scale member may comprise a series of features that the first non-contact sensor / readhead can read to determine its relative position. The scale member may comprise an incremental scale track (with or without one or more reference markings). The incremental scale track typically comprises a periodic series of features that form a signal in the non-contact sensor / readhead that varies with relative movement between the non-contact sensor / readhead and the scale. In the case of an optical encoder device, the signal may be a fringe field (e.g., interference fringes) that moves with relative movement between the non-contact sensor / readhead and the scale, or, for example, one or more points of light whose intensity varies with relative movement between the non-contact sensor / readhead and the scale. The scale member may comprise an absolute scale track. An absolute scale track typically comprises a non-periodic series of features that encode unique position information along the length of the track. The scale member may comprise two or more scale tracks, optionally comprising a combination of incremental and absolute scale tracks. The scale member / series of features / one or more of its tracks may extend about said first axis. A non-contact sensor / readhead may read the scale member to determine / provide a direct measurement of the relative position of the first and second members about the first axis.

[0016] The non-contact sensor may sense the region via one or a combination of optical, inductive, magnetic, and capacitive means. Thus, the non-contact sensor may comprise an optical sensor, a magnetic sensor, an inductive sensor, and / or a capacitive sensor. Thus, in the case of an encoder device, the encoder device may comprise an optical, inductive, magnetic, or capacitive encoder device (any of which may use a combination of sensing technologies; for example, incremental optical encoder devices are known to include magnetic reference marks).

[0017] The first non-contact sensor is not necessarily part of the encoder device. Rather, the first non-contact sensor does not necessarily sense the scale member, but may sense, for example, a structural portion of the first member. The first non-contact sensor may comprise a position sensitive device (PSD) whose output depends on the relative spatial positions of the first member and the second member when locked together. In other embodiments, the first non-contact sensor may comprise a height / separation sensor (e.g., an inductive, magnetic, and / or capacitive sensor) configured to sense the height / separation between the sensor and the first body.

[0018] The device can be configured such that when the first and second members lock together in the indexed position, the first non-contact sensor is used to establish information about the state of engagement between the first and second bodies. For example, a signal provided by the first non-contact sensor can be used to establish information about the state of engagement between the first and second members.

[0019] The first non-contact sensor can be used to obtain a (“current”) measurement of the relative spatial configuration of the first and second members. Information obtained from this measurement can be compared to calibration information. The calibration information can be obtained from at least one other (i.e., previous) measurement of the relative spatial configuration of the first and second members (e.g., taken by the first non-contact sensor) when the first and second members were locked at an earlier time in the index position (or when the first and second members were previously locked at the index position). The comparison can be performed to establish information about the state of engagement between the first and second members. Preferably, the calibration information was obtained from at least one other measurement of the relative spatial configuration of the first and second members taken by the first non-contact sensor (e.g., when the first and second members were locked at an earlier time in the index position).

[0020] The first non-contact sensor can help verify that the first and second members have properly locked together, and can help verify that they have locked together in substantially the same relative configuration as at a previous / earlier point in time, such as during a calibration phase, etc. Thus, the first non-contact sensor can help verify the repeatability of the index position at which the first and second members lock together.

[0021] The device may be configured to respond in a predetermined manner depending on the determined state of engagement between the first and second members, such as depending on the results of the aforementioned comparison. For example, if the determined state of engagement (e.g., the comparison) indicates that the first and second members are not properly locked together, the device may be configured to respond by unlocking the first and second members. Optionally, the device may be configured to respond by unlocking and re-locking the first and second members at the same indexed position. Optionally, this may include re-locking the first and second bodies from a slightly different position (e.g., from a slightly different relative rotational orientation). Responding in a predetermined manner may additionally or alternatively include recording and / or reporting (e.g., outputting to a controller device) an error or warning condition.

[0022] As will be understood, references to a "previous measurement" and "previously locked at the index position" do not necessarily refer to the immediately preceding or most recent measurement or the time they were locked at the index position. Rather, the terms "previous" and "previously" are used to refer to some earlier point in time. Thus, the first and second members may have been locked at the index position multiple times between the current time and the time the calibration information was obtained.

[0023] The apparatus may be configured to determine that the first and second members are not properly locked together if the comparison indicates that the current relative spatial configuration of the first and second members at the indexed position differs from the relative spatial configuration of the first and second members represented by the calibration information by more than a predetermined threshold. The predetermined threshold may be 100 μm (microns) or less, for example 50 μm or less, optionally 20 μm or less, but may also be as small as 1 μm or less, for example 100 nm (nanometers) or less, 50 nm or less, or 10 nm or less.

[0024] As described above, the first non-contact sensor may be part of an encoder device (e.g., a readhead). As also described above, the scale track may comprise a series of features, such as a series of generally periodic features. The scale track may have a characteristic pitch distance (or a "characteristic pitch angle" for some rotary systems, such as a circular scale where the scale features are easily located). The signal from the non-contact sensor / readhead may be used to interpolate between intervals of the scale pitch to generate a position measurement with a much finer resolution than the scale pitch. (The readhead may generate a spatially periodic signal, and in some embodiments, the readhead signal period has a higher frequency (shorter wavelength) than the scale period. In these cases, interpolation may still be used to generate a position measurement with a much finer resolution than the signal period.) "Information derived from said measurement" and "calibration information" may include relative position information at a resolution much finer than the scale period. Such relative position information may be referred to as a "phase reading" because the information relates to the "phase" position between periodic features of the scale. Thus, "information derived from the measurement" and "calibration information" may include phase readings. Thus, in such an embodiment, the device may be configured such that a phase reading obtained from the first non-contact sensor when the first and second members are locked together at an index position is compared to a phase reading obtained by the first non-contact sensor when the first and second members are locked at an earlier point in time at the index position to establish information about the state of engagement between the first and second members.

[0025] It may be preferable for the device to include a second non-contact sensor configured to provide a signal dependent on the spatial configuration of the first and second members when they are in their locked state. Preferably, the second non-contact sensor is mounted to the second member and configured to sense an area on the first member at an annular position about the first axis different from the annular position sensed by the first non-contact sensor (thereby providing a signal dependent on the spatial configuration of the first and second members when they are in their locked state). As will be appreciated, a further (e.g., third) non-contact sensor configured to provide a signal dependent on the spatial configuration of the first and second members when they are in their locked state may be provided. Furthermore, the (e.g., third) non-contact sensor may be particularly beneficial if the non-contact sensor includes a height / separation sensor used to sense the height / separation between the sensor and the first body. For example, in such a case, it may be beneficial to provide three height / separation sensors equiangularly spaced about the axis of rotation. Preferably, each height / separation sensor is configured such that its sensor window is located within ±15° of a radial line along which its nearest engagement feature on the second member is located.

[0026] All statements made above in connection with the first non-contact sensor can also apply to the second non-contact sensor (and any additional non-contact sensors). Thus, statements made above in connection with the type (e.g., whether it is a readhead of an encoder device or another type of sensor, such as a height / separation sensor), location (e.g., its location relative to an engagement feature of the second member), and use of the first non-contact sensor can also apply to the second non-contact sensor. Thus, for example, with respect to the use of the second non-contact sensor, in embodiments in which at least a first non-contact sensor and a second non-contact sensor are provided, the device can be configured such that when the first and second members lock together at an indexed position, the first and second non-contact sensors are used to sense the first member and thereby provide a signal dependent on the spatial configuration of the first and second members when in their locked state. The signals provided by the first and second non-contact sensors can be used to establish information about the state of engagement between the first and second members. For example, the first and second non-contact sensors may be used to obtain a measurement of the relative spatial configuration of the first and second members, and information obtained from said measurement may be compared to calibration information obtained from at least one other (i.e., previous) measurement of the relative spatial configuration of the first and second members (e.g., taken by the first and second non-contact sensors) when the first and second members were locked at an earlier time in the index position (i.e., when the first and second members were previously locked at the index position) to establish information about the state of engagement between the first and second members.Optionally, in accordance with the above description where the non-contact sensor is part of the encoder device, the measurement device may be configured such that the first and second non-contact sensors read the scale members (the same or different scale members) when the first and second members are locked together in the index position, and the first and second phase readings obtained from the first and second readheads, respectively, are compared with first and second phase readings obtained by the first and second readheads, respectively, when the first and second members were locked at an earlier point in time in the index position, to establish information about the state of engagement between the first and second members.

[0027] It may be preferred that the second non-contact sensor be configured to sense an area of the first member at a location about the first axis that is less than 180° from where the at least one first non-contact sensor senses the first member, such as between 45° and 135° from where the at least one first non-contact sensor senses the first member, for example, between 115° and 125° from where the at least one first non-contact sensor senses the first member, for example, 120° from where the at least one first non-contact sensor senses the first member. As described in more detail below, it may be preferred that the second non-contact sensor be configured to sense an area of the first member at a location that is 90° from where the at least one first non-contact sensor senses the first member.

[0028] As mentioned above, statements regarding the position of the first non-contact sensor also apply to the second non-contact sensor. Thus, for example, the second non-contact sensor may be positioned so that the radial line on which its sensing window is located is within ±15° of the radial line on which the second of the engagement features of the second member is located, preferably within ±10° of the radial line on which the second of the engagement features of the second member is located, more preferably within ±5° of the radial line on which the second of the engagement features of the second member is located, and most preferably within ±1° of the radial line on which the second of the engagement features of the second member is located. However, this need not be the case, and the second non-contact sensor may be intentionally positioned so that the radial line on which its sensing window is located is not within ±15° of the radial line on which any of the engagement features of the second member are located. For example, it may be advantageous to position the second non-contact sensor at substantially 90° to each other about the first axis to be able to extract the most accurate information about the spatial configuration of the first and second members in multiple (e.g., orthogonal) dimensions, and the second engagement feature may not be positioned at or near 90° to the first engagement feature about the first axis (e.g., they may be positioned 120° apart).

[0029] The area of the first member sensed by the second non-contact sensor need not be the same area sensed by the first non-contact sensor. For example, if the first and second non-contact sensors are part of an encoder device (e.g., a readhead) and the area includes a scale member, the second non-contact sensor may sense a different track on the scale member or may sense a different scale member. Furthermore, the first non-contact sensor can be part of the encoder device (and therefore can sense the scale member), while the second non-contact sensor is not part of the encoder device (and therefore does not sense the scale member).

[0030] The indexing arrangement of the first and second members can be unlocked (or their interlocking features can be disengaged) by relative axial movement of the first and second members along the first axis in a first direction such that the first and second members are free to rotate relatively about the first axis. The first and second members can be locked (or their interlockable features can be reengaged) by relative axial movement of the first and second members along the axis in a second direction.

[0031] The device may further include a motor (eg, an electric motor) for driving the first member and the second member about the first axis when unlocked.

[0032] The device may further include a primary encoder device configured to monitor the relative rotational position of the first and second members about the first axis when unlocked. Optionally, in those embodiments in which the non-contact sensor is part of the encoder device (classified herein as a "secondary encoder device"), the primary encoder device may be the same encoder device as the non-contact sensor / secondary encoder device. Stated differently, the non-contact sensor may be configured such that it can be used (and, optionally, is used) to detect / monitor the relative rotational position of the first and second members about the first axis when unlocked. Optionally, the primary encoder device may share some common parts with the non-contact sensor / secondary encoder device (e.g., they may share the same scale as the primary encoder device with a different readhead than the first non-contact sensor / readhead). However, it may be preferred for the primary encoder device to be an entirely different encoder device from the non-contact sensor / secondary encoder device with a different readhead and different scale.

[0033] The device may include a motor mechanism for actuating the first and second members between their locked and / or unlocked states ("lock / unlock" motor mechanism). The lock / unlock motor mechanism may include a member (e.g., a post) actuable by the motor to unlock (index configurations of) the first and second members (e.g., by separating them along a first axis). For example, the post may be actuable between a retracted configuration in which the first and second members are in their locked state and an extended configuration in which the first and second members are held apart by the post along the first axis so that the first and second members are unlocked and thereby permit relative rotation of the first and second members. For example, the member body may include a post, and the post and second member may be magnetically biased toward each other to magnetically grip the first and second members. A secondary biasing member (eg, a magnetic material) may be configured to bias the post toward its retracted configuration.

[0034] For example, the first member (or, in an alternative embodiment, the second member) can include a component (e.g., a post) configured to be rotatable relative to the first (or second) member about a first axis when at least in an unlocked configuration, and to be engaged and rotatably fixed relative to the second (or first) member when the first and second members are disengaged and rotated relative to one another. The component can be actuatable (e.g., by a motor) between a retracted configuration in which the first and second members are in their locked state, and an extended configuration in which the first and second members are held apart by the component / post along the first axis to unlock the first and second members and thereby permit relative rotation of the first and second members. The primary encoder device may include a readhead on one of the first (or second) member and the part, and a scale member on the other, such that the readhead provides a measurement of the relative rotational position of the first (or second) member and the part. The device may be configured to use the output of the primary encoder device to control the rotation of the first and second members when unlocked. The part / post may be coupled to the second member (or, in alternative embodiments, to the first member) when in its extended configuration (via corresponding engagement features as described in more detail below) and may be uncoupled from the second (first) member when in its retracted configuration.

[0035] The metrology device may include a rotary table with an indexing articulating joint, on which the artefact to be inspected is mounted. The metrology device may include a probe head with an indexing articulating joint. The probe head may be configured to support the measurement probe on a coordinate positioning device such that the measurement probe may be positioned in a plurality of different indexed rotational orientations. Suitable measurement probes include contact measurement probes and non-contact measurement probes. Suitable measurement probes include probes for measuring dimensions of artefacts. Suitable measurement probes include touch-trigger, scanning or "analog" measurement probes.

[0036] The metrology device (e.g., rotary table / probe head) may be configured to be mounted to a positioning device, in particular a coordinate positioning device, such as a coordinate measuring machine (CMM). The metrology device (e.g., rotary table / probe head) may be mounted to a positioning device configured to facilitate repositioning of the metrology device in at least two, e.g., three orthogonal linear degrees of freedom. The metrology device (e.g., rotary table / probe head) may be removably mounted to the positioning device (e.g., Z-column or quill of a CMM) via one or more releasable fasteners, such as one or more bolts.

[0037] The measurement apparatus may include a storage device containing the calibration information. The storage device may be located in a part of the apparatus separate from the indexing articulation joint (e.g., in the control unit). Preferably, a part of the indexing articulation joint (e.g., the first body or the second body) includes the storage device. In embodiments where the apparatus includes a probe head (or a rotary table), the probe head (or the rotary table) may include the storage device.

[0038] The measurement apparatus may include a processing device configured to perform the aforementioned comparison. The processing device may be located in a part of the apparatus separate from the indexing articulation joint (e.g., in the control unit). Optionally, a part of the indexing articulation joint (e.g., the first member or the second member) includes the processing device. In embodiments where the apparatus includes a probe head (or a rotary table), the probe head (or the rotary table) may include the processing device.

[0039] Thus, the device may be configured such that the aforementioned information about the state of engagement (e.g., the comparison) is implemented within a portion of the measurement device that comprises the indexing articulation joint itself (e.g., within the probe head or rotary table).

[0040] The calibration information can be stored in a lookup table. Optionally, the calibration information may be represented by a function. Thus, the storage device may include a lookup table and / or a function that includes / represents the calibration information. The lookup table may include calibration information for at least a subset of the possible index positions of the first body and the second body. The lookup table may include calibration information for each of the possible index positions of the first body and the second body. For example, the lookup table may include at least one element / data cell for each index position. Each element / data cell may include calibration information for the index position with which the element / data cell is associated. The lookup table may include multiple elements / data cells for each index position. This may be useful in situations where there are two or more non-contact sensors.

[0041] "Information obtained from said measurements" and "calibration information" may include relative position information (eg, as opposed to absolute position information).

[0042] The calibration information (e.g., a lookup table or function) may be updated over time. This may be done continuously or at regular intervals. This may be done as part of a dedicated calibration process or may be done between measurement operations. For example, each time the first and second members lock together at any given index position and the comparison indicates that the first and second members are properly locked together (e.g., the comparison indicates that the current relative spatial configuration of the first and second members at the index position does not differ from the relative spatial configuration of the first and second bodies represented by the calibration information by more than a predetermined threshold), information obtained from the measurement provided by the first non-contact sensor (and optionally the second non-contact sensor) of the current relative spatial configuration of the first and second members may be used to update (e.g., stored) the calibration information (e.g., used to update / replace information stored in a particular element / data cell in the lookup table associated with the index position).

[0043] As will be understood, "information derived from said measurements" may mean that information is derived from measurements obtained by the non-contact sensor itself, or that information is derived from measurements obtained by the non-contact sensor and also from other sources of data. Thus, information does not necessarily or solely have to be derived / derived from measurements obtained by the non-contact sensor. Nevertheless, it may be preferred that "information derived at least from said measurements" is solely measurements obtained by the non-contact sensor. Thus, "information derived from said measurements" may be measurements obtained by the non-contact sensor, e.g., solely output from the non-contact sensor.

[0044] Similarly, "calibration information obtained from at least one other / prior measurement of the relative spatial configuration of the first member and the second member" may mean that the calibration information is obtained from at least one other / prior measurement obtained by the non-contact sensor itself, or that the calibration information is obtained from at least one other / prior measurement obtained by the non-contact sensor and also from other sources of data. Thus, the calibration information does not necessarily or need to be obtained / derived solely from at least one other / prior measurement obtained by the non-contact sensor. Nevertheless, it may be preferable for the "calibration information" to be solely measurements obtained by the non-contact sensor at an earlier point in time. Thus, the "calibration information" may be measurements obtained by the non-contact sensor, e.g., solely output from the non-contact sensor.

[0045] Thus, the apparatus may be configured to compare a current measurement of the relative spatial configuration of the first and second members obtained by the first non-contact sensor (and optionally the second non-contact sensor) with other / previous measurements of the relative spatial configuration of the first and second members taken by the first non-contact sensor (and optionally the second non-contact sensor) when the first and second members were locked at an earlier point in time in said indexed position, in order to establish information about the state of engagement between the first and second members.

[0046] As will be appreciated, the interlocking features of the indexing arrangement can provide a plurality of predetermined angular index positions at which the first and second members can be locked relative to one another. The interlocking features of the indexing arrangement can provide index increments of 10° or less, such as index increments of 5° or less, such as 4° or less. The interlocking features of the indexing arrangement can provide index increments of at least 0.5°, such as at least 1°. For example, the interlocking features of the indexing arrangement can provide index increments of approximately 2.5°.

[0047] The device may include a motor configured to drive the first member and the second member about an axis of rotation (the first axis) when the first member and the second member are unlocked (a "reorientation" motor mechanism).

[0048] The series of features on a first member that is part of the indexing arrangement may comprise a series of teeth, preferably tapered teeth. The tapered teeth in the series may comprise protrusions with straight or curved sides. For example, the teeth may have a spherical (e.g., hemispherical) shape, a cylindrical (e.g., semi-cylindrical) shape, or a substantially triangular shape. The series of teeth may provide a face spline member. When the device is in a locked state (for each possible index position), engagement features on the other of the second member may engage with a subset of the series of teeth on the first member at multiple (e.g., three) discrete, equiangularly spaced locations.

[0049] The engagement feature of the second member may comprise one or more teeth.

[0050] The engagement feature of the second member may comprise only one tooth. Optionally, the engagement feature of the second member may comprise multiple teeth, such as, for example, groups of teeth arranged at different annular locations about the first axis (e.g., a small series of teeth extending annularly about the first axis). Such a group of teeth may comprise five or fewer teeth, such as, for example, two, three, or four teeth. Preferably, the angle formed by the arc subtended by the first and last teeth in said group / series of teeth of the engagement feature is 25° or less, more preferably 20° or less, and especially more preferably 10° or less, e.g., 5° or less.

[0051] As described above in connection with the teeth of the first member, the teeth of the engagement feature of the second member may comprise tapered teeth. The tapered teeth may comprise protrusions with straight or curved sides. For example, the teeth may have a spherical (e.g., hemispherical) shape, a cylindrical (e.g., semi-cylindrical) shape, or a substantially triangular shape.

[0052] The device may be configured such that when in a locked state (for each possible index position), the indexing features on the first member and the engagement features on the second member provide a kinematic mount / location / connection / coupling between the first and second members. As will be understood, a kinematic mount is one having elements on one part positioned to cooperate with elements on another part to provide highly repeatable positioning. The elements are positioned to cooperate with one another to constrain relative movement between the parts in all six degrees of freedom (i.e., three perpendicular linear degrees of freedom and three perpendicular rotational degrees of freedom), preferably by six points of contact or constraint. In a specific embodiment, features on one of the members may be positioned to provide pairs of converging surfaces at each of three spaced locations in a manner that provides all six points of rigid contact with features on the other member. This constrains the six possible degrees of freedom of one part relative to the other. Such kinematic mountings are sometimes known as Boys supports and are described, for example, in Non-Patent Document 1. Further details of example configurations for providing such kinematic mountings / locations / connections are provided below.

[0053] The metrology device may comprise a third member, and may be provided with a second indexing arrangement configured to provide a plurality of angularly indexed positions of i) the third member and ii) either the first member or the second member (depending on which it is mounted) about a second axis (hereinafter referred to as the "second pair of relatively reorientable members"). Features described above in relation to the first member and second member are equally applicable to the second pair of relatively reorientable members. Thus, for example, there may be provided: a) a series of features on one of the second pair of relatively reorientable members, the series extending annularly about the second axis; and b) engagement features on the other of the second pair of relatively reorientable members at at least three discrete annularly spaced locations, the engagement features configured to interlock with a subset of the series of features when in a locked state, thereby providing a stable, repeatable relative rest position of the second pair of relatively reorientable members at each index position. Further, a first non-contact sensor may be provided and configured to provide a measurement of the relative spatial configuration of the second pair of relatively reorientable members. Preferably, the first non-contact sensor is mounted on the member on which the engagement features are provided and is configured to sense an area on the other member (on which the series of features are provided) and thereby provide a signal dependent on the spatial configuration of the second pair of relatively reorientable members when in their locked state.

[0054] As will be understood, references herein to a "subset" mean a "suitable subset" in the mathematical sense (in the sense that each engagement feature on the second member cannot and does not engage with all of the features in the series of features on the first member). [Brief explanation of the drawings]

[0055] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Figure 1]FIG. 1 is a diagram of an indexing head according to the present invention mounted on a coordinate measuring machine (CMM). [Figure 2] FIG. 2 is a cutaway view of the indexing head of FIG. 1. [Figure 3] FIG. 2 is a cross-sectional view of the indexing head of FIG. 1 in its locked configuration. [Figure 4] FIG. 2 is a cross-sectional view of the indexing head of FIG. 1 in its unlocked configuration. [Figure 5a] FIG. 2 is a diagram of an indexing configuration of the indexing head of FIG. 1. [Figure 5b] FIG. 5b shows the indexing arrangement of FIG. 5a with one part broken away. [Figure 6] FIG. 5b is a detailed view of the indexing arrangement shown in FIG. 5a. [Figure 7a] FIG. 5c is a view of a single tooth of the indexing arrangement part shown in FIG. 5b. [Figure 7b] FIG. 5c is a view of a single tooth of the indexing arrangement part shown in FIG. 5b. [Figure 7c] FIG. 5c is a view of a single tooth of the indexing arrangement part shown in FIG. 5b. [Figure 8] FIG. 2 is an exploded view of the different parts of the indexing mechanism and unlocking mechanism of the indexing head of FIG. 1. [Figure 9] FIG. 2 is a cutaway view of the different parts of the indexing mechanism and unlocking mechanism of the indexing head of FIG. 1; [Figure 10a] FIG. 2 is a cutaway view of the different parts of the indexing mechanism and unlocking mechanism of the indexing head of FIG. 1; [Figure 10b] FIG. 2 shows the underside of one of the articulating components of the indexing arrangement of the indexing head of FIG. 1. [Figure 11a] 2A-2C are schematic cross-sectional views of different parts of the indexing and unlocking mechanisms of the indexing head of FIG. 1 at different stages during the unlocking and locking operation; [Figure 11b] 2A-2C are schematic cross-sectional views of different parts of the indexing and unlocking mechanisms of the indexing head of FIG. 1 at different stages during the unlocking and locking operation; [Figure 11c]2A-2C are schematic cross-sectional views of different parts of the indexing and unlocking mechanisms of the indexing head of FIG. 1 at different stages during the unlocking and locking operation; [Figure 11d] 2A-2C are schematic cross-sectional views of different parts of the indexing and unlocking mechanisms of the indexing head of FIG. 1 at different stages during the unlocking and locking operation; [Figure 12] 10A-10C are schematic cross-sectional views of different parts of an indexing mechanism and an unlocking mechanism according to an alternative embodiment, specifically with a different magnet configuration; [Figure 13] 10A-10C are schematic cross-sectional views of different parts of an indexing mechanism and an unlocking mechanism according to an alternative embodiment, specifically with a different magnet configuration; [Figure 14] 10A-10C are schematic cross-sectional views of different parts of an indexing mechanism and an unlocking mechanism according to an alternative embodiment, specifically with a different magnet configuration; [Figure 15] 10A-10C are schematic cross-sectional views of different parts of an indexing mechanism and an unlocking mechanism according to an alternative embodiment, specifically with a different magnet configuration; [Figure 16] 12 is a graph showing support and retention forces for the three ring magnet embodiments of FIGS. 3, 4, and 11. FIG. [Figure 17] 13 is a graph showing the post force and holding force for the two ring magnet embodiments of FIG. 12. [Figure 18] 10 is a graph showing post force and break torque for two disc magnet embodiments. [Figure 19] FIG. 2 is a first isometric view of the indexing configuration of the indexing head of FIG. 1 along with an associated non-contact sensor; [Figure 20] FIG. 2 is a second isometric view of the indexing configuration of the indexing head of FIG. 1 along with an associated non-contact sensor. [Figure 21] FIG. 10 is a plan view of the first member of the articulating head showing the relative positions of the non-contact sensor and the teeth of the indexing arrangement; [Figure 22a] 1A and 1B are side cross-sectional views of two different non-contact optical sensors for determining the spatial configuration of a first member 102 and a second member 104. [Figure 22b] 1A and 1B are side cross-sectional views of two different non-contact optical sensors for determining the spatial configuration of a first member 102 and a second member 104. [Figure 23a] 1 is a cross-sectional side view of a non-contact inductive sensor for determining the spatial configuration of a first member 102 and a second member 104. FIG. [Figure 23b] 1 is a cross-sectional side view of a non-contact inductive sensor for determining the spatial configuration of a first member 102 and a second member 104. FIG. [Figure 24] FIG. 10 is a plan view of a portion of a first member of the articulating head showing engagement features of an indexing arrangement comprising a plurality of teeth; [Figure 25] FIG. 1 is a cross-sectional view of the articulating head 100 taken in the ZX plane with the first axis "D" and the second axis "E" both in an unlocked configuration. DETAILED DESCRIPTION OF THE INVENTION

[0056] Referring to FIG. 1, an articulating head 100 according to the present invention is shown mounted on a positioning device 200 .

[0057] The positioning apparatus 200 comprises a moving structure, in this case in the form of a coordinate measuring machine ("CMM"). The CMM 200 comprises a base 202 supporting a frame 204, which further carries a carriage 206, which in turn carries a quill 208 (or "Z-column"). Motors (not shown) are provided for moving the quill 208 along three mutually orthogonal axes X, Y, and Z (e.g., by moving the frame along the Y-axis, moving the carriage 206 along the X-axis, and moving the quill 208 along the Z-axis).

[0058] The quill 208 holds the articulating head 100, which in turn holds the probe 300. In this embodiment, the articulating head 100 facilitates repositioning of the probe 300 mounted thereto about a first axis of rotation D and a second axis of rotation E, as will be described in more detail below.

[0059] The combination of the two rotational axes (D, E) provided by the articulating head 100 and the three linear (X, Y, Z) axes of translation of the CMM 200 allows the probe 300 to be moved / positioned in five degrees of freedom (two rotational and three linear).

[0060] Although not shown, measurement encoders may be provided for measuring the relative positions of the base 202, frame 204, carrier 206, quill 208, and parts of the articulating head 100 so that the position of the measurement probe 300 relative to a workpiece positioned on the base 202 can be determined.

[0061] A controller 220 is provided for controlling the operation of CMM 200, such as controlling the position and orientation of probe 300 within the space of the CMM (either manually, such as via an input device such as joystick 216, or automatically, such as under the control of an inspection program), and for receiving information (e.g., measurement information) from CMM 200. A display device 218 may be provided to assist in user interaction with controller 220. Controller 220 may, for example, be a dedicated electronic control system and / or may comprise a personal computer.

[0062] In the embodiment shown, the probe 300 is a contact probe comprising a probe body 302 and a stylus 304. The stylus 304 has a spherical tip 306 for contacting the articulation workpiece being inspected, and in this embodiment the stylus 304 is deflectable relative to the probe body 302. The contact probe 300 may be what is commonly referred to as a touch-trigger probe, or may be a scanning (or analogue) probe. It will be appreciated that other types of probes may be mounted to the articulating head 100, including non-contact probes.

[0063] In the current embodiment, the articulating head 100 includes a probe mount 108 for facilitating the exchange of different probes therein. Specifically, this may be a mount that facilitates automatic exchange of probes with racks in the working space of the CMM. For example, the probe mount 108 and the probe body 302 may include magnets for gripping the probe on the mount.

[0064] It is possible for the articulating head 100 to have built-in sensor components for detecting deflection of the stylus 304 of a contact probe mounted thereon. However, in this embodiment all such sensor components are provided within the body 302 of the probe 300 itself. The probe 300 is configured to send stylus deflection signals to the controller 220. Typically this can be done by a contact signal interface between the probe 300 and the probe mount 108, so that such signals are relayed to the controller 220 via cables in the articulating head 100 and CMM 200. Such an interface may also be used to supply power to the probe 300. It will therefore be appreciated that the articulation head 100 will have its own signal interface with the quill 208 (e.g., one or more corresponding electrical contacts on the articulation head and quill) which may be used not only to relay probe signals but also to receive power and motor control commands to control the articulation head 100 (e.g., to control the operation of the powered locking motors 190, 190' and drive motors 192, 192' described in more detail below). It will therefore be appreciated that, although not shown for simplicity of illustration, in the described embodiment the CMM 200 and articulation head 100 will comprise electrical wires to relay signals from the articulation head 100 (e.g., probe signals, position information, error messages, etc.) and / or to provide power and commands / signals to the articulation head 100 (e.g., to control the motors of the articulation head to control the rotational position of the relatively rotatable members). It will be appreciated that in other embodiments the articulation head may comprise one or more of its own power sources (e.g., one or more batteries) to power the articulation head. It will also be appreciated that in other embodiments, the articulating head may communicate with the controller 220 wirelessly.

[0065] 2 to 24, the articulating head 100 will now be described in more detail.

[0066] As shown in Figure 2, the articulating head 100 comprises a first member 102 or "mounting plate", a second member 104 that is articulatable / rotatable relative to the first member 102 about a first axis of rotation "D", and a third member in the form of a probe arm 106 that, in this embodiment, is articulatable / rotatable relative to the second member 104 about a second axis of rotation "E". The second axis of rotation "E" is orthogonal to the first axis of rotation "D". In the embodiment being described, the first axis of rotation "D" is aligned parallel to the Z axis of the CMM, although this need not be the case.

[0067] The first member / mounting plate 102 includes holes 103 through which bolts can pass to fasten the articulating head 100 to the quill 208 of the CMM 200. The probe arm 106 includes a probe mount 108 to which a probe (such as contact probe 300) can be replaceably mounted.

[0068] In alternative embodiments, the probe arm 106, along with its probe mount 108, may itself be a replaceable member. For example, rather than being part of the articulating head 100, the probe arm 106 may be provided as part of the probe such that it can be replaced (e.g., automatically) along with the probe. In this case, a third member of the articulating head 100 may comprise a mounting member 106' for the probe / probe arm 106, the mounting member 106' being articulatable / rotatable relative to the second member 104 about a second axis of rotation "E." The mounting member 106' and the probe arm 106 may be provided with cooperating mounting features such that the probe arm 106 can be removably mounted to the mounting member 106'. Such cooperating mounting features may comprise, for example, features 150 (see FIG. 25 ) that define a kinematic mount. One or more magnets may be provided for gripping the probe arm 106 on the mounting member 106'.

[0069] Figures 3 and 4 show cross-sectional views of the articulating head 100 taken in the ZY plane, and Figure 25 shows a cross-sectional view of the articulating head 100 taken in the ZX plane. Figures 3 and 4 are substantially identical and represent a common view of the same articulating head, except that in Figure 3 the articulating head 100 is shown with the first member / mounting plate 102 and second member 104 in their locked position, and in Figure 4 the articulating head 100 is shown with the first member / mounting plate 102 and second member 104 in their unlocked position. Many numerals have been omitted from Figure 4 to aid in the visualization of the various features of the articulating head 100.

[0070] The locking / unlocking mechanisms, rotation mechanisms, and indexing arrangements of the first axis "D" and second axis "E" (i.e., of the first member / mounting plate 102 and second member 104) will now be described. In this embodiment, the locking / unlocking mechanisms and indexing arrangements of the second axis "E" (i.e., of the second member 104 and third member 106 / 106') are substantially the same (but are disposed perpendicular to that of the first axis "D"). Thus, in this embodiment, the locking / unlocking mechanisms, rotation mechanisms, and indexing arrangements of the first axis "D" and second axis "E" have substantially identical components. For brevity and clarity, the following description will focus primarily on the first shaft "D", but it will be understood that much of the description of the first shaft "D" is also applicable to the second shaft "E", and parts of the second shaft "E" that are the same as those of the first shaft "D" are marked in the drawings with the same reference numerals as the first shaft "D", but suffixed with a prime symbol. In this embodiment, the locking / unlocking of the first shaft "D" and the locking / unlocking of the second shaft "E" can be controlled / actuated independently of each other. In other words, the articulation head can be operated to unlock the first shaft "D" but keep the second shaft "E" locked, or vice versa. Of course, it is still possible to control the articulation head to unlock the first shaft "D" and the second shaft "E" together / simultaneously, if desired. Furthermore, it will be appreciated that other configurations are possible, including those in which the locking / unlocking of the first and second shafts cannot be independently controlled / actuated, for example as described in U.S. Patent No. 5,929,999 or U.S. Patent No. 5,929,999.

[0071] The indexing configuration of the first axis "D" comprises an arrangement of interengageable features on the first member / mounting plate 102 and the second member 104. Specifically, a first annular member 110 is provided having a continuous series of tapered teeth 112 (see, e.g., FIGS. 5a and 6 for detailed views). The teeth 112 extend substantially radially, in that the extension of the teeth extends primarily along the radial direction (with respect to the radius of the first annular member and also with respect to the first axis "D"). Thus, in this embodiment, the first annular member 110 is in the form of a "facial spline member" and is substantially referred to as such (specifically, in the described embodiment, the facial spline member has the configuration of a Hirth coupling member). The teeth of the facial spline member 110 are radially elongated and have a generally tapered cross-sectional profile (taken perpendicular to their length). In this embodiment, each side 111 of tooth 112 is substantially flat / planar, although this need not be the case (e.g., it may be curved or crowned, such as crowned tooth 118 described below).

[0072] The indexing arrangement further includes a second annular member 114 having engagement features 117 configured to intermesh with the teeth 112 of the facial spline member 110. The second annular member 114 has engagement features 117 configured to engage with only a subset (i.e., a "proper subset" in the mathematical sense) of the continuous series of teeth provided on the facial spline member 110 (see FIGS. 5b and 6 for detailed views). Thus, instead of the second annular member 114 providing a continuous series of interengaging teeth, the second annular member 114 only includes engagement features 117 configured to intermesh with the teeth 112 of the facial spline member 110 at three discrete, equiangularly (120°) spaced locations 116. In this particular embodiment, the engagement feature 117 at each of the locations 116 comprises a single tooth 118, each of which is configured to fit snugly between and engage two adjacent teeth 112 of the facial spline member 110. Each tooth 118 is radially elongated and has a generally tapered profile (cut perpendicular to their length), thereby providing two curved engagement side surfaces 120 that are configured to engage with the side surfaces 111 of the teeth 112 on the facial spline member 110. Because the engagement side surfaces 120 are curved, the teeth 118 may be described as "crowned" teeth 118. As shown in Figures 7a-7c, the engaging side surfaces 120 of the crowned teeth 118 are curved not only along their length (in this embodiment, along the radial dimension, or along the X-axis, as shown in Figures 7a and 7c), but also along the contour of their cross section (taken perpendicular to their length / radial dimension, as shown in Figure 7b). This configuration (i.e., the crowned teeth 118 engaging with the flat / planar teeth 112 on the facial spline member 110) ensures that the engaging side surface 120 of each of the crowned teeth 118 exhibits a crest region 122. It is the crest region 122 that tends to engage with the side surfaces 111 of the teeth 112 on the facial spline member 110. It has been found that providing the crest region provides a more repeatable seating position between the first annular member / facial spline member 110 and the second annular member 114. This is because the provision of the apex region 112 means that for any given pair of teeth on the first annular member / facial spline member 110 and the second annular member 114, the teeth in the pair are significantly more likely to engage in the same area on their side surfaces 111, 120 each time they come together (compared to if the side surfaces 111, 120 of the teeth on both the first annular member / facial spline member 110 and the second annular member 114 were both substantially flat / planar), thereby helping to ensure that the first annular member / facial spline member 110 and the second annular member 114 will seat together in the same position each time they come together at a given angular orientation.

[0073] As will be appreciated, the teeth of the indexing arrangement can be provided by other types of features. For example, the teeth on the second member 104 can comprise a series of spherical members (e.g., "rings of balls") extending annularly about the rotation axis D, and the engagement features 117 can each comprise a pair of cylindrical members, such as those described in U.S. Patent No. 5,627,297 or U.S. Patent No. 5,627,297. However, the arrangement described in this embodiment has been found to provide superior kinematic coupling between the first annular member / facial spline member 110 and the second annular member 114 compared to other arrangements. This is particularly true when the indexing increments are small (e.g., less than 7.5°, and particularly less than 5°, e.g., closer to 2.5°). This is because the intermeshing features become smaller as the indexing increments become smaller. Not only can a ring of balls with a sufficiently small diameter be difficult to accurately manufacture and assemble, but the very small points of contact between the very small diameter balls with the corresponding cylindrical members can result in very high Hertzian contact pressures, overstressing them, which in turn can result in excessive wear and / or failure of the indexing arrangement.

[0074] For example, in the embodiment described herein, the first annular member / facial spline member 110 and the second annular member 114 have an outer diameter of 75 mm and are provided with teeth sized to provide 2.5° indexing increments, and the articulation head 100 is configured such that when in the locked position, the first annular member / facial spline member 110 and the second annular member 114 are held together by a force of approximately 120 N (Newtons). The radius of curvature R' of the truncated crowned tooth in a plane perpendicular to its length (e.g., in the ZY plane in Figure 7b) is 1.8 mm, and the radius of curvature R" of the truncated crowned tooth in a plane along its length (e.g., in the ZX plane in Figure 7c) is 23 mm. In contrast, if spherical balls were used instead of the crowned teeth, the balls would need to have a radius of curvature of less than 0.75 mm in order to fit between the teeth 112 of the first annular member / face spline member. Not only would such small balls be difficult to incorporate into the articulation head, but they would also present very small contact points, resulting in extremely high Hertzian contact pressures.

[0075] As will be appreciated, the same effect can be achieved by crowning the teeth 112 on the first annular member / face spline member 110 and providing flat flanks on the teeth 118 on the second annular member 114, but this may be more difficult to manufacture. Alternatively, the teeth 112, 118 on both the first annular member / face spline member 110 and the second annular member 114 may be crowned, but with increased difficulty in manufacture, the tooth dimensions would need to be adjusted (specifically, increased) to avoid undesirable Hertzian contact pressures.

[0076] In the previously described embodiment, the teeth 112 are provided on the annular member 110 which is subsequently attached to the body 105 / top surface 115 of the second member 104, and the teeth 118 are also provided on the annular member 114 which is subsequently attached to the first member 102. However, this does not necessarily have to be the case, and for example, the teeth 112 may be provided directly on the body 105 / top surface 115 of the second member 104 and / or the teeth 118 may be provided directly on the first member / mounting plate 102.

[0077] A mechanism for locking and unlocking the indexing configuration of the first axis "D" will now be described. In summary, in the specific embodiment being described, the locking / unlocking mechanism relies solely on magnets to provide a gripping force between the first annular member / facial spline member 110 and the second annular member 114 having crowned teeth 118, and a motor-driven actuator is used to push the first member / mounting plate 102 and the second member 104 away from each other to separate the first annular member / facial spline member 110 and the second annular member 114 having crowned teeth 118. This mechanism is described in more detail immediately below.

[0078] In the described embodiment, the locking / unlocking mechanism comprises three stacked magnets. Specifically, a first annular magnet 140 is mounted on the top surface 115 of the housing 105 of the second member 104, a second annular magnet 142 is mounted on the contact plate 134 (described in more detail below) of the support 130, and a third magnet 144 is mounted on the first member / mounting plate 102. The first, second, and third annular magnets 140, 142, and 144 are identical in shape and size and are stacked coaxially with one another, with both the first and third annular magnets 140, 144 positioned to attract the second magnet 142 sandwiched between them. The poles of the annular magnets are coaxially positioned (i.e., positioned so that two magnetic poles are at the top and bottom of the flat surfaces of the annulus). Specifically, the ring magnets are configured such that the north pole of the first magnet 140 faces the south pole of the second magnet 142, and such that the north pole of the second magnet 142 faces the south pole of the third magnet 144. As will be described in more detail below, when in the locked and unlocked positions, the second member 104 is held together solely by magnetic forces, specifically by the magnetic attractive forces between the third magnet 144, the second magnet 142, and the first magnet 140.

[0079] The locking / unlocking mechanism comprises a post 130 having a shaft 132 and a "head" or "contact plate" 134. The shaft 132 of the post 130 is supported in a linear cylindrical bearing housing 107 provided by a member on the top surface 115 of the housing 105 of the second member 104. Bearings (in this case an array of ball bearings 109) are provided between the shaft 132 and the cylindrical bearing housing 107 to facilitate relative linear and rotational movement between the shaft 132 and the cylindrical bearing housing 107 (i.e., along and about the first axis "D"). The contact plate 134 comprises a radially extending surface that is sandwiched between the body of the first member / mounting plate 102 and the body of the second member 104.

[0080] A motor-driven lever 170 is provided for effecting linear / axial movement of the shaft 132 along the first axis "D." The lever 170 is pivotally mounted at a first end to a flexure 178 that is anchored to the second member housing 105 (in this embodiment, to the top plate 115) via a mounting block 179. The lever 170 is attached at a second end to a lead screw mechanism 172 configured to raise and lower the second end of the lever 170. The lever is attached at a point between its first and second ends to the end of the shaft 132 distal to the contact plate 134 via a bobbin 146 (which facilitates relative rotation between the shaft 132 and the lever 170). A powered locking motor 190 is configured to drive the lead screw mechanism 172. Specifically, locking motor 190 is configured to drive first gear wheel 171, which is configured to engage and turn second drive gear wheel 173, which turns lead screw 174. As lead screw 174 is turned, it causes nut 176 (attached to lever 170 via pin 175) to advance axially along lead screw 174. Lead screw 174 is capable of rotating about its axis of rotation, but is also captive to second member housing 105 (in this embodiment, to cylindrical bearing housing 107) via mounting bracket 177 and bearing 179 such that lead screw 174 is fixed relative to second member housing 105 in the Z dimension (as shown in FIGS. 3 and 4 ).

[0081] This can be advantageous if the drive mechanism for the support 130 resists backdriving (in other words, is not easily manually backdriven), especially if the three-magnet design described below is not used. This is because, if the net external force on the support 130 is small enough, the drive mechanism, which is not easily manually backdriven, will attempt to maintain its position even when the motor / power supply is not activated. This can avoid the need to servo the drive mechanism / motor to maintain a fixed position, thereby reducing the power consumption of the articulating head. This can therefore reduce the amount of heat generated by the drive mechanism / motor, which can further improve the metrology performance of the articulating head by reducing thermal distortion. A lead screw mechanism with a large gear pitch is an example of a drive mechanism that is not easily backdriven.

[0082] As will be explained in more detail below, shaft 132 is provided with a powered drive motor 192 having a gearing arrangement (not shown) configured to engage and drive a drive gear 148 provided towards its end distal to contact plate 134, and operable to rotate / turn housing 105 of first member 104 (and anything connected thereto) about shaft 132 about first axis "D". A first (or "primary") rotary encoder device 135 (e.g., a magnetic, absolute rotary encoder device) is provided for measuring / monitoring the relative angular position of housing 105 of first member 104 and shaft 132 about first axis "D".

[0083] The support contact plate 134 and the first member / mounting plate 102 have corresponding engagement elements. Specifically, the corresponding engagement elements comprise features configured to provide a repeatable, specifically kinematic, coupling between the support contact plate 134 and the first member / mounting plate 102 when coupled. In the described embodiment, the support contact plate 134 includes three engagement balls 152 positioned 120° apart from each other, and the first member / mounting plate 102 has three pairs of engagement balls 154, with the pairs positioned 120° apart from each other (see FIG. 10b). Each pair of engagement balls 154 on the first member / mounting plate 102 defines a passage or groove for receiving one of the engagement balls 152 positioned on the contact plate 134.

[0084] As also shown in FIGS. 3 and 4 , a second rotary encoder device is provided, comprising an annular scale 162 on the top surface 115 of the housing 105 of the second member 104 and a first readhead 160 and a second readhead 161 (not shown in FIGS. 4 and 5 ) on the underside of the first member / mounting plate 102. In the described embodiment, the first readhead 160 and the second readhead 161 are annularly spaced 120° apart from each other. In the described embodiment, the second rotary encoder device is an incremental optical rotary encoder device. In the specific described embodiment, the second encoder device is a high-resolution encoder capable of establishing the relative position of the first member / mounting plate 102 and the body 105 of the second member 104 to within 10 nm (nanometers). Its purpose is described in more detail later in this document.

[0085] The unlocking / reorienting / locking process for the first member / mounting plate 102 and the second member 104 will now be described. FIG. 3 shows the first member / mounting plate 102 and the second member 104 in a locked state. In the locked state, the probe 300 mounted on the probe mount 108 can be held in a constant, well-defined angular position so that it can be used in a measurement operation to inspect for artifacts. However, it may be desirable to reorient the probe mounted on the probe mount 108, for example, for access reasons. To do so, it is necessary to unlock the first member / mounting plate 102 and the second member 104, reorient them relative to each other, and then lock them together in the new orientation.

[0086] Unlocking the first member / mounting plate 102 and the second member 104 involves driving the post 130 axially along the first axis "D" toward the first member / mounting plate 102. In the described embodiment, this is facilitated by the controller 220 sending a command to the articulation head 100 to operate the locking motor 190 to drive the lead screw 174 so as to drive the lead screw nut 176 upward in the Z dimension (in the orientation shown in FIGS. 3 and 4). In addition, the lever 170 (coupled to, and therefore actuated by, the lead screw nut 176) pushes the post shaft 132 upward in the Z dimension (in the orientation shown in FIGS. 3 and 4). After a short distance, the engagement balls 152 on the contact plate 134 of the support 130 contact and engage with the pair of engagement balls 154 on the first member / mounting plate 102, and then subsequent actuation of the lead screw 174 causes the lever 170 and lead screw mechanism 172 to push the housing 105 axially downward (via the cylindrical bearing housing 107 in which the lead screw 174 is anchored), thereby separating the housing 105 of the second member 104 and the first member / mounting plate 102. The lead screw 174 is operated to separate the second member 104 and the first member / mounting plate 102 by a controlled, predetermined amount sufficient to clear the teeth 112 from the crown teeth 118, but not too much, as will be explained in more detail below, since it is desired that the first magnet 140 remain close enough to the second magnet 142 to have a reasonable amount of pull on the second magnet 142, even in the unlocked state. FIG. 4 shows the index head 100 in such an unlocked state. In this embodiment, the locking motor 190 of the first axis "D" can be operated independently of the powered locking motor 190' of the second axis "E" so that the first axis "D" is unlocked while the second axis "E" can remain locked.

[0087] Once the unlocked state is reached, the locking motor 190 which drives the lead screw mechanism 172 is stopped and the drive motor 192 of the first axis "D", which is engaged with the drive gear 148 of the shaft 132, is operated to effect a change in the rotational position of the second member 105 of the articulating head 100. As previously mentioned, in the unlocked state the post 130 is engaged with the first member / mounting plate 102 via the engagement balls 152, 154 and is therefore rotatably fixed (in the unlocked state) relative thereto. Thus, when the drive motor 192 engaged with the drive gear 148 of the shaft 132 is operated, it drives the entire housing 105, 107, 115 of the second member 104 (and all components connected thereto, including the aforementioned motor) about the shaft 132, which in turn causes the entire housing 105, 107, 115 of the second member 104 (and all components connected thereto) to rotate about the first axis "D".

[0088] The relative rotational position of i) the housings 105, 107, 115 of the second member 104 and ii) the shaft 132 (and by extension the first member / mounting plate 102) is known from the first (“primary”) encoder device 135. The controller 220 can therefore use the output from the first encoder device 135 to control a motor (not shown) engaged with the shaft's drive gear 148, thereby bringing the first member / mounting plate 102 and second member 104 to the desired relative orientation. As will be appreciated, the rotational position needs to be controlled with sufficient precision so that, when in the new desired relative orientation, the crown tooth 118 on the second annular member 114 will be located opposite the valley between the teeth 112 on the first annular member / facial spline member 110, such that the crown tooth 118 fits neatly and snugly between the two teeth 112 of the first annular member / facial spline member 110 when they are locked together.

[0089] The process of locking the first member / mounting plate 102 and the second member 104 will now be described. In the described embodiment, this is effected by operating the locking motor 190 to drive the lead screw 174 so as to drive the lead screw nut 176 downward (in the orientation shown in FIGS. 3 and 4 ). This will pull the housing 105 of the second member 104 up toward the first member / mounting plate 102 until the crowned teeth 118 on the second annular member 114 engage the teeth 112 of the facial spline member 110, after which continued operation of the locking motor 190 retracts the posts 130 away from the first member / mounting plate 102, thereby disengaging the engagement balls 152, 154 on the contact plate 134 and the first member / mounting plate 102. Thus, at the time of disengagement of the engagement balls 152, 154, the first member / mounting plate 102 and the second member 104 are held together via the kinematic constraints provided by the six points of rigid contact between the three crowned teeth 118 and the teeth 112 of the first annular member / facial spline member 110.

[0090] The manner in which the first magnet 140, second magnet 142, and third magnet 144 interact with one another is described with reference to Figures 11a-11d, which schematically show the support shaft 132 and contact plate 134, the second member top plate 115, the first member / mounting plate 102, the second annular member 114 (as three crowned teeth 118), the first annular member / facial spline member 110 (having a continuous series of teeth 112), the first annular magnet 140, the second annular magnet 142, and the third annular magnet 144. Figure 11a shows the first member / mounting plate 102 and second member 104 in a locked position, i.e., where the teeth 112 on the first annular member / facial spline member 110 and the teeth 118 on the second annular member 114 are fully engaged. Figure 11b shows the first member / mounting plate 102 and second member 104 in a locked position but at a point when the posts 130 have been actuated to the point where the engagement balls 152 on the contact plate 134 have engaged the engagement balls 154 on the first member / mounting plate 102 and are beginning to separate the teeth 112 on the first annular member / facial spline member 110 and the teeth 118 on the second annular member 114. Figure 11c shows the first member / mounting plate 102 and second member 104 as they begin to separate but have not yet reached their fully unlocked configuration. FIG. 11d shows the first member / mounting plate 102 and the second member 104 in an unlocked position where the teeth 112 on the first annular member / facial spline member 110 and the teeth 118 on the second annular member 114 are completely clear of each other so that the first member / mounting plate 102 and the housing 105 of the second member 104 are free to rotate relative to each other about the first axis "D".

[0091] In the configuration shown in FIG. 11 a, the third magnet 144 is attracted to both the second magnet 142 and the third magnet 144, thereby pulling the first member / mounting plate 102 toward the support post 130 and the housing 105 of the second member 104. In the described embodiment, the device is configured such that when in the locked position (with appropriate head dimensions, specifically the indexing configuration and the diameter and location of the annular magnet provide a break torque of 2 Nm), there is a total locking force of approximately 120 N between the first member / mounting plate 102 and the second member 104. As will be appreciated, break torque is the moment that can be exerted before the first and second bodies begin to peel away from each other. This can be important because articulating heads are often eccentrically mounted. As will also be appreciated, break torque depends on factors other than gripping / holding / locking force, such as the diameter of the facial spline member 110 or the diameter of the engaging balls 152, 154.

[0092] To transition to the unlocked state, the post 130 needs to be moved toward the first member / mounting plate 102. While the presence of the first magnet 140 will likely increase, at least initially, the work required of the locking motor 190 to do so (compared to its absence), it should be noted that in the locked state shown in FIG. 11 a, the device is configured such that the post's contact plate 134 is held in a predetermined position that places the second magnet 142 halfway between the first magnet 140 and the third magnet 144. This ensures that the pull of the first magnet 140 on the second magnet 142 is significantly less than if they were in contact with each other. Also, the third magnet 144 has some magnetic pull on the second magnet 142. Therefore, the work / power required to move the second magnet (and thus the contact plate 134) away from the first magnet 140 is significantly less than if the first magnet 140 and the second magnet 142 were in contact.

[0093] Specifically, in the described and illustrated embodiment, the contact plate 134 of the support post is held in a predetermined position that places the second magnet 142 slightly closer to the first magnet 140 than the third magnet 144, but approximately halfway between the first magnet 140 and the third magnet 144. This means that the magnetic forces on the second magnet exerted by the first magnet 140 and the third magnet 144 are nearly (though not perfectly) balanced. Thus, very little work / power is required for the locking motor 190 to move the support post 130 toward the first member / mounting plate 102. In fact, when the second magnet 142 reaches a point halfway between the first magnet 140 and the third magnet 144, the magnetic pull of the third magnet 144 on the second magnet 142 becomes greater than that of the first magnet 140. As the contact plate 134 advances toward the first member / mounting plate 102, the magnetic attraction of the third magnet 144 to the second magnet 142 gradually increases.

[0094] Therefore, when the support post 130 is moved to the configuration shown in Figure 11b, it is necessary for the locking motor 190 to pull the teeth 118 on the body 105 / 115 of the second member 104 from the teeth 112 on the first member / mounting plate 102. At this point, there is a large enough holding / gripping force (at least 160 N) to hold the second member 104 on the first member / mounting plate 102 (via the engagement balls 152, 154), but the locking motor 190 needs to exert less than the holding / gripping force because it now only needs to exert enough force (which in this embodiment is about 95 N) to overcome the attractive force between the pull of the first magnet 140 to the second magnet 142 and the pull of the first magnet 140 to the third magnet 144.

[0095] The locking motor 190 continues to drive the post 130 until the housing 105 of the second member 104 moves away from the first member / mounting plate 102 enough that the teeth 112 of the first annular member 110 clear the teeth 118 on the second annular member 114, as shown in FIG. 11 d. At this point, the gripping force holding the post 130 and the housing 105 of the second member 104 to the first member / mounting plate 102 is approximately 160 N. A greater gripping force is achieved in the configuration shown in FIG. 11 d compared to FIG. 11 a by controlling the gap between the first magnet 140, second magnet 142, and third magnet 144. Specifically, in the unlocked state of FIG. 11d, there is a relatively small gap between second magnet 142 and third magnet 144 (compared to the gap between first magnet 140 and second magnet 142 when in the locked state of FIG. 11a) so as to achieve a greater overall gripping force despite the relatively large gap between first magnet 140 and second magnet 142. A greater overall gripping force is desirable in the unlocked position because the diameter S of the rings of engagement balls 152, 154 is smaller than the diameter S' of first annular member 110 and second annular member 114, which means that they require a greater pulling / gripping force to ensure the same or similar break torque of approximately 2 Nm (Newton-meters).

[0096] When in the unlocked state shown in FIG. 11d, the first member 102 and the second member 104 can be relatively rotated about the D axis to a new relative rotational position / orientation. As described above, this involves driving a drive motor 192 (not shown) that engages the shaft drive gear 148 to rotate the body 105 of the second member 104 about the shaft 132. The output of the first encoder device 135 is used to measure / monitor the relative position of the body 105 of the second member 104 and the shaft 132 (and thus the first member / contact plate 102, whose rotational orientation about D is fixed). When the output of the first encoder device 135 indicates that the body 105 of the second member 104 is now at the desired index position, the drive motor 192 is stopped and the first member 102 and second member 104 are locked together as described below.

[0097] To lock the first member 102 and second member 104 in their new rotational position / orientation, the locking motor 190 is operated to drive the lead screw mechanism 172 to drive the lead screw nut 176 down on the lead screw 174. This initially pulls the housing 105 of the second member 104 up towards the first member / mounting plate 102. As can be seen, very little power is required for the locking motor 190 because the housing 105 of the second member 104 is already being pulled towards the first member / mounting plate 102 by the first magnet 140, the second magnet 142, and the third magnet 144. This continues until the teeth 112 of the first annular member 110 engage the teeth 118 of the second annular member 114 (shown in FIG. 11b), at which point the locking motor 190 and lead screw mechanism 172 must begin to push against the magnetic force to separate the support contact plate 134 from the first member / mounting plate 102. However, at this point the first magnet 140 is much closer to the second magnet 142 and therefore exerts a relatively large force on it. In fact, at this point the net force on the second magnet 142 in the state shown in FIG. 11b is only 95 N. Thus, the locking motor 190 and lead screw mechanism 172, assisted by the magnets, can much more easily pull the second magnet 142 (and thus the support 130) away from the third magnet 144 (and thus the first member / mounting plate 102) until the contact plate 134 reaches the predetermined axis / Z position shown in FIG. 11a.

[0098] The output of the first rotary encoder 135 indicates what rotational index positions the first member / mounting plate 102 and second member 104 are in. This can also be useful for verifying that the first member / mounting plate 102 and second member 104 are properly locked together. This can be achieved in a variety of ways, such as by using one or more sensors that can verify separation between opposing surfaces of the first member / mounting plate 102 and second body 105, and if the separation is greater than a fixed threshold amount (which is the same for all index positions), corrective action can be taken (e.g., an error / alarm can be reported and / or action can be taken to try to rectify the problem, such as by attempting an unlock / relock operation from a different position / orientation, and / or by attempting to recalibrate the request).

[0099] In the described embodiment, the second rotary encoder device is configured to measure and provide information about the relative spatial configuration of the first body and the second body when locked. Specifically, the outputs of the first readhead 160 and the second readhead 161 of the second rotary encoder device are used to ensure that the first member / mounting plate 102 and the second member 104 are properly locked together. Specifically, when locked, the outputs of the first readhead 160 and the second readhead 161 are passed to electronics 400 within the readhead, which may include, for example, a processing device 402 (e.g., a CPU (Central Processing Unit), FPGA (Field Programmable Gate Array), or ASIC (Application Specific Integrated Circuit)) and a memory device 404. The processing device 402 compares the values received from the first readhead 160 and the second readhead 161 with values stored in a lookup table in the memory device 404. Specifically, the processing device 402 compares the outputs of the first readhead 160 and the second readhead 161 to determine whether they are substantially the same as the values stored in the lookup table elements associated with that particular index position. If the output of either or both of the first readhead 160 or the second readhead 161 is substantially different from the values stored in the lookup table (e.g., by more than 100 nm), this may be an indication that something is wrong, specifically that the teeth 112 / 118 of the first member / mounting plate 102 and the second body 105 are not properly locking together / interdigitating and therefore are not seated in substantially the same position as when the value stored in the lookup table associated with that particular index position was obtained. This may be because, for example, the teeth 112 / 118 have collided, there is debris between the teeth 112 / 118, there is excessive wear between the teeth 112 / 118, etc. Thus, a device (eg, a controller) can take corrective action in such situations.Such corrective action may include re-unlocking and re-locking the first member / mounting plate 102 and the second body 105, outputting a warning signal to an operator and / or other process, halting the current operation, etc.

[0100] As described above, the second rotary encoder device is an incremental encoder device. As such, the outputs of the first readhead 160 and the second readhead 161 do not contain any absolute position information. Therefore, rather than comparing absolute position information, the processing unit 402 compares relative (position) data / information. Specifically, for example, as will be understood by those skilled in the art of position measurement encoders, the scale of an incremental position encoder typically comprises an array of regularly spaced features arranged at specific intervals or “periods” (20 μm in the described embodiment, although it will be understood that scales of other periods may be used). A readhead can read the features (e.g., optically, magnetically, inductively, depending on the technology used), and the readhead or its output is typically used to “count” the relative position of the readhead and the scale as they move relative to one another. Signals received by and / or output by the readhead can be interpolated to provide a measurement of the relative position of the readhead and the scale to a resolution much finer than the actual period of the scale. Such interpolated readings are often referred to as "phase" readings. For example, typically quadrature (e.g., sine and cosine) signals are generated from the scale signals and / or output by the readhead. Such quadrature (e.g., sine and cosine) signals may be interpolated to provide such "phase" readings. In the described embodiment, it is the interpolated or "phase" readings that are used by the processing unit 402 and compared to pre-stored "phase" readings stored in lookup table elements associated with specific values.

[0101] Thus, it is not necessary for the first readhead 160 or the second readhead 161 to read the scale 162 as the first member 104 / mounting plate 102 and second body 105 move relative to one another as the index position is changed (although this can be done if the configuration allows). Rather, when the locking operation is completed, single readings can be taken and output by the first readhead 160 and the second readhead 161, and the interpolated or "phase" values of those readings can be compared to pre-stored "phase" readings stored in lookup table elements associated with specific values. If either or both of the phase readings differ by more than a predetermined amount (e.g., 100 nm as in the example above), corrective action can be taken as described above.

[0102] The data elements in the lookup table can therefore be said to be a "phase signature" for each of the calibration index positions, and if the values of the phase readings of the first readhead 160 and the second readhead (not shown) differ significantly from the phase signature in the lookup table for a given index position, corrective action can be taken.

[0103] The look-up table is populated before the articulating head 100 is used for a measurement operation (for example, it may be populated during a calibration procedure). This may involve locking the first member 104 / mounting plate 102 and the second body 105 relative to each other at a given index position, and recording / saving the phase readings of the first readhead 160 and the second readhead (not shown) in elements / data cells associated with the given index position. This is then reproduced for each of the index positions of the articulating head (or at least for the index positions where the head is used and where such verification is desired).

[0104] Optionally, the lookup table may be updated over time to allow for small degrees of drift over time. This may be done continuously or at regular intervals. This may be done as part of a dedicated calibration process or may be done between measurement operations. For example, each time the first member / mounting plate 102 and second body 105 successfully lock together at any given index position (e.g., each time they pass the 100 nm test described above), the phase readings output by the first readhead 160 and second readhead (not shown) may be saved in the lookup table in place of the previous values.

[0105] As will be appreciated, if desired, the lookup table may be replaced with a function that represents the values in the lookup table, however, a lookup table may be preferred because of its ease of generation and because it is easy to keep up to date.

[0106] As will be appreciated, a single readhead may be used rather than two, or three or more readheads may be used. It is not necessary for the multiple readheads to be positioned 120° apart from each other around the scale 162. However, providing multiple readheads that are not diametrically opposed to each other (i.e., not at 180°) has been found to be particularly advantageous, as it can provide information about the spatial configuration of the first member / mounting plate 102 and second body 105 in multiple dimensions. While providing them at substantially / approximately 90° to each other about the first axis may be preferred for reasons of efficiency and optimal performance, for the reasons described above, locating them substantially on the same radial line as the engagement tooth 118 may also be preferred for different reasons. Thus, depending on the specific situation, when the engagement teeth are not at 90° to each other (such as the described embodiment where they are positioned 120° from each other), the benefit of positioning each of the read heads on the same radial line as their respective engagement teeth may outweigh the benefit of positioning the read heads at 90° to each other.

[0107] The second rotary encoder device described above is an incremental encoder, but it will be appreciated that it may alternatively be an absolute encoder device.

[0108] In the described embodiment, the first read head 160 (and optional second read head 161) may be referred to as a "verification" sensor, as it is used to confirm / verify that the first member / mounting plate 102 and the second body 105 are properly locked together.

[0109] In the previously described embodiment, an optical rotary encoder device is used to determine the spatial configuration of the first member 102 and the second member 104 when locked together. However, this need not be the case. Other types of non-contact sensors may be used, such as a position sensing device (PSD) mounted to the first member 102 whose output depends on the relative spatial position of the first member / mounting plate 102 and the second body 105 when they are locked together. In this case, a lookup table may be populated during a calibration phase to record the output of the PSD for each index position of interest (which may be, for example, all index positions or only those intended to be used during subsequent measurement operations). Subsequently, during use, when the first member / mounting plate 102 and the second body 105 are locked together at a particular index position, the PSD may provide an output to the processing unit 402 that is later compared with a value stored in a particular element of the lookup table stored in the memory device 404 that is associated with the particular index position. If the outputs of the PSD differ by more than a threshold amount, corrective action can be taken.

[0110] In an alternative embodiment, a (so-called "verification") non-contact sensor is configured to measure only the relative height / separation of the first and second bodies (e.g., via a capacitive or inductive sensor, as described in more detail below in connection with FIG. 23). Advantageously, when the first and second bodies are locked together, the output of the height sensor is compared to a pre-stored value in an element of a look-up table associated with the particular index position at which the first and second bodies are locked together.

[0111] As will be appreciated, further variations and alternative embodiments of the articulation joint described above are possible. For example, one or two of the first magnet 140, second magnet 142, and third magnet 144 may be replaced with a magnetically attractable (e.g., ferrous) material. This would be similar to providing three magnets, but would provide a weaker effect. Therefore, the remaining one or two magnets would need to remain stronger and therefore larger, which may also mean a higher maximum motor force is required (depending on the configuration).

[0112] In other similar embodiments, the first magnet 140 is positioned elsewhere. For example, the first magnet 140 may be positioned at / towards the end of the shaft 132 distal to the contact plate 134. Again, this provides a similar effect in terms of assisting the locking motor 190 during the locking / unlocking process, however, because the first magnet 140 is positioned farther away from the first member / mounting plate 102, it provides less, if any, gripping force, thereby necessitating the provision of a larger / stronger second magnet 142 and / or third magnet 144.

[0113] 12 schematically illustrates an alternative embodiment in which the first magnet 140 is omitted so that the second member 104 is magnetically gripped at the first member / mounting plate 102 by only one pair of magnets: a third magnet 144 on the first member / mounting plate 102 and a second magnet 142 on the contact plate 134 of the post 132. While this is possible, the second magnet 142 and the third magnet 144 would need to provide all of their own locking / gripping forces, so either or both of them would need to be much stronger than the three-magnet configuration previously described, and therefore would require the locking motor 190 to work much harder during the locking process when the post's contact plate 134 is pulled away from the first member / mounting plate 102 (i.e., in the transition from FIG. 11b to FIG. 11a). Also, without the first magnet 140, all of the force gripping the housing 105 of the second member 104 to the first member / mounting plate 102 would have to be transmitted through the support post 130, lever 170, and lead screw mechanism 172 and associated bearings. This would require those components to be larger / stronger, and would ideally require a motor that prevents backdriving.

[0114] 13 shows another alternative embodiment in which the third magnet 144 is omitted so that the second member 104 is magnetically gripped at the first member / mounting plate 102 by only a pair of magnets: a first magnet 140 on the housing top plate 115 and a second magnet 142 on the support post contact plate 134. In this case, the first member / mounting plate 102 (at least a portion thereof) must be made from a material that can be attracted by a magnet (e.g., a ferrous material). A disadvantage of this embodiment is that the holding / gripping and breaking torque is smaller than when the third magnet is present (so that if the same holding / gripping and breaking torque is desired, a larger / stronger first magnet 140 and / or second magnet 142 are required).

[0115] 14 shows another alternative embodiment in which the second magnet 144 is omitted so that the second member 104 is magnetically gripped at the first member / mounting plate 102 by only a pair of magnets: a first magnet 140 on the housing top plate 115 and a third magnet 144 on the first member / mounting plate 102. A disadvantage of this embodiment is that the holding / gripping and breaking torque is less than if the second magnet were present (so if the same holding / gripping and breaking torque is desired, a larger / stronger first magnet 140 and / or third magnet 144 would be required). In this embodiment, the contact plate 134 may comprise a material that can be attracted by a magnet (e.g., a ferrous material) to aid in magnetic holding, but this is not as good as a contact plate 134 comprising a magnet.

[0116] Figure 15 shows another alternative embodiment, illustrating that the magnets do not necessarily need to be directly stacked in line with one another. For example, Figure 15 illustrates alternative locations for the first magnet 140 and / or the third magnet 144 (e.g., they could be positioned further radially outward than the second magnet 142).

[0117] It is also possible for the magnets to be used in a configuration where they repel each other to provide the required locking / gripping force.

[0118] However, it has been found that the described configuration, having at least three stacked magnets, all arranged to attract each other, as in the embodiment of Figures 1-11, can be advantageous. Specifically, it has been found that this not only significantly reduces the work required of the locking motor 190, which controls the linear position of the strut 130 when in the locked state, but also helps reduce the maximum work required of the locking motor 190 during the locking process. This can not only reduce the required size of the locking motor 190 and help keep the articulation head compact and lightweight, but also reduce the thermal output of the locking motor 190 (which can further improve the metrology performance of the articulation head by reducing / avoiding thermal distortion). In fact, the magnets in the embodiment of Figures 1-11 can be configured to provide a pulling force of 120 N when locked and 160 N when unlocked (to provide a break torque of 2 Nm) because of the three stacked magnets, but the locking motor 190 only needs to generate a maximum force of 95 N.

[0119] Figure 16 is a graph showing the support force and holding force for the three magnet embodiment of Figures 3, 4, and 11, and Figure 17 is a graph showing the support force and holding force for the two magnet embodiment of Figure 12 (equivalent in all respects except that first magnet 140 is omitted). The holding force (also referred to above as the "gripping force" or "locking force") is the net force pulling first member 102 and second member 104 together. The support force is the net magnetic force experienced by / exerted on support 130. Therefore, this is the magnetic force that needs to be overcome to hold support 130 in its position. Such forces can be overcome by a combination of the force exerted on the strut by the locking motor 190 and any friction in the gearing / motor / strut system (it will be appreciated that if friction in the gearing / motor / strut system is eliminated, the strut force will be proportional to the work required of the locking motor 190, e.g., proportional to the motor current).

[0120] As shown in FIG. 16 , when the first member / mounting plate 102 and the second member 104 are in their locked state, the support force is very small (less than 10 N). Therefore, the force required to hold the support 130 in place is small. In fact, it is so small that, depending on the gearing / motor / support system, friction may be sufficient to hold the support 130 in place (e.g., if there is significant resistance to backdriving). Therefore, very little or no motor power is required to hold the support 130 in place. Furthermore, as previously described, the configurations of FIGS. 1 through 11 are arranged such that, in the locked position, the support 130 is positioned such that the magnetic force urging the second magnet 142 toward the first magnet 140 is greater than the magnetic force urging the second magnet 142 toward the third magnet 144. Thus, even if the locking motor 190 that controls the linear position of the support 130 is stopped, and even if friction is not sufficient to hold the support 130 in place against the magnetic bias, all that happens is that the support 130 will retract further until the contact plate 134 abuts the top surface 115 of the housing, which abutment has no detrimental effect on the engagement of the teeth 112, 118 of the first member / mounting plate 102 and second member 104.

[0121] This is in contrast to the pillar force experienced by the post 130 in the two-magnet embodiment of FIG. 12. As shown in FIG. 17, when in the locked state (as shown in FIG. 12), there is a significant net magnetic force (approximately 110 N) urging the second magnet 142 toward the third magnet 144. Thus, in the locked position, significant work / power is required from the locking motor 190 to hold the post 130 in place. In fact, the pillar force is so great that even the friction of the lead screw mechanism, which is highly resistant to backdriving, is not sufficient to overcome it; therefore, when the locking motor 190 is powered off, the post 130 will slowly creep toward the first member / mounting plate 102 until they touch, thereby interfering with the engagement of the teeth 112, 118 of the first member / mounting plate 102 and second member 104.

[0122] As can be seen from the graphs in Figures 16 and 17, there is some disadvantage to the three-magnet embodiment in that there is a significant support force when the posts 130 and first member / mounting plate 102 are engaged. Therefore, significant motor work / power is required to push against the support force to separate the first member / mounting plate 102 and second body 104 (e.g., between the state shown in Figure 11b and the state shown in Figure 11d) and to hold the first member / mounting plate 102 and second body 104 in their unlocked state (e.g., the state shown in Figure 11d). In contrast, the two-magnet embodiment of Figure 12 has zero support force, and therefore very little motor work / power is required to separate the first member / mounting plate 102 and second body 104 once the posts 130 and first member / mounting plate 102 are engaged.

[0123] However, under normal circumstances, the amount of time the articulation head spends in its unlocked state is significantly less than the amount of time it spends in its locked state, so the benefit of the three magnet embodiment requiring significantly less (or even no) motor power in the locked state outweighs the cost of having to work harder in the unlocked state.

[0124] 1-11 also has the benefit of requiring less maximum motor work / power from the locking motor 190 than the two-magnet embodiment. In the two-magnet embodiment, the maximum amount of work required by the locking motor 190 is when re-locking the first member / mounting plate 102 and second body 104; specifically, maximum motor work / power is required when the teeth 112, 118 of the first member / mounting plate 102 and second body 104 engage and the locking motor 190 attempts to separate the post contact plate 134 and first member / mounting plate 102. At this point, the locking motor 190 must itself overcome all of the attractive pull of the second magnet 142 and third magnet 144 (and overcome any friction in the gearing / motor / post system), thereby exerting a force in excess of 150 N. In contrast, in the three-magnet embodiment, when the teeth 112, 118 of the first member / mounting plate 102 and second body 104 are engaged and the locking motor 190 attempts to separate the post contact plate 134 and the first member / mounting plate 102 during the locking operation (i.e., at the time represented by FIG. 11b), the first magnet 140 and the second magnet 142 are brought closer together (compared to when fully unlocked as shown in FIG. 11d). Thus, the first magnet 140 is close enough to the second magnet 142 to exert a significant amount of pull on the second magnet 142, thereby assisting the locking motor 190 in separating the separate post contact plate 134 and the first member / mounting plate 102. This results in the locking motor 190 only needing to exert approximately 95 N to effect such separation (see point A in FIG. 16).

[0125] As will be appreciated, alternative means may be provided for gripping the first member / mounting plate 102 and the second member 104 and / or for gripping the third member 106 / 106′ and the second member 104. For example, a mechanical spring may be used to pull the housing 105 of the second member 104 and the first member / mounting plate 102 together, and / or a mechanical spring may be used to pull the third member 106 / 106′ and the second member 104 together. In other embodiments, one or more mechanical rods (such as those described in U.S. Patent No. 6,277,999) can be used to pull the housing 105 of the second member 104 and the first member / mounting plate 102 together (i.e., to lock / unlock the D-axis), and / or a mechanical rod (again, such as that described in U.S. Patent No. 6,277,999) can be used to pull the third member 106 / 106′ and the housing 105 of the second member 104 together (i.e., to lock / unlock the E-axis). However, it has been found that due to potential issues with friction-induced hysteresis, magnets may be preferred to such mechanical solutions (which may avoid the need for any moving parts between the first member / mounting plate 102 and the second member 104 and / or between the third member 106 / 106′ and the second member 104).

[0126] Advantageously, the above embodiments rely on the use of ring magnets. While it is possible that one or more of the ring magnets could be replaced by disk-shaped magnets, somewhat counterintuitively, the inventors have determined that ring magnets have a substantially different force / distance profile compared to disk-shaped magnets, which may be significantly advantageous in this situation (specifically, ring magnets appear to provide a more efficient design for a given surface area compared to disk-shaped magnets). Indeed, in this configuration, ring magnets can provide much greater force (approximately 50% greater) than disk-shaped magnets of the same outer diameter and depth (measured orthogonally to the diameter of the ring). FIG. 18 is a graph showing post force and breakaway torque for two disk-shaped magnet embodiments equivalent in all respects to that shown in FIG. 12, except that second magnet 142 and third magnet 144 are disk-shaped magnets instead of rings (the outer diameter of the disk-shaped magnets is the same as the outer diameter of the ring magnet). As can be seen, the bracing force and, crucially, the breakaway torque, is significantly less than the equivalent ring magnet embodiment.

[0127] This discovery allows articulating heads to be provided with very large gripping / locking forces, which in turn allows larger loads / moments to be supported by the articulating head before the magnetic coupling fails. For example, it may be desirable to carry very heavy probes, such as camera / video probes, and / or to support very long styli that impart large moments to the magnetic coupling, particularly while interrogating. The need for such large forces has previously led designers of articulating heads suitable for supporting large loads / moments to avoid the use of magnets. For example, the articulating heads disclosed in U.S. Pat. Nos. 5,629,999 and 5,729,999 use mechanical rods to provide the locking force. However, the present inventors have discovered that the use of ring magnets can provide a reasonably large gripping load without requiring physically large magnets, and thus can be properly fitted to articulating heads mounted on positioning devices such as CMMs.

[0128] As an alternative to a continuous ring magnet, a series of small disc magnets arranged in a ring shape can offer advantages over a single disc magnet having the same diameter as the ring shape, but it has been found that a continuous ring provides the most efficient design (for a given surface area).

[0129] As previously described, the planar teeth 112 of the first annular member / facial spline member 110 and the coronal teeth 118 of the second annular member 114 provide stable, repeatable positioning of the first member / mounting plate 102 and the second member 104. When in a locked condition, the only physical / mechanical constraints between the first member / facial spline member 102 and the second member 104 are the points of contact between the planar teeth 112 of the first annular member / facial spline member 110 and the coronal teeth 118 of the second annular member 114. It is a particular advantage of this configuration that, at each of the index positions, the second member 104 is constrained in all six degrees of freedom with respect to the first member / mounting plate 102 by the six points of contact provided by the coronal teeth 118 of the second annular member 114 and the planar teeth 112 of the first annular member / facial spline member 110, thereby providing kinematic constraints. This is true for each of the possible index positions, which provides maximum position repeatability for the probe 300 mounted to the articulating head 100 at each index position. It is also advantageous that the facial spline member 110 and the second annular member 114 have the dual function of being both an indexing element and a gripping element.

[0130] 3, 4, and 24, a safety catch 136 or "pin" is provided. The safety catch 136 is provided solely to act as a safety mechanism to prevent complete separation of the first and second members 102, 104 in the event of a failure of the magnetic gripping mechanism (e.g., due to overloading of the second member 104 due to a collision, etc.). One end of the safety catch 136 is fixed to the contact plate 134 of the support mast, and the other "head" end sits loosely within a cavity 138 in the first member / mounting plate 102. Because it sits loosely in the cavity in the first member / mounting plate 102, it does not act as a constraint between the first member / mounting plate 102 and the posts 130 / second member 104 (and therefore does not interfere with the previously described kinematic coupling between the first member / mounting plate 102 and the second member 104 when in the locked configuration, and does not interfere with the kinematic coupling between the first member / mounting plate 102 and the posts 130 when in the unlocked configuration). However, the safety catch 136 has an enlarged head member 137 that engages with a protrusion 139 in the cavity, thereby preventing further separation of the first member / mounting plate 102 and the second member 104, in the event of a failure of the magnetic coupling between the second magnetic ring 142 and the third magnetic ring 144.

[0131] In the previously described embodiment, the facial spline member 110 is provided on the second member 104 of the articulating head and the crown teeth 118 are provided on the first member / mounting plate 102. However, this does not necessarily have to be the case and they may be provided vice versa.

[0132] In the previously described embodiment, the first member / mounting plate 102 and the second member 104 are magnetically gripped via a magnet arrangement, meaning that it is not necessary to use mechanical means (e.g., arms / levers) to draw and hold the first member / mounting plate 102 and the second member 104 together. Thus, when in the locked state, the only mechanical constraint between the first member / mounting plate 102 and the second member 104 is provided by the teeth of the facial spline member 110 and the teeth of the second annular member 114. As such, when in the locked configuration, the posts 130 are decoupled from the first member / mounting plate 102 so as not to interfere with the previously described kinematic coupling between the first member / mounting plate 102 and the second member 104. However, this does not necessarily have to be the case. For example, in other embodiments, a mechanical push / pull lever arm mechanism may be provided, with one end of the arm resting in a bearing on the first member / mounting plate 102 and the other end of the arm resting in a bearing on the second member 104.

[0133] According to the invention, the first read head 160 and the second read head 161 are mounted on the first member 102, i.e. on a member having crowned teeth 118 positioned at three discrete, annularly spaced locations about the axis of rotation. This has been found to be particularly advantageous and can help to improve the reliability of the information provided by the first read head 160 and the second read head 161, as will be explained in the following paragraphs.

[0134] The original design for the articulating head had non-contact ("verification") sensors (e.g., first readhead 160 and second readhead 161) mounted on the second member 104, i.e., the member provided with a continuous series of tapered teeth. However, a problem was identified in that the relationship between the position information provided by the first readhead 160 and second readhead 161 and the actual relative spatial configuration of the first member 102 and second member 104 was found to be affected by changes in temperature. In other words, it has been found that even if the teeth 112 / 118 of the first member / mounting plate 102 and second body 105 are actually seated in substantially the same position as when the value stored in the look-up table associated with that index position was obtained, if the current temperature of the articulating head is different from the temperature at which the value stored in the look-up table associated with that index position was obtained, the first readhead 102 and / or second readhead 104 will provide a reading that indicates that the teeth 112 / 118 of the first member / mounting plate 102 and second body 105 are not seated in substantially the same position as when the value stored in the look-up table associated with that index position was obtained. Furthermore, it has been found that this temperature effect is difficult to predict and varies, particularly from one index position to another. Such adverse information from the read heads 102, 104 can lead to two concerns: i) the "corrective action" described above being taken unnecessarily, and ii) the "corrective action" not being taken when it should be (e.g., teeth 112 / 118 are not actually properly seated and are therefore seated in different relative positions, but thermal effects compensate for such differences).

[0135] The inventors have determined that the unpredictability of the temperature effects described above can be significantly reduced if the readhead is instead provided on a member having engagement features 117 positioned at discrete annularly spaced locations 116. This is because the positional relationship between the first readhead 160 (and second readhead 161) and the engagement features 117 on the first member 102 is the same regardless of index position, so any temperature effects are repeatable and can be calibrated / error mapped.

[0136] Furthermore, the inventors have determined that the above-described temperature effects can be substantially eliminated by positioning the read head on the same radial line as one of the engagement features 117 on the first member 102, or on a radial line close to the radial line on which one of the engagement features 117 is located. The embodiments described herein are configured such that the first read head 160 is positioned on the same radial line 119 as one of the engagement features 117 on the first member 102, and the second read head 161 is positioned on the same radial line 121 as the other engagement feature 117 on the first member 102. This is most clearly shown in Figures 19-21.

[0137] As will be appreciated, what is important here is the location of the sensing window / area of the readhead relative to the radial line 119 / 121 on which the engagement features 117 are located. Referring to Figures 22a and 22b, different first and second optical implementations 160a, 160b of the first readhead 160 are shown, along with their sensors 163a / 163b and optical components 165a / 165b (which may comprise, for example, one or more lenses and / or diffraction gratings) for forming the optical signal at the sensors. As shown in Figure 22a, the sensor 163a of the readhead 160a is positioned so that it is located on the same radial line 119 as one of the engagement features 117 on the first member 102. As shown in Figure 22b, readhead 160b is nominally optically identical to that of Figure 22a, except that a mirror 169 is used to fold the optical scheme so that sensor 163b is positioned off to the side, which may be beneficial to minimize the height of the readhead, but results in sensor 163b being positioned farther away from the radial line 119 of engagement feature 117. However, the effect of any change in temperature on their readings will be substantially the same, because the angle between i) the radial direction in which their sensing windows / regions 167a, 167b are positioned and ii) the radial line in which teeth 118 are positioned is the same (in the illustrated embodiment, such angle is substantially 0°, so that the readings of both readheads 160a, 160b are substantially unaffected by any change in temperature).

[0138] While optimal performance may be achieved by positioning the readhead sensing window / region on the same radial line 119 / 121 as the engagement features 117 (i.e., by positioning such that the angle between i) the radial line on which those sensing windows 167a, 167b are located and ii) the radial line on which the engagement features 117 are located is 0°), this is not necessarily the case. As mentioned earlier, there are benefits to having the readhead mounted on a member (in this embodiment, member 102) that has engagement features 117 positioned at discrete, annularly spaced locations, regardless of its location relative to the engagement features 117. Nevertheless, it has been found that the closer the readhead sensing window 167 is to the radial line 119 / 121 on which the engagement features 117 are located, the smaller the temperature effects described above. Specifically, the inventors believe that positioning the readhead such that its sensing window 167 is located on a radial line that is within ±15° of the radial line 119 / 121 of the nearest engagement feature 117 (as shown in FIG. 21 ) should sufficiently minimize the above temperature effects (e.g., to the extent that any calibration / mismapping of temperature effects should become unnecessary). The magnitude of the above temperature effects decreases as the readhead sensing window 167 approaches the radial line 119 / 121 of the engagement feature 117, so that it may be particularly preferred for the angle between i) the radial line on which the readhead sensing window 167 is located and ii) the radial line 119 / 121 on which the engagement feature 117 is located to be ±5° or less, more preferably ±1° or less.

[0139] In the previously described embodiment, both the first readhead 160 and the second readhead 161 are disposed such that the angle between i) the radial line on which its sensing window 167 is located and ii) the radial line 119 / 121 on which its nearest engagement feature 117 is located is the same for both readheads (specifically, is 0° for both readheads), but it will be understood that this need not necessarily be the case. For example, one of the readheads may be disposed such that the angle between i) the radial line on which its sensing window 167 is located and ii) the radial line 119 / 121 on which its nearest engagement feature 117 is located is 0°, and the other readhead may be disposed such that the angle between i) the radial line on which its sensing window 167 is located and ii) the radial line 119 / 121 on which its nearest engagement feature 117 is located is 5°.

[0140] Furthermore, as will be appreciated (and as previously described), two read heads need not be provided, one read head would suffice, although two read heads may be advantageous.

[0141] In the previously described embodiment, the engagement feature 117 at each of the locations 116 comprises only a single tooth 118 (and thus the radial line 119 / 121 of the engagement feature 117 may be the same as the radial line of such single tooth 118). However, this need not necessarily be the case; the engagement feature 117 may comprise, for example, multiple teeth, or, stated differently, a “group” of teeth, such as two, three, or four teeth. This may help distribute the load between the first member 102 and the second member 104 at each engagement location, but may be detrimental to the repeatability of the connection. The teeth of the tooth group of the engagement feature 117 may be positioned to engage consecutive teeth 112 of the facial spline member 110, or may be spaced apart to engage non-consecutive teeth 112. In those embodiments in which the engagement feature 117 comprises two teeth, the two teeth may be configured to engage with opposite sides of the same tooth 112 on the second member 104 .

[0142] In the event that the engagement feature 117 comprises a group of teeth, the radial line of the engagement feature may include a radial line that is midway between the first and last teeth in the group of teeth of the engagement feature. Such an example is shown in FIG. 24, where the engagement feature 117 comprises a group of three teeth 118a, 118b, and 118c. As shown, the radial line 121 of the engagement feature is a radial line that is midway between the first and last teeth 118, 118c in the group. In this embodiment, the radial line 121 coincides with a tooth of the engagement feature 117, i.e., the middle tooth 118b, but this need not necessarily be the case. For example, if the engagement feature 117 comprises a group of teeth that includes only two teeth (e.g., 118a and 118c), the radial line of such an engagement feature would not coincide with either of those teeth but would instead be located between the two teeth.

[0143] It is possible for an engagement feature to comprise multiple teeth, but in such cases it may be preferred that the angle formed by the arc subtended by the first and last teeth in the group of teeth of the engagement feature be 25° or less. Again, this is shown in Figure 24, which shows the angle α formed by the arc subtended by the first tooth 118a and the last tooth 118c in the group of teeth of engagement feature 117.

[0144] It may be preferable for the indexing arrangement to be configured to provide a kinematic coupling between the first member 102 and the second member 104. Optionally, the engagement features may be configured such that, in total, there are only six points of contact between the first member 102 and the second member 104 when locked together in the indexed position. This may be achieved by providing only one tooth on the first member 102 that contacts opposite sides of an opposing tooth 112 on the second member 104 at each of three discrete locations 116 (as in the embodiment shown), or in other embodiments, this may be achieved by providing two teeth on the first member that contact opposite sides of a single tooth 112 on the second member at each of three discrete locations 116 (such as in the spherical ball and cylindrical roller embodiments described in U.S. Patent No. 5,629,999 or U.S. Patent No. 5,629,999).

[0145] As mentioned above, the present invention is not limited to optical sensors. For example, inductive or capacitive sensors could be used. Figures 23a and 23b show a non-contact ("verification") sensor 180 having an inductive sensor 182 mounted on the first member 102 and positioned to sense an area of the top surface 115 of the second member 104. The output of the inductive sensor 182 varies depending on the separation "s" between the inductive sensor 182 and the top surface 115. As shown in Figures 23a and 23b, the separation "s" varies depending on the resting position of the teeth 118 of the first member 102 relative to the teeth 112 of the second member 104. Thus, the output of the inductive sensor 182 can be used in the same manner as described above in connection with the optical encoder embodiment, for example, by comparing the output of the inductive sensor 182 with a pre-stored value to verify that the first member 102 and the second member 104 have locked together at a given index position in substantially the same spatial configuration as at an earlier point in time (e.g., during a calibration phase).

[0146] At each index position, the material of at least those portions of the first member 102 and the second member 104 that mechanically couple the readhead 160 / 161 on the first member 102 and the scale 162 on the second member, thereby defining their relative spatial configuration, has a CTE of at least 2 ppm / °C. In the described embodiment, the first member 102 and the top surface 115 are made of metal, specifically an aluminum alloy having a CTE of 24 ppm / °C, and the first and second annular members 110 and 114 (and their associated teeth 112, 118) are made of metal, specifically a stainless steel alloy having a CTE of 10 ppm / °C. Thus, at each index position, those portions of the first member 102 and second member 104 that mechanically couple the readhead 160 / 161 on the first member 102 and the scale 162 on the second member, thereby defining their relative spatial configuration, have a CTE greater than 2 ppm / ° C. Other suitable materials include ceramics (e.g., tungsten carbide, zirconia, silicon nitride).

[0147] The disengagement and engagement of the first axis "D" is described in detail above. In the described embodiment, the locking / unlocking mechanism and indexing arrangement of the second axis "E" (i.e., of the second member 104 and third member 106 / 106') are substantially the same as those of the first axis "D", and therefore the above description of the first axis "D" also applies to the second axis "E". Thus, in summary, similar to the first axis "D" above, the controller 220 can send commands to the articulation head 100 to operate the lock motor 190' of the second axis "E" to drive a lead screw (not shown) to drive the lead screw nut 176' horizontally in the X dimension (in the orientation shown in FIG. 25). Furthermore, the lever 170' (which is coupled to, and therefore actuated by, the lead screw nut 176') pushes the support shaft 132 upward in the Z dimension (in the orientation shown in Figures 3 and 4). After a short distance, the engagement ball 152' on the contact plate 134' of the support 130' contacts and engages the pair of engagement balls 154' on the third member 106 / 106', after which subsequent actuation of the lead screw 174' causes the lever 170' and lead screw mechanism 172' to push the third member 106 / 106' axially outward, thereby separating the housing 105 of the second member 104 and the third member 106 / 106'. The lead screw 174' is operated to separate the second member 104 and the third member 106 / 106' by a controlled and predetermined amount sufficient to clear the coronal teeth 118' of the third member 106 / 106' from the matching teeth 112' on the second member 104, but not too great, as described above in connection with the first axis "D", since it is desired that the first magnet 140' remain close enough to the second magnet 142' so as to have a reasonable amount of pull on the second magnet 142' even in the unlocked state. Figure 25 shows the second axis "E" of the articulating head 100 in such an unlocked state.

[0148] Once the unlocked condition is reached, the locking motor 190' driving the lead screw mechanism 172' is stopped and the second axis "E" electric drive motor 192' engaged with the drive gear 148' of the shaft 132' is operated to effect a change in the rotational position of the third member 106 / 106' relative to the second member 105. In the unlocked condition, the post 130' is engaged with the third member 106 / 106' via the engagement balls 152', 154' and is therefore rotatably fixed relative thereto (in the unlocked condition). Thus, when the second axis "E" drive motor 192' engaged with the drive gear 148' of the shaft 132' is operated, it drives the third member 106 / 106' about the axis of rotation E defined by the shaft 132'.

[0149] The relative rotational position of i) the housing 105, 107, 115 of the second member 104 and ii) the shaft 132′ of the second axis “E” (and thus the third member 106 / 106′) is known from the second axis “E” first (“primary”) encoder device 135′. The controller 220 can therefore use the output from the second axis “E” first encoder device 135′ to control the second axis “E” drive motor 192′ which is engaged with the shaft drive gear 148′, thereby bringing the second member 104 and the third member 106 / 106′ into the desired relative orientation. As will be appreciated, the rotational position needs to be controlled with sufficient precision so that, when in the new desired relative orientation, the crown tooth 118' on the third member 106 / 106' is located opposite the valley between the teeth 112 on the second axis "E" annular member / facial spline member 110', so that when they are locked together the crown tooth 118' fits neatly and snugly between the two teeth 112' of the second axis "E" annular member / facial spline member 110'.

[0150] Similar to the first axis "D," the first magnet 140', second magnet 142', and third magnet 144' are provided on the housing 105 of the second member 104, the support post 130', and the third member 106 / 106' of the second axis "E," thereby benefiting from the magnetically assisted unlocking and locking configuration of the first axis "D." The indexing configuration of the second axis "E" is also similar to that of the first axis "D" in that it comprises a first annular member 110' (e.g., in the form of a "facial spline member") having a continuous series of tapered teeth extending substantially radially, and a second annular member 114' having three crowned teeth configured to engage subsets of the continuous series of teeth provided on the facial spline member 110' at three equiangularly spaced locations. Additionally, similar to the first axis "D", a second rotary encoder device is provided for the second axis "E" which includes an annular scale 162' on the housing 105 of the second member 104 and a first read head 160' and a second read head (not shown) on the third member 106 / 106' and which can be used in the same manner as described above in connection with the first axis "D" to verify / confirm that the second member 104 and the third member 106 / 106' have properly locked together.

[0151] In the previously described embodiments, separate locking motors are provided for the first axis "D" and the second axis "E." However, this need not be the case. For example, a single "locking" motor may be provided, and means may be provided for selecting which of the axes is to be separated. For example, a gear system, such as a gearbox, may be provided and configured so that the motor can be selected to control the locking / unlocking of only the first axis "D," the locking / unlocking of only the second axis "E," or the locking / unlocking of both the first axis "D" and the second axis "E" together. However, for simplicity, the use of separate motors may be preferred, as if only one motor is provided, it needs to be of sufficient size / power to accommodate unlocking both axes simultaneously when required. Similarly, in the previously described embodiments, separate drive motors are provided for the first axis "D" and the second axis "E." However, this need not be the case. For example, there may be one "drive" motor and means (e.g., a gearbox) for selecting which of the axes is to be driven. Again, separate drive motors may be preferred.

[0152] The above describes a biaxial articulating head. However, it should be understood that the present invention is not limited to biaxial articulating heads. For example, the articulating head can be a triaxial articulating head, and a fourth member coupled to the third member is provided such that its orientation relative to the third member about the third axis can be varied (e.g., can be varied between a plurality of predetermined indexable orientations and can be locked in one of the plurality of predetermined indexable orientations). In such cases, the above-described features can be applied to the third and fourth members, as appropriate. The first, second, and third axes can be mutually orthogonal. In such cases, the fourth member can include a tool mount for receiving a tool. In such cases, the first, second, and third axes of rotation can be mutually orthogonal. The present invention is also applicable to single-axis devices, such as a rotary table having one axis of rotation.

Claims

1. 1. An apparatus comprising: a first relatively reorientable member; a second relatively reorientable member; and an indexing arrangement configured to provide a plurality of angularly indexed lockable positions of the first member and the second member about a first axis, the indexing arrangement comprising: i) a series of features on the first member, the series of features extending annularly about the first axis; ii) engagement features on the second member at at least three discrete locations spaced annularly about the first axis, the engagement features configured to interlock with a subset of the features on the first member when in a locked state, thereby providing a stable relative rest position of the first and second members at each index position; Equipped with The device further comprises at least a first non-contact sensor mounted to the second member, the first non-contact sensor configured to sense an area on the first member and thereby provide a signal dependent on a spatial configuration of the first member and the second member when they are in a locked state.

2. 2. The apparatus of claim 1, wherein at each index position, the material of at least those portions of the first member and the second member that mechanically couples the first non-contact sensor in the second member to the region in the first member, thereby defining the relative spatial configuration of the first member and the second member, has a CTE of at least 2 ppm / °C.

3. 3. The device of claim 1 or 2, wherein the first non-contact sensor is positioned so that the radial line on which its sensing window is located is within ±15° of the radial line on which the first of the engagement features of the second member is located.

4. 4. The apparatus of claim 3, wherein the first non-contact sensor is positioned such that the radial line along which its sensing window is located is within ±1° of the radial line along which the first engagement feature is located.

5. 5. The apparatus of claim 1, wherein the area of the first member sensed by the first non-contact sensor comprises a scale member having a series of features that can be read by the first non-contact sensor to determine the relative position of the first non-contact sensor.

6. 6. The device of claim 1, wherein the first non-contact sensor is configured to be used to establish information about the state of engagement between the first body and the second body when the first member and the second member lock together in an indexed position.

7. 7. The device of claim 6, configured to respond in a predetermined manner depending on the determined state of engagement between the first member and the second member.

8. 8. The apparatus of claim 7, wherein said responding in a predetermined manner includes causing said first member and said second member to unlock and optionally relock at the same indexed position.

9. 9. The apparatus of claim 1, further comprising a second non-contact sensor mounted on the second member, the second non-contact sensor configured to sense an area on the first member at an annular position about the first axis that is different from an annular position sensed by the first non-contact sensor, thereby providing a signal that is dependent on the spatial configuration of the first and second members when they are in their locked state.

10. 10. The device of claim 9, wherein the second non-contact sensor is positioned such that the radial line along which its sensing window is located is within ±15° of the radial line along which the second one of the engagement features of the second member is located.

11. 11. The device of claim 1, wherein at each index position, the engagement features on the second member are configured to interlock with a subset of the features on the first member when in a locked state, thereby providing a kinematic coupling between the first member and the second member.

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