Encoder device

Abstract

A position encoder device comprising a scale and a reading head equipped with at least one scale signal sensor configured to detect a scale signal, wherein at least one position signal indicating the relative position of the scale and the reading head along the measurement direction of the scale is generated and output from the scale signal, and the encoder device is configured to compensate for changes in the relative pitch of the reading head and the scale to reduce positional displacement errors in the at least one position signal due to pitch.

Description

[Technical Field]

[0001] The present invention relates to an encoder device, and more particularly to a position measuring encoder device comprising a scale and a reading head that are movable relative to each other. [Background technology]

[0002] As is well known, position measuring encoder devices typically have a scale with a set of features, and a reading head can read the set of features to determine and measure the relative position (and / or its derivatives, such as velocity and / or acceleration). Encoders are typically classified as either incremental or absolute. The scale of an incremental encoder has a series of generally periodic features, and the reading head detects the series of generally periodic features to determine the relative position and movement of the scale and the reading head. One or more reference marks can be provided on the scale to provide a reference position from which the relative position of the scale and the reading head can be counted. The scale of an absolute encoder has features that define unique positions along the scale length, for example, a regular (e.g., consecutive) series of unique absolute positions, which can allow the reading head to determine its absolute position at startup without requiring relative motion.

[0003] Position encoder devices can be made sensitive to changes in the relative pitch between the scale and the reading head. For example, changes in the relative pitch between the scale and the reading head can cause changes / displacements in the signal detected by the reading head, which may be misinterpreted and reported as changes in the relative position / displacement of the reading head and scale along the measurement dimension of the position encoder device, thereby leading to position displacement errors due to the pitch.

[0004] To ensure understanding, the term “pitch” in this document adopts the usual meaning of “pitch” in the established right-hand rule for “pitch,” “roll,” and “yaw” rotations of an object movable along the direction of movement (in this case, along the measuring dimension of the reading head / position encoder). Thus, the relative “pitch” of the scale and the reading head is the rotation around the transverse axis / lateral axis, i.e., the axis that extends perpendicular to the measuring dimension / direction of movement and also perpendicular to the “vertical” or “ride height” dimension of the reading head (i.e., the separation dimension between the reading head and the scale, which, as understood, is orthogonal / perpendicular to the scale).

[0005] Therefore, when a reading head and scale are installed, the change in relative pitch is a change in the relative angular direction of the reading head and scale, at least at the measurement point on the scale, around an axis that is parallel to the scale at the measurement point and extends perpendicular to the measurement direction of the scale.

[0006] Furthermore, as can be understood, changes in the relative pitch of the scale and the reading head may be caused by rotation of the reading head and / or the entire scale, or by changes in the slope of the scale along the measurement direction, for example, due to scale undulations. In fact, once a position encoder device is installed, the most likely cause of changes in the relative pitch of the scale and the reading head at the measurement point is changes in the slope of the scale due to scale undulations.

[0007] The reading head of a position encoder device may have a "pitch-insensitive axis," which is an axis from which the relative pitch between the scale and the reading head can be changed without generating positional displacement errors in the signal detected by the reading head, and therefore without generating positional measurement / displacement errors. For example, when installed, the pitch-insensitive axis will typically be parallel to the scale and extend perpendicular to the measuring direction of the scale. As can be understood, in the case of a rotary encoder with a ring scale (i.e., a scale formed on the circumference of a ring), the orientation of the pitch-insensitive axis relative to the scale will be as measured at the current reading position of the reading head. As explained in the preceding paragraph, a possible cause of relative pitch is a local change in the slope of the scale. Therefore, the pitch-insensitive axis may be called a "scale slope insensitive axis."

[0008] Some known encoders, such as the TONiC® and ATOM® encoders available from Renishaw plc, have a design in which the “pitch dead axis” lies on the surface of the scale when properly installed. Therefore, theoretically, changes in the relative pitch of the scale and the reading head, for example, variations in the slope of the scale along the length of the scale, should not cause pitch-related positional displacement errors. However, even if such encoder devices are configured so that the accuracy of their position signals is not sensitive to changes in the relative pitch of the scale and the reading head, changes in the relative configuration of the reading head and scale during use (for example, changes in the “ride height” of the reading head, which is the distance between the scale and the reading head measured orthogonally to the scale) can cause the pitch dead axis to move away from the scale, thus making the encoder more susceptible to positional displacement errors caused by scale pitch. Other known encoders have a design in which, when properly installed, the “pitch dead axis” lies above the surface of the scale, thereby causing the undulation of the scale to move the scale signal (e.g., fringe) that reaches the sensor of the reading head across the sensor, leading to erroneous position readings.

[0009] As taught in Patent Document 1 (published as Patent Document 2), it is known that the reading head is aligned with the scale during installation to minimize the angular misalignment between the reading head and the scale in order to help generate an optimal signal (e.g., optimize signal intensity / amplitude). Such installation solutions include using an angle sensor in the reading head to determine the angular alignment and providing an output indicating the angular misalignment that can be used by the user to determine how to physically adjust the angle of the reading head and / or scale to reduce the angular misalignment. Also, as described in Patent Document 2, the angle sensor can be used in a calibration routine to compensate for the effect of the angular misalignment on the electrical signal amplitude.

[0010] The inventors desired to provide an improved encoder, in particular an encoder with improved accuracy, and especially an encoder with reduced positional displacement error. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] International Patent Application No. PCT / GB2005 / 002431 Specification [Patent Document 2] International Publication No. 2005 / 124283 brochure [Patent Document 3] International Publication No. 2002 / 084223 Pamphlet [Patent Document 4] U.S. Patent No. 7499827 [Patent Document 5] U.S. Patent No. 5279044 [Patent Document 6] International Publication No. 2005 / 124282 brochure [Patent Document 7] European Patent No. 0826138 [Patent Document 8] U.S. Patent No. 5861953 [Patent Document 9] U.S. Patent No. 8525102 [Patent Document 10] U.S. Patent No. 9952068 [Patent Document 11] International Publication No. 2010 / 139964 Pamphlet [Patent Document 12] U.S. Patent Application Publication No. 2012 / 0072169 [Overview of the Initiative]

[0012] According to a first aspect of the present invention, there is provided a position encoder device comprising a scale and a reading head provided with at least one scale signal sensor configured to detect a scale signal, wherein at least one position signal indicating the relative position of the scale and the reading head along the measurement direction of the scale is generated from the scale signal and output, and this encoder device is configured to compensate for a change in the relative pitch of the reading head and the scale to reduce a position displacement error caused by pitch in the at least one position signal.

[0013] The present invention helps to reduce the influence of the position displacement error caused by such a pitch as described above.

[0014] The above compensation occurs during the generation of at least one position signal / as part of its generation, and can reduce the measurement error caused by the scale pitch in the position signal before the position signal is generated. Optionally, the above compensation is performed after at least one position signal is generated, and thus the above compensation functions to correct the generated position signal, thereby reducing the measurement error caused by the scale pitch in the position signal.

[0015] The above compensation can be performed within the reading head or within a device external to the reading head, for example, in a controller or in an interface unit separate from the reading head. The reference to the controller includes, for example, the controller of the machine in which the position encoder device is installed. In other words, the reading head, or a device external to the reading head, can be configured to perform the above compensation.

[0016] A position encoder device (e.g., a read head, or at least one of devices outside the read head, such as a controller and / or interface unit) may include means, e.g., a “compensator,” e.g., electronic equipment and / or software (e.g., one or more processing devices) configured to compensate for changes in the relative pitch of the read head and scale in order to reduce pitch-related position displacement errors in at least one position signal. As understood, the processing devices may include custom-configured processors for specific applications (e.g., field-programmable gate arrays “FPGAs”) and more general-purpose processors that can be programmed (e.g., via software) according to the needs of the application in which they are used.

[0017] The encoder device (e.g., a reading head and / or the device outside the reading head) may be configured to detect, for example, a change in the relative pitch between the reading head and the scale (e.g., at least an indication of change).

[0018] The above identification / detection / monitoring can be performed within the reading head or in a device outside the reading head, for example, in a controller or in an interface unit separate from the reading head. Thus, the encoder device (e.g., the reading head, and / or the above device outside the reading head) can be configured to identify / detect / monitor changes in the relative pitch of the reading head and the scale (e.g., at least an indication of change). In a preferred embodiment, the encoder device (e.g., the reading head, and / or the above device outside the reading head) is configured to continuously monitor changes in the relative pitch of the reading head and the scale (e.g., at least an indication of change) as the scale and the reading head move relative to each other.

[0019] Therefore, the encoder device (e.g., a reading head) may include a “pitch identifier,” “pitch detector,” or “pitch monitor” configured to identify, detect, or monitor changes (indications) in the relative pitch between the reading head and the scale. In a preferred embodiment, the encoder device (e.g., a reading head) may include a “pitch monitor” configured to continuously monitor changes (indications) in the relative pitch between the reading head and the scale as the scale and the reading head move relative to each other.

[0020] The pitch discriminator / detector / monitor described above may include electronic and / or software (e.g., one or more processing devices) for determining the change (indication of) in the relative pitch of the reading head and scale. The pitch discriminator / detector / monitor described above may include one or more sensors (one or more "pitch discriminator / detector / monitor" sensors) through which the change (indication of) in the relative pitch of the reading head and scale can be determined. Thus, an encoder device (e.g., a reading head) may include one or more sensors (one or more "pitch discriminator / detector / monitor" sensors) through which the change (indication of) in the relative pitch of the reading head and scale can be determined. In a preferred embodiment, the reading head includes the one or more "pitch discriminator / detector / monitor" sensors described above. This may be beneficial for reasons of cost, assembly, and / or measurement performance. One or more such sensors (one or more “pitch discriminator / detector / monitor sensors”) may be configured to detect a signal in which at least one aspect of its nature (e.g., its position, shape, size, or form when it reaches the sensor) is affected by the relative pitch of the reading head and scale. Thus, the above-mentioned discriminating / detecting / monitoring may include providing an output that depends on at least one aspect of the signal reaching the sensor (one or more “pitch discriminator / detector / monitor” sensors) (the above-mentioned pitch discriminator / detector / monitor may be configured to provide this output) (where the above-mentioned output is suitable for compensating for changes in the relative pitch of the reading head and scale to reduce pitch-induced positional displacement errors in at least one position signal). Thus, an encoder device (e.g., a reading head, controller, interface unit) may be configured to use the above-mentioned output to compensate for changes in the relative pitch of the reading head and scale to reduce pitch-induced positional displacement errors in at least one position signal.

[0021] One or more of the above (pitch discriminator / detector / monitor) sensors may include at least one of the above scale signal sensors. This may be beneficial for reasons of cost, assembly, and / or measurement performance. In other words, at least one sensor may be used to determine both i) at least one position signal and ii) the above indication of change in the relative pitch of the scale and the reading head. For example, one or more of the above (pitch discriminator / detector / monitor) sensors and at least one of the above scale signal sensors may be the same sensor. In fact, the same signal used to determine at least one position signal indicating the relative position of the scale and the reading head along the measurement direction of the scale may be used to determine the above indication of change in the relative pitch of the reading head and the scale. Optionally, one or more of the above (pitch monitor) sensors may differ from the above at least one scale signal sensor.

[0022] The above compensation may be based on the output / results of the above-mentioned pitch monitoring / supervision (or above-mentioned pitch identification / detection). In particular, the above compensation may be based on the current / instantaneous / real-time output of the above-mentioned pitch monitoring / supervision (or above-mentioned pitch identification / detection). This is in contrast to using a pre-generated error map, for example, which is used to reduce pitch-related positional displacement errors in at least one position signal based on the current reading position.

[0023] Naturally, in alternative embodiments, the output of the pitch monitoring / supervision (or pitch identification / detection) described above may be used to update an error map or function, which is subsequently used to compensate for changes in the relative pitch of the reading head and scale to reduce pitch-related positional displacement errors in at least one position signal. Such updates to the error map or function may occur during calibration routines (e.g., while the reading head is in calibration mode) and / or during normal operation of the reading head (e.g., while the reading head is in its normal position reporting mode, e.g., while the output of the reading head is used to control the operation of the machine in which the reading head is installed).

[0024] Compensating for changes in the relative pitch of the read head and scale to reduce pitch-induced positional displacement errors in at least one position signal may include advancing or delaying the relative position indicated by at least one position signal. The degree to which at least one position signal is advanced or delayed may depend on the results of the above monitoring / output of the above pitch monitor. For example, the degree to which at least one position signal is advanced or delayed may depend on the current / instantaneous / real-time output of the above pitch monitor / monitoring, for example, on the output of the above at least one (pitch monitor) sensor. Optionally, the degree to which at least one position signal is advanced or delayed may be derived, for example, from an error map or function whose content / form is generated / derived from the above monitoring / pitch monitor (which may occur during calibration routines and / or normal operation of the read head, as described in the preceding paragraphs).

[0025] The above monitoring may include analyzing the distortion / deformation of the signal reaching the sensor (e.g., at least one scale signal sensor and / or at least one (pitch monitor) sensor) (for example, the pitch monitor may be configured to analyze this distortion / deformation of the signal). In particular, the above monitoring may include determining the distortion / deformation of the signal along the measuring dimension.

[0026] An encoder device can be configured such that the signal reaching a sensor (e.g., at least one scale signal sensor and / or at least one (pitch monitor) sensor) has periodic content. As described above, optionally, the at least one scale signal sensor and at least one (pitch monitor) sensor can be the same sensor. As understood, the signal produced by an incremental scale typically and inherently has periodic content. Similarly, the signal produced by an absolute scale may also have periodic content. For example, an absolute scale may include at least one periodic scale track that generates a periodic scale signal. Even if the absolute scale has its absolute position information embedded within a non-periodic structure (e.g., the absolute scale embeds the absolute position within it by omitting a selection scale line from a periodic arrangement of scale lines, as described in Patent Document 3), the signal reaching the sensor (e.g., at least one scale signal sensor and / or at least one (pitch monitor) sensor) may still have periodic content.

[0027] In embodiments where the signal arriving at the sensor (e.g., at least one scale signal sensor and / or at least one (pitch monitor) sensor) has periodic content, determining the distortion / deformation of the signal may include analyzing the periodic content of the signal arriving at the sensor (e.g., at least one scale signal sensor and / or at least one (pitch monitor) sensor). For example, this may include analyzing how the period of the signal changes across the entire sensor (i.e., along the measurement dimension). For example, this may be done by analyzing the positions of the maximum and minimum values ​​of the periodic structure of the entire sensor to analyze, for example, how the relative positions of the maximum and minimum values ​​change across the entire sensor, and optionally, how much this relative position changes (e.g., whether their intervals are constant or whether their intervals vary across the sensor).

[0028] The above monitoring may include analyzing the deformation / strain of a signal (e.g., a scale signal) that reaches at least one scale signal sensor (which is configured to detect a scale signal, from which at least one position signal indicating the relative positions of the scale and the reading head is generated) (the pitch monitor above may be configured to analyze the deformation / strain of this signal). Thus, the same sensor can be used for both i) determining the relative positions of the scale and the reading head along the measurement direction of the scale, and ii) determining the strain / deformation of the signal reaching the sensor (the sensor is used to determine the change in the relative pitch of the reading head and the scale). The above monitoring may include analyzing the deformation / strain of a scale signal (the pitch monitor above may be configured to analyze the deformation / strain of a scale signal), from which at least one position signal indicating the relative positions of the scale and the reading head is generated. Thus, the same signal can be used for both i) determining the relative positions of the scale and the reading head along the measurement direction of the scale, and ii) determining the strain / deformation of the signal reaching the sensor (the sensor is used to determine the change in the relative pitch of the reading head and the scale).

[0029] A position encoder device can be configured to output a signal (e.g., a "pitch signal") indicating the relative pitch between the reading head and the scale. The position encoder device can be configured to output the above signal indicating the relative pitch between the reading head and the scale to (or in a suitable format) another system / device / processor device, and / or to output the above signal indicating the relative pitch between the reading head and the scale to (or in a suitable format) (e.g., in the form of a human-perceptible signal (e.g., a visual signal or an auditory signal)).

[0030] The position encoder device (e.g., at least one of a reading head, controller, and interface unit) may be further configured to determine a separation signal indicating the separation between the reading head and the scale (e.g., in a dimension perpendicular to both the measurement dimension and the axis on which the pitch is determined). Optionally, the separation signal is taken into consideration to compensate for changes in the relative pitch of the reading head and the scale as described above, thereby reducing pitch-related positional displacement errors in at least one position signal. In other words, compensating for changes in the relative pitch of the reading head and the scale to reduce pitch-related positional displacement errors in at least one position signal may include using the separation signal described above.

[0031] Therefore, the position encoder device (e.g., at least one of a reading head, a controller, and / or an interface unit) may include means configured to determine a separation signal indicating separation between the reading head and the scale, e.g., a "separation monitor," e.g., electronic equipment and / or software (e.g., one or more processing devices). The position encoder device (e.g., the reading head) may be configured to continuously determine the separation signal as the scale and the reading head move relative to each other. Optionally, the position encoder device (e.g., the reading head) may be configured to determine the separation signal at regular or irregular intervals.

[0032] A position encoder device (e.g., a reading head, e.g., a "separation monitor") may comprise one or more sensors (one or more "separation sensors"), via which a separation signal indicating separation between the reading head and the scale can be determined. Thus, an encoder device (e.g., a reading head) may comprise one or more sensors (one or more separation sensors), via which a separation signal indicating separation between the reading head and the scale can be determined. Such one or more sensors (one or more separation sensors) may be configured to detect a signal in which at least one aspect of its nature (e.g., its position, shape, size, or form when it reaches the sensor) is affected by the relative separation between the reading head and the scale. Determining the separation signal may include providing an output that depends on at least one aspect of the signal reaching the separation monitor sensor (the above separation sensor / monitor may be configured to provide this output).

[0033] The one or more (separated) sensors described above may comprise at least one scale signal sensor described above. In other words, at least one sensor may be used to determine both i) at least one position signal and ii) the separated signal described above (and optionally, additionally, iii) the indication of change in the relative pitch of the scale and the reading head). For example, the one or more (separated) sensors described above and the at least one scale signal sensor described above may be the same sensor. In fact, the same signal used to determine at least one position signal indicating the relative position of the scale and the reading head along the measuring direction of the scale may be used to determine the separated signal described above (and optionally, the indication of change in the relative pitch of the reading head and the scale). Optionally, the one or more (separated) sensors described above may be different from the at least one scale signal sensor described above (and optionally, different from one or more “pitch monitor” sensors).

[0034] In a particularly preferred embodiment, the signal indicating separation between the sensor and the reading head is determined from the apparent size of the features of the scale signal that reach one or more separation sensors. For example, the encoder device may be configured to determine the separation signal based on the periodic size of the scale signal.

[0035] A position encoder device can be configured to output a signal (e.g., a "separation signal") indicating the relative separation between the reading head and the scale. The position encoder device can be configured to output the above-mentioned signal indicating the relative separation between the reading head and the scale to another system / device / processor device (or in a format suitable therefor), and / or to a user / human (or in a format suitable therefor) (e.g., in the form of a human-perceptible signal (e.g., a visual signal or an auditory signal)).

[0036] The position encoder device may include an incremental position encoder device. Therefore, the scale may include an incremental scale track containing a series of periodic features. One or more reference marks may be embedded within the incremental scale track or positioned adjacent to the incremental scale track.

[0037] The position encoder device may include an absolute position encoder device. Thus, the scale may include an absolute scale track containing a set of regular (e.g., continuous) position features that define the absolute scale track. As can be understood, the absolute scale track differs from an incremental scale track (with or without a reference mark) in that its features define a set of unique positions along the length of the scale that can be read by the read head, allowing the relative positions of the read head and the scale to be determined (e.g., at startup) at any position along the scale without requiring movement to a reference position (e.g., a reference mark). Examples of absolute scales include those described in Patent Documents 4 and 5. The scale may include separate incremental scale tracks and absolute scale tracks. Optionally, incremental scale features and absolute scale features are combined into a single track. For example, absolute scale features may be superimposed on periodic incremental scale features. As is known, and as explained in the prior art described above in this paragraph, absolute position information can be encoded into a scale track by omitting selected position features from a series of periodic position features.

[0038] The position encoder device / scale may be magnetic, optical, capacitive, or inductive. As can be understood, in the case of an optical position encoder device / scale, the reading head may include at least one light source configured to illuminate the scale. In the case of an optical position encoder device / scale, the scale signal may include an optical scale signal. Such an optical scale signal may include an optical scale pattern, such as a fringe pattern. Such an optical scale signal may include a shaded representation of a portion of the scale (i.e., the portion currently at the reading position by the reading head). Such an optical scale signal may include an image of the scale (e.g., a Talbot image).

[0039] Preferably, the scale is a reflective scale member (in this case, the light source and sensor are on the same side of the scale member). To be understood, references to “light” and “optical” in this specification encompass any electromagnetic radiation (EMR) ranging from infrared to ultraviolet. For example, the light source may be an infrared light source.

[0040] A second aspect of the present invention provides a position encoder device comprising a scale and a reading head equipped with at least one scale signal sensor configured to detect a scale signal, wherein a position signal indicating the relative position of the scale and the reading head along the measuring direction of the scale can be generated from the scale signal, and the position encoder device is configured to generate at least one such position signal from the scale signal detected by the scale signal sensor, and to perform i) a separation signal indicating separation between the reading head and the scale, and / or ii) a pitch signal indicating the relative pitch between the reading head and the scale. In particular, the position encoder device can be configured to determine a separation signal indicating separation between the reading head and the scale, and / or to determine a pitch signal indicating the relative pitch between the reading head and the scale, based on the scale signal.

[0041] As can be understood, the features described above in relation to the first aspect of the present invention are applicable to the second aspect of the present invention, and vice versa.

[0042] For example, as described above in relation to a first aspect of the present invention, a position encoder device (e.g., at least one of a reading head, a controller, and / or an interface unit) may include means configured to determine a separation signal indicating separation between the reading head and the scale, e.g., a “separation monitor”, e.g., electronic equipment and / or software (e.g., one or more processing devices). Similarly, as described above in relation to a first aspect of the present invention, a position encoder device (e.g., at least one of a reading head, a controller, and / or an interface unit) may include means configured to identify, detect, or monitor the relative pitch (indication of change) between the reading head and the scale, e.g., a “pitch identifier”, a “pitch detector”, or a “pitch monitor”. A position encoder device (e.g., a reading head) may be configured to continuously determine i) a separation signal indicating separation between the reading head and the scale, and / or ii) a pitch signal indicating the relative pitch between the reading head and the scale, as the scale and the reading head move relative to each other. Optionally, the position encoder device (e.g., a reading head) may be configured to determine, at regular intervals (regular or irregular), i) a separation signal indicating the separation between the reading head and the scale, and / or ii) a pitch signal indicating the relative pitch between the reading head and the scale.

[0043] A position encoder device (e.g., a reading head, e.g., a "separation monitor") may comprise one or more sensors (one or more separation sensors) through which a separation signal can be determined. Such one or more sensors (e.g., one or more separation sensors) may be configured to detect a signal whose at least one aspect (e.g., its position, shape, size, or form when it reaches the sensor) is affected by the relative separation between the reading head and the scale. Determining a separation signal may include providing an output that depends on at least one aspect of the signal reaching such one or more sensors (the one or more separation sensors / monitors described above may be configured to provide this output). Similarly, a position encoder device (e.g., a reading head, e.g., a "pitch monitor") may comprise one or more sensors (one or more pitch sensors) through which a pitch signal can be determined. One or more such sensors (e.g., one or more pitch sensors) may be configured to detect a signal whose at least one aspect (e.g., its position, shape, size, or form when it reaches the sensor) is affected by the relative pitch of the reading head and scale. Determining the pitch signal may include providing an output that depends on at least one aspect of the signal reaching such one or more sensors (the one or more pitch sensors / monitors described above may be configured to provide this output).

[0044] An encoder device (e.g., at least one of a reading head, controller, and interface unit) can be configured to determine the separation signal and / or pitch signal based on a scale signal sensed by the at least one scale signal sensor. In other words, it is preferable that the signal indicating separation between the reading head and the scale (and / or the signal indicating relative pitch) is determined from the same sensor used to determine the generation of the at least one position signal indicating the relative position of the scale and the reading head along the measuring direction of the scale. Advantages include cost reduction, a more compact reading head, and improved measurement performance. Nevertheless, as is understood, the separation signal and / or pitch signal, and the position signal, may be determined from scale signals sensed by separate / different sensors.

[0045] The encoder device may be configured to determine the at least one position signal based on a) the separation signal and / or the pitch signal, and b) the same scale signal sensed by the at least one scale signal sensor.

[0046] The encoder device may be configured to determine the separation signal and / or the pitch signal based on the size of the features of the scale signal (when the scale signal reaches a sensor that detects the scale signal, for example, at least one scale signal sensor that determines at least one position signal). For example, the encoder device may be configured to determine the separation signal and / or the pitch signal based on the periodic size of the scale signal.

[0047] A position encoder device can be configured to output signals (e.g., “separation signal” and / or “pitch signal”) indicating the relative separation between the reading head and the scale and / or the relative pitch. The position encoder device can be configured to output the above signals indicating the relative separation between the reading head and the scale and / or the relative pitch to another system / device / processor device (or in a format suitable therefor), and / or can be configured to output the above signals indicating the relative separation between the reading head and the scale and / or the relative pitch to a user / human (or in a format suitable therefor) (e.g., in the form of a human-perceptible signal (e.g., a visual signal or an auditory signal)).

[0048] Furthermore, as described above in relation to the first aspect of the present invention, the separation signal and / or the pitch signal may be used by a position encoder device. For example, the separation signal and / or the pitch signal may be used to compensate for errors in the position signal. For example, the separation signal may be used to determine the relative pitch between the scale and the reading head. [Brief explanation of the drawing]

[0049] Herein, embodiments of the present invention will be described only as examples, with reference to the following drawings.

[0050] [Figure 1] This figure shows a position encoder device according to the present invention. [Figure 2(a)] This diagram schematically shows some of the components of the read head shown in Figure 1. [Figure 2(b)] This diagram schematically shows some of the components of the read head shown in Figure 1. [Figure 3] This diagram illustrates the relationship between scale pitch / gradient and measurement error. [Figure 4] This figure shows the effect of changes in the gradient on the signal received by the read head. [Figure 5]This is a Lissajous representation of the orthogonal sine and cosine signals output by the sensor in the reading head. [Figure 6] This figure shows the effect of changes in the gradient on the signal received by the reading head along a wavescale. [Figure 7] This figure schematically shows the functional components of an exemplary read head and interface unit according to a first embodiment of the present invention. [Figure 8] This figure schematically illustrates how the correction signal from the pitch gradient monitor of the reading head shown in Figure 7 can be used to correct the reading head's position signal, according to the first embodiment of the present invention. [Figure 9] This figure schematically illustrates how the correction signal from the pitch gradient monitor of the reading head shown in Figure 7 can be used to correct the reading head's position signal, according to a second embodiment of the present invention. [Figure 10] This figure schematically shows the functional components of an exemplary read head and interface unit according to a second embodiment of the present invention. [Figure 11] This figure shows how, according to an exemplary embodiment of the present invention, the correction signal from the pitch gradient monitor of the reading head in Figure 10 can be used by the interface unit to correct the reading head's position signal. [Figure 12] This figure shows an exemplary embodiment for determining a pitch (e.g., gradient) compensation signal. [Figure 13] This diagram illustrates the mechanism by which a scale tilted at an angle α reads a portion of the optical signal and deflects it by different amounts to different parts of the head sensor. [Figure 14] This diagram shows an encoder system that displays a magnification that changes with ride height. [Figure 15] This figure shows the distortion effect that causes a periodic signal I(x) to have the positions of its maximum and minimum values ​​shifted relative to the maximum and minimum values ​​of the same undistorted signal I0(x). [Figure 16]This figure schematically shows the functional components of an exemplary read head and interface unit according to a third embodiment of the present invention. [Figure 17] This figure schematically illustrates how the correction signal from the pitch gradient monitor of the reading head shown in Figure 16 can be used to correct the reading head's position signal, according to a third embodiment of the present invention. [Modes for carrying out the invention]

[0051] Referring to Figures 1 and 2, a first exemplary linear position encoder device 2 according to the present invention is shown. As can be understood, the present invention is also applicable to rotary scale devices including ring encoders (i.e., edge-reading rotary encoders) and disk encoders (i.e., surface-reading rotary encoders).

[0052] The linear encoder device in Figure 1 comprises a reading head 4 and a scale 6. Although not shown, in use, the reading head 4 can be fixed to a part of the machine, and the scale 6 can be fixed to another part of the machine, and these parts are linearly movable relative to each other along the X dimension. The reading head 4 is used to measure the relative position / displacement (and / or its derivatives, such as velocity and / or acceleration) of the reading head 4 itself and the scale 6 along the X dimension, and can therefore be used to provide a measure of the relative position / displacement (and / or its derivatives, such as velocity and / or acceleration) of the two movable parts of the machine along the X dimension. As will be understood by those skilled in the art, the relative positions of the reading head 4 and the scale 6 in the Z and Y dimensions, and the relative pitch (i.e., any relative rotation around an axis parallel to the Y dimension), yaw (i.e., any relative rotation around an axis parallel to the Z dimension), and roll (i.e., any relative rotation around an axis parallel to the X dimension) of the reading head and the scale are constrained by the machine. Naturally, as the reading head and scale move relative to each other along the X dimension, some relative displacement in the Z and Y dimensions, and / or some relative pitch, roll, and / or yaw of the scale and reading head may occur, for example, due to bearing misalignment, nonlinearity, or other defects that restrict the movable parts of the machine. Also, as will be understood by those skilled in the art, and as will be described in more detail below, changes in the relative pitch of the reading head 4 and scale 6 may occur due to the undulation of scale 6 and due to changes in the slope of the scale along the length of the scale.

[0053] The reading head 4 communicates with an external device, such as a controller 8, via a wired communication channel and / or a wireless communication channel (as shown in the figure). The reading head 4 reports signals from its detector (described in more detail below) to the controller 8, which then processes these signals to determine position / displacement information. Additionally, or alternatively, the reading head 4 itself can process signals from its detector and transmit position / displacement information to the controller 8.

[0054] As shown in Figure 1, the signals transmitted from the read head 4 to the controller 8 can take many different forms. For example, as is known in the field of position measuring encoders, the read head can output digital quadrature (A, B) signals, analog quadrature (SIN, COS) signals, and / or serial data representing position information.

[0055] In another embodiment, an intermediate unit, such as an interface unit, may be positioned between the read head 4 and the controller 8. The interface unit can facilitate communication between the read head 4 and the controller 8. For example, the interface unit may be configured to process the read head signals and provide position information to the controller 8 (e.g., in the form of digital quadrature (A, B) signals, analog quadrature (SIN, COS) signals, and / or serial data).

[0056] In this embodiment, the position encoder is an optical, diffraction-based, incremental position encoder. Thus, the scale 6 comprises an incremental track 10 including a series of periodic scale marks 14 that form a diffraction grating. The incremental track 10 may be commonly referred to as an amplitude scale or a phase scale. As understood, in the case of an amplitude scale, the feature is configured to control the amplitude of light transmitted toward the incremental detector of the reading head (e.g., by selectively absorbing, scattering, and / or reflecting light), whereas in the case of a phase scale, the feature is configured to control the phase of light transmitted toward the incremental detector of the reading head (e.g., by delaying the phase of light). In this embodiment, the incremental track 10 is an amplitude scale, but in either case, light interacts with the periodic scale marks 14 to produce diffracted orders, as will be described in more detail below. In the embodiment described, the period of the periodic scale marks 14 is 20 microns (μm).

[0057] Although not shown, the scale 6 may have one or more reference marks. For example, the scale may have a separate reference mark track having reference marks that define a reference position. The reference marks may serve to enable the reading head 4 to position its absolute position relative to the scale 6 in a unique incremental scale period, and the incremental position may be counted from a unique incremental scale period. As can be understood, there are numerous methods for providing reference marks. For example, the reference marks may be embedded within an incremental feature (as described, for example, in Patent Document 6). Optionally, the reference marks may be non-optical features. For example, the reference marks may be magnetic features, as described in Patent Document 7.

[0058] Referring to Figures 2(a) and 2(b), the read head 4 is shown in more detail. Note that in Figure 2(a), the read head is shown upside down compared to the configuration in Figures 1 and 2(b). The read head 4 comprises a light source 20, a first sensor 22 (hereinafter referred to as the "scale track sensor 22"), a second sensor 42 (hereinafter referred to as the "scale gradient sensor 42"), a processing device 24, an input / output unit 26, a printed circuit board (PCB) 28, and a housing 30. To be understood, references to "processing device / unit" in this specification include custom-configured processing devices / units for specific applications (e.g., field-programmable gate arrays "FPGAs") and more general-purpose processing devices / units that can be programmed (e.g., via software) according to the needs of the application in which they are used. Accordingly, suitable processing devices / units include, for example, a CPU (central processing unit), an FPGA (field-programmable gate array), or an ASIC (application-specific integrated circuit).

[0059] Light emitted by the light source 20 passes through the window 32 of the housing 30 and illuminates an area on the scale 6. The light is reflected by the periodic scale marks 14 and forms a pattern on the scale track sensor 22. As understood, the pattern may be formed in various ways, for example, by diffraction (e.g., forming a Talbot image) or by projection of the shadow of the scale pattern. The pattern formed on the scale track sensor may be a fringe pattern. The pattern changes in response to the relative movement of the reading head 4 and the scale 6 along the X dimension (measurement dimension) (e.g., the pattern moves). Such changes in the pattern are monitored by the processing device 24 to maintain a count of the relative positions of the reading head 4 and the scale 6.

[0060] The illuminated area on the scale 6, where light is reflected and diffracted to form a pattern on the scale track sensor 22, can be referred to as the scale track sensor's "reading footprint" (roughly highlighted as area 21 in Figure 2(b)). As can be understood, the actual area on the scale 6 illuminated by the light source 20 may be larger than the scale track sensor's "reading footprint" 21, but rays reaching the scale 6 outside the scale track sensor's reading footprint 21 do not contribute to the pattern detected by the scale track sensor 22 (for example, because those rays are reflected and diffracted beyond the range of the scale track sensor 22).

[0061] In the described embodiment, the scale track sensor 22 outputs analog quadrature sine and cosine signals. As will be understood by those skilled in the art, the amplitudes of the sine and cosine signals change in accordance with the relative movement of the scale 6 and the reading head 4, and cycling of these signals can be used to record a count of the relative locations of the scale 6 and the reading head 4. In the particular embodiment described, the scale track sensor 22 comprises an array of photodiodes. In particular, as will be understood by those skilled in the art, in embodiments in which a pattern is generated in the scale track sensor 22, the scale track sensor 22 can take the form of a so-called "electro-grating," which is, in other words, an optical sensor array comprising two or more (e.g., four) sets of interlocked / interlaced photosensitive sensors, each set detecting a different phase of the fringe. Such electro-gratings are known in the field of position measuring encoders and are described in more detail in Patent Documents 8, 9 and 10.

[0062] In the embodiments described, the light source 20 emits electromagnetic radiation (EMR) in the infrared range, but as will be understood by those skilled in the art, this is not necessarily the case, and the light source may emit EMR in other ranges, for example, any range from infrared to ultraviolet. As will be understood, the selection of a suitable wavelength for the light source 20 may depend on many factors, including the availability of suitable gratings and detectors that operate at EMR wavelengths.

[0063] As described above, the position encoder device is sensitive to errors caused by pitch, thereby causing the relative pitching of the reading head 4 and the scale 6 to displace the fringe field along the scale track sensor 22. Thus, both i) the relative motion of the scale 6 and the reading head 4 along the X dimension, and ii) the relative pitching of the reading head 4 and the scale 6 displace the fringe field along the scale track sensor 22. Therefore, without the present invention, the relative pitching of the scale 6 would be misinterpreted and reported by the encoder device as relative motion along the X dimension. As described above, and as will be discussed in more detail below, such relative pitching of the reading head 4 and the scale 6 can be caused, for example, by changes in the slope of the scale along the length of the scale due to the undulation of the scale.

[0064] The relationship between scale pitch / gradient and measurement error is explained with reference to Figure 3. When the portion of the scale located within the reading footprint 21 of the reading head is tilted by an angle α, a light ray emitted from a source perpendicular to the reading head is reflected at an angle 2α, and within the sensor plane, distance

[0065] Δx = u tan 2α (1)

[0066] It is displaced by only that much.

[0067] The actual measurement / position error Δe (i.e., the magnitude of the position error along the X dimension) caused by the scale pitch / gradient depends on the magnification M of the optical system and can therefore be determined from Δx by the following:

[0068]

number

[0069] Therefore, changes in the gradient of the scale 6 in the reading footprint 21 of the scale track sensor will adversely affect the pattern that reaches the scale track sensor 22. For example, changes in the gradient of the scale in the reading footprint 21 of the scale track sensor (as the reading head 4 and scale 6 move relative to each other) may affect the position of the pattern that reaches the scale track sensor 22. Without this invention, this would lead to the subsequent processing of the pattern to assume that more or less relative motion along the X dimension occurred (depending on the direction of the gradient) than actually occurred, thereby leading to measurement errors due to pitch. As can be understood, even a relatively small gradient angle α can induce significant measurement errors (for example, if we estimate M=2, then α=0.1°, u=3mm, then Δe=5μm).

[0070] For example, as shown in Figure 4(a), the scale 6 has a substantially constant gradient along most of its length, but there is a region 7 in the scale 6 with a small ridge, which changes the gradient of the scale, and this effectively causes a change in the relative pitch between the scale 6 and the reading head 4 at the point of the measurement / reading footprint 21, and therefore causes a shift in the fringe formed in the scale track sensor 22 as the reading head 4 passes over the ridge 7. Thus, if the gradient of the scale at position A of the reading head is taken as the reference gradient or datum gradient, the position shift effect on the fringe formed in the detector is zero (0) as the reading head 4 moves from left to right from position A to position B, and therefore the position measurement error due to the gradient of the scale is zero. However, between position B and position F, the gradient of the scale 6 (and therefore the relative pitch between the scale 6 and the reading head 4) changes, which has a position shift effect on the fringe formed in the scale track sensor 22.

[0071] For example, there is a relative rise in the scale gradient, as shown at position C, and a relative fall in the scale gradient, as shown at position E. As shown in Figures 4(a) and 4(b), the relative rise (with respect to the direction of travel) has a negative (-ve) position shift effect on the pattern reaching the scale track sensor 22, which (without the present invention) is reported to the controller 8 and leads to a negative position error in the position signal used by the controller 8.

[0072] For example, referring to Figure 5, the Lissajous figures of the analog quadrature sine and cosine signals output by the scale track sensor 22 of the reading head are shown. The dashed circle 50 schematically shows the ideal Lissajous shape generated by the sine and cosine signals as the reading head 4 and scale 6 move relative to each other. Spot 52 schematically shows the expected relative amplitude of the sine and cosine signals when the reading head is at reading head position C, and spot 54 schematically shows the actual relative amplitude of the sine and cosine signals when the reading head is at reading head position C. As shown, spot 54 is slightly delayed / behind the expected position (if there were no change in the scale gradient at position C) due to the positional shift of the pattern caused by the change in the gradient.

[0073] As shown in Figures 4(a) and 4(b), the downward slope of the scale (with respect to the direction of travel) has a positive (+ve) position shift effect on the pattern reaching the scale track sensor 22, which in turn shifts the position of the pattern on the scale track sensor 22 forward, and consequently leads to a positive position error in the position signal reported to and used by the controller 8 (if this invention were not present). Referring again to Figure 5, spot 56 schematically shows the expected relative amplitudes of the sine and cosine signals when the reading head is at reading head position E, and spot 58 schematically shows the actual relative amplitudes of the sine and cosine signals when the reading head is at reading head position E. However, due to the phase shift of the pattern caused by the change in the slope (in other words, caused by the change in the relative pitch of the scale and the reading head), the Lissajous curve is slightly advanced / forward than the expected position (if there was no downward slope in the scale at position E). For clarity of the illustration, as can be understood, spots 54 and 56 are shown as being located in opposing Lissajous quadrants, although this is not necessarily the case (each spot may be in any Lissajous quadrant).

[0074] Figure 6 shows that this is a problem not only with isolated bumps or depressions on a generally flat scale, but also with wavy scales. For example, at positions H and J, there is a relative rise in the scale gradient (with respect to the direction of travel), which has a negative (-ve) position shift effect with respect to the pattern reaching the scale track sensor 22, and at positions I and K, there is a relative fall in the scale gradient (with respect to the direction of travel), which has a positive (+ve) position shift effect with respect to the pattern.

[0075] The present invention overcomes errors caused by changes in the relative pitch of the scale and the reading head, for example, errors caused by changes in the scale gradient, by continuously monitoring at least one indication of changes in the relative pitch of the scale and the reading head in the reading footprint 21 of the scale track sensor (for example, by continuously monitoring at least one indication of changes in the scale gradient), and by dynamically compensating for such changes in response to indications of such changes, thereby reducing measurement errors caused by the scale gradient, and / or by providing an output indicating the scale gradient / relative pitch of the scale and the reading head.

[0076] Figure 7 shows one embodiment of a reading head 4 configured according to the present invention. As shown, the reading head 4 comprises a pitch monitor 40 (e.g., “scale gradient monitor”), the pitch monitor 40 comprises a pitch sensor 42 (e.g., “scale gradient sensor”), and is configured to determine and provide at least one signal based on the output of the pitch sensor 42 (as will be described in more detail in relation to Figures 8 and 9), which can be used to correct the position determined from a first (scale pattern) sensor 22. For example, the at least one signal output by the pitch monitor may include a measurement of the scale and relative pitch (e.g., gradient) of the reading head in the reading footprint 21 of the reading head, and / or a pitch (e.g., gradient) correction / compensation signal which can be used directly by a processing device 24 to correct the position, e.g., position signal, determined from the first (scale pattern) sensor 22.

[0077] In the embodiment shown in Figure 7, the reading head processing device 24 is configured to receive analog sine and cosine quadrature signals from the scale track sensor 22 and, based on these signals, output a position signal indicating the relative position of the reading head 4 and the scale (not shown in Figure 7) to an external device such as a controller 8. As shown in Figure 7, such an output signal can be one or a variety of different formats, for example, the output signal can be analog sine and cosine quadrature signals, digital quadrature signals, or digital serial data. According to the present invention, in this embodiment, the processing device 24 is configured to receive a pitch (e.g., scale gradient) compensation signal from a pitch (e.g., scale gradient) monitor 40 and use it to compensate for errors caused by changes in the relative pitch of the reading head and the scale (e.g., changes in the gradient of the scale) to reduce measurement errors caused by pitch (e.g., scale gradient) in the signal output by the reading head 4 to the controller 8. Further details on how the processing device 24 uses the pitch (e.g., scale gradient) compensation signal from the pitch (e.g., scale gradient) monitor 40 to compensate for errors caused by changes in the relative pitch (e.g., changes in its gradient) of the scale 6 and the reading head 4 will be explained below with reference to Figures 8 and 9.

[0078] In the embodiment shown in Figure 8, the processing device 24 is configured to output corrected analog quadrature sine and cosine signals. In this embodiment, this is achieved by shifting the analog sine and cosine signals forward along the sine wave (in other words, "advancing" the analog sine and cosine signals) or backward (in other words, "delaying" the analog sine and cosine signals). This is equivalent to rotating the Lissajous diagram described above counterclockwise or clockwise by the required compensation amount. Such rotation can be achieved by analog multipliers 60a to 60d and adder amplifiers 62a and 62b configured to implement the following equations.

[0079] COS(rotated) = [COS(input) × COS(rotation angle)] - [SIN(input) × SIN(rotation angle)] (4) SIN(rotated) = [COS(input) × SIN(rotation angle)] + [SIN(input) × COS(rotation angle)] (5)

[0080] The rotation angle is generated by the pitch monitor 40. In particular, in this embodiment, the pitch gradient monitor 40 includes a pitch sensor 42 that outputs a signal dependent on the scale and the relative pitch of the read head (for example, by measuring the gradient of the scale) in the reading footprint 21 of the read head (an example of which is described in more detail below in relation to Figures 12 to 15). Optionally, if necessary, an analog-to-digital converter (ADC) 44 may be provided to convert the output of the pitch sensor 42 into a digital signal. In this embodiment, the pitch monitor 40 includes an additional processing device 46 configured to convert the output of the pitch sensor 42 into a signal that can be used directly by the processing device 24 of the read head. In this embodiment, the processing device 46 of the pitch monitor 40 uses a vector rotation lookup table 46 to supply the cosθ and sinθ rotation angles to the processing device 24 based on the output of the pitch sensor 42. The processing device 24 receives the cosθ and sinθ rotation angles via digital-to-analog converters (DACs) 64a and 64b, converts them into analog signals for use by analog multipliers 60a to 60d, which implement the above equations to compensate for the effects of changes in the scale and the relative pitch of the reading head.

[0081] The embodiment in Figure 9 is similar to the embodiment in Figure 8 in that the reading head processing device 24 receives analog sine quadrature and cosine quadrature signals from the scale track sensor 22. However, in this embodiment, the reading head and processing device 24 are not configured to output analog quadrature sine and cosine signals, but are configured to output digital quadrature (A, B) position signals. In this embodiment, the processing device 24 is equipped with ADCs 70a and 70b for converting the analog sine and cosine signals from the scale track sensor 22 into digital values, which are then supplied to the cycle count and interpolation unit 72. The cycle count and interpolation unit 72 uses the outputs of ADCs 70a and 70b to record a count of the relative positions of the reading head and scale. The relative positions of the scale and reading head can be established by interpolation to a degree finer than the period of the scale feature (a fraction of the cycle count). The cycle count and the interpolated value are combined to provide a position value, which is supplied to the position compensation unit 74.

[0082] The position compensation unit 74 obtains position values ​​from the cycle count and interpolation unit 72 and also obtains compensation values ​​from the pitch monitor 40 to determine whether to advance or delay the position values, and by how much, to compensate for variations in the scale and the relative pitch of the read head in the read footprint 21 of the read head. Similar to the embodiment in Figure 8, the pitch monitor 40 includes a pitch 42 that outputs a signal dependent on the scale and the relative pitch of the read head. If necessary, an analog-to-digital converter (ADC) 44 can be provided to convert the output of the pitch 42 into a digital signal. In this embodiment, the pitch monitor 40 includes an additional processing device 46', which includes a position compensation value lookup table used to determine an appropriate compensation value to the processing device 24 based on the output of the pitch sensor 42, and which includes a position compensation value lookup table used by the position compensation unit 74 to advance or delay the position values ​​to compensate for variations in the slope of the scale in the read footprint 21 of the read head. The compensated position values ​​are then supplied to the orthogonal generation unit 76, which is configured to generate and output digital orthogonal (A, B) signals based on the compensated position values.

[0083] As shown in Figure 9, in an alternative embodiment, instead of generating a digital quadrature signal from the compensated position values ​​supplied by the position compensation unit 74, unit 76 may be configured to generate serial data for transmission over a serial communication link. Alternatively, unit 76 may be configured to convert the compensated position signal received from the position compensation unit 74 into an analog quadrature (SIN, COS) signal, for example, via a DAC.

[0084] In the embodiments shown in Figures 8 and 9, the gradient correction signal is determined by an auxiliary processing device 46 separate from the main processing device 24 of the read head, and this gradient correction signal is then supplied to the main processing device 24. However, this is not necessarily the case. For example, the read head 4 may be configured such that the main processing device 24 receives the raw output from the pitch sensor 42 and determines how to compensate the position signal based on the raw output from the pitch sensor 42.

[0085] Referring now to Figure 10, another embodiment of the present invention is shown. In contrast to the embodiment in Figure 7, the encoder device in Figure 10 is configured such that the read head processing device 24 does not receive a pitch compensation signal from the pitch monitor 40. Instead, the read head processing device 24 and the read head 4 itself output an uncompensated position signal (for example, this position signal can be in the form of a digital quadrature signal, an analog sine and cosine signal, or serial data) to an external device. The raw output of the pitch sensor 42, and / or the pitch compensation signal obtained therefrom, is also output to an external device, which uses the raw output of the pitch sensor 42 (and / or the pitch compensation signal, whichever is supplied) to correct the position signal received from the read head 4. In particular, in this embodiment, the position signal and the raw output of the pitch sensor 42 (and / or the pitch compensation signal, whichever is supplied) can be passed to the interface 7, which is configured to use the raw output of the pitch sensor 42 (and / or the pitch compensation signal, whichever is supplied) to compensate the position signal for errors caused by changes in the relative pitch of the scale and the reading head, thereby reducing measurement errors in the position signal that are passed to the controller 8.

[0086] As shown in Figure 10, the format of the position signal output to the controller 8 by the interface 7 can be the same as the format received from the read head 4, or the interface 7 may be configured to convert the position signal to a different format. Also, as shown in Figure 10, the interface 7 may be part of a controller device 8' that incorporates the interface 7 and the controller 8.

[0087] Figure 11 schematically shows how interface 7 of the apparatus in Figure 10 may be configured. For example, in an embodiment in which the read head 4 provides digital quadrature A, B signals, interface 7 may include a quadrature decoding unit 80 that generates position values ​​from the digital quadrature A, B signals. The position values ​​are passed to a position compensation unit 82, which also receives a pitch compensation signal from the read head 4 and uses the pitch compensation signal to compensate the position values ​​for errors caused by changes in the relative pitch of the read head and scale (e.g., due to changes in the slope of the scale). The compensated position values ​​are then supplied to a quadrature generation unit 84, which is configured to generate and output digital quadrature (A, B) signals based on the compensated position values. In an alternative embodiment, as shown in Figure 11, instead of generating digital quadrature signals from the compensated position values ​​supplied by the position compensation unit 82, unit 84 may be configured to generate serial data for transmission over a serial communication link. Alternatively, unit 84 may be configured to convert the compensated position values ​​received from position compensation unit 82 into analog quadrature (SIN, COS) signals.

[0088] In the embodiments shown in Figures 10 and 11, the pitch monitor 40 is configured to determine a pitch compensation signal and pass it to the interface unit 7. As can be understood, this is not necessarily required; for example, the pitch monitor 40 may simply output the raw signal from the pitch sensor 42, and the interface unit 7 may be configured to determine the pitch compensation signal itself.

[0089] In an optional alternative embodiment, interface 7 may be omitted, and the controller 8 itself may be configured to receive the uncompensated position signal and the raw output of the pitch sensor 42 (and / or the pitch-compensated signal, whichever is supplied) and determine a corrected position signal from there.

[0090] As will be understood by those skilled in the art, the pitch sensor 42 can measure the relative pitch of the scale and the read head in / within the read footprint 21 of the read head (for example, as in the embodiments of Figures 13 and 14 described below). Alternatively, the pitch sensor 42 may measure the relative pitch of the scale and the read head at a point outside the read footprint of the read head, from which the relative pitch in the read footprint of the read head can be determined / estimated. For example, as in the embodiment of Figure 12 described below, the pitch sensor 42 may measure the relative pitch of the scale and the read head at at least one point that coincides along the measurement dimension (i.e., along the X dimension) but is offset in a direction perpendicular to the measurement direction (i.e., along the Y dimension). In another alternative embodiment, the pitch sensor 42 may be configured to measure the relative pitch of the scale and the read head at at least one point in front of and / or after the read footprint of the read head, from which the relative pitch of the scale and the read head in the read footprint of the read head can be determined.

[0091] As can be understood, the above-described configuration for correcting the position signal of the read head, as shown in and related to Figures 7 to 11, may be applied to an absolute position encoder / read head. As can be understood, an absolute position encoder is a device having a scale (provided by one or more scale tracks) that has the characteristic of encoding a series of consecutive unique positions along the length of the scale, which allows the encoder device to determine the absolute position of the scale and the read head without requiring relative movement of the scale and the read head at startup. Compare this, for example, to an incremental encoder device that requires relative movement with respect to a reference mark.

[0092] For example, with respect to Figures 16 and 17, in an absolute system, the absolute encoder read head 4' / its processor 24' may be configured to output serial data (e.g., a codeword) indicating the absolute position of the scale 2 and the read head 4', instead of a cycle count. Thus, the processor unit 72' may be configured to receive and decode the absolute codeword detected by the sensor 22, and to find and determine the absolute position based on the decoded signal (for example, an exemplary technique is described in Patent Document 11). As can be understood, in addition to simply decode the codeword that reaches the sensor, the processor unit 72' may also fine-tune the determined position based, for example, the position of the codeword on the sensor (e.g., how far the codeword was along the sensor at the point where the codeword was read). In particular, in embodiments in which the absolute position is embedded within the absolute scale by omitting a selection scale line from a periodic arrangement of scale lines, such as that described in Patent Document 3, the fine-tuned position may include, for example, scale phase position information. As can be understood, the scale phase position information can identify where the read head / sensor is relative to the scale, as a division of the (e.g., nominal) scale period. For example, scale phase position information may include the position of a scale feature detected by the sensor of the reading head relative to a predetermined point on the sensor. This can be determined by examining the phase offset between the signal output by the sensor of the reading head and a predetermined reference signal (as described in more detail, for example, Patent Document 12). In an alternative embodiment (and in accordance with the content described in Patent Document 12), the absolute position can be determined from i) a coarse position extrapolated from a previously determined position and ii) fine / phase position information determined / extracted from the current scale reading. In any case, the absolute position can be compensated by the position compensation unit 74' according to a compensation value from the pitch monitor 40, as in the embodiments described above.In an optional embodiment in which scale phase position information is determined from signals received by the sensor, the step of compensating for the absolute position may include manipulating the scale phase position information (for example, before or after the rough position represented by a codeword is combined with the scale phase position information).

[0093] The compensated position measurement can then be output by the serial communication circuit 76'.

[0094] Figure 12 shows a first exemplary embodiment for determining a pitch (e.g., gradient) compensation signal. In this embodiment, a beam staircase 80 is positioned in front of the light source 20, thereby splitting the light emitted by the light source 20 into a first beam and a second beam, whose central rays 82 and 84 are shown as thick lines in Figure 12. The light of the first beam is reflected and diffracted by the incremental feature 14 in the incremental track 10, forming a pattern (e.g., a fringe field) on the scale track sensor 22. The reading footprint 21 of the scale track sensor on the scale 6 is schematically shown in Figure 12. The light of the second beam reaches a mirror-like surface on the scale 6 adjacent to the incremental track 10, and is thereby reflected onto a splitting detector 42, which is shown in Figure 12 as having two halves A and B. The footprint 23 of the second beam on the scale 6 is schematically shown in Figure 12. In the illustrated embodiment, the footprint of the second beam reaches the scale 6 at a position that is laterally offset (i.e., offset in the Y dimension with respect to the reading footprint 21 of the scale track sensor), but at substantially the same position along the measurement dimension of the scale (i.e., the X dimension) as the reading footprint 21 of the scale track sensor.

[0095] The relative pitch between the scale 6 and the reading head 4 (e.g., the slope of the scale 6 at the point where the second beam reaches the scale 6) affects the reflection angle toward the segmented detector 42, and therefore affects the position where the reflected second beam reaches the segmented detector 42. Accordingly, the outputs of the two halves A and B of the segmented detector 42 can be used to determine the relative pitch between the scale 6 and the reading head 4 (e.g., the slope of the scale 6 at the measurement point). For example, in one theoretical setting as shown in Figure 12, if the slope of the scale with respect to the horizontal (XY) plane is 0°, the outputs A and B of the segmented detector 42 may be equal, but if the slope of the scale changes, the output of A or B will increase relative to the other output. For example, a signal linearly proportional to the position of the spot reaching the segmented detector 42 can be determined as follows.

[0096]

number

[0097] In the described embodiments, the output S of the split detector 42 is directly linked to Δx, so the output S of the split detector 42 can be used to determine errors caused by changes in the relative pitch of the read head and scale (e.g., changes in the gradient of the scale), as described above in relation to Figures 7 to 11, for example, and / or can be used to determine a pitch (e.g., gradient) compensation signal. As can be understood, the relationship between S and Δx depends on the configuration of the split detector, but can be easily determined through calibration or from the manufacturer's specifications of the split detector. Thus, for example in relation to the embodiment of Figure 8, S (or Δx derived therefrom) can be used by the processor 46 of the pitch monitor to determine appropriate cosθ and sinθ rotation angles to pass to the processing device 24. In the described embodiments, the relationship between S and Δx is constant regardless of the ride height (distance between the read head and scale in the Z dimension), so it is not necessary for the ride height to be known or estimated in order to determine Δx from S. If the encoder configuration is such that the relationship between S and Δx changes with the ride height, the ride height can be estimated or determined / measured and incorporated into Δx from S, as described below in relation to the embodiment of Figure 13.

[0098] As can be understood, a two-dimensional splitting detector may be used as an alternative to the one-dimensional splitting detector 42 in Figure 12.

[0099] The segmented detector 42 described above requires a finite-width spot spanning two detectors A and B to enable the generation of a signal S that is linearly proportional to the pitch error. However, as is understood, if the spot is only on A or B, the segmented detector practically "stops working". Other types of optical position sensors (OPS) and position-sensitive devices (PSDs) (e.g., position-sensitive photodiodes) are well known, which have the ability to return a continuously variable signal with a linear offset across the entire width of the device's active region.

[0100] Furthermore, in another embodiment, it may not be necessary to provide separate sensors for determining the relative pitch / scale gradient and relative position of the reading head 4 and scale 6 along the measurement direction x. For example, a one-dimensional or two-dimensional sensor array, such as a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor, may be used, and its output may be analyzed to determine both the relative pitch / scale gradient and relative position of the reading head 4 and scale 6 along the measurement direction x.

[0101] As can be understood (and with reference to the above description of absolute encoder systems in relation to Figures 7 and 9), the embodiments described above in relation to Figure 12 are equally applicable to pitch-sensitive absolute position encoder devices (e.g., shadowcast absolute encoder devices) as to incremental position encoder devices.

[0102] Here, a second exemplary embodiment for determining the gradient compensation signal is described with reference to Figures 13 to 15. In addition to the changes in the scale gradient that cause the positional displacement errors described above, it has been identified that there are more subtle effects in that the shape of the optical signal is also distorted / deformed as a result of changes in the relative pitch of the scale and the reading head (for example, due to changes in the scale gradient). This is shown in Figure 13. The solid, dashed, and dotted rays represent the portions of the optical signal and their reflection paths at points directly below, diagonally to the left, and diagonally to the right of the reading head light source, respectively. The dashed line represents the corresponding reflected ray path on the level scale. Figure 13 shows that the scale gradient / pitch deflects the central ray (solid line) by Δx1, where Δx1 corresponds to a large displacement Δx of the optical signal according to the basic pitch error mechanism described above in relation to Figure 3. On the other hand, Figure 13 clearly shows that the oblique rays (dashed and dotted lines) are deflected by amounts greater than Δx1, Δx2 and Δx3. Therefore, the light signal is not only deflected, but also distorted / deformed.

[0103] Furthermore, the magnitude of this strain / deformation effect depends on the degree of the scale gradient / relative pitch. The strain / deformation "D" is known to depend continuously on the position along the sensor "x", the scale gradient / pitch α, and the read headride height h, and takes the following form:

[0104] D(x, α, h) = F1(h)αx 2 +F2(h)α (8)

[0105] F1(h) and F2(h) are known different scaling constants that depend on the ride height / separation "h" between the sensor and the scale and the reading head geometry.

[0106] Here, how F1(h) and F2(h) can be established will be explained with reference to Figures 14 and 15. Figure 14 schematically shows an exemplary system configuration in which the sensing surface 22' of the scale track sensor 22 and the light-emitting surface 20' of the light source 20 are intentionally not placed on the same plane. The distance from the sensing surface 22' of the scale track sensor 22 to the scale is h, and the vertical offset between the sensing surface 22' and the light-emitting surface 20' is t. t is known from the manufacturing / design of the reading head. When t is positive, the sensing surface 22' is closer to the scale than the light-emitting surface 20', and when t is negative, the sensing surface 22' is further from the scale than the light-emitting surface 20'. When t=0, the sensing surface 22' and the light-emitting surface 20' are on the same plane.

[0107] In the described embodiment, F1(h) and F2(h) take the following forms:

[0108]

number

[0109] F2(h) = -2h (10)

[0110] These equations hold without loss of generality even when t>0, t=0, or t<0. Therefore, the strain scaling constant depends on two factors, t and h.

[0111] A typical encoder will have a specified nominal ride height h0 (i.e., the distance from the datum feature of the reading head to the surface of the scale), but additionally, it will be able to operate correctly within a specified tolerance window of the ride height range Δh.

[0112] The value of h can be estimated. For example, the read head may be estimated to be operating at its nominal ride height such that h = h0. In this case, the subsequent pitch correction coefficient may be incomplete due to the discrepancy between the actual h and h0. This strategy may be acceptable for lower-performance encoder systems and / or when simplified computational workloads are preferred, and / or when the operating ride height range Δh is relatively small.

[0113] Optionally, the true current value of h may be determined and used to calculate the values ​​of F1(h) and F2(h). In this case, more calculations are required, but the resulting pitch compensation signal takes into account the deviation between h and h0.

[0114] An exemplary scheme for determining h in this manner is described in relation to Figure 14(a). (As can be understood, there are alternative approaches to independently determining h, for example, by adding an additional encoder or an alternative height measuring device). In the case of the encoder shown in Figure 14(a), the size L of the scale feature scale (For example, the repetition period of the scale, or the size of a typical non-periodic feature) is the corresponding size L of the feature image on the sensor. sensor The following are related:

[0115] L sensor =ML scale (11)

[0116] The magnification M is determined by the system geometry as follows:

[0117]

number

[0118] Figure 14 shows the case where t > 0 (when the sensing surface 22' is closer to the scale than the light-emitting surface 20'), but the above equation for M holds the same way when t < 0 (meaning the light-emitting surface 20' is closer to the scale than the sensing surface 22').

[0119] The vertical offset t is known (from the design / manufacturing of the read head), and M can be determined through analysis of the detector signal, for example, by frequency domain analysis of a periodic or quasi-periodic structure. For example, M can be determined by decomposing the spatially resolved detector signal into a corresponding sum of periodic signals, such as sine waves of different frequencies, via techniques known to those skilled in the art, such as Fourier transforms. Thus, M is determined by identifying the frequency component with the maximum amplitude within a suitable range. Thus, h can be determined by rearranging equation 12 as follows:

[0120]

number

[0121] Since h is known, F1(h) and F2(h) can be determined via equations 9 and 10.

[0122] In the embodiments described, the measurement of h is incorporated into pitch compensation. However, in other embodiments, this is not necessarily required. For example, the device may be configured to determine the isolation / ride height "h" and use / output it independently. For example, the isolation / ride height measurement may simply be output as information (to be used, for example, during the installation phase). In fact, according to another embodiment, no pitch compensation / calculation is performed at all. Thus, the device may be configured to determine the position measurement (along the measurement axis) and the isolation / ride height measurement.

[0123] In the described embodiment, a local strain D(x,α,h) is measured across the entire range of the sensor to determine the scale gradient / pitch angle α. In the described embodiment, the sensor is a one-dimensional sensor array of pixels that generate an electrical signal proportional to the light reaching each pixel. In the described embodiment, the signal reaching the sensor has at least some periodic content, for example, due to the periodicity of an incremental scale design, or by a non-periodic absolute scale having an embedded periodic structure (such as described in Patent Document 3). In such a signal, the periodic structure contributes to regularly spaced maximum and minimum values, and the strain effect shifts the positions of these maximum and minimum values ​​by a varying amount D(x,α,h) across the width of the sensor, as shown in Figure 15. Thus, this embodiment is applicable to both incremental scales and absolute scales having at least some periodic content (for example, an absolute scale in which absolute positions are embedded by omitting a selection scale line from a periodic arrangement of scale lines, such as described in Patent Document 3).

[0124] In the described embodiment, the strain D(x,α,h) is determined by measuring the maximum and minimum shifts of the periodic structure over the entire range of the sensor. One method to achieve this is to perform a single-point discrete Fourier transform of a windowed portion of I(x) evaluated at a spatial frequency 1 / P0, which is reasonably selected to be close to the nominal spatial frequency of the signal. The window function W(x) has the property that it is non-zero over a narrow range compared to the length scale over which the strain changes, and zero elsewhere. Thus, the single-point Fourier transform is evaluated as follows:

[0125]

number

[0126] The phase of this complex-valued Fourier transform can then be evaluated.

[0127]

number

[0128] however,

[0129]

number

[0130] and

[0131]

number

[0132] These represent the real and imaginary parts of Γ(x), respectively. The complex phase Θ(x) is periodic every 2π radians and therefore needs to be unwrapped using standard numerical methods known to those skilled in the art.

[0133] Θ unwrap (x) = unwrap{Θ(x)} (16)

[0134] As can be understood, the closer the selected spatial frequency 1 / P0 is to the nominal spatial frequency of the signal, the coarser the discrete sampling interval can be without causing the unwrapping algorithm to fail.

[0135] Finally, the unwrapped phase Θ unwrap (x) is transformed into distortion as follows:

[0136]

number

[0137] In the encoder application, the relative movement between the scale and the read head, i.e., the desired measurement object of the system, causes a large displacement of the optical signal across the entire sensor. Here, therefore, in addition to the scale gradient distortion, an additional term δx can be added to Equation (8) for D(x,α,h) in order to consider the influence of the relative movement between the scale and the read head.

[0138] Δd(p,α,h)=F1(h)αp 2 +F2(h)α+δx (18)

[0139] Since all other relevant quantities are known, it is possible here to use this equation to calculate the scale gradient / pitch angle α. There are various techniques with advantages and disadvantages that can achieve this. One approach is, for example, to use standard least squares error minimization techniques to fit a quadratic polynomial curve of the form D(x,α,h)=Ax 2 +B to the measured D(x,α,h) and determine α from the fitting parameters as follows.

[0140] α=A / F1(h) (19)

[0141] Another approach is to evaluate D(x,α,h) only at specific locations, for example,

[0142] - At the center of the image p = 0, D(0,α,h) is obtained - At the point x = x sample is typically placed near the end of the sensor where the distortion is maximum, and Δx(x sample ,α,h) is obtained

[0143] Under this approach, α is evaluated as follows.

[0144]

Number

[0145] Alternatively, a variation of this latter approach may be adopted, in which a small number of sampling points x=x sample1 x=x sample2 x=x sample3 ...The process is repeated, the results are averaged, and the accuracy of the results is improved.

[0146] Once the scale gradient angle α is determined by an approximate or suitable approximation, the final step is to determine a pitch compensation signal / coefficient that can be used to apply a corrective calibration to the encoder output position. It should be noted that the presence of a local scale pitch causes a macroscopic shift of the optical signal across the entire sensor (see, for example, Figure 3), in addition to the distortion effect employed by the present invention to characterize α (see, for example, Figure 13). In fact, we have already taught the mathematical form (see, for example, Equation 8), where the F2 term corresponds to the global shift and the F1 term corresponds to the distortion.

[0147] Therefore, the measurement error caused by the scale pitch is represented by the F2(h)α = -2hα term in the equation for D(x,α,h). Note that this corresponds to a simplified discussion of the problem surrounding Figure 3 before the influence of strain is introduced.

[0148] Therefore, the pitch compensation signal Δe corr This can be calculated.

[0149] Δe corr = +2hα (21)

[0150] As mentioned above, an estimate of h may be applied to determine the approximate correction coefficient, or alternatively, a nearly accurate correction coefficient may be applied based on an independent determination of h.

[0151] Finally, this pitch compensation signal Δe corrThis is applied to the encoder's position output, and for example, as described above in relation to Figures 7 to 11, a pitch-corrected position output can be obtained.

[0152] In the embodiments of Figures 13 and 14, there is a single sensor 22 used to determine both the scale pitch / gradient (and separation / ride height) along the measurement direction x and the relative position of the reading head 4 and the scale 6. Advantages of such a configuration include cost-effectiveness (no additional sensors are required) and improved accuracy because the pitch and / or separation / ride height measurements are taken at the same points as the position measurements. However, as can be understood, separate sensors may be used as needed, as described above in relation to Figure 12 (for example, a separate sensor may be provided for determining the distortion of a pattern on the scale, which may be, for example, a scale feature or an adjacent pattern).

[0153] It should be noted that, as described above, if the ride height is accurately determined by an approach such as a ride height-dependent magnification, the encoder can provide multiple measured outputs, including any combination of compensated position, ride height, and local scale pitch / gradient.

[0154] This could offer users additional benefits, such as tracking installation and execution tolerances.

[0155] It should also be noted that the sensor used to characterize pitch / scale gradient via scale pattern distortion as described above does not need to be an additional single-purpose sensor "bolted" to the read head to compensate for pitch errors. It is entirely possible to use a single sensor for both determining the read head position and compensating for scale gradient / pitch.

[0156] Alternative embodiments of the present invention include using height / distance sensors positioned forward and backward along the measurement dimension to measure the separation between the scale and the reading head. For example, the reading head may include a first capacitance sensor configured to detect the separation between the reading head and the scale at a forward position, and a second capacitance sensor configured to detect the separation between the reading head and the scale at a backward position. As understood, any change in the relative pitch of the scale and the reading head will cause a change in the difference between the output of the first capacitance sensor and the output of the second capacitance sensor, which can be used to determine the scale of the relative pitch (for example, as in the embodiment of the segmented detector 42 in Figure 12).

[0157] In the embodiments described above, the encoder device is an optical encoder device, but this is not necessarily required. For example, the encoder device may be a magnetic, inductive, or capacitive encoder device.

Claims

1. A position encoder device comprising a scale and a reading head equipped with at least one scale signal sensor configured to detect a scale signal, wherein at least one position signal indicating the relative position of the scale and the reading head along the measurement direction of the scale is generated and output from the scale signal, and the encoder device is configured to compensate for changes in the relative pitch of the reading head and the scale to reduce positional displacement errors in the at least one position signal caused by the pitch. Position encoder device.

2. The encoder device is configured to continuously monitor indications of changes in the relative pitch of the reading head and the scale as the scale and the reading head move relative to each other, and the compensation is based on the results of the monitoring. The position encoder device according to claim 1.

3. Compensating for changes in the relative pitch of the reading head and the scale to reduce pitch-related positional displacement errors in the at least one position signal includes advancing or delaying the relative position indicated by the at least one position signal based on the results of the monitoring. The position encoder device according to claim 2.

4. The monitoring includes analyzing the deformation of the scale signal reaching the sensor. A position encoder device according to any one of claims 1 to 3.

5. The monitoring includes analyzing a deformation of a signal reaching the at least one scale signal sensor configured to detect a scale signal, and from the scale signal, at least one position signal is generated indicating the relative position of the scale and the reading head. The position encoder device according to claim 4.

6. The monitoring includes analyzing the deformation of the scale signal, from which at least one position signal indicating the relative position of the scale and the reading head is generated. The position encoder device according to claim 5.

7. Further configured to determine a separation signal indicating separation between the reading head and the scale, A position encoder device according to any one of claims 1 to 6.

8. The separation signal indicating the separation between the reading head and the scale is taken into consideration to compensate for changes in the relative pitch of the reading head and the scale, thereby reducing positional displacement errors in the at least one position signal due to pitch. The position encoder device according to claim 7.

9. The position encoder device is an absolute type encoder device. A position encoder device according to any one of claims 1 to 8.

10. The reading head is configured to perform the compensation. A position encoder device according to any one of claims 1 to 9.

11. A position encoder device comprising a scale and a reading head equipped with at least one scale signal sensor configured to detect a scale signal, wherein at least one position signal indicating the relative position of the scale and the reading head along the measurement direction of the scale can be generated from the scale signal, and the position encoder device is configured to generate at least one such position signal from the scale signal detected by the scale signal sensor, and i) Based on the scale signal, determine a separation signal indicating the separation between the reading head and the scale, and / or ii) Determine a pitch signal indicating the relative pitch between the reading head and the scale based on the scale signal. A position encoder device configured as follows.

12. The encoder device is configured to determine the separation signal and / or the pitch signal based on the scale signal sensed by the at least one scale signal sensor. The position encoder device according to claim 11.

13. The encoder device, a) The separation signal and / or the pitch signal, b) Based on the same scale signal sensed by the at least one scale signal sensor, the at least one position signal and Configured to determine, The position encoder device according to claim 11 or 12.

14. The encoder device is configured to determine the separation signal and / or the pitch signal based on the size of the features of the scale signal. A position encoder device according to any one of claims 11 to 13.

15. The encoder device is configured to determine the separation signal and / or the pitch signal based on the period size of the scale signal. A position encoder device according to any one of claims 11 to 14.

16. The encoder device is configured to determine the pitch signal based on a transformation of the scale signal. A position encoder device according to any one of claims 11 to 15.

17. The encoder device is configured to determine the pitch signal based on the deformation of the scale signal along the measurement dimension. The position encoder device according to claim 16.

18. The scale signal has a periodic content, and determining the separation signal and / or the pitch signal includes analyzing the periodic content of the signal reaching the sensor. A position encoder device according to any one of claims 11 to 17.

19. The position encoder according to claim 18, wherein determining the pitch signal depends on analyzing how the period of the signal changes over the entire sensor, particularly along the measurement dimension.

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