Coordinate positioning device, method for controlling coordinate positioning device, and computer program causing method to be performed

Abstract

To provide a coordinate positioning device.SOLUTION: A coordinate positioning device includes: a structure movable within a working volume of the device; a drive arrangement for moving the structure around the working volume with fewer degrees of freedom than six degrees of freedom; and a measuring arrangement for measuring the position of the structure within the working volume with more degrees of freedom than the drive arrangement. The measuring arrangement includes a plurality of measurement transducers and the drive arrangement includes a plurality of actuators. The device further includes a controller operable to request data values from the measurement transducers of the measuring arrangement and to control the actuators of the drive arrangement to position the movable structure within the working volume using the data values received from the measurement transducers of the measuring arrangement.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to coordinate positioning apparatus, including, for example, coordinate measuring machines (CMMs) and machine tools. [Background technology]

[0002] A non-Cartesian coordinate positioning apparatus 1 is illustrated diagrammatically in Figure 1 of the accompanying drawings. The coordinate positioning apparatus 1 generally comprises a first structure 2 and a second structure 4 which are supported and moved relative to each other by a plurality of telescoping or extendable legs 6 provided therebetween. The first structure 2 and the second structure 4 may also be referred to as a platform or stage, and the extendable legs 6 may also be referred to as struts or rams. When there are six such extendable legs 6 (as illustrated in Figure 1), the apparatus is generally referred to as a hexapod.

[0003] The extendable legs 6 are typically mounted on structures 2 and 4 via ball joints 8, with each leg 6 either having its own ball joint 8 at one or both ends (as shown in FIG. 1 ) or sharing a ball joint 8 with an adjacent leg 6 at one or both ends. Each extendable leg 6 is typically formed as a pair of tubes, with one tube telescopically moved within the other by a drive mechanism (e.g., a motor) to provide extension and retraction of the extendable leg 6.

[0004] Various relative positions between the first structure 2 and the second structure 4 can be achieved by extending the legs 6 by different amounts, as illustrated in FIG. 1 by the arrows 13. The relative position at any instant is monitored by a number of length measuring transducers 10, for example one transducer associated with each extendable leg 6. Each length measuring transducer 10 may include an encoder scale paired with a read head, the encoder scale being suitably mounted on one of the pair of telescopic tubes and the read head being suitably mounted on the other. Thus, extension of the leg 6 moves the encoder scale past the read head, thereby allowing the length of the extendable leg 6 to be measured. The computer controller 5 operates to set the length of each extendable leg 6, providing the required relative movement between the structure 2 and the structure 4. By having six such length measuring transducers 10, the relative position can be measured in six corresponding respective degrees of freedom (three translational degrees of freedom and three rotational degrees of freedom).

[0005] One of structure 2 and structure 4 is typically provided as part of the fixed structure of the coordinate positioning apparatus 1, and the other of structure 4 and structure 2 involves movements 12 and 11 relative to the fixed structure. A tool (e.g. a measurement probe or drill) can be mounted on the moving structure and a workpiece can be mounted on the fixed structure, or vice versa, allowing an operation to be performed on the workpiece (e.g. measuring, probing or scanning in the case of a coordinate measuring machine, or machining in the case of a machine tool).

[0006] 1, the lower structure 4 is fixed and the upper structure 2 is movable, with a workpiece 9 mounted on the lower structure 4 and a probe component 3 mounted on the upper structure 2. A working volume 14 is defined between the upper structure 2 and the lower structure 4 when in their most spaced apart position, and the probe component 3 is positioned within the working volume 14 by operation of the extendable legs 6. Although arrow 11 is shown to indicate translational movement, by appropriate control of the various legs 6, the structure 2 may also be tiltable.

[0007] Alternatively, the upper structure 2 may be fixed and the lower structure 4 may be movable, with the probe mounted to the lower surface of the lower structure 4 and the workpiece mounted below a portion of the fixed structure, such that the working volume (or operating volume) of the apparatus is below the lower structure 4 rather than above it.

[0008] Various types of non-Cartesian coordinate positioning devices are described in more detail in U.S. Patent Nos. 5,233,633; ...

[0009] For example, US Pat. No. 5,399,633 describes a hexapod machine tool comprising an upper movable structure which is attached to a base by six hydraulically extendable legs, similar in principle to that illustrated in FIG. 1 described above. The extendable legs are attached to the base and to the movable structure via ball joints. The extendable legs are hydraulic and include piston rods which are movable in cylinders. The amount of leg extension is measured by mounting a magnetic scale on the cylinder and a suitable read head on the piston rod. Leg extension thus moves the scale past the read head, thereby allowing the leg length to be measured. A computer controller operates to set the length of each leg and provide the required movement.

[0010] US Patent No. 5,399,633 discloses the use of a delta robot to move an end effector and an imaging detector to capture image data of at least a portion of the end effector. From the captured image data, and in particular from reference points on the end effector, the position of the end effector can be determined by photogrammetry. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 91 / 03145 Brochure [Patent Document 2] International Publication No. 95 / 14905 Brochure [Patent Document 3] International Publication No. 95 / 20747 Brochure [Patent Document 4] International Publication No. 92 / 17313 Brochure [Patent Document 5] International Publication No. 03 / 006837 Brochure [Patent Document 6] International Publication No. 2004 / 063579 Brochure [Patent Document 7] International Publication No. 2007 / 144603 Brochure [Patent Document 8] International Publication No. 2007 / 144573 Brochure [Patent Document 9] International Publication No. 2007 / 144585 Brochure [Patent Document 10] International Publication No. 2007 / 144602 Brochure [Patent Document 11] International Publication No. 2007 / 144587 Brochure [Patent Document 12] European Patent Application Publication No. 3054265A1 [Patent Document 13] US Patent Application Publication No. 2003 / 0005786 [Patent Document 14] International Publication No. 2017 / 021733 Brochure [Patent Document 15] International Application No. PCT / GB2017 / 050909 [Patent Document 16] U.S. Pat. No. 4,976,582 [Patent Document 17] US Patent Application Publication No. 2009 / 0066100A1 [Non-patent literature]

[0012] [Non-Patent Document 1] HJJ Braddick, “Mechanical Design of Laboratory Apparatus”, Chapman & Hall, London, 1960, pages 11-30 Summary of the Invention [Means for solving the problem]

[0013] According to a first aspect of the invention there is provided a coordinate positioning apparatus comprising a structure movable within a working volume of the apparatus, a hexapod metrology arrangement for measuring a position of the structure in the working volume, and a non-hexapod drive arrangement for moving the structure around the working volume.

[0014] According to a second aspect of the invention there is provided a coordinate positioning apparatus comprising a structure movable in a working volume of the apparatus, a drive arrangement for moving the structure around the working volume with fewer than six degrees of freedom, and a metrology arrangement for measuring a position of the structure in the working volume with more degrees of freedom than the drive arrangement.

[0015] According to a third aspect of the present invention there is provided a coordinate positioning apparatus comprising a structure movable within a working volume of the apparatus, a metrology arrangement for measuring a position of the structure in the working volume and a drive arrangement for moving the structure around the working volume, the metrology arrangement including a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurement values ​​from which a position of the movable structure is determinable, and the drive arrangement including a plurality of mechanical linkages arranged in parallel between the movable structure and a fixed structure of the apparatus, each mechanical linkage being actuated by a drive mechanism acting between the fixed structure and the mechanical linkage.

[0016] According to a fourth aspect of the invention, there is provided a coordinate positioning apparatus comprising a structure movable in a working volume of the apparatus, a metrology arrangement for measuring a position of the structure in the working volume, and a drive arrangement for moving the structure around the working volume, the metrology arrangement including a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurement values ​​from which a position of the movable structure is determinable, and the drive arrangement including a plurality of actuators in a parallel arrangement of a different type than that of the metrology arrangement. For example, the parallel actuator arrangement may be a tri-glide type arrangement or a cable robot type arrangement, while the parallel measurement transducer arrangement may be a hexapod type arrangement.

[0017] The metrology arrangement may include a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurements from which a position of the moveable structure is determinable.

[0018] The measurement transducer may be a length measuring transducer.

[0019] The measurements can relate to different respective distances between the moveable structure and a fixed structure of the apparatus.

[0020] The measurement transducer may be adapted to provide a direct measurement of distance.

[0021] The measurement transducer may be adapted to provide a direct measurement of the change in distance as the structure moves around the working volume, from which the distance may be determined.

[0022] Each of the measurement transducers may include an encoder scale and an associated readhead.

[0023] The metering arrangement may include a plurality of extendable legs arranged in parallel, the extendable legs corresponding in number to a number of measurement transducers, each of the plurality of measurement transducers being associated with a different respective one of the plurality of extendable legs.

[0024] The coordinate positioning apparatus may be equipped with six such measurement transducers.

[0025] The multiple measurement transducers may be multiple independent measurement transducers.

[0026] The multiple measurement transducers described herein are to be contrasted with image-based, or photogrammetric, metrology arrangements, for example in which each image capture device does not make any direct or independent measurement of length or distance, or of the position of at least a portion of the movable structure, and a determination of the position of the movable structure can only be made based on a photogrammetric combination of images from all of the image capture devices, in which distances can only be indirectly estimated from image data.

[0027] The drive arrangement may be adapted to maintain the moveable structure in a substantially constant orientation as it moves about the working volume.

[0028] The drive arrangement may include a number of mechanical linkages connected in parallel between the moveable structure and a fixed structure of the apparatus.

[0029] Each mechanical linkage can be actuated (or driven) by a drive mechanism acting between the fixed structure and the mechanical linkage. Here, a distinction is made between such drive mechanisms and those that act, for example, between two parts (or links) of the mechanical linkage. For example, in a typical hexapod drive arrangement, the motor for each extendable leg acts between the two parts of the extendable leg, pushing the two parts away from each other to extend the leg and doing the opposite to retract the leg. The motor does not act between the extendable leg (mechanical linkage) and the fixed structure. As such, in embodiments of the present invention, it is possible to consider that each mechanical linkage can be actuated (or driven) by a drive mechanism acting directly between the fixed structure and the mechanical linkage.

[0030] Movement of a driven part of a mechanical linkage can be caused by the drive mechanism associated with that mechanical linkage, rather than by one or more other drive mechanisms associated with other mechanical linkages of the drive arrangement.

[0031] The driven parts of the mechanical linkage may be carriages, which are arranged to move linearly along corresponding tracks. A number of such tracks may be arranged substantially parallel to one another. In the case of a tri-glide drive arrangement, three such tracks are provided.

[0032] The drive mechanism may be a rotary drive mechanism, such as that found in a delta robot.

[0033] Such a drive mechanism may be arranged to drive a driven part of a mechanical linkage in a substantially rotary manner relative to a fixed structure.

[0034] The rotary drive may be a direct rotary drive.

[0035] The drive mechanism may be a tri-glide arrangement, or a linear drive mechanism such as those found in cable robots.

[0036] Such a drive mechanism can be arranged to drive a driven part of the mechanical linkage in a substantially linear manner relative to the fixed structure, e.g., along a substantially linear feature of the fixed structure, e.g., along a substantially linear track of the fixed structure.

[0037] The linear drive mechanism may be a direct linear drive mechanism.

[0038] The linear drive mechanism may be arranged to translate the end of the mechanical linkage in a substantially linear manner.

[0039] The linear drive mechanism may include a linear motor.

[0040] Each mechanical linkage can include at least two substantially parallel rods and maintains the moveable structure in a substantially constant orientation as it moves about the working volume.

[0041] The drive arrangement may include three such mechanical linkages.

[0042] The drive arrangement may be a tri-glide configuration.

[0043] The metrology arrangement may include a plurality of mechanical linkages arranged in parallel between the fixed structure and the movable structure, with a corresponding plurality of measurement transducers respectively associated with the plurality of mechanical linkages. There may be six such mechanical linkages of the metrology arrangement, and six corresponding respective measurement transducers.

[0044] Each mechanical linkage of the metering arrangement can be respectively connected between a point on the fixed structure and the moveable structure and can be adapted to allow the distance between those points to be changed.

[0045] A measurement transducer associated with the mechanical linkage of the metering arrangement may be adapted to provide an output that is distance dependent.

[0046] Each mechanical linkage of the metering arrangement may be a telescoping or extendable leg.

[0047] The mechanical linkage may also be referred to or thought of as a kinematic chain, or a mechanical assembly.

[0048] The drive mechanisms may be provided separately for each mechanical linkage of the drive arrangement.

[0049] A drive mechanism associated with the mechanical linkage can be positioned to act between the fixed structure and an end of the mechanical linkage.

[0050] Each mechanical linkage may include at least one rigid rod.

[0051] Each mechanical linkage can include at least two substantially parallel rods to maintain the moveable structure in a substantially constant orientation as it moves about the working volume.

[0052] Each of the mechanical linkages may be of substantially the same configuration or design.

[0053] A drive arrangement having three mechanical linkages, each with a linear drive mechanism, is known as a tri-glide configuration.

[0054] The drive arrangement may include, be in the form of, or provide a delta robot configuration.

[0055] The drive arrangement may include or be in the form of or provide a linear delta robot configuration.

[0056] The drive arrangement can be adapted to move the structure around the working volume with fewer than six degrees of freedom, and the metrology arrangement can be adapted to measure the position of the structure within the working volume with more degrees of freedom than the drive arrangement.

[0057] The metering arrangement may be a hexapod metering arrangement.

[0058] The drive arrangement may be a non-hexapod type drive arrangement.

[0059] The actuation arrangement may be adapted to move the structure around the working volume in three degrees of freedom.

[0060] The three degrees of freedom can be three translational degrees of freedom.

[0061] The metrology arrangement can be adapted to measure the position of the structure in six degrees of freedom (three translational and three rotational degrees of freedom, ie position and orientation).

[0062] The metering arrangement and the drive arrangement may be respectively arranged between (eg connected to) the moveable structure and a fixed structure of the apparatus.

[0063] The metering arrangement and the drive arrangement can be connected or coupled to both a movable structure and a fixed structure, respectively.

[0064] The drive arrangement may include multiple actuators in a parallel arrangement, which contrasts with the serial arrangement of actuators present, for example, in a conventional 3-axis (x, y, z) coordinate measuring machine (CMM).

[0065] The actuation arrangement may include multiple actuators in a parallel arrangement of a different type to that of the metering arrangement.

[0066] The actuation arrangement may include a plurality of measurement transducers for providing corresponding respective measurement values, separate from those of the metrology arrangement, from which a position of the moveable structure is determinable independently from a position determined based on measurements from the metrology arrangement, in other words the actuation arrangement may be encoded independently from the metrology arrangement.

[0067] The drive arrangement may be coupled to the weighing arrangement via a coupling arrangement, which prevents at least some strain associated with the drive arrangement from being transmitted to the weighing arrangement.

[0068] The coupling arrangement may be a kinematic coupling arrangement or a quasi-kinematic coupling arrangement.

[0069] The coupling arrangement may include a plurality of balls.

[0070] The coupling arrangement may include a number of resilient spacers or pads.

[0071] The movable structure may include a drive part associated with the drive arrangement and a metering part associated with the metering arrangement, the drive part of the movable structure being coupled to the metering part of the movable structure via the coupling arrangement.

[0072] The drive parts of the movable structure may be coupled to a drive arrangement.

[0073] The metering part of the movable structure may be coupled to the metering arrangement.

[0074] The coordinate positioning apparatus may comprise a fixed structure having a drive part associated with the drive arrangement and a measurement part associated with the measurement arrangement, the drive part of the fixed structure being coupled to the measurement part of the fixed structure via a coupling arrangement.

[0075] The drive parts of the fixed structure may be coupled to a drive arrangement.

[0076] The metering part of the fixed structure may be coupled to the metering arrangement.

[0077] The movable structure can carry the operating tool, in other words the operating tool can be moved by the movable structure while the weighing arrangement is coupled to the movable structure. A coordinate measuring machine in such a configuration is, for example, ready for operational use (the working tool is in a suitable location) rather than simply ready to be calibrated (in which case the working tool is not present in a suitable location).

[0078] The apparatus may be a coordinate measuring machine.

[0079] The device may be a comparator.

[0080] The operational tool may be a surface sensing device, or a measurement probe.

[0081] The apparatus may be a machine tool.

[0082] The operating tool may be a mechanical tool for shaping or machining a material.

[0083] The metering arrangement may include measurement transducers arranged exclusively in parallel with one another, ie no measurement transducers are arranged in series with one another.

[0084] The metrology arrangement may include six measurement transducers in a parallel arrangement to provide six corresponding respective measurements from which the position of the moveable structure can be determined. A metrology arrangement having fewer than six measurement transducers in a parallel arrangement is not a hexapod type metrology arrangement (e.g. a tripod is not a hexapod).

[0085] The metering arrangement can be considered to be for measuring different positions of the movable structure in the working volume resulting from respective different states or configurations of the drive arrangement. In other words, the metering arrangement measures a position of the movable structure (first position) in a first configuration of the drive arrangement, the drive arrangement then moves to a second configuration different from the first one, and the metering arrangement measures a position of the movable structure (second position) in the second configuration of the drive arrangement. The drive arrangement can be considered to include all parts of the apparatus used to move the movable structure from the first position to the second position.

[0086] The weighing arrangement may include six extendable legs arranged in parallel, six measurement transducers being associated with the six extendable legs respectively.

[0087] The measurement transducer may be an interferometric measurement transducer.

[0088] The drive arrangement may include fewer than six actuators in a parallel configuration.

[0089] The drive arrangement can be a parallel kinematic arrangement.

[0090] The drive arrangement may be a non-Cartesian arrangement.

[0091] The drive arrangement may include fewer than six actuators.

[0092] The parallel arrangement of the actuators associated with the drive arrangement may be different to the parallel arrangement of the measurement transducers associated with the metering arrangement.

[0093] The drive arrangement may include a plurality of measurement transducers to provide corresponding respective measurement values ​​from which the position of the moveable structure can be determined.

[0094] The measurement transducer may be a mechanical measurement transducer, as opposed to an optical, or image-based, or photogrammetric measurement transducer.

[0095] The measurement transducer may be a length measuring transducer. Measuring the length of a part of the device, such as an extendable leg, can be considered equivalent to measuring the distance between two parts of the device, such as the ends of an extendable leg. The transducer may not measure absolute length or distance, but may measure changes in length or distance, from which absolute length or distance can be determined (e.g., based on a geometric model of the device). Examples of sensors that do not measure length but can be used in combination with other sensor data, for example to determine position, are accelerometers (acceleration sensors), tilt sensors, and gyroscopes.

[0096] The measurement transducer can be sampled at a first clock rate that is comparable to (e.g. at least half of, at least equal to, or substantially the same as) a second clock rate used to control the drive arrangement.

[0097] Measurements received from the measurement transducer may enable the position of the structure to be determined without reference to other sensor or transducer data (e.g., photogrammetric data from an image sensor, etc.), which may be acquired at a third clock rate lower than the first clock rate.

[0098] The first clock rate may be greater than 1 kHz, more preferably greater than 10 kHz, and more preferably greater than 15 kHz.

[0099] The metering arrangement can for example be optically and non-contactably coupled to the movable structure, in which case the metering arrangement is an optical metering arrangement.

[0100] For example, the metering arrangement may be mechanically coupled to the moveable structure, such as when the metering arrangement is a hexapod metering arrangement.

[0101] The metrology arrangement may for example be a mechanical metrology arrangement, as opposed to an optical, or image-based, or photogrammetric metrology arrangement.

[0102] The metering arrangement may be mechanically coupled to the moveable structure.

[0103] The extendable legs may include any mechanical arrangement (e.g., a mechanical linkage) that allows the distance between a point on a fixed structure and a point on a movable structure to be changed.

[0104] The movable structure can be adapted to support or carry an object that is moved around the work volume. The object can be something that is picked up and / or placed in the work volume. The object can be a tool for interacting with or operating on another object (such as, for example, a workpiece) positioned in the work volume. The tool can be a surface-sensing device. The surface-sensing device can be a measurement probe. The measurement probe can be a contact probe. The contact probe can include a stylus that, in use, makes physical contact with the workpiece surface to take measurements. The measurement probe can be a non-contact probe. The non-contact probe can be an optical probe. The tool can include a camera for imaging the surface of the workpiece. The tool can be a mechanical tool typically found in machine tools for forming or machining metals or other rigid materials.

[0105] The movable structure can be adapted to move the motion tool with the metrology arrangement and the drive arrangement also coupled to the movable structure. The coordinate positioning apparatus can be set up with the motion tool already coupled to the movable structure and ready to perform its intended motion. In other words, the coordinate positioning apparatus can be set up for motion rather than just for calibration.

[0106] The hexapod weighing arrangement may be coupled to the moveable structure via a different attachment than that used to attach the operating tool to the moveable structure.

[0107] The hexapod weighing arrangement may be directly coupled to the moveable structure.

[0108] A "transducer" may be considered herein to be either a device that converts a variation in a physical quantity into an electrical signal (a "sensor", such as a measurement transducer described herein that measures or senses a change in length) or vice versa, a device that converts an electrical signal into a physical quantity (an "actuator", such as a motor and associated drive linkage described herein that provides movement to a structure based on an input or drive signal).

[0109] Measuring the "position" of a structure is understood as measuring the position and / or orientation of the structure up to an appropriate number of degrees of freedom. For example, if the position is measured in six degrees of freedom, then both the position and orientation of the structure are determined. However, if the position is only measured in three degrees of freedom, then this may or may not include determining the orientation of the structure. The term "measuring the position" is interpreted accordingly.

[0110] According to a fifth aspect of the present invention there is provided a method of controlling a coordinate positioning apparatus according to any of the aspects described above, comprising the steps of coupling a tool to a moveable structure, moving the tool around a working volume using a drive arrangement with a metrology arrangement also coupled to the moveable structure, and performing an operation with the tool.

[0111] The method may comprise the step of determining a position of the tool in the working volume for operation using a metrology arrangement.

[0112] The method may comprise the step of associating the determined location with an action to be performed.

[0113] The operation may be a measurement operation. The operation may be a machining operation.

[0114] The tool may be a measurement probe and the operation may be a measurement operation, such as making a touch-trigger measurement of a workpiece positioned within the working volume.

[0115] The method can be performed based on the location of the structure, rather than the tool, or a combination thereof.

[0116] According to a sixth aspect of the invention there is provided a controller for a coordinate positioning apparatus, the controller being configured to carry out a method according to the fifth aspect of the invention.

[0117] According to a seventh aspect of the present invention there is provided a computer program which, when executed by a coordinate positioning device controller, causes the controller to perform a method according to the fifth aspect of the present invention or which, when loaded into a coordinate positioning device controller, causes the coordinate positioning device controller to become a coordinate positioning device controller according to the sixth aspect of the present invention. The program may be embodied on a carrier medium. The carrier medium may be a storage medium. The carrier medium may be a transmission medium.

[0118] According to an eighth aspect of the present invention there is provided a computer readable medium having stored therein computer program instructions for controlling a coordinate positioning apparatus controller to perform a method according to the fifth aspect of the present invention. [Brief description of the drawings]

[0119] Reference is now made, by way of example, to the accompanying drawings in which: [Figure 1] FIG. 2 is a schematic illustration of a hexapod coordinate positioning device having six extendable legs, as discussed above; [Diagram 2] FIG. 2 is a schematic side view of the hexapod coordinate positioning device of FIG. 1; [Diagram 3]FIG. 1 shows a schematic side view of a coordinate positioning apparatus embodying the invention having a weighing arrangement and a separate drive arrangement; [Figure 4] FIG. 1 shows a schematic side view of a coordinate positioning apparatus embodying the invention in which the metrology arrangement is somewhat decoupled from the drive arrangement; [Diagram 5] FIG. 5 is a first perspective view of a practical embodiment of the coordinate positioning apparatus of FIG. 4; [Figure 6] FIG. 6 is a second perspective view of the embodiment of FIG. 5. [Figure 7] FIG. 6 is a side view of the embodiment of FIG. 5. [Figure 8] FIG. 6 is a top view of the embodiment of FIG. 5. [Figure 9A] FIG. 2 is a schematic illustration of the operation of an embodiment of the present invention. [Figure 9B] FIG. 2 is a schematic illustration of the operation of an embodiment of the present invention. [Figure 9C] FIG. 2 is a schematic illustration of the operation of an embodiment of the present invention. [Figure 9D] FIG. 2 is a schematic illustration of the operation of an embodiment of the present invention. [Figure 9E] FIG. 2 is a schematic illustration of the operation of an embodiment of the present invention. [Figure 10] FIG. 5 is a schematic illustration of a top-down variant of the coordinate positioning apparatus of FIG. 4; [Figure 11] FIG. 11 is a schematic illustration of a variation of the top-down coordinate positioning apparatus of FIG. [Figure 12] FIG. 9 shows a slight modification of the coordinate positioning apparatus of FIGS. 5 to 8; [Figure 13] FIG. 13 shows a top-down variant of the coordinate positioning apparatus of FIG. 12; [Figure 14] FIG. 14 shows a variation of the top-down coordinate positioning apparatus of FIG. 13. [Figure 15] FIG. 14 shows another variation of the top-down coordinate positioning apparatus of FIG. 13. [Figure 16A] 1A-1C are schematic diagrams illustrating embodiments having different types of non-hexapod drive arrangements. [Figure 16B] 1A-1C are schematic diagrams illustrating embodiments having different types of non-hexapod drive arrangements. [Figure 17] FIG. 16C illustrates a practical embodiment of the coordinate positioning apparatus of FIGS. 16A and 16B. [Figure 18A] FIG. 16C illustrates, in a simplified manner, a variation of the embodiment of FIGS. 16A and 16B in which the metering arrangement is somewhat decoupled from the drive arrangement. [Figure 18B] FIG. 16C illustrates, in a simplified manner, a variation of the embodiment of FIGS. 16A and 16B in which the metering arrangement is somewhat decoupled from the drive arrangement. [Figure 19] FIG. 16C illustrates a schematic variation of the embodiment of FIGS. 16A and 16B with a bottom-up, rather than top-down, hexapod metering arrangement. [Figure 20] FIG. 16C is a schematic illustration of a variation of the embodiment of FIGS. 16A and 16B in which fixed length weighing struts are used in a hexapod type weighing arrangement. [Figure 21] FIG. 21 illustrates a schematic representation of a variation of the embodiment of FIG. 20 in which an offset pivot plate is used on the weighing strut. [Figure 22] FIG. 13A illustrates a schematic representation of an embodiment having a delta robot type non-hexapod drive arrangement. [Figure 23] FIG. 23 illustrates, in a simplified manner, a variation of the embodiment of FIG. 22 having an increased amount of decoupling between the metering arrangement and the drive arrangement. [Figure 24] FIG. 23 illustrates, in a simplified manner, a variation of the embodiment of FIG. 22 having a reduced amount of decoupling between the metering arrangement and the drive arrangement. [Diagram 25] FIG. 2 illustrates a schematic representation of an embodiment having a non-hexapod drive arrangement of the serial kinematic type. [Figure 26]FIG. 13 illustrates a schematic diagram of another embodiment having a non-hexapod drive arrangement of the serial kinematic type. [Figure 27] 14A-14D illustrate the concept of providing a drive arrangement with fewer degrees of freedom than the metering arrangement. [Figure 28] FIG. 1 illustrates a dual hexapod arrangement in which the hexapod drive arrangement has constrained movement. [Figure 29] 4 is a flow diagram illustrating a method of controlling a coordinate positioning apparatus embodying the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0120] A side view of the coordinate positioning apparatus 1 discussed above with reference to Figure 1 is illustrated diagrammatically in Figure 2. The coordinate positioning apparatus 1 comprises an upper structure 2, which is movable within a working volume 14 of the apparatus 1. Six extendable legs 6 form both a hexapod-type drive arrangement 18 (shown in solid lines) for moving the upper structure 2 around the working volume 14, and also a hexapod-type weighing arrangement 16 (shown in dotted lines) for measuring the position of the upper structure 2 within the working volume 14. The coordinate positioning apparatus 1 thus has a combined drive and weighing strut.

[0121] By way of comparison, a coordinate positioning apparatus 21 according to an embodiment of the invention is illustrated diagrammatically in Figure 3. Like the apparatus of Figure 2, the coordinate positioning apparatus 21 comprises an upper structure 22 movable within a working volume 34 of the apparatus 21, a drive arrangement 28 (shown in solid lines) for moving the upper structure 22 around the working volume 34, and a metrology arrangement 26 (shown in dotted lines) for measuring the position of the movable upper structure 22 within the working volume 34.

[0122] Whereas in the coordinate positioning apparatus 1 shown in Figure 2 the hexapod weighing arrangement 16 and the hexapod drive arrangement 18 are combined, in a coordinate positioning apparatus 21 embodying the invention as shown in Figure 3 the drive arrangement 28 is distinct from and separate from the weighing arrangement 26. Technical advantages are realised by separating the weighing arrangement 26 from the drive arrangement 28 in such a way, because it allows these two different components to be designed with very different (and sometimes even contradictory) technical considerations in mind.

[0123] Having the weighing arrangement 26 separated and distinct from the drive arrangement 28 enables the drive arrangement 28 to be made relatively light and fast, allowing the structure 22 to be moved quickly around the working volume 34, with high accelerations and rapid changes of direction. Focusing on factors such as weight and speed can sacrifice some positional accuracy within the drive arrangement 28, but this is overcome by providing a weighing arrangement 26 that is instead designed with positional accuracy in mind.

[0124] Because the weighing arrangement 26 is passive and there is no need for any moving components (which add weight and generate heat), weighing errors caused by inertia and thermal deformation of parts (including the measuring scale used to measure distances) can thereby be controlled and reduced.

[0125] The use of a weighing arrangement 26 that is separate from and distinct from the drive arrangement 28 provides a coordinate positioning apparatus 21 in which the movable structure can be rapidly driven around the working volume while still maintaining the accuracy required for demanding positioning applications.

[0126] Such a design also makes it possible to choose a relatively inexpensive off-the-shelf drive mechanism for the drive arrangement 28, in particular a drive mechanism that has not been designed with high precision in mind, knowing that it will be coupled with a dedicated weighing arrangement 26 to provide the required precision, which therefore allows production costs to be reduced.

[0127] Also, mechanical weighing arrangements benefit from having low friction joints, whereas drive arrangements typically require more robust and substantial joints, which in particular have a higher degree of friction when under load, and therefore there is a design conflict that is overcome by separating the weighing arrangement 26 from the drive arrangement 28. The joints of the weighing arrangement 26 can be of a lower friction type and are also not under the same loads as the drive arrangement 28. Hysteresis effects can lead to different measurements being recorded depending on the direction in which the workpiece is approached, and therefore hysteresis effects can be reduced by separating the weighing arrangement 26 from the drive arrangement 28.

[0128] In the coordinate positioning apparatus 21 of Fig. 3, the metrology arrangement 26 is a hexapod arrangement, while the drive arrangement 28 is a non-hexapod arrangement (i.e. other than or different from a hexapod arrangement). The use of a hexapod-based metrology arrangement 26 is particularly advantageous, since a hexapod provides a robust mechanical system with a parallel arrangement of measurement transducers, which provides a direct measurement of distance from which highly accurate and reliable position determinations in six degrees of freedom can be derived.

[0129] The hexapod-based metrology arrangement 26 also has advantages over some image-based metrology arrangements in terms of the rate at which the position of the moving structure 22 can be determined or sampled. For image-based (photogrammetric) metrology arrangements, the sampling rate is essentially limited by the sampling rate of the image sensor and further limited by the time it takes to perform complex calculations based on large amounts of image information to derive the position of the moving platform. For example, it is stated by the image-based system of US Pat. No. 5,399,433 that "the frame rate provided by the imaging detector is typically only up to a maximum of a few hundred Hertz." This is described as being beneficial because it avoids detecting undesirable high frequency movements such as vibrations.

[0130] However, applicants have recognized that much higher dynamic bandwidths can be achieved by direct sampling of data from measurement transducers associated with the extendable legs of a hexapod weighing arrangement. For example, a machine controller can typically request position data from an absolute encoder every 65 μs (15 kHz), although higher sampling rates are also possible. Incremental encoder systems produce a continuous sinusoidal output, which still allows for finer motion control, limited only by the rate at which the continuous output can be sampled. Unlike image-based systems, the calculations required to determine the position of the movable structure from these values ​​are not overly time-consuming.

[0131] It is also noted that the image-based system of US Pat. No. 6,299,433 does not describe the use of a parallel arrangement of measurement transducers that independently provide a corresponding set of measurements, each measurement in the set being directly representative of or related to a distance or separation between a different point on the moving platform and a point on a fixed structure, from which the position and / or orientation of the moving platform can be determined. In particular, US Pat. No. 6,299,433 does not describe the use of a hexapod type weighing arrangement.

[0132] With a hexapod drive arrangement, such as that shown in FIG. 1, each of the six struts requires a motor, which necessarily forms part of the associated strut, i.e., the motor moves with the strut. Thus, when the hexapod is actuated to move the mobile structure around the working volume, the weight of the relatively heavy motor part is also moved around. Having to move this excess mass around reduces the potential speed (or acceleration) of the drive arrangement and generates additional heat in the device, which has a negative effect when it reaches the weighing arrangement. By providing a non-hexapod drive arrangement, such as that shown in FIG. 3, these problems can be overcome, because it allows the motor part to be removed from the part that is moving.

[0133] Moreover, by using a non-hexapod type drive arrangement that provides movement to the movable structure 22 with fewer than six degrees of freedom, fewer actuators are required (i.e., fewer than the six actuators required in a hexapod), reducing cost and complexity, and also reducing the amount of heat generated due to the fewer number of heat-generating motor parts, thus improving metering results.

[0134] The hexapod weighing arrangement 26 of Figure 3 is generally similar to the hexapod arrangement of Figures 1 and 2, but does not have any actuation or motor components required to provide drive. In this embodiment, the drive arrangement 28 is a so-called "tri-glide" arrangement, such as that disclosed in US Pat. No. 5,399,363, and has three carriages 56 moving along three corresponding respective linear tracks 51 arranged substantially in parallel. These arrangements are described in more detail below with reference to Figures 5 to 8.

[0135] 3, the coordinate positioning apparatus 21 comprises a lower structure 24, which forms part of the fixed structure of the apparatus 21, with a workpiece 29 mounted on the lower structure 24. A measurement probe 30 is supported on the upper structure 22 such that it can be moved around a working volume 34. The working volume 34 is defined between the upper structure 22 and the lower structure 24 when in their most spaced position, with the probe component 30 positioned in the working volume 34 by operation of the drive arrangement 28.

[0136] Also illustrated diagrammatically in Figure 3 is a controller C for controlling the drive arrangement 28 to cause the desired movement of the structure 22. The controller C can be implemented in hardware or software, or a combination thereof. Purely for the sake of clarity and brevity, the controller C is omitted from the subsequent figures.

[0137] 4, to provide even further isolation between the drive arrangement 28 and the weighing arrangement 26, the drive arrangement 28 may be coupled to the weighing arrangement 26 via a coupling arrangement 38 that prevents at least some strain associated with the drive arrangement 28 from being transmitted to the weighing arrangement 26. The coupling arrangement 38 includes a first coupling 38a associated with the moveable structure 22 and a second coupling 38b associated with the fixed structure 24.

[0138] In the schematic embodiment illustrated in Fig. 4, the movable structure 22 comprises a metering part 22a associated with the metering arrangement 26 and a drive part 22b associated with the drive arrangement 28, with the metering part 22a of the movable structure 22 being coupled to the drive part 22b of the movable structure 22 via a first coupling 38a. The metering part 22a of the movable structure 22 is coupled to the metering arrangement 26. The drive part 22b of the movable structure 22 is coupled to the drive arrangement 28.

[0139] Similarly, the fixed structure 24 includes a metering part 24a associated with the metering arrangement 26 and a drive part 24b associated with the drive arrangement 28, with the metering part 24a of the fixed structure 24 being coupled to the drive part 24b of the fixed structure 24 via a second coupling 38b. The metering part 24a of the fixed structure 24 is coupled to the metering arrangement 26. The drive part 24b of the fixed structure 24 is coupled to the drive arrangement 28.

[0140] In this example, each coupling 38a and 38b of the coupling arrangement 38 is in the form of a kinematic or quasi-kinematic coupling. In the context of positioning a body part relative to another, kinematic design considerations are met by constraining the degrees of freedom of movement of the body part using a minimum number of constraints, in particular involving avoiding over-constraining. Over-constraining can result in multiple points of contact between the two body parts, allowing one body part to be placed in more than one position relative to the other. The location of the body part is therefore not repeatable, since it is not known in which of several positions the body part will be placed. In particular, when over-constraining is present, there is a conflict between the constraints in place, such that it is not possible to determine with any certainty which combination of constraints determines the actual position of the body part. These concepts are explained in "A Study of the Mechanical Design of a Body Part by Using a Kinematic Coupling," ...

[0141] Such a kinematic coupling, with a minimum number of contact points (or point-like contacts) to provide an ideal constraint, is also very effective in isolating the strains in one half of the coupling being transmitted to the other half of the coupling. Thus, the first coupling 38a helps to prevent strains of the drive part 22b of the movable structure 22 (resulting from forces acting on that part from the drive arrangement 28) from being transmitted to the weighing part 22a (and thereby to the weighing arrangement 26), and similarly for the second coupling 38b with respect to the fixed structure 24. This provides a clearly delimited weighing frame 36 with a good degree of mechanical isolation from the drive arrangement 28.

[0142] In particular, in this embodiment, each coupling 38a and coupling 38b includes a set of three balls, providing three points of contact according to kinematic design principles (only two are shown in the schematic illustration of FIG. 4). It is also beneficial to use multiple elastic spacers or pads instead of rigid balls, for example three such spacers arranged at the corners of a triangle. This provides some degree of kinematic coupling even if the contact points are not point-like but instead spread over a small area of ​​the elastic spacers. The use of elastic spacers (for example made of rubber) is beneficial because they act to absorb some vibrations from the drive arrangement 28 so that they are not transmitted to the weighing arrangement 26.

[0143] It will also be appreciated that such coupling may be provided at both ends (i.e., in association with the movable structure 22 and the fixed structure 24), or only at one end (i.e., in association with only one of the movable structure 22 and the fixed structure 24), or not provided at all (i.e., in neither the movable structure 22 nor the fixed structure 24).

[0144] The embodiment will now be described in more detail with reference to Figures 5 to 8, which show more detailed representative views of the device structure than the highly schematic illustrations of Figures 3 and 4.

[0145] The hexapod weighing arrangement 26 shown in Figures 5 to 8 comprises six extendable legs 60, which are generally of the same construction and are arranged between the upper structure 22 and the lower structure 24. According to Figure 4, the upper structure 22 comprises a weighing part 22a associated with the weighing arrangement 26 and a drive part 22b associated with the drive arrangement 28, the weighing part 22a of the mobile structure 22 being connected to the drive part 22b of the mobile structure 22 via a first coupling 38a. The weighing part 22a of the mobile structure 22 is connected to the weighing arrangement 26 via a ball joint 68. The drive part 22b of the mobile structure 22 is connected to the drive arrangement 28 via a ball joint 58.

[0146] Similarly, the fixed structure 24 includes a metering part 24a associated with the metering arrangement 26 and a drive part 24b associated with the drive arrangement 28, the metering part 24a of the fixed structure 24 being coupled to the drive part 24b of the fixed structure 24 via a second coupling 38b. The metering part 24a of the fixed structure 24 is coupled to the metering arrangement 26. The drive part 24b of the fixed structure 24 is coupled to the drive arrangement 28.

[0147] Each of the six extendable legs 60 includes an upper tube 64 and a lower tube 62 that slide telescopically within the upper tube 64. The extendable legs 60 are generally of similar construction to those described in U.S. Pat. Nos. 5,993,333 and 5,993,351, except that in this embodiment, there is no need for the extendable legs to be driven, and therefore no need for any motor-related components. However, the general construction of the extendable legs 60 is generally similar.

[0148] In the example shown in Figures 5 to 8, the lower structure 24 is fixed and the upper structure 22 is movable relative to the lower structure 24 by the action of six extendable legs 60, with the measurement probe 30 attached to the lower surface of the upper structure 22. In this configuration, the workpiece (not shown in Figures 5 to 8) is mounted on the metering part 24a of the lower structure 24, such that the working volume of the device 21 is between the metering parts 22a and 24a of the upper and lower structures 22 and 24, respectively. The measurement probe 30 includes a stylus with a tip in contact with the workpiece, with the measurement probe 30 connected to the metering part 22a of the moving structure 22 via a quill 32.

[0149] The extendable legs 60 are for positioning (i.e., determining the position of) a component (in the illustrated example, the component is a measurement probe 30) or at least a portion of a particular part of the component (e.g., the tip of the measurement probe) within the working volume of the apparatus, which is supported by the movable structure 22.

[0150] The upper and lower ends of each extendable leg 60 are connected to the upper structure 22 (specifically, the metering part 22a of the upper structure 22) and the lower structure 24 (specifically, the metering part 24a of the lower structure 24), respectively, via respective ball joints 68. The upper and lower tubes 62 and 64 of each extendable leg 60 surround an elongated member 66 (shown in dotted outline in one of the extendable legs in FIG. 5) with an encoder scale 10 affixed to the elongated member 66. The elongated member 66 is itself extendable, for example, by a telescoping arrangement. Each elongated member 66 extends from its upper joint 68 to its lower joint 68, and it is the length of each of the elongated members 66 that determines the precise positioning and orientation of the metering part 22a of the upper structure 22 (and thus the measurement probe 30). It is therefore the length of elongated member 66 that must be precisely measured during a measurement or scanning operation on a workpiece to determine the precise location when the tip of the stylus is in contact with the workpiece surface.

[0151] In this embodiment, the drive arrangement 28 is a so-called "tri-glide" arrangement, as described, for example, in US Pat. No. 5,399,363. The tri-glide arrangement is provided by three mechanical linkages 50 of substantially identical design connected in parallel between the mobile structure 22 and the fixed structure 24. Each mechanical linkage 50 includes two substantially parallel rigid rods 52 and 54 of fixed length, which act to maintain the mobile structure 22 in a substantially constant orientation as it moves around the working volume 34. Each mechanical linkage 50 also includes a carriage 56 to which the rods 52 and 54 are pivotally connected at their lower ends via ball joints 58 and at their upper ends to the drive part 22b of the mobile structure 22.

[0152] The three linear tracks 51 are arranged substantially vertically (substantially parallel) on the drive part 24b of the fixed structure 24, with the three carriages 56 respectively arranged to move (up and down) along the three linear tracks 51. The three linear tracks 51 effectively form part of the fixed structure of the coordinate positioning device 21 and can be considered as an extension of the fixed structure 24 (specifically the drive part 24b of the fixed structure 24). Each carriage 56 is driven substantially linearly along its corresponding respective track 51 by a linear drive mechanism, the position of which is marked diagrammatically by reference numeral 29 in FIG. 3. The linear drive mechanism may include a linear motor. The linear drive mechanism may include a stepper motor.

[0153] Thus, each mechanical linkage 50 is actuated by a drive mechanism acting between the fixed structure (linear track) 51 and the mechanical linkage 50. More specifically, the drive mechanism acts between the fixed structure (linear track) 51 and the end of the mechanical linkage 50 (i.e., carriage 56). In other words, the drive mechanism effectively couples the mechanical linkage directly to the fixed structure ("coupled to ground") and provides a force therebetween that acts to push or pull the mechanical linkage (for a linear drive mechanism) or to rotate the mechanical linkage (for a rotary drive mechanism). There is no additional movable linkage between the drive mechanism and the fixed structure, where such additional linkage is capable of causing movement of the driven part of the mechanical linkage that is not generated by the drive mechanism itself.

[0154] For example, in the "tri-glide" drive arrangement illustrated in Figures 5 to 8 (and more generally in Figures 3 and 4), each carriage 56 is driven linearly up and down its respective track 51 by the action of the motor (not shown) associated with that carriage 56 and not by the action of any other motors associated with other carriages 56. In the case of a typical hexapod drive arrangement (as shown in Figure 1), the motor in a particular extendable leg can only extend or retract the leg linearly along its length, and in use each leg is moving laterally as well. Any lateral movement of a leg (and its associated motor) must result from the action of other motors in other legs, such that each motor is actually moving the weight of other struts (along with their respective motors).

[0155] Thus, in such a drive arrangement as illustrated in Figures 3 to 8, the moving parts can be kept relatively light, in this example the thin and light rods 52 and 54, rather than each motor moving around the weight of the other motors (as is the case in a typical hexapod drive arrangement having six extendable legs such as that shown in Figure 1). This allows for a lightweight drive arrangement that can move quickly, with high accelerations and rapid changes of direction.

[0156] Returning to a more schematic format, operation of the Tri-Glide embodiment will now be described with reference to Figures 9A-9E, each of which uses a representation similar to that used in Figure 3, with the two carriages labelled 56a and 56b respectively, and the two linear tracks labelled 51a and 51b respectively.

[0157] Due to the constraints provided by the parallel rods 52 and the parallel rods 54, described above with reference to Figures 5 to 8, the movement of the moveable structure 22 (by operation of the tri-glide drive arrangement 28) is limited to three translational degrees of freedom, such that the moveable structure 22 maintains a substantially constant orientation as it moves about the working volume. This constraint to movement in three degrees of freedom is illustrated by the arrows labeled 3DOF in Figure 9A.

[0158] On the other hand, six extendable legs 60 of the hexapod weighing arrangement 26, including six corresponding measurement transducers in a parallel arrangement, provide six corresponding respective measurement values ​​from which the position of the movable structure can be determined in all six degrees of freedom, as shown by the arrows labeled 6DOF in FIG. 9A.

[0159] As shown in FIG. 9B, by lowering carriage 56a and raising carriage 56b along their respective tracks 51a and 51b, the moveable structure 22 (and along it, the measurement probe 30) is moved leftward and downward through the working volume 34, maintaining substantially the same orientation, since the rods 52 and 54 of the mechanical linkage 50 are of fixed length. This shortens the extendable leg 60 closest to carriage 56a and lengthens the extendable leg 60 closest to carriage 56b, with these changes in length being measured by the measurement transducers (e.g., encoders) 10 in the extendable legs 60. From those transducer measurements, the position of the moveable structure 22 within the working volume 34 can be determined, and also the position of the measurement probe 30 (and thus the probe tip) can be determined, since the measurement probe 30 is in a known, fixed spatial relationship to the moveable structure 22. FIG. 9C is the same as FIG. 9B but without the translation indications for clarity, thereby showing the final positions of the components after the translation operations.

[0160] Similarly, as shown in FIG. 9D, the movable structure 22 (and thus the measurement probe 30) can be moved rightward through the working volume 34 by raising the carriage 56a and lowering the carriage 56b along their respective tracks 51a and 51b, again maintaining substantially the same direction. This lengthens the extendable legs 60 closest to the carriage 56a and shortens the extendable legs 60 closest to the carriage 56b, with these changes in length being measured by the measurement transducers (e.g., encoders) 10 in the extendable legs 60. From those transducer measurements, the positions of the movable structure 22 and the measurement probe 30 within the working volume 34 can be determined. FIG. 9E is the same as FIG. 9D, but without the movement indications for clarity, thereby showing the final positions of the components after the movement operations.

[0161] In the tri-glide embodiment described above, the extendable legs 60 of the hexapod weighing arrangement 26 and the rods 50 of the drive arrangement 28 extend from the bottom up, and therefore this embodiment can be described as a "bottom-up" arrangement. Figure 10 shows an alternative "top-down" arrangement, which is generally the same as the "bottom-up" arrangement, except that the extendable legs 60 of the hexapod weighing arrangement 26 and the rods 50 of the drive arrangement 28 extend from the top down (hence the "top-down" arrangement). To enable this, a frame 25a is provided to support the hexapod weighing arrangement 26, allowing it to effectively "hang" from the top. The frame 25a effectively forms part of the fixed structure of the coordinate positioning apparatus 21, as an extension to the fixed structure 24 (in this case the hexapod part 24a of the fixed structure 24). As in the previous embodiment, a coupling arrangement 38 a and a coupling arrangement 38 b are provided to isolate the metering arrangement 26 from the drive arrangement 28 .

[0162] Yet another "top-down" arrangement is diagrammatically illustrated in Fig. 11. It differs from the embodiment of Fig. 10 in that the hexapod weighing arrangement 26 is supported from a frame 25 extending around the top and is provided inside the tri-glide drive arrangement 28. The frame 25 effectively forms part of the fixed structure of the coordinate positioning apparatus 21 as an extension to the fixed structure 24 with the vertical linear track 51, which also becomes part of the frame 25. Moreover, the embodiment of Fig. 11 does not provide any coupling arrangement 38 for isolating the weighing arrangement 26 from the drive arrangement 28.

[0163] For comparison with Figures 13 to 15, Figure 12 is provided to show a practical tri-glide embodiment, which corresponds closely to that described above with reference to Figures 5 to 8, differing mainly in having a closed frame, with extra rigidity and stability provided by the upper plate of the frame to the vertical track 51. As with the previous embodiment, the weighing arrangement of Figure 12 is at least to some extent decoupled from the drive arrangement, both at the top (i.e. in a movable structure) and at the bottom (i.e. in a fixed structure).

[0164] Figure 13 shows a practical embodiment of the "top down" arrangement shown diagrammatically in Figure 11, but differs from the embodiment of Figure 11 by some decoupling of drive and metering in the moveable structure. Figure 14 is a variation of Figure 13, providing decoupling of drive and metering in both the moveable and fixed structures. Figure 15 is a further variation, having a separate metering frame located within the drive frame, with decoupling of the drive frame from the metering frame in both the moveable and fixed structures.

[0165] It is understood that the present invention is not limited to embodiments in which the drive arrangement 28 is in the form of a tri-glide. Figure 16A illustrates, in a schematic manner, an embodiment in which a hexapod metering arrangement 26 is coupled with a different type of non-hexapod drive arrangement 28. Rather than a fixed length rod 52 (one end of which is driven linearly along a track 51 by a carriage 56, as in the tri-glide embodiment), in the embodiment of Figure 16A, a rod extending to a fixed length is instead driven through a pivoting guide 76 by a suitable linear drive mechanism mounted within the guide 76, thereby varying the separation indicated by the arrows in Figure 16A, thereby moving the structure 22.

[0166] In Figures 16A and 16B, similar to Figure 11, the weighing arrangement 26 and the drive arrangement 28 are supported top-down from a frame 25, which forms part of the fixed structure of the coordinate positioning apparatus 21. From the position as shown in Figure 16A, the structure 22 can be moved to the position as shown in Figure 16B when both rods are driven downwards through their respective guides 76. As before, the position of the structure 22 is measured by the hexapod weighing arrangement 26.

[0167] It will be appreciated that, similar to the tri-glide configuration, each mechanical linkage of the drive arrangement 28 in the embodiment of FIG. 16A is actuated by a drive mechanism acting between a fixed structure and the mechanical linkage, and thus this embodiment shares the same advantages in terms of speed and acceleration.

[0168] Figure 17 illustrates a practical embodiment of the arrangement diagrammatically illustrated in Figure 16A. The embodiment of Figure 17 is closely based on a non-Cartesian type coordinate measuring machine sold under the trademark EQUATOR by the applicant Renishaw plc. The hexapod weighing arrangement 26 is generally similar to that of Figure 5 and includes six extendable legs, each having an upper tube 64 and a lower tube 62, with the lower tube 62 telescopically sliding within the upper tube 64. In this embodiment, the extendable legs are supported in a top-down arrangement from the frame 25 to the weighing platform 22a (part of the movable structure 22). The pivot guide 76 is hidden in Figure 17 by the structure of the frame 25. Three fixed length drive rods 72 pass through the three pivot drive guides 76, respectively, and are connected at their lower ends to the drive plate 22b (part of the movable structure 22). In this embodiment, the two parts 22a and 22b of the moveable structure 22 are spatially separated by a rigid column 23. Three sets of parallel rod pairs 72 and 74 are arranged to constrain movement in three degrees of freedom, similar to rods 52 and 54 in FIG.

[0169] Returning to a more schematic representation, Figures 18A and 18B show a modification of the device of Figures 16A and 16B in which the metering arrangement 26 is further isolated from the drive arrangement 28. This is similar to the tri-glide embodiment described above with reference to Figure 10 and therefore no further explanation is necessary. Figure 19 shows an alternative to the arrangement of Figure 16A, having a bottom-up hexapod-type metering arrangement 26 instead of a top-down arrangement.

[0170] FIG. 20 illustrates a schematic variation of the embodiment of FIGS. 16A and 16B, in which fixed length metering struts are used in the hexapod metering arrangement 26, as well as the fixed length struts of the drive arrangement 28 of that embodiment. The six fixed length extending struts as illustrated in FIG. 20 are considered to be functionally equivalent to the six extendable struts of the previous embodiment, with the variable length parts of the struts being as indicated by the arrows in FIG. 20, which parts are equivalent to the extendable struts of the previous embodiment. Thus, the terms "extendable leg" and "extending leg" should be understood herein as equivalent and mean any type of mechanical arrangement or linkage between two points, which allows the separation between those points to vary. The drive arrangement 28 is still a non-hexapod drive arrangement, since it has only three extending struts, as shown in more detail in FIG. 17. FIG. 21 illustrates generally a variation of the embodiment of FIG. 20 in which a fixed support (pivot plate) 25a is used for the weighing struts, which is spatially offset from a fixed support (pivot plate) 25b used for the drive struts.

[0171] Above have been described embodiments in which two different types of non-hexapod drive arrangements are used: a tri-glide linear drive arrangement (e.g., FIG. 5), and a pivoting linear drive arrangement (e.g., FIG. 17). Many other possibilities for drive arrangements exist, only some of which are briefly described here, others will be apparent to those skilled in the art.

[0172] FIG. 22 illustrates a schematic representation of an embodiment having a non-hexapod drive arrangement of the delta robot type. Delta robots are a type of parallel robot, an example of which is described in detail in US Pat. No. 5,399,363. FIG. 23 illustrates a schematic representation of a variant of the embodiment of FIG. 22, having an increased amount of decoupling between the weighing arrangement and the drive arrangement. FIG. 24 illustrates a schematic representation of a variant of the embodiment of FIG. 22, having a reduced amount of decoupling between the weighing arrangement and the drive arrangement. It will be appreciated that, like the tri-glide configuration, in the delta robot configuration each mechanical linkage is actuated by a drive mechanism acting between a fixed structure and the mechanical linkage, and therefore these delta robot embodiments share the same advantages in terms of speed and acceleration (in the delta robot configuration the drive mechanism is a rotary drive mechanism, whereas in the tri-glide configuration the drive mechanism is a linear drive mechanism). Moreover, with appropriate constraints (such as those described in U.S. Pat. No. 5,399,323), the delta robot drive arrangement 28 can be adapted to provide movement to the structure 22 in three degrees of freedom (i.e., fewer degrees of freedom than are measured by the hexapod weighing arrangement 26).

[0173] The position of the rotary drive mechanism is indicated by reference numeral 27 in Fig. 22, while the position of the linear drive mechanism is indicated by reference numeral 29 in Fig. 3. In each case, the drive mechanism acts directly between the fixed structure and the drive arrangement. In the case of Fig. 3, the drive mechanism acts to drive the carriage 56 (which forms part of the drive arrangement, for example as part of the mechanical linkage 50 as shown in Fig. 5), while in the case of Fig. 22, the drive mechanism acts to drive (rotate) the upper part of the mechanical linkage, which is mounted between the mobile structure and the fixed structure.

[0174] Another example of a non-hexapod drive arrangement suitable for use in the embodiments of the present application is a cable-driven robot configuration (otherwise known as a cable-suspended robot, or simply a cable robot, or a wire-driven robot). This is a type of parallel manipulator (parallel kinematic configuration) in which multiple flexible cables are used as actuators. One end of each cable is wrapped around a rotor that is rotated by a corresponding respective motor, and the other end is connected to an end effector. An example of a cable robot is disclosed in US Pat. No. 5,399,433. The end effectors of a cable robot can achieve high accelerations and velocities, since the cables are typically much lighter than the rigid linkages of serial or parallel robots. Due to the high measurement rates and dynamic bandwidths, as well as the high precision, that can be achieved with a hexapod weighing arrangement, the combination of a hexapod weighing arrangement with a cable-driven arrangement is particularly advantageous.

[0175] Other types of non-hexapod drive arrangements are also envisaged. For example, FIG. 25 illustrates diagrammatically an embodiment having a non-hexapod drive arrangement of the serial kinematic type (as opposed to parallel kinematic) with multiple segments or links connected in series by a revolute joint, one end of the drive arrangement being attached to the ground and the other end to the weighing arrangement. As with the embodiment illustrated in FIG. 4, the drive arrangement illustrated in FIG. 25 is attached to the weighing arrangement via a coupling that helps prevent drive-related distortions from being transferred to the weighing arrangement. FIG. 26 illustrates diagrammatically another embodiment having a serial kinematic type of non-hexapod drive arrangement, with three parts connected in series that are movable along orthogonal axes x, y and z (as marked in FIG. 26), respectively. Thus, the embodiment of FIG. 26 has a Cartesian type serial drive arrangement, whereas the embodiment of FIG. 25 has a non-Cartesian type serial drive arrangement. These types of drive arrangements are well known and further description thereof is not required here.

[0176] As explained in detail above with reference to FIG. 9A, the drive arrangement 28 provides three translational degrees of freedom to the movable structure 22, while the hexapod weighing arrangement 26 is adapted to measure in six degrees of freedom. According to one aspect of the invention, a coordinate positioning apparatus is proposed, comprising a structure movable in a working volume of the apparatus, a drive arrangement for moving the structure around the working volume with fewer than six degrees of freedom, and a weighing arrangement for measuring the position of the structure in the working volume with more degrees of freedom than the drive arrangement. This is illustrated diagrammatically in FIG. 27. One or both of the drive arrangement and the weighing arrangement can be parallel kinematic arrangements, such as, for example, a hexapod arrangement, a triglide arrangement, or a delta robot arrangement. In particular, it should be noted that in this aspect, the weighing arrangement does not have to be a hexapod weighing arrangement.

[0177] It is not uncommon to provide measurement (specifically direct measurement) in more degrees of freedom than movement. Typically there will be N driven parts (rotary or linear), each separately encoded to give N corresponding measurements. For example, for a 3-axis CMM there are three driven, linear axes, each with a position encoder, and therefore three corresponding measurements (i.e., both driven and measured in three degrees of freedom). For a hexapod there are six variable length struts, each with a position encoder, and six corresponding measurements (i.e., both driven and measured in six degrees of freedom).

[0178] However, applicants have recognized the desirability and advantage of being able to provide a relatively inaccurate driver, constrained to move in a limited number of degrees of freedom (e.g., three), coupled by a separate weighing arrangement that is highly accurate and capable of measuring in all six degrees of freedom, and thus capable of compensating for any inaccuracies in the mechanically constrained drive arrangement. For example, if a moving platform is constrained to translate in the working volume without rotation, there may be some unintended rotation of the platform caused by distortions or other types of inaccuracies in the structure, and at least some of the inaccuracies may be caused by dynamic effects associated with high speed motion. Such rotations are detected by measuring in more degrees of freedom than are driven. It is also possible to apply the scheme of FIG. 27 to a dual hexapod type arrangement, as shown diagrammatically in FIG. 28, in which the drive hexapod is constrained to move in fewer than six degrees of freedom by appropriate mechanical constraints.

[0179] As will be apparent to one skilled in the art, there are many other forms of non-hexapod drive arrangements, or drive arrangements constrained to fewer than six degrees of freedom. For example, there are many possible variations of the tri-glide arrangement shown. One variation is to provide an arrangement having more than three drives and associated mechanical linkages. And instead of a vertical track 51 as shown in FIG. 3, the track may instead be positioned horizontally from a point, e.g., radially outward, such that movement of the structure 22 is also affected by movement of the carriage 56 along the horizontal track. Many other such possibilities exist.

[0180] Although embodiments of the invention have been described primarily in relation to the use of contact probes, where a stylus of the contact probe makes physical contact with the workpiece surface to make measurements, it will be appreciated that the invention is not limited to contact probes. The same concepts are equally applicable to non-contact probes, such as optical probes, where the surface is sensed without making physical contact. The invention is generally applicable to any surface sensing device adapted to sense a surface, whether by contact or non-contact. The invention can also be applied to positioning components other than surface sensing devices, for example to orient component parts of an article during the manufacture of the article. Or the component can be a tool or part thereof, such as a tool typically found in a machine tool for forming or machining metals or other rigid materials. The component can be the movable structure itself. The component may include a camera for imaging the surface of the workpiece. The component may include an eddy current probe for detecting and / or measuring eddy currents at or near the surface of the workpiece. Many other possibilities will become apparent to those skilled in the art.

[0181] It should be noted that in an embodiment of the present invention, the hexapod weighing arrangement 26 is not provided purely for calibration purposes, but is temporarily coupled to the mobile structure to perform a calibration of the combined drive and weighing arrangement, and is then removed for operational use of the device. Rather, the hexapod weighing arrangement is intended to remain coupled to the mobile structure and provide position measurements regarding the mobile structure during operational use. In contrast to a calibration-only weighing arrangement, in an embodiment of the present invention, the mobile structure is adapted to carry an operational tool, with the weighing arrangement and the drive arrangement also coupled to the mobile structure. The hexapod weighing arrangement can be coupled to the mobile structure via a different attachment than that used to attach the operational tool to the mobile structure. The hexapod weighing arrangement can be directly coupled to the mobile structure (e.g., rather than via an attachment primarily intended for the operational tool).

[0182] A method for controlling a coordinate positioning apparatus is illustrated by the flow chart of FIG. 29. In step S1, the metrology arrangement 26 is coupled to the movable structure (or platform) 22. In step S2, the drive arrangement 28 is coupled to the movable structure (or platform) 22. In step S3, a tool (e.g., a measurement probe 30 or a cutting tool) is coupled to the movable structure (or platform) 22. Thus, at this point, all three are coupled to the movable structure (or platform) 22. In step S4, the drive arrangement 28 is used to move the tool around the working volume 34 (the metrology arrangement 26 also remains coupled to the movable structure). In step S5, an operation is performed with the tool, such as, for example, performing a touch trigger operation on the workpiece surface with the measurement probe 30 and performing a machining operation on the workpiece surface with a cutting tool or a machining tool. In step S6, the metrology arrangement 26 is used to determine the position of the tool within the working volume 34 when the operation occurs (e.g. to enable the position of the tip of the measurement probe 30 or cutting tool to be determined). In step S7, the determined position is associated with the performed operation (e.g. so that a touch trigger event can be associated with a position measurement for that event).

[0183] It will be appreciated that operation of the coordinate measuring machine 21 may be controlled by a program running on the machine 21, and in particular by a program running on a coordinate measuring machine controller, such as the controller C shown diagrammatically in Figure 3. It will be appreciated that control of the extendable legs may be provided by a program running on the controller C. Such an operating program may be stored on a computer readable medium or may be embodied in a signal, such as, for example, a downloadable data signal provided from an internet website. The appended claims are to be understood as covering an operating program by itself, or as a record on a carrier, or as a signal, or in any other form.

[0184] Although the above embodiments have been described primarily in the context of a coordinate measuring machine, the concepts are more generally applicable to any type of coordinate positioning apparatus, such as comparators, scanning apparatus, machine tools, positioning devices (e.g. for optical components), prototype manufacturing apparatus, and a variety of other uses.

Claims

1. 1. A coordinate positioning apparatus comprising: a structure movable within a working volume of the apparatus; a drive arrangement for moving the structure around the working volume with fewer than six degrees of freedom; and a metrology arrangement for measuring a position of the structure within the working volume with more degrees of freedom than the drive arrangement, 1. A coordinate positioning apparatus comprising: a measuring arrangement, the measuring arrangement comprising a plurality of measurement transducers, the drive arrangement comprising a plurality of actuators, the apparatus further comprising a controller operable to request data values ​​from the measurement transducers of the measuring arrangement and to control the actuators of the drive arrangement using the data values ​​received from the measurement transducers of the measuring arrangement to position the moveable structure within the working volume.

2. 2. The coordinate positioning apparatus of claim 1, wherein the metrology arrangement comprises a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurement values ​​from which the position of the movable structure is determinable.

3. The coordinate positioning apparatus of claim 2 , wherein the plurality of measurement transducers is a plurality of independent measurement transducers.

4. 4. A coordinate positioning apparatus according to claim 2, wherein the measurement transducer is a length measuring transducer.

5. A coordinate positioning apparatus according to any one of claims 2 to 4, wherein the measurements relate to different respective distances between the moveable structure and a fixed structure of the apparatus.

6. 6. A coordinate positioning apparatus according to claim 5, wherein the measurement transducer is adapted to provide a direct measurement of the distance, or a direct measurement of a change in the distance as the structure moves around the working volume, from which the distance is determinable.

7. A coordinate positioning apparatus according to any one of claims 2 to 6, wherein each of the measurement transducers comprises an encoder scale and an associated readhead.

8. 8. The coordinate positioning apparatus according to claim 2, wherein the weighing arrangement comprises a number of extendable legs arranged in parallel, the number of the extendable legs corresponding to a number of measurement transducers, each of the plurality of measurement transducers being associated with a different respective one of the plurality of extendable legs.

9. A coordinate positioning apparatus according to any one of claims 2 to 8, comprising six such measurement transducers.

10. A coordinate positioning apparatus according to any preceding claim. A coordinate positioning apparatus according to any preceding claim, wherein the controller is operable to request data values ​​from the measurement transducer at a particular sampling rate.

11. 11. The coordinate positioning apparatus of claim 10, wherein the sampling rate is at least 15 kHz and the controller requests a data value from the measurement transducer every 65 [mu]s or less.

12. 12. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is adapted to maintain the moveable structure in a substantially constant orientation as it moves around the working volume.

13. 13. A coordinate positioning apparatus according to any one of claims 1 to 12, wherein the drive arrangement comprises a plurality of mechanical linkages connected in parallel between the movable structure and a fixed structure of the apparatus, each mechanical linkage being actuated by a drive mechanism acting between the fixed structure and the mechanical linkage.

14. 14. The coordinate positioning apparatus of claim 13, wherein the drive mechanism is a rotary drive mechanism.

15. 14. The coordinate positioning apparatus of claim 13, wherein the drive mechanism is a linear drive mechanism.

16. 16. A coordinate positioning apparatus according to any one of claims 13 to 15, wherein each mechanical linkage comprises at least two substantially parallel rods to maintain the moveable structure in a substantially constant orientation as it moves around the working volume.

17. A coordinate positioning apparatus according to any one of claims 13 to 16, wherein the drive arrangement comprises three such mechanical linkages.

18. 18. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is a tri-glide arrangement.

19. 19. A coordinate positioning apparatus according to any preceding claim, wherein the weighing arrangement is a hexapod weighing arrangement.

20. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is a non-hexapod drive arrangement.

21. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is adapted to move the structure around the working volume in three degrees of freedom.

22. 22. A coordinate positioning apparatus according to claim 21, wherein said three degrees of freedom are three translational degrees of freedom.

23. 23. A coordinate positioning apparatus according to any preceding claim, wherein the metrology arrangement is adapted to measure the position of the structure in six degrees of freedom.

24. 24. A coordinate positioning apparatus according to any preceding claim, wherein the metrology arrangement and the drive arrangement are respectively arranged between the moveable structure and a fixed structure of the apparatus.

25. 25. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement comprises a plurality of actuators in a parallel arrangement.

26. 26. The coordinate positioning apparatus according to claim 25, wherein the drive arrangement comprises a plurality of actuators in a parallel arrangement of a different type to that of the weighing arrangement.

27. 27. A coordinate positioning apparatus according to any one of claims 1 to 26, wherein the drive arrangement comprises a plurality of measurement transducers for providing corresponding respective measurement values ​​separate from those of the metrology arrangement, from which the position of the movable structure is determinable independently from the position determined based on the measurements from the metrology arrangement.

28. 28. The coordinate positioning apparatus according to any one of the preceding claims, wherein the drive arrangement is coupled to the weighing arrangement via a coupling arrangement, the coupling arrangement preventing at least some distortion associated with the drive arrangement from being transmitted to the weighing arrangement.

29. 29. A coordinate positioning apparatus according to claim 28, wherein the coupling arrangement is a kinematic coupling arrangement or a quasi-kinematic coupling arrangement.

30. 30. The coordinate positioning apparatus according to claim 28 or 29, wherein the movable structure comprises a drive part associated with the drive arrangement and a weighing part associated with the weighing arrangement, the drive part of the movable structure being coupled to the weighing part of the movable structure via the coupling arrangement.

31. 31. The coordinate positioning apparatus according to any one of claims 28 to 30, comprising a fixed structure having a drive part associated with the drive arrangement and a weighing part associated with the weighing arrangement, the drive part of the fixed structure being coupled to the weighing part of the fixed structure via the coupling arrangement.

32. 32. A coordinate positioning apparatus according to any preceding claim, wherein the movable structure carries a motion tool.

33. 33. The coordinate positioning apparatus according to claim 32, wherein the weighing arrangement is coupled to the moveable structure via a mounting different from that used to mount the operative tool to the moveable structure.

34. 34. A coordinate positioning apparatus according to any preceding claim, wherein the apparatus is a coordinate measuring machine or a comparator.

35. 35. A coordinate positioning apparatus according to claim 34 when dependent on claim 32 or 33, in which the operating tool is a surface sensing device or a measurement probe.

36. 34. A coordinate positioning apparatus according to any preceding claim, wherein the apparatus is a machine tool.

37. 35. A coordinate positioning apparatus according to claim 34 when dependent on claim 32 or 33, in which the operating tool is a mechanical tool for shaping or machining material.

38. 38. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is a parallel kinematic arrangement, such as a hexapod arrangement, a triglide arrangement, or a delta robot arrangement.

39. A coordinate positioning apparatus according to any preceding claim, wherein the drive arrangement is a serial kinematic drive arrangement, such as one having a number of segments or rings connected in series by rotary and / or linear joints.

40. 40. A coordinate positioning apparatus according to any preceding claim, wherein the weighing arrangement is a mechanical weighing arrangement.

41. 1. A coordinate positioning apparatus comprising: a movable structure within a working volume of the apparatus; a metrology arrangement for measuring a position of the structure within the working volume; and a drive arrangement for moving the structure around the working volume, the metrology arrangement including a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurement values ​​from which the position of the movable structure is determinable, the drive arrangement including a plurality of mechanical linkages arranged in parallel between the movable structure and a fixed structure of the apparatus, each mechanical linkage being actuated by a drive mechanism, the drive mechanism acting between the fixed structure and the mechanical linkage.

42. 1. A coordinate positioning apparatus comprising: a structure movable within a working volume of the apparatus; a metrology arrangement for measuring a position of the structure in the working volume; and a drive arrangement for moving the structure around the working volume, wherein the metrology arrangement comprises a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurement values ​​from which the position of the movable structure can be determined, and wherein the drive arrangement comprises a plurality of actuators in a parallel arrangement of a different type to that of the metrology arrangement.

43. 43. A method of controlling a coordinate positioning apparatus according to any one of claims 1 to 42, comprising the steps of: moving the operating tool around the working volume using the drive arrangement with an operating tool coupled to the movable structure, the metrology arrangement also being coupled to the movable structure; performing an operation using the operating tool; and controlling the actuators of the drive arrangement using the data values ​​received from the metrology transducers of the metrology arrangement to determine a position of the operating tool within the working volume.

44. A method according to claim 43 when dependent on claim 34 or 35, in which the operation is a measurement operation.

45. 44. The method of claim 43 when dependent on claim 36 or 37, wherein the operation is a machining operation.

46. A computer program which, when executed by a controller for a coordinate positioning apparatus, causes the controller to perform a method according to any one of claims 43 to 45.

47. 46. ​​A computer readable medium having stored therein computer program instructions for controlling a coordinate positioning apparatus controller to perform a method according to any one of claims 43 to 45.

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