Coordinate positioning machine

Abstract

The coordinate positioning machine includes a drive frame (37) and a metrology frame (36). The drive frame (37) includes a drive unit (28) for moving the movable structure (22) within the working volume (34) of the machine. The metrology frame (36) includes a metrology unit (26) for measuring the position of the movable structure (22) within the working volume (34). In one embodiment, the metrology unit (26) is a hexapod metrology unit and the drive unit (28) is a non-hexapod metrology unit. The metrology frame (36) has a thermal expansion coefficient that is lower than the thermal expansion coefficient of the drive frame (37). The drive frame (37) is coupled to the metrology frame (36) via a coupling unit (38), which prevents at least some strain associated with excess thermal expansion and contraction of the drive frame (37) from being transferred to the metrology frame (36). In another aspect, the drive arrangement (28) moves the movable structure (22) in the working volume (34) with fewer than six degrees of freedom, and the measurement arrangement (26) measures the position of the movable structure (22) within the working volume (34) with more degrees of freedom than the drive arrangement (28).

Description

[Technical field]

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

[0002] A non-Cartesian coordinate positioning machine 1 is shown diagrammatically in Figure 1 of the accompanying drawings. The coordinate positioning machine 1 generally comprises a first structure 2 and a second structure 4 supported and movable relative to each other by a plurality of telescoping or extendable legs 6 disposed therebetween. The first structure 2 and second structure 4 may also be referred to as a platform or stage, and the telescoping legs 6 may also be referred to as a post or ram. Where there are six such telescoping legs 6 (as illustrated in Figure 1), the apparatus will generally be Hexapod It is called.

[0003] The extendable legs 6 are typically attached to the structures 2, 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 at one or both ends with an adjacent leg 6. Each extendable leg 6 is typically formed as a pair of tubes, one telescopically moved within the other by a drive mechanism (e.g., a motor) to cause the extendable leg 6 to extend and retract.

[0004] The respective relative positions between the first structure 2 and the second structure 4 can be achieved by extending the legs 6 by different amounts, as indicated by the arrows 13 in FIG. 1. 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. In this way, extension of the leg 6 causes the encoder scale to move past the read head, thereby measuring the length of the extendable leg 6. The computer controller 5 operates to set the length of each extendable leg 6, resulting in the required relative movement between the structure 2 and the structure 4. Having six such length measuring transducers 10 allows the relative position to 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 a fixed structure of the coordinate positioning machine 1, while the other of structure 2 and structure 4 performs 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 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] For example, as illustrated in Figure 1, the lower structure 4 is fixed and the upper structure 2 is movable, with the workpiece 9 mounted on the lower structure 4 and the 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 positions, and the probe component 3 is positioned within the working volume 14 by operation of the extendable legs 6. Although the arrow 11 is shown to point to a translational movement, by appropriate control of the individual legs 6, the structure 2 may also be tilted.

[0007] Alternatively, the upper structure 2 can be fixed and the lower structure 4 movable, in which case the probe would be mounted to the underside of the lower structure 4 and the workpiece would be mounted to a part of the fixed structure below it, with the working volume (or operating volume) of the apparatus being below the lower structure 4 rather than above it.

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

[0009] For example, Patent Document 1 states: Hexapod Type This describes the machine tools, Hexapod TypeThe machine tool comprises an upper movable structure which is attached to a base by six hydraulically extendable legs similar in principle to those shown in FIG. 1 above. The extendable legs are attached to the base and to the movable structure via ball joints. The extendable legs are hydraulic and comprise piston rods which are movable in cylinders. The amount of leg extension is measured by mounting a magnetic scale to the cylinder and a suitable read head to the piston rod. In this way, leg extension causes the scale to move past the read head, whereby the length of the leg can be measured. A computer controller operates to set the length of each leg and to effect 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 which the position of the end effector can be determined by photogrammetry, in particular from reference points on the end effector. [Prior art documents] [Patent documents]

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

[0012] [Non-Patent Document 1] H. J. J. Braddick, “Mechanical Design of Experimental Apparatus”, Chapman & Hall, London, 1960, pp. 11-30. Summary of the Invention

[0013] According to a first aspect of the invention there is provided a coordinate positioning machine including a drive frame and a metrology frame, the drive frame comprising a drive arrangement for moving a moveable structure in a working volume of the machine, and the metrology frame comprising a metrology arrangement for measuring the position of the moveable structure within the working volume, the metrology arrangement comprising: Hexapod Type The driving device is a non-measuring device. Hexapod TypeThere is provided a coordinate positioning machine comprising a drive mechanism, the metrology frame having a lower coefficient of thermal expansion than a coefficient of thermal expansion of the drive frame, thereby causing excess thermal expansion and contraction of the drive frame (during use) compared to the metrology frame, and the drive frame is coupled to the metrology frame via a coupling arrangement that prevents (adapted to prevent) at least a portion of the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), or at least a portion of the distortion associated with the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), from being transferred to the metrology frame.

[0014] According to a second aspect of the present invention there is provided a coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising drives for moving a moveable structure in a working volume of the machine with fewer than six degrees of freedom, and the metrology frame comprising metrology for measuring the position of the moveable structure in the working volume with more degrees of freedom than the drives, the metrology frame having a lower thermal expansion coefficient than that of the drive frame, thereby resulting in excess thermal expansion and contraction of the drive frame (in use) compared to the metrology frame, and the drive frame is coupled to the metrology frame via a coupling arrangement that prevents (adapted to prevent) at least part of the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), or at least part of the distortion associated with the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), from being transferred to the metrology frame.

[0015] According to a third aspect of the present invention there is provided a coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising a drive for moving a movable structure in a working volume of the machine, and the metrology frame comprising a metrology arrangement for measuring a position of the movable structure within the working volume, the metrology arrangement comprising a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements by which a position of the movable structure can be determined, the drive arrangement comprising a plurality of mechanical linkages arranged in parallel between the movable structure and a fixed structure of the machine, each mechanical linkage being connected between the fixed structure and the mechanical linkage. a drive mechanism acting on said metrology frame, said metrology frame having a lower thermal coefficient of expansion than a thermal coefficient of expansion of said drive frame, thereby causing excess thermal expansion and contraction of said drive frame (during use) compared to said metrology frame, and said drive frame being coupled to said metrology frame via a coupling arrangement that prevents (or is adapted to prevent) at least a portion of the excess thermal expansion and contraction of said drive frame (compared to said metrology frame), or at least a portion of the distortion associated with the excess thermal expansion and contraction of said drive frame (compared to said metrology frame), from being transferred to said metrology frame.

[0016] According to a fourth aspect of the present invention there is provided a coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising a drive arrangement for moving a moveable structure in a working volume of the machine and the metrology frame comprising a metrology frame for measuring a position of the moveable structure within the working volume, the metrology frame comprising a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements by which a position of the moveable structure can be determined, the drive arrangement comprising a plurality of actuators in a parallel arrangement, the metrology frame having a lower thermal expansion coefficient than that of the drive frame, thereby resulting in excess thermal expansion and contraction of the drive frame (in use) compared to the metrology frame, and the drive frame is coupled to the metrology frame via a coupling arrangement that prevents (adapted to prevent) at least a portion of the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), or at least a portion of the distortion associated with the excess thermal expansion and contraction of the drive frame (compared to the metrology frame), from being transferred to the metrology frame.

[0017] The extra thermal expansion and contraction referred to above can be considered as the extra thermal expansion or contraction of the drive frame compared to the metrology frame for the same (or at least similar or a given) change in temperature of both the drive frame and the metrology frame. Such a change in temperature can be caused, for example, by a change in the environmental temperature in which the machine is operating, where the temperatures of the drive frame and the metrology frame closely follow the environmental temperature. Unless the environment in which the machine is operating (such as a manufacturing facility) is very tightly controlled, the environmental temperature will constantly change throughout the day, sometimes significantly as a result of heat from other machines operating nearby or due to surrounding heating and / or cooling systems.

[0018] An embodiment of the present invention ensures that thermal effects causing excess expansion or contraction of the drive frame do not result in distortion of the metrology frame, or at least ensure that such effects are reduced. Isolating the metrology frame from the drive frame in such a way therefore prevents or at least reduces substantial distortion of the metrology frame due to differences in thermal expansion / contraction of the drive frame compared to the metrology frame, thereby providing more accurate metrology measurements, or at least more consistent measurements over different operating temperatures, compared to previously considered metrology arrangements.

[0019] The metrology frame advantageously has a low coefficient of thermal expansion (CTE), advantageously less than 15 ppm / c, more preferably less than 10 ppm / c, more preferably less than 5 ppm / c, more preferably less than 3 ppm / c, more preferably less than 2 ppm / c, more preferably less than 1 ppm / c, where 1 ppm / c means 1×10-6 per degree C.

[0020] The metrology frame is at least partially formed of a composite material such as carbon fiber, which typically has a coefficient of thermal expansion of less than 5 ppm / C, depending on the exact composition. The metrology frame is at least partially formed of INVAR®, which is an iron-nickel based alloy having a coefficient of thermal expansion of approximately 0.5-1.5 ppm / C, depending on the exact composition. The metrology frame is at least partially formed of a glass-ceramic material such as ZERDUR®, which has a coefficient of thermal expansion of approximately 0.02 ppm / C or less, depending on the exact composition. Such a low coefficient of expansion can be more than an order of magnitude lower than traditional materials such as aluminum (24 ppm / C), brass (19 ppm / C) or steel (13 ppm / C) used to form at least a portion of the drive frame.

[0021] The coefficient of thermal expansion of the measurement frame may be less than half the coefficient of thermal expansion of the drive frame. The coefficient of thermal expansion of the measurement frame may be less than one third the coefficient of thermal expansion of the drive frame. The coefficient of thermal expansion of the measurement frame may be less than one quarter the coefficient of thermal expansion of the drive frame. The coefficient of thermal expansion of the measurement frame may be less than one fifth the coefficient of thermal expansion of the drive frame. The coefficient of thermal expansion of the measurement frame may be less than one eighth the coefficient of thermal expansion of the drive frame. The coefficient of thermal expansion of the measurement frame may be less than one tenth the coefficient of thermal expansion of the drive frame.

[0022] A metrology frame made from a material with a low coefficient of thermal expansion (e.g., carbon fiber and / or INVAR and / or ZERODUR) can prevent or at least reduce substantial metrology errors caused by temperature changes. However, such materials are often difficult to machine and are typically significantly more expensive than standard factory materials such as aluminum. Thus, one embodiment of the present invention combines the metrology advantages of a low CTE material such as carbon fiber with the ease of manufacture associated with materials such as aluminum. For example, a metrology frame made primarily of a low CTE material such as carbon fiber can be combined with a drive frame made primarily of a traditional material such as aluminum (with certain parts, such as guide rails, requiring higher hardness and / or stiffness and / or durability that may be formed from other materials such as steel).

[0023] It is not necessary that the thermal expansion coefficient of each part of the metrology frame be lower than that of the drive frame. Indeed, for practical and engineering reasons, parts of the metrology frame will usually need to be made of materials that are easier to handle and / or more wear-resistant or durable, such as aluminum, steel or plastic. For example, the bearing between the two parts of a telescopically extendable metrology leg may be made of steel.

[0024] Importantly, it may be considered that the representative coefficient of thermal expansion of the metrology frame is lower than the representative coefficient of thermal expansion of the drive frame. The representative coefficient of thermal expansion may be the result of considering the relevant frame as a single structure with a single overall (e.g. average) coefficient of thermal expansion. The representative coefficient of thermal expansion may be (or a measure of) the coefficient of thermal expansion of that or those portions of the relevant frame that contributes most (e.g. contributes at least 25% or at least 50% or at least 75%) to the thermal expansion and contraction of that frame. The representative coefficient of thermal expansion may be (or a measure of) the coefficient of thermal expansion of the material that forms the largest part (e.g. by weight and / or volume and / or length and / or width and / or depth) of the relevant frame. The representative coefficient of thermal expansion may be (or a measure of) the coefficient of thermal expansion of that or those portions of the relevant frame that has a significant effect on the measurement results from the coordinate positioning machine for a given temperature change. The metrology result may be a position determined by a coordinate positioning machine or a measured result, for example, a distance between two points on a representative workpiece occupying at least one-quarter (or at least one-half) of the working volume of the machine. The significant effect may be at least 0.01% of the metrology result, at least 0.1% of the metrology result, at least 1% of the metrology result, or at least 5% of the metrology result. The predetermined temperature change may be 1°C or 5°C or 10°C.

[0025] The metrology device may comprise a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurements based on which the position of the movable structure can be determined.

[0026] The measurement transducer may be a length measurement transducer.

[0027] The measurements may relate to the respective differences in separation distances between movable and fixed structures in the machine.

[0028] The measurement transducer may be configured to provide a direct measurement of the distance separation.

[0029] The measurement transducer may be configured to provide a direct measurement of the change in separation distance as the structure moves in the working volume, from which the separation distance can be determined.

[0030] Each of the measurement transducers may include an encoder scale and an associated readhead.

[0031] The measurement device may comprise 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.

[0032] The coordinate positioning apparatus may be equipped with six such measurement transducers.

[0033] The multiple measurement transducers may be multiple independent measurement transducers.

[0034] The multiple measurement transducers described herein are to be contrasted with image-based or photogrammetric measurement apparatus, in which, for example, each image capture device does not provide a direct or independent measurement of any length or distance interval, or a direct or independent measurement of the position of at least a portion of the movable structure. A determination of the position of the movable structure can only be made based on a photogrammetric combination of images from all image capture devices. In image-based or photogrammetric measurement apparatus, distances can only be indirectly inferred from image data.

[0035] The drive device can be a non-telescopic drive device, in which parts of the device telescopically move inwardly or at least linearly along other parts, and during this movement the parts remain substantially aligned with each other.

[0036] The metrology setup may be a top-down metrology setup or a bottom-up metrology setup, where the metrology setup is connected between a movable structure and a fixed structure in the machine, in a top-down metrology setup the movable structure is located below the fixed structure and the metrology setup extends downwards from the fixed structure to the movable structure, whereas in a bottom-up metrology setup the movable structure is located above the fixed structure and the metrology setup extends upwards from the fixed structure to the movable structure.

[0037] The drive can be a top-down drive or a bottom-up drive, where the drive is connected between a movable structure and a fixed structure in the machine, in a top-down drive the movable structure is located below the fixed structure and the drive extends downwards from the fixed structure to the movable structure, whereas in a bottom-up drive the movable structure is located above the fixed structure and the drive extends upwards from the fixed structure to the movable structure.

[0038] The drive may be configured to maintain the moveable structure in a substantially constant orientation as it moves in the working volume.

[0039] The drive arrangement may include a number of mechanical linkages connected in parallel between the movable and fixed structures of the machine.

[0040] Each mechanical linkage may be actuated (or driven) by a drive mechanism acting between the fixed structure and the mechanical linkage. Here, such a drive mechanism is different from one that acts between, for example, two parts (or links) of the mechanical linkage. For example, in a typical hexapod drive, 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 does the opposite to retract the leg. The motor does not act between the extendable leg (mechanical linkage) and the fixed structure. Therefore, in embodiments of the present invention, it can be considered that each mechanical linkage may be actuated (or driven) by a drive mechanism that acts directly between the fixed structure and the mechanical linkage.

[0041] Movement of the driven portion of the 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.

[0042] The driven portion of the mechanical linkage may be a carriage arranged for linear movement along a corresponding track. A plurality of such tracks may be arranged substantially parallel to one another. In the case of a three-slide drive, three such tracks are provided.

[0043] The drive mechanism may be a rotary drive mechanism such as found in a delta robot.

[0044] Such a drive mechanism may be configured to drive a driven portion of the mechanical linkage in a substantially rotational manner relative to the fixed structure.

[0045] The rotary drive may be a direct rotary drive.

[0046] The drive mechanism can be a three-slide mechanism or a linear drive mechanism such as found in a cable robot.

[0047] Such a drive mechanism may drive the driven portion of the mechanical linkage substantially linearly relative to the fixed structure, such as along a substantially linear feature of the fixed structure and along a substantially linear trajectory of the fixed structure.

[0048] The linear drive mechanism may be a direct linear drive mechanism.

[0049] The linear drive mechanism may be configured to translate the end of the mechanical linkage in a substantially linear manner.

[0050] The linear drive mechanism may include a linear motor.

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

[0052] The drive may include three such mechanical linkages.

[0053] The drive may be a three-slide device.

[0054] The metrology arrangement may comprise a plurality of mechanical linkages arranged in parallel between the fixed structure and the movable structure such that a corresponding plurality of measurement transducers are associated with the plurality of mechanical linkages respectively. There may be six such mechanical linkages and six corresponding respective measurement transducers in the metrology arrangement.

[0055] Each mechanical linkage of the metrology device may be connected between a respective point on the fixed structure and the movable structure and may be configured to allow the distance separation between those points to be varied.

[0056] A measurement transducer associated with the mechanical linkage of the measurement device may be configured to produce an output that is dependent on the distance interval.

[0057] Each mechanical linkage of the metrology device may be a telescoping or extension leg.

[0058] A mechanical linkage may also be called or considered as a kinematic chain or a mechanical assembly.

[0059] A separate drive mechanism may be provided for each mechanical linkage of the drive arrangement.

[0060] A drive mechanism associated with the mechanical linkage may be configured to act between a fixed structure and an end of the mechanical linkage.

[0061] Each mechanical linkage may include at least one rigid rod.

[0062] Each mechanical linkage may include at least two substantially parallel rods for maintaining the moveable structure in a substantially constant orientation as it moves about the working volume.

[0063] Each of the mechanical linkages may be of substantially the same construction or design.

[0064] A drive system that has three mechanical linkages, each with a linear drive, is known as a three-slip system.

[0065] The drive may include, be in the form of, or effect a delta robot configuration.

[0066] The drive may include, be in the form of, or effect a linear delta robot configuration.

[0067] The actuators may be configured to move the structure around the working volume with fewer than six degrees of freedom, and the metrology device may be configured to measure the position of the structure within the working volume with more degrees of freedom than the actuators.

[0068] The measuring device is Hexapod Type It can be a measuring device.

[0069] The drive unit is Hexapod Type It may be a drive device.

[0070] The actuators may be configured to move the structure about the working volume in three degrees of freedom.

[0071] The three degrees of freedom may be three translational degrees of freedom.

[0072] The metrology device can be configured to measure the position of the structure in six degrees of freedom (three translational and three rotational degrees of freedom, i.e. position and orientation).

[0073] The metrology device and the drive device may each be located (eg, connected) between a movable structure and a fixed structure in the machine.

[0074] The metrology device and the drive device may each be connected or coupled to both the movable structure and the fixed structure.

[0075] The metrology device may be formed primarily (eg, greater than 50% or 75% by weight) of a composite material such as carbon fiber, or other materials such as INVAR or ZERODUR.

[0076] A drive may include multiple actuators in a parallel arrangement, as opposed to the serial arrangement of actuators present, for example, in a conventional 3-axis (x, y, z) coordinate measuring machine (CMM).

[0077] The drive arrangement may comprise multiple actuators in a parallel arrangement of a different type than the configuration of the metrology arrangement.

[0078] The drive may be formed primarily (eg, greater than 50% or 75% by weight) of an engineering material such as aluminum or steel.

[0079] The drive may comprise a number of measurement transducers separate from that of the metrology device in order to provide corresponding respective measurements from which the position of the movable structure can be determined independently of a position determined based on measurements of the metrology device, in other words the drive may be encoded independently of the metrology device.

[0080] The driver may be coupled to the measurement device via a coupling device that prevents at least some strain associated therewith from being transmitted to the measurement device.

[0081] The coupling device can be a kinematic or pseudo-kinematic coupling device. Such coupling devices constrain six degrees of freedom between the coupled parts without introducing redundant constraints. Kinematic coupling devices can include, for example, cones, V-grooves, and plate arrangements.

[0082] The coupling device may include a plurality of balls.

[0083] The coupling device may include a number of resilient spacers or pads.

[0084] The movable structure may comprise a drive portion associated with the drive device and a measurement portion associated with the measurement device, the drive portion of the movable structure being coupled to the measurement portion of the movable structure via the coupling device.

[0085] The measurement part of the movable structure may have a lower coefficient of thermal expansion than the drive part of the movable structure. Suitable representative values ​​(relative and absolute) of these coefficients of thermal expansion are described above with reference to the metrology frame and the drive frame. The measurement part of the movable structure may be made mainly (e.g., more than 50% or 75% by weight) of a composite material such as carbon fiber or other materials such as INVAR or ZERODUR. The drive part of the movable structure may be made mainly (e.g., more than 50% or 75% by weight) of an engineering material such as aluminum or steel.

[0086] The drive of the movable structure is coupled to a drive device.

[0087] The metrology portion of the movable structure is coupled to a metrology device.

[0088] The metrology frame may be considered to include the metrology portion of the moveable structure.

[0089] The drive frame may be considered to include the drive portion of the movable structure.

[0090] The coordinate positioning machine may comprise a fixed structure having a drive section associated with the drive device and a measurement section associated with the measurement device, the drive section of the fixed structure being coupled to the measurement section of the fixed structure via the coupling device.

[0091] The measurement portion of the fixed structure may have a lower coefficient of thermal expansion than the drive portion of the fixed structure. The measurement portion of the fixed structure may be made primarily (e.g., greater than 50% or 75% by weight) of a composite material such as carbon fiber or other materials such as INVAR or ZERODUR. The drive portion of the fixed structure may be made primarily (e.g., greater than 50% or 75% by weight) of an engineering material such as aluminum or steel.

[0092] The drive portion of the stationary structure is coupled to a drive device.

[0093] The measurement portion of the fixed structure is coupled to a measurement device.

[0094] The metrology frame may be considered to include a metrology portion of a fixed structure.

[0095] The drive frame may be considered to include the drive portion of the fixed structure.

[0096] The drive frame can be considered to consist of those parts of the fixed structure that are not part of the metrology frame.

[0097] The driving frame can be considered to consist of those parts of the machine that are not part of the measuring frame.

[0098] The metrology frame may be provided entirely within (e.g. within its boundaries) the drive frame. Conversely, the drive frame may be provided entirely outside (e.g. outside its boundaries) the metrology frame. The metrology frame may be spatially separated (e.g. not overlapping) within the drive frame. There may be no part of the drive frame between the metrology portion of the fixed structure and the metrology portion of the movable structure. The metrology portion of the fixed structure may be provided between the drive portion of the fixed structure and the movable structure. Such an arrangement provides a simple machine structure that facilitates assembly and maintenance of the machine and provides an effective thermal barrier between the metrology frame and the heat generating drive means (e.g. a motor) within the drive frame.

[0099] The movable structure supports the operating tool, in other words the operating tool is supported by the movable structure at the same time that the measuring device is connected to the movable structure, such that the coordinate measuring machine is ready for operating use (the operating tool is in the appropriate place) rather than, for example, merely ready to be calibrated (in which case the operating tool is not present in the appropriate place).

[0100] The machine may also be a coordinate measuring machine.

[0101] The machine may also be a comparator.

[0102] The manipulation tool may be a surface sensing device or a measurement probe.

[0103] The machine may be a machine tool.

[0104] The manipulation tool may be a mechanical tool for shaping or machining the material.

[0105] The measurement device may include measurement transducers arranged exclusively in parallel with one another, i.e. the measurement transducers are not arranged in series with one another.

[0106] The metrology device may include six measurement transducers in a parallel arrangement to provide six corresponding respective measurements based on which the position of the movable structure can be determined. A metrology device having fewer than six measurement transducers in a parallel arrangement may include Hexapod Type It is not a measuring device (e.g., a tripod device is Hexapod Type (not the device).

[0107] The metrology device can be considered to be for measuring different positions of the movable structure in the working volume resulting from each different state or configuration of the drive. In other words, the metrology device measures the position of the movable structure in a first configuration of the drive (first position), then the drive moves to a second configuration different from the first configuration, and the metrology device measures the position of the movable structure in the second configuration of the drive (second position). The drive can be considered to include all parts of the machine used to move the movable structure from the first position to the second position.

[0108] The measurement device may comprise six extendable legs arranged in parallel, with six measurement transducers associated with each of the six extendable legs.

[0109] The measurement transducer may be an interferometric measurement transducer.

[0110] The drive may comprise fewer than six actuators in a parallel arrangement.

[0111] The drive may be a parallel kinematic device.

[0112] The drive may be a non-Cartesian device.

[0113] The drive unit may comprise fewer than six actuators.

[0114] The parallel arrangement of the actuators associated with the drive device may differ from the parallel arrangement of the measurement transducers associated with the metrology device.

[0115] The drive may include a number of measurement transducers for providing corresponding respective measurements from which the position of the movable structure can be determined.

[0116] The measurement transducer may be a mechanical measurement transducer, as opposed to an optical or image-based or photogrammetric measurement transducer.

[0117] The measurement transducer may be a length measuring transducer. Measuring the length of a part of the machine, such as an extendable leg, may be considered equivalent to measuring the distance interval between two parts of the machine, such as the ends of an extendable leg. The transducer may not be able to measure an absolute length or interval, but may measure a change in length or interval, from which an absolute length or interval can be determined based on the length, etc. (e.g., based on a geometric model of the machine). Examples of sensors that do not measure length, but can determine position, for example in combination with other sensor data, are accelerometers (acceleration sensors), tilt sensors, and gyroscopes.

[0118] The measurement transducer may sample at a first clock rate that is comparable to (eg, at least half, or at least the same, or substantially the same as) a second clock rate used to control the drive.

[0119] Measurements received from the measurement transducer may allow the position of the structure to be determined without reference to other sensor or transducer data (such as photogrammetric data from an image sensor), which may be acquired at a third clock rate that is lower than the first clock rate.

[0120] The first clock rate may be greater than 1 kHz, more preferably greater than 10 kHz, more preferably greater than 15 kHz.

[0121] The metrology device may be coupled to the moveable structure in a non-contact manner, such as optically where the metrology device is an optical metrology device.

[0122] The measuring device is Hexapod Type It may be mechanically coupled to a movable structure, such as may be the case with a metrology device.

[0123] The metrology device may be, for example, a mechanical metrology device, as opposed to an optical or image-based or photogrammetric device.

[0124] The metrology device may be mechanically coupled to the movable structure.

[0125] The extendable legs may include a mechanical device (eg, a mechanical linkage) that allows for varying the distance separation between a point on a fixed structure and a point on a movable structure.

[0126] The movable structure can be configured to support or carry an object to be moved around the working volume. The object can be an object to be taken and / or placed in the working volume. The object can be a tool for interacting with or manipulating another object, such as a workpiece, located in the working space. 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 perform the measurement. 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 a machine tool for forming or machining metals or other rigid materials.

[0127] The movable structure may be configured to carry an operating tool with a measuring device and a drive device coupled to the movable structure. The coordinate positioning machine may be set up with the operating tool already coupled to the movable structure and ready to perform the intended operation. In other words, the coordinate positioning machine may be set up for operation and not just for calibration.

[0128] Hexapod Type The metrology device may be coupled to the moveable structure by a mounting different from that used to attach the manipulation tool to the moveable structure.

[0129] Hexapod Type The metrology device may be directly coupled to the movable structure.

[0130] A "transducer" may be considered herein to be a device that converts a variation in a physical quantity into an electrical signal (such as a "sensor" as described herein that measures or senses a change in length) or vice versa from an electrical signal to a physical quantity (such as a "actuator" as described herein that effects a movement on a structure based on an input or drive signal). Measuring the "position" of a structure should be understood as measuring the position and / or orientation of the structure in an appropriate number of degrees of freedom. For example, if the position is measured in six degrees of freedom, then both the position and the orientation of the structure are determined. However, if the position is measured in only three degrees of freedom, this may or may not include determining the orientation of the structure. The term "measuring position" should be interpreted accordingly.

[0131] According to a fifth aspect of the present invention there is provided a method of controlling a coordinate positioning machine according to any of the above aspects, the method comprising the steps of coupling a tool to a movable structure and using a drive device to move the tool within a working volume having a metrology device also coupled to the movable structure to perform an operation using the tool.

[0132] The method may include determining a position of a tool for operation within the working volume using a metrology device.

[0133] The method may include associating the determined location with the performed operation.

[0134] The operation may be a measurement operation.The operation may be a machining operation.

[0135] 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 disposed in the working volume.

[0136] The method may be performed based on the position of the structure rather than the tool, or a combination of the tool and the structure.

[0137] According to a sixth aspect of the present invention there is provided a controller for a coordinate positioning machine, the controller configured to carry out a method according to the fifth aspect of the present invention.

[0138] According to a seventh aspect of the present invention there is provided a computer program which, when executed by a coordinate positioning machine 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 machine controller, causes the coordinate positioning machine controller to become a coordinate positioning machine controller according to the sixth aspect of the present invention. The program may be carried on a carrier medium. The carrier medium may be a storage medium. The carrier medium may be a transmission medium.

[0139] According to an eighth aspect of the present invention there is provided a computer readable medium having stored thereon computer program instructions for controlling a coordinate positioning machine controller to perform a method according to the fifth aspect of the present invention. [Brief description of the drawings]

[0140] Reference will now be made, by way of example, to the accompanying drawings in which: [Figure 1] FIG. 1, as previously mentioned, is a schematic diagram of a hexapod coordinate positioning machine having six extendable legs. [Diagram 2] FIG. 2 is a schematic side view of the hexapod coordinate positioning machine of FIG. [Diagram 3] FIG. 3 is a schematic side view of a coordinate positioning machine on which an embodiment of the present invention is based, the coordinate positioning machine having a metrology device and a separate drive device. [Figure 4] FIG. 4 is a schematic side view of a coordinate positioning machine embodying the invention, with the metrology arrangement shown somewhat separated from the drive arrangement. [Diagram 5] FIG. 5 shows a first perspective view of a practical embodiment of the coordinate positioning machine of FIG. [Figure 6] FIG. 6 shows a second perspective view of the embodiment of FIG. [Figure 7]FIG. 7 shows a side view of the embodiment of FIG. [Figure 8] FIG. 8 shows a top view of the embodiment of FIG. [Figure 9A] FIG. 9A is a diagram illustrating the operation of an embodiment of the present invention. [Figure 9B] FIG. 9B is a diagram illustrating the operation of an embodiment of the present invention. [Figure 9C] FIG. 9C is a diagram illustrating the operation of an embodiment of the present invention. [Figure 9D] FIG. 9D is a diagram illustrating the operation of an embodiment of the present invention. [Figure 9E] FIG. 9E is a diagram illustrating the operation of an embodiment of the present invention. [Figure 10] FIG. 10 is a schematic diagram of a top-down variant of the coordinate positioning machine of FIG. [Figure 11] FIG. 11 is a schematic diagram of a variation of the top-down coordinate positioning machine of FIG. [Figure 12] FIG. 12 shows a slight modification of the coordinate positioning machine of FIGS. [Figure 13] FIG. 13 shows a top-down variation of the coordinate positioning machine of FIG. [Figure 14] FIG. 14 shows a variation of the top-down coordinate positioning machine of FIG. [Figure 15] FIG. 15 shows another variation of the top-down coordinate positioning machine of FIG. [Figure 16A] FIG. 16A is a schematic diagram of an embodiment having a different type of non-hexapod drive. [Figure 16B] FIG. 16B is a schematic diagram of an embodiment having a different type of non-hexapod drive. [Figure 17] FIG. 17 shows a practical embodiment of the coordinate positioning machine of FIGS. 16A and 16B. [Figure 18A] FIG. 18A illustrates, in schematic form, a variation of the embodiment of FIGS. 16A and 16B, showing some separation of the metrology arrangement from the drive arrangement. [Figure 18B] FIG. 18B illustrates, in schematic form, a variation of the embodiment of FIGS. 16A and 16B, showing some separation of the metrology arrangement from the drive arrangement. [Figure 19] FIG. 19 illustrates a variation of the embodiment of FIGS. 16A and 16B, in a bottom-up rather than top-down hexapod metrology setup. [Figure 20] FIG. 20 illustrates a schematic variation of the embodiment of FIGS. 16A and 16B, showing the use of fixed length metrology struts in a hexapod metrology device. [Figure 21] FIG. 21 shows a schematic variation of the embodiment of FIG. 20, illustrating the use of offset fulcrum plates on the measurement struts. [Figure 22] FIG. 22 shows a schematic of an embodiment having a delta robot type non-hexapod drive. [Figure 23] FIG. 23 shows, in schematic form, a variation of the embodiment of FIG. 22, illustrating an increased amount of separation between the metrology arrangement and the drive arrangement. [Figure 24] FIG. 24 shows a schematic variation of the embodiment of FIG. 22, illustrating a reduced amount of separation between the metrology arrangement and the drive arrangement. [Diagram 25] FIG. 25 shows a schematic of an embodiment having a non-hexapod drive of the series kinematic type. [Figure 26] FIG. 26 shows diagrammatically another embodiment having a non-hexapod drive of the series kinematic type. [Figure 27] FIG. 27 illustrates the concept of providing a drive mechanism with fewer degrees of freedom than the measurement mechanism. [Figure 28] FIG. 28 shows a dual hexapod device where the motion of the hexapod drive is constrained. [Figure 29] FIG. 29 is a flow diagram illustrating a method of controlling a coordinate positioning machine embodying the present invention. [Diagram 30]FIG. 30 is a perspective view of a kinematic coupling suitable for use between a metrology device and a drive device in one embodiment of the present invention. [Diagram 31] FIG. 31 is a top view of the first portion of the coupling shown in FIG. [Diagram 32] FIG. 32 is a perspective view of a first portion of the coupling of FIG. 30. FIG. [Diagram 33] FIG. 33 shows how the ball for the second part of the joint of FIG. 30 seats within the V-groove of the first part. [Diagram 34] FIG. 34 shows the six contact points made by a ball seated in a V-groove. [Diagram 35] FIG. 35 illustrates how the contact points move when the first and second parts of the joint are subjected to different thermal expansions. [Diagram 36] FIG. 36 illustrates another form of kinematic coupling suitable for use between a metrology device and a drive device in an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0141] A side view of the coordinate positioning machine 1 described above with reference to Figure 1 is shown diagrammatically in Figure 2. The coordinate positioning machine 1 comprises an upper structure 2 which is movable within a working volume 14 of the machine 1. Six extendable legs 6 provide support for moving the upper structure 2 within the working volume 14. Hexapod Type A drive 18 (shown in solid lines) and a similar mechanism for measuring the position of the upper structure within the working volume 14. Hexapod Type The coordinate positioning machine 1 thus forms both a drive strut and a metrology strut.

[0142] By way of comparison, a coordinate positioning machine 21 on which an embodiment of the invention is based is shown diagrammatically in Figure 3. Like the machine of Figure 2, the coordinate positioning machine 21 comprises an upper structure 22 movable within a working volume 34 of the machine 21, a drive device 28 (shown in solid lines) for moving the upper structure 22 in the working volume 34, and a metrology device 26 (shown in dotted lines) for measuring the position of the movable upper structure 22 within the working volume 34.

[0143] In the coordinate positioning machine 1 shown in FIG. Hexapod Type Measuring device 16 and Hexapod Type Whereas the drive 18 is combined with the measuring device 26, in a coordinate positioning machine 21 embodying the invention shown in Figure 3 the drive 28 is distinct and separate from the measuring device 26. Separating the measuring device 26 from the drive 28 in this manner provides technical advantages, as these two different devices can be designed with very different (and sometimes competing) technical considerations in mind.

[0144] By separating and distinguishing the metrology arrangement 26 from the drive arrangement 28, the drive arrangement 28 can be made relatively light and fast, thereby allowing the structure 22 to be moved quickly in the working volume 34 with high accelerations and rapid changes of direction. The focus on factors such as weight and speed may come at the expense of some positional accuracy of the drive arrangement 28, but this is overcome by providing a metrology arrangement 26 that is instead designed with positional accuracy in mind.

[0145] Because the measurement device 26 is passive and does not require moving components that add weight and generate heat, measurement errors caused by inertia and thermal distortion of components (including the measurement scale used to measure distance) can be controlled and reduced.

[0146] By using a measuring device 26 that is separate and distinct from the drive device 28, it is possible to obtain a coordinate positioning machine 21 that can rapidly drive a movable structure through a working volume while still maintaining the accuracy required for demanding positioning applications.

[0147] Such a design also allows the selection of relatively inexpensive off-the-shelf drives for drive 28 that are not specifically designed with high accuracy in mind, and coupled with dedicated metrology 26 to provide the required accuracy, thus reducing production costs.

[0148] While mechanical metrology devices may benefit from having low friction joints, drives typically require more robust and substantial joints that will necessarily have higher friction, especially under load, thus presenting a design conflict that is resolved by isolating the metrology device 26 from the drive 28. The joints of the metrology device 26 can be of a lower friction type and are not subject to the same loads as the drive 28. As a result, isolating the metrology device 26 from the drive 28 reduces hysteresis effects that would result in different measurements being recorded depending on the direction of approaching the workpiece.

[0149] In the coordinate positioning machine 21 of FIG. Hexapod Type device, whereas the drive device 28 is not Hexapod Type Equipment (i.e. Hexapod Type Other than the device or Hexapod Type (different from the device). Hexapod By using the base measurement device 26, Hexapod However, this is particularly advantageous as it gives a robust mechanical system with parallel arranged measurement transducers which allows direct measurement of distance from which a very accurate and reliable determination of position in six degrees of freedom can be derived.

[0150] HexapodThe image-based metrology device 26 is also superior to some image-based metrology devices in terms of the speed at which it can determine or sample the position of the moving structure 22. In the case of image-based (photogrammetry) metrology devices, 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, in the image-based system of US Pat. No. 5,233,633, it is stated that "the frame rate provided by the image detector is typically only a few hundred Hertz," which is described as an advantage since it avoids detecting undesirable high-frequency movements such as vibrations.

[0151] However, the applicant Hexapod Type We recognize that a much higher dynamic bandwidth can be achieved by direct sampling of data from measurement transducers associated with the extendable legs of the metrology device. For example, a machine controller might typically request position data from an absolute encoder every 65 μs (15 kHz), although higher sampling rates are possible. Relative encoder systems produce a continuous sinusoidal output, which allows for even 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.

[0152] The '122 patent for an image-based system does not describe using a parallel arrangement of measurement transducers that independently provide a set of corresponding measurements, where each measurement in the set directly represents or relates to a distance or distance interval between a different point on the moving platform and a point on the fixed structure, from which the position and / or orientation of the moving platform can be determined. In particular, the '122 patent: Hexapod Type There is no mention of using a measuring device.

[0153] As shown in Figure 1 Hexapod TypeThe drive system requires a motor for each of the six struts, which necessarily forms part of the associated strut, i.e. moves with the strut. Hexapod When the motor is actuated to move a moving structure in the working volume, the weight of the relatively heavy motor components is also moved. When this extra mass needs to be moved, the potential speed (or acceleration) of the drive system is reduced, and additional heat is generated in the machine, which can have detrimental effects when it is passed on to the measurement system. Hexapod Type By providing a drive arrangement, these problems can be eliminated by shifting the motor components away from the moving parts.

[0154] Furthermore, non-linear motions that give the movable structure 22 less than six degrees of freedom are possible. Hexapod Type By using a drive mechanism, fewer actuators are required (i.e. Hexapod The motor is driven by a 3.5-kV motor, which is rated at 1.5 V and is rated at 2.5 V. This reduces cost and complexity; in addition, less heat is generated in the motor components, resulting in improved measurement results.

[0155] Figure 3 Hexapod Type The measuring device 26 is shown in FIG. Hexapod Type The drive device 28 in this example is generally similar to the drive device 20 in the prior art, but without any actuator or motor components required to provide drive. The drive device 28 in this example is a so-called "three-slide" device, such as that disclosed in U.S. Pat. No. 5,399,323, and has three carriages 56 that move along three corresponding respective linear tracks 51 arranged substantially in parallel. These devices are described in more detail below with reference to Figures 5 to 8.

[0156] 3, the coordinate positioning machine 21 comprises a lower structure 24 which forms part of the fixed structure of the machine 21 and on which a workpiece 29 is mounted. A measurement probe 30 is supported on the upper structure 22 and thereby movable through a working volume 34. The working volume 34 is defined between the upper structure 22 and the lower structure 24 when they are in their farthest apart positions and the probe component 30 is positioned within the working volume 34 by operation of the drive 28.

[0157] 3 also shows, diagrammatically, a controller C, which may be implemented in hardware or software or a combination thereof, for controlling the drives 28 to cause the desired movement of the structure 22. For the sake of clarity and brevity, the controller C is omitted from subsequent figures.

[0158] One embodiment of the present invention is shown in Figure 4. To provide additional separation between the actuator 28 and the measurement device 26, the actuator 28 is coupled to the measurement device 26 via a coupling device 38, which prevents at least some thermally induced strain associated with the actuator 28 from being transferred to the measurement device 26 (as described in more detail below). The coupling device 38 includes a first coupling portion 38a associated with the movable structure 22 and a second coupling portion 38b associated with the fixed structure 24.

[0159] In the schematic embodiment shown in Fig. 4, the movable structure 22 includes a measurement portion 22a associated with the measurement device 26 and a drive portion 22b associated with the drive device 28, where the measurement portion 22a of the movable structure 22 is coupled to the drive portion 22b of the movable structure 22 via a first coupling portion 38a. The measurement portion 22a of the movable structure 22 is coupled to the measurement device 26. The drive portion 22b of the movable structure 22 is coupled to the drive device 28.

[0160] Similarly, the fixed structure 24 includes a measurement section 24a associated with the measurement device 26 and a drive section 24b associated with the drive device 28, where the measurement section 24a of the fixed structure 24 is coupled to the drive section 24b of the fixed structure 24 via a second coupling section 38b. The measurement section 24a of the fixed structure 24 is coupled to the measurement device 26. The drive section 24b of the fixed structure 24 is coupled to the drive device 28.

[0161] In this example, each of the couplings 38a, 38b of the coupling device 38 is in the form of a kinematic or pseudo-kinematic coupling. In terms of positioning an object relative to another object, kinematic design considerations can be addressed by constraining the degrees of freedom of the object's motion with a minimum number of constraints, and in particular over-constraints should be avoided. Over-constraints can result in multiple contact points between the two objects, with one object coming to rest in multiple positions relative to the other. As a result, the object's position cannot be reproduced because it is not known where the object will come to rest in several positions. In particular, when there are over-constraints, there are conflicts between the constraints for a given position, and as a result it is not possible to determine with certainty which combination of constraints determines the actual position of the object. These ideas are described in non-patent document 1.

[0162] Such a kinematic coupling has a minimum number of contact points (or point-like contacts) to provide ideal constraints, and is also very effective in isolating the strains of one side of the coupling when they are transmitted to the other side of the coupling. Thus, the first coupling 38a helps to prevent strains of the drive part 22b of the movable structure 22 (due to forces acting on it from the drive device 28) from being transmitted to the measurement part 22a (and therefore to the measurement device 26), as well as the second coupling 38b with respect to the fixed structure 24. This will be explained in more detail below with reference to Figures 30 to 36.

[0163] This results in a clearly delineated metrology frame 36 (i.e., metrology portion 24a of the fixed structure 24, metrology portion 22a of the movable structure 22, and metrology device 26) with good mechanical isolation from the drive frame 37 (i.e., the other parts of the metrology frame 36, including drive portion 22b of the movable structure 22, drive portion 24b of the fixed structure 24, drive device 28, linear actuation 51 and carriage 56).

[0164] The mechanical isolation provided by the first and second joints 38a, 38b is effective not only to isolate drive or load related distortions of the drive frame 37 from the metrology frame 36, but also to isolate the effects of thermal expansion and contraction of the drive frame 37 from the metrology frame 36. Furthermore, the metrology frame 36 is formed from a material with a low coefficient of thermal expansion (CTE), thereby allowing it to remain substantially unaffected by changes in the thermal environment. The first and second joints 38a, 38b between the metrology frame 36 and the drive frame 37 also ensure that distortions of the metrology frame 37 are not induced by thermal expansion or contraction of those portions of the apparatus that make up the drive frame 37, and at least ensure that such distortions of the metrology frame 37 are at least reduced.

[0165] For example, the extendable legs of the metrology device 26 (described in more detail below with reference to Figures 5-8) are made of a composite material such as carbon fiber, while the measurement portion 24a of the fixed structure 24 and the measurement portion 22a of the movable structure 22 can be made of materials such as INVAR® or ZERODUR®, or composite materials such as carbon fiber. However, such low CTE materials are often difficult to machine and are typically significantly more expensive than standard, factory materials such as aluminum. Although aluminum has a much higher CTE than materials such as carbon fiber, INVAR, ZERODUR, the mechanical / thermal isolation provided by the first and second coupling portions 38a, 38b allows a significant portion of the drive frame 37 to be made of conventional materials such as aluminum without introducing distortions into the metrology frame 36 due to differences in thermal expansion / contraction between the drive frame 37 and the metrology frame 36.

[0166] For example, one or both of the measurement portion 24a of the fixed structure 24 and the measurement portion 22a of the movable structure 22 can be made of carbon fiber (and the extendable legs of the measurement device 26), while one or both of the drive portion 24b of the fixed structure 24 and the drive portion 22b of the movable structure 22 can be made of aluminum or steel. For example, the drive platform 22b (made of aluminum or steel) expands and contracts with changes in temperature more than the measurement platform 22a (made of carbon fiber) to which it is coupled (through the coupling 38a), but the properties of the coupling 38a allow one to expand and / or contract relative to the other without inducing differential forces. That is, the shape of one side of the coupling allows the feature surface of the other side of the coupling to slide without inducing or transmitting stress. The same is true for the measurement platform 24a at the other end and the coupled drive platform 24b. This has been found to provide significant advantages for measurements in environments with changing ambient temperatures.

[0167] In this embodiment, each coupling 38a, 38b includes a set of three spheres (only two are shown in the schematic diagram of FIG. 4) that provide three contact points according to kinematic design principles. It is also beneficial to use multiple elastic spacers or pads instead of rigid spheres, for example three such spacers arranged at the corners of a triangle. This provides a degree of kinematic coupling, even if the contact points are not point-like but spread over a small area of ​​the elastic spacers. The use of an elastic spacer (e.g. made of rubber) is beneficial because it acts to absorb vibrations and / or thermal expansion / contraction of the drive frame 37, so that these are not transmitted to the metrology frame 36 (and especially the metrology device 26). Instead of using rigid spheres for the kinematic couplings 38a, 38b, it is also possible to use somewhat flexible spheres, for example made of rubber, that at least partially absorb the differences in expansion and / or contraction in the normal direction to the mobile structure 22 and / or the fixed structure 24.

[0168] It will also be understood that such couplings may be provided at both ends (i.e., in relation to the movable structure 22 and the fixed structure 24), or only at one end (i.e., in relation to only one of the movable structure 22 and the fixed structure 24), or not at all (i.e., not in either the movable structure 22 or the fixed structure 24).

[0169] The embodiment will now be described in more detail with reference to Figures 5 to 8, which show a more detailed view of the mechanical structure than the highly schematic views of Figures 3 and 4.

[0170] As shown in Figs. Hexapod TypeThe measurement device 26 comprises six extendable legs 60 of generally identical construction disposed between the upper works 22 and the lower works 24. As shown in FIG. 4, the movable structure 22 includes a measurement section 22a associated with the measurement device 26 and a drive section 22b associated with the drive unit 28, where the measurement section 22a of the movable structure 22 is coupled to the drive section 22b of the movable structure 22 via a first coupling section 38a. The measurement section 22a of the movable structure 22 is coupled to the measurement device 26 via a ball joint 68. The drive section 22b of the movable structure 22 is coupled to the drive unit 28 via a ball joint 58.

[0171] Similarly, the fixed structure 24 includes a measurement section 24a associated with the measurement device 26 and a drive section 24b associated with the drive device 28, where the measurement section 24a of the fixed structure 24 is coupled to the drive section 24b of the fixed structure 24 via a second coupling section 38b. The measurement section 24a of the fixed structure 24 is coupled to the measurement device 26. The drive section 24b of the fixed structure 24 is coupled to the drive device 28.

[0172] Each of the six extendable legs 60 comprises an upper tube 64 and a lower tube 62, which can slide telescopically within the upper tube 64. The extendable legs 60 are generally of similar structure to those described in US Pat. No. 5,999,333 and International Patent Application No. PCT / GB2017 / 050909, but in this embodiment, the extendable legs do not need to be driven and therefore no motor-related arrangement is required. However, the overall structure of the extendable legs 60 is generally similar.

[0173] In the example shown in Figures 5-8, the lower structure 24 is fixed, the upper structure 22 is movable relative to the lower structure 24 by operation of six extendable legs 60, and the measurement probe 30 is mounted on the underside of the upper structure 22. In this configuration, a workpiece (not shown in Figures 5-8) is mounted on the measuring portion 24a of the lower structure 24, whereby the working volume of the machine 21 is between the measuring portions 22a, 24a of the upper and lower structures 22, 24, respectively. The measurement probe 30 includes a stylus with a tip that contacts the workpiece, and the measurement probe 30 is connected to the measuring portion 22a of the movable structure 22 via a shaft 32.

[0174] The extendable legs 60 are for positioning (i.e., determining the location) of a component supported by the movable structure 22 (in the illustrated example, the component is the measurement probe 30), or at least a particular portion of the component (e.g., the tip of the measurement probe) within the working volume of the machine.

[0175] The upper and lower ends of each extendable leg 60 are connected to the upper structure 22 (specifically, the measuring portion 22a of the upper structure 22) and the lower structure 24 (specifically, the measuring portion 24a of the lower structure 24), respectively, via respective ball joints 68. The upper and lower tubes 62, 64 of each extendable leg 60 enclose an elongated member 66, shown in dashed outline on one of the extendable legs in FIG. 5, to which the encoder scale 10 is attached. The elongated member 66 is itself extendable, for example, by a telescopic arrangement. Each elongated member 66 extends from its upper joint 68 to its lower joint 68, and it is the respective length of the elongated members 66 that determines the precise positioning and orientation of the measuring portion 22a of the upper structure 22 (and thus the measurement probe 30). It is therefore the length of the elongated member 66 that must be precisely measured during a measurement or scanning operation of a workpiece in order to determine the precise position of the tip of the stylus when it contacts the surface of the workpiece.

[0176] The drive 28 in this embodiment is a so-called "three-slide" arrangement, as described, for example, in US Pat. No. 5,399,363. The three-slide arrangement relies on 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, 54 of fixed length and acts to maintain the mobile structure 22 in a substantially constant orientation as it moves through the working volume 34. Each mechanical linkage 50 also includes a carriage 56, with the rods 52, 54 pivotally coupled at their lower ends to the carriage 56 and at their upper ends to the drive portion 22b of the mobile structure 22 via ball joints 58, respectively.

[0177] The three linear tracks 51 are arranged substantially vertically (substantially parallel) on the drive part 24b of the fixed structure 24, where the three carriages 56 are 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 machine 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, where the position of the linear drive mechanism is marked diagrammatically by reference number 29 in FIG. 3. The linear drive mechanism may include a linear motor. The linear drive mechanism may include a stepper motor.

[0178] 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., the carriage 56. In other words, the drive mechanism couples the mechanical linkage directly to the fixed structure ("couples to ground") and provides a force therebetween to push or pull the mechanical linkage (in the case of a linear drive mechanism) or to rotate the mechanical linkage (in the case of a rotary drive mechanism). There are no additional movable linkages between the drive mechanism and the fixed structure, and such additional linkages may cause movement of the driven portion of the mechanical linkage not produced by the drive mechanism itself.

[0179] For example, in the "three-slide" drive configuration shown in Figures 5-8 (and more diagrammatically in Figures 3 and 4), each carriage 56 is driven linearly up and down its respective track 51 by the action of a motor (not shown) associated with that carriage 56, and not by the action of other motors associated with other carriages 56. Hexapod Type In the case of a drive arrangement (as shown in FIG. 1), the motor of each individual extendable leg can only extend or retract the leg linearly along its length, but in use each leg also moves laterally - that is, the lateral movement of a leg (and associated motor) is due to the action of other motors in other legs, so that each motor is actually moving the weight of other struts (with their respective motors).

[0180] Thus, a drive such as that shown in Figures 3-8 can keep the moving parts relatively light, which in this example can be thin, light rods 52, 54 (as opposed to a typical six extendable legs drive such as that shown in Figure 1). Hexapod Type Each motor does not move the weight of the other motors (as in a drive train). This allows for a lightweight drive train that can move quickly with high acceleration and quick changes of direction.

[0181] The telescopic structure of each of the six extendable legs 60 of the measuring device 26 should be contrasted with the non-telescopic structure of each of the mechanical linkages 50 of the drive device 28. In a telescopic device, one part of the device telescopically slides or moves inwardly or at least linearly along another part, and during this movement the parts remain substantially aligned with each other. However, this is not the case with the drive device 28 of this embodiment. Because each carriage 56 moves linearly along its corresponding respective track 51, one part of the device (rigid rods 52, 54) does not remain substantially aligned with the other part of the device (track 51), or even at a constant angle relative to the other part, and the angle between these two parts changes when the drive device 28 is actuated. The drive device 28 shown in Figures 3 to 8 can therefore be described as a non-telescopic drive device. However, each of the parallel linkages of drive 28, even if it is not telescopically extendable (i.e., even if it is not a telescopically extendable leg), can still be considered to be or result in a telescopic leg, in the same way that a human leg can be described as telescopic even though it is not telescopic. A telescopic device can be considered to be any type of device that is capable of changing the distance separation between two points.

[0182] Returning to a more schematic view, operation of the three slide embodiment is described with reference to Figures 9A-9E. Each of Figures 9A-9E uses a similar representation to that used in Figure 3, with the two carriages identified as 56a and 56b, respectively, and the two linear tracks identified as 51a and 51b, respectively. For simplicity, the coupling device 38 described above with reference to Figure 4 is not shown in Figures 9A-9E, but is present (on either or both of the movable structure 22 or the fixed structure 24) in embodiments of the invention.

[0183] 5-8, the movement of the movable structure 22 (by operation of the three-slide drive 28) is limited to three translational degrees of freedom, such that the movable structure 22 maintains a substantially constant orientation as it moves through the working volume. This constraint on the three degrees of freedom of movement is illustrated by the arrows labeled 3DOF in FIG. 9A.

[0184] Meanwhile, it contains six corresponding measurement transducers in a parallel arrangement Hexapod Type The six extendable legs 60 of the measurement device 26 provide six corresponding respective measurements based on 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.

[0185] As shown in FIG. 9B, lowering carriage 56a and raising carriage 56b along their respective tracks 51a, 51b causes the movable structure 22 (along with the measurement probe) to move leftward and downward within the working volume, maintaining substantially the same orientation, due to the fixed length of rods 52, 54 of mechanical linkage 50. This causes the extendable leg 60 closest to carriage 56a to shorten and the extendable leg 60 closest to carriage 56b to lengthen, with these changes in length being measured by measurement transducers (e.g., encoders) 10 within extendable legs 60. Based on those transducer measurements, the position of movable structure 22 within working volume 34 can be determined, as can the position of measurement probe 30 (and probe tip) since it is in a known, fixed spatial relationship to movable structure 22. FIG. 9C is the same as FIG. 9B, but without the indication of movement for clarity, thereby showing the final positions of the components after the movement operation.

[0186] Similarly, as shown in Figure 9D, by raising carriage 56a and lowering carriage 56b along their respective tracks 51a, 51b, the movable structure 22 (and with it the measurement probe 30) can be moved to the right within the working volume 34, again maintaining substantially the same orientation. This lengthens the extendable leg 60 closest to carriage 56a and shortens the extendable leg 60 closest to carriage 56b, and these changes in length are measured by measurement transducers (e.g., encoders) 10 in the extendable legs 60. Based on the measurements of these transducers, the positions of the movable structure 22 and the measurement probe 30 within the working volume 34 can be determined. Figure 9E is the same as Figure 9D, but without the indication of movement for clarity, thereby showing the final positions of the components after the movement operation.

[0187] In the three-slide embodiment described above, Hexapod Type The extendable legs 60 of the measuring device 26 and the rods 50 of the driving device 28 extend upward from the bottom, and therefore the embodiment can be described as a "bottom-up" device. Figure 10 shows an alternative "top-down" device, which is Hexapod Type It is generally the same as the "bottom-up" system, except that the extendable legs 60 of the measuring system 26 and the rods 50 of the driving system 28 extend from top to bottom (hence the "top-down" system). Hexapod Type A frame 25a is provided to support the metrology device 26, so that the frame can effectively be "hung" from above. The frame 25a effectively forms part of the fixed structure of the coordinate positioning machine 21, as an extension to the fixed structure 24 (in this case the hexaped section 24a of the fixed structure 24, i.e. the stationary part of the metrology frame 36). As in the previous embodiment, coupling devices 38a, 38b are provided to decouple the metrology frame 36 (in particular the metrology device 26) from the drive frame 37 (in particular the drive device 28).

[0188] Yet another "top-down" device is shown diagrammatically in FIG. Hexapod TypeIt differs from the embodiment of FIG. 10 in that the measuring device 26 is supported by a frame 25 extending around the top and is provided inside the three-slide drive 28. The frame 25 effectively forms part of the fixed structure of the coordinate positioning machine 21, as an extension to the fixed structure 24 with a vertical linear track 51 also being part of the frame 25. The embodiment of FIG. 11 again provides a coupling device 38 for separating the measuring device 26 from the drive 28, but in the embodiment of FIG. 11 the coupling device is only present in the mobile structure 22 and not in the fixed structure 24. The measuring probe 30 is attached to the underside of the measuring part 22a of the mobile structure 22 and extends through the drive part 22b of the mobile structure 22 without contacting it (for example through an opening formed in the drive part 22b of the mobile structure).

[0189] For comparison with Figures 13-15, Figure 12 is intended to show an actual three-slide embodiment which corresponds closely to that described above with reference to Figures 5-8, differing primarily in having a closed drive frame, with additional stiffness and stability provided to the vertical track 51 by the upper plate of the drive frame. As with the previous embodiments, the metrology frame of Figure 12 is decoupled, at least to some extent, from the drive frame, both at the upper (i.e. movable structure) and lower (i.e. fixed structure) portions.

[0190] Figure 13 shows a practical embodiment of the "top down" apparatus shown diagrammatically in Figure 11, but differs from the example of Figure 11 in that it provides some separation of the drive frame and metrology frame at the movable structure. Figure 14 is a variation of Figure 13, providing separation of the drive frame and metrology frame at both the movable and fixed structures. Figure 15 is a further variation, having a separate metrology frame located within the drive frame, here separating the drive frame from the metrology frame at both the movable and fixed structures.

[0191] It will be understood that the present invention is not limited to embodiments in which the drive 28 is in a three-slide configuration. Hexapod Type 26 measuring devices of different types Hexapod Type16A is a schematic diagram of an embodiment coupled to a drive 28. In the embodiment of Fig. 16A, rather than a fixed length rod 52 having one end driven linearly along a track 51 by a carriage 56, as in the three-slide embodiment, a fixed length extension rod is instead driven through a guide 76 by a suitable linear drive mechanism mounted within a pivoting guide 76, which changes the distance indicated by the arrows in Fig. 16A, thereby allowing movement of the structure 22.

[0192] In Figures 16A and 16B, similar to Figure 11, the metrology device 26 and drive device 28 are supported in a top-down manner by a frame 25, which forms part of the fixed structure of the coordinate positioning machine 21. From the position shown in Figure 16A, when both rods are driven downwards through their respective guides 76, the structure 22 can be moved to the position shown in Figure 16B. As before, the position of the structure 22 can be determined by: Hexapod Type 16A and 16B is also a non-telescopic drive because each drive strut has a fixed length rod that extends to either side and moves through a (much shorter) guide, and therefore is not considered a telescopic drive.

[0193] As with the three-slide system, each mechanical linkage of the drive system 28 in the embodiment of FIG. 16A is actuated by a drive mechanism acting between a fixed structure and the mechanical linkage, and as a result, this embodiment has the same advantages in terms of speed and acceleration.

[0194] Figure 17 shows a practical embodiment of the apparatus shown diagrammatically in Figure 16 A. The embodiment of Figure 17 is closely based on a coordinate measuring machine of the non-Cartesian type sold under the trademark EQUATOR by the applicant, Renishaw. Hexapod Type The measurement device 26 is substantially similar to that of FIG. 5 and includes six telescopic legs each with an upper tube 64 and a lower tube 62, where the lower tube 62 slides telescopically within the upper tube 64. In this embodiment, the telescopic legs are supported in a top-down configuration from the frame 25 to the measurement platform 22a (part of the movable structure 22). The pivot guide 76 is hidden in FIG. 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 coupled at their lower ends to the drive plate 22b (part of the movable structure 22). In this embodiment, the two parts 22a, 22b of the movable structure 22 are spatially separated by a rigid column 23. Similar to the rods 52, 54 of FIG. 5, the three parallel rod pairs 72, 74 are arranged to constrain the motion in three degrees of freedom.

[0195] Returning to a more schematic representation, Figures 18A and 18B show a modification of the machine of Figures 16A and 16B, where the metrology device 26 (part of metrology frame 36) is further separated / decoupled from the drive device 28 (part of drive frame 37). This is similar to the three-slide embodiment described above with reference to Figure 10 and therefore no further explanation is necessary. Figure 19 shows an alternative to the Figure 16A device, where instead of a top-down device, it is a bottom-up type. Hexapod Type A measuring device 26 is shown.

[0196] FIG. 20 illustrates a schematic of a variation of the embodiment of FIGS. 16A and 16B in which fixed length metrology struts, like the fixed length struts of driver 28 of that embodiment, Hexapod Type20 are used in the measuring device 26. The six fixed length extending struts as shown in FIG. 20, with the variable length portions of the struts indicated by the arrows in FIG. 20, are considered to be functionally equivalent to the six extendable struts of the previous embodiment. That is, the struts are equivalent to the extendable struts of the previous embodiment. Thus, the terms "extendable leg" and "extending leg" should be understood as equivalent herein, which means any type of mechanical device or linkage between two points that allows for a change in the distance interval between these two points. However, each of the fixed length extending struts of the measuring device 26 of FIG. 20 cannot be described as an extendable device. That is, each measuring strut is a non-extendable device. As shown in more detail in FIG. 17, the driving device 28 is still a non-extendable device since it has only three extendable struts. Hexapod Type Fig. 21 shows a schematic variation of the embodiment of Fig. 20 in which a fixed support (pivot plate) 25a is used for the metrology struts that is spatially offset from a fixed support (pivot plate) 25b used for the drive struts, with a coupling 38b (e.g. three spheres forming a kinematic coupling according to the previous embodiment) between these plates 25a, 25b providing thermal / mechanical decoupling between the drive frame and the metrology frame. Of course, the embodiment of Fig. 21 can be modified to provide decoupling also in the movable platform 22 (as in Fig. 20).

[0197] Two different types of non Hexapod Type Above have been described embodiments in which the drives are employed: a three-slip linear drive (e.g., FIG. 5) and a pivoting linear drive (e.g., FIG. 17), both of which are non-telescopic. There are many other possible implementations of the drives, only a few of which are briefly described. Others will be apparent to those skilled in the art.

[0198] Figure 22 shows a delta robot type non- Hexapod TypeFIG. 23 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 24 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 25 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 26 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 27 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 28 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 29 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 30 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 31 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 32 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and a drive mechanism for measuring the length of the metrology frame. FIG. 33 shows a schematic of an embodiment having a drive mechanism for measuring the length of the metrology frame and ... Hexapod Type It may be configured to provide movement to structure 22 in fewer degrees of freedom than are being measured by metrology device 26 .

[0199] The position of the rotary drive mechanism is indicated by reference number 27 in Figure 22, while the position of the linear drive mechanism is indicated by reference number 29 in Figure 3. In both cases the drive mechanism acts directly between the fixed structure and the drive. In the case of Figure 3, the drive mechanism acts to drive a carriage 56 (forming part of the drive mechanism, for example as part of the mechanical linkage 50 as shown in Figure 5), while in the case of Figure 22, the drive mechanism acts to drive (rotate) the upper part of a mechanical linkage that is mounted between the mobile structure and the fixed structure.

[0200] Non-limiting examples suitable for use in the embodiments of the present application Hexapod TypeAnother example of a drive is a cable-driven robotic device (alternatively 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 device) that uses multiple flexible cables 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. 5,399,633. The cables are typically much lighter than the rigid linkages of serial or parallel robots, so the end effectors of cable robots can achieve high accelerations and velocities. Hexapod Type The high measurement rate and dynamic bandwidth as well as the high accuracy achievable with the measuring device Hexapod Type The combination of a measuring device and a cable drive is particularly advantageous.

[0201] Other types of non Hexapod Type Drives are also envisaged. For example, FIG. 25 shows a non-linear actuator of the series kinematic type (as opposed to parallel kinematic). Hexapod Type FIG. 25 shows a schematic representation of a (and non-telescopic) actuator having multiple segments or links connected in series by revolute joints, with one end of the actuator attached to ground and the other end attached to a measurement device. As with the embodiment shown in FIG. 4, the actuator shown in FIG. 25 is attached to the measurement device via a coupling that helps prevent strains associated with the actuator from being transferred to the measurement device. FIG. 26 shows a non-telescopic actuator of the series kinematic type having three serially connected parts that are respectively movable along orthogonal axes x, y and z (as shown in FIG. 26). Hexapod Type 26 and 27 show schematic diagrams of alternative embodiments having serial drives. Thus, the embodiment of Fig. 26 has a Cartesian type serial drive, whereas the embodiment of Fig. 25 has a non-Cartesian type serial drive. These types of drives are well known and need not be described further here.

[0202] As described above, particularly with reference to FIG. 9A, the actuator 28 provides the movable structure 22 with three translational degrees of freedom, while: Hexapod Type The measuring device 26 is configured to measure in six degrees of freedom. According to one aspect of the invention there is proposed a coordinate positioning machine comprising a structure movable within a working volume of the machine, a drive arrangement for moving the structure within the working volume with fewer 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. This is shown diagrammatically in Figure 27. One or both of the drive arrangement and the measuring arrangement may be Hexapod Type The metrology device may be a parallel kinematic device such as a robotic device, a three-slide device or a delta robotic device. Hexapod Type It should be noted that it need not be a metrology device.

[0203] It is not standard to measure, especially directly, in more degrees of freedom than movement. Typically there are N driving parts (rotary or linear), each of which is individually encoded resulting in N corresponding measurements. For example, in a 3-axis CMM there are 3 driven linear axes, each with a position encoder, and therefore 3 corresponding measurements (i.e. both driving and measuring in 3 degrees of freedom). Hexapod In the case of , there are six variable length struts, each with a position encoder, and six corresponding measurements (i.e., both actuated and measured in six degrees of freedom).

[0204] However, Applicant has come to realise that it would be desirable and advantageous to be able to provide a drive that is relatively imprecise and constrained to move in limited degrees of freedom (e.g., three degrees of freedom) in combination with a separate metrology device that is highly accurate and capable of measuring in all six degrees of freedom, thus making it possible to compensate for any inaccuracies of the mechanically constrained drive. For example, if a moving platform is constrained to translate within a working volume without rotation, structural distortions and other types of inaccuracies may cause the platform to inadvertently rotate, at least in part due to dynamic effects associated with high speed operation. Such rotations may be detected by measuring in more degrees of freedom than there are degrees of freedom of movement. The concept of FIG. 27 can be further developed into a dual-axis drive, as shown diagrammatically in FIG. 28. Hexapod Type It is also possible to apply the device to the driving Hexapod is constrained to movement in fewer than six degrees of freedom by appropriate mechanical constraints.

[0205] As will be apparent to those skilled in the art, Hexapod Type There are many other forms of drive, or drive that are constrained to fewer than six degrees of freedom. For example, there are many possible variations of the three-slide arrangement shown. One variation is to provide an arrangement with more than two drives and associated mechanical linkages. And instead of a vertical track 51 as shown in FIG. 3, the track could instead be arranged horizontally, for example radially outward from a point, so that movement of the structure 22 is also effected by movement of the carriage 56 along the horizontal track. There are many other such possibilities.

[0206] Although embodiments of the invention have been described primarily in relation to the use of contact probes in which the stylus of the contact probe makes physical contact with the workpiece surface to perform measurements, it is understood that the invention is not limited to contact probes. The same concepts are equally applicable to non-contact probes, such as optical probes, in which the surface is detected without physical contact. The invention is generally applicable to any surface sensing device configured to sense a surface, whether by contact or not. The invention can also be applied to positioning components other than surface sensing apparatus, for example to orient components of an article during manufacture of the article. Alternatively, the component may be a tool or part of it, such as a tool typically found in a machine tool for forming or machining metals or other rigid materials. The component may be the movable structure itself. The tool 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 may be apparent to those skilled in the art.

[0207] In an embodiment of the present invention, Hexapod Type It should be noted that the metrology device 26 is not provided purely for calibration purposes, but is temporarily coupled to the movable structure to perform a calibration of the drive and metrology device combination, and is then removed for use in operating the machine. Hexapod Type The metrology device remains coupled to the movable structure and is intended to provide position measurements related to the movable structure during use for operation. In contrast to a calibration-only metrology device, in an embodiment of the invention, the movable structure is configured to support an operating tool, where the metrology device and a drive device are also coupled to the movable structure. Hexapod Type The metrology device may be coupled to the moveable structure by a mounting different from that used to attach the manipulation tool to the moveable structure. Hexapod Type The metrology device may be coupled directly to the movable structure (eg, rather than primarily by a mounting for a manipulation tool).

[0208] A method for controlling coordinate positioning is shown in the flow chart of FIG. 29. In step S1, the metrology device 26 is coupled to the movable structure (or platform) 22. In step S2, the drive device 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, so that at this point all three are coupled to the movable structure (or platform) 22. In step S4, the drive device 28 is used to move the tool in the working volume 34 (with the metrology device 26 now also coupled to the movable structure). In step S5, an operation with the tool is performed, such as a touch trigger operation on the workpiece surface with the measurement probe 30 or machining the workpiece surface with a cutting tool or machine tool. In step S6, the metrology device 26 is used to determine the position of the tool in the working volume 34 when the operation is performed (e.g. so that the position of the tip of the measurement probe 30 or the cutting tool can be determined). In step S7, the determined position is associated with the performed operation (eg, a touch trigger event is associated with a position measurement of that event).

[0209] As explained above with reference to Fig. 4, the kinematic coupling is extremely effective in isolating strain at one of the couplings that can be transmitted to the other of the couplings. Thus, the first coupling 38a helps to prevent strain of the driving part 22b of the movable structure 22 (due to the force acting on that part from the driving device 28) from being transmitted to the measuring part 22a (and therefore to the measuring device 26), and the same is true for the second coupling 38b with respect to the fixed structure 24. Next, such kinematic coupling will be explained in more detail with reference to Figs. 30 to 35.

[0210] FIG. 30 is a perspective view of a kinematic coupling 80 having a first portion 81 and a second portion 82. The kinematic coupling 80 can be applied to either or both of the couplings 38a and 38b of FIG. 4. For example, when applied to the (lower) coupling 38b, the first and second portions 81, 82 of FIG. 30 correspond to or at least form part of the metrology and drive portions 24a, 24b of FIG. 4, respectively. The first portion 81 has three V-grooves 83a, 83b, 83c that engage with three corresponding respective balls 84a, 84b, 84c of the second portion 82 (when the first and second portions 81, 82 are coupled together). Also shown in FIG. 30 are a biasing feature 85 and an orientation feature 86. In this example, the biasing feature 85 is a hole through which a biasing member (such as a bolt) can pass, and the bolt engages with a corresponding threaded hole (not shown) in the second portion 82. To avoid providing additional constraints, the biasing members are positioned so as not to contact the sides of the holes, whereby they act to bias the first portion 81 against the second portion 82, ensuring that the two remain in kinematic contact with one another and do not move apart. However, biasing may alternatively be provided by, for example, gravity or magnetic means. The optional orientation features 86 in this example are two off-center holes through which corresponding protruding pins (not shown) on the second portion 82 protrude to ensure that the first portion 81 is correctly oriented on the second portion 82. For example, in the case of an arrangement in which the V-grooves are not rotationally symmetric, the orientation features are not required. In practice, the features of the kinematic coupling 80 can be further apart than suggested by the figure. The spacing of the features depends on the overall size of the relevant platform, taking into account mechanical stability and force transmission requirements.

[0211] Top and perspective views of the first part 81 are shown in Figures 31 and 32. In use, the first part 81 is placed on the second part 82 such that each of the three spheres 84a, 84b, 84c of the second part 82 is seated in and against the three V-grooves 83a, 83b, 83c of the first part 81, as shown most clearly in Figure 33. When coupled, each sphere 84a, 84b, 84c makes point-like contact with the opposing faces of the V-grooves 83a, 83b, 83c in which it rests, making a total of six point-like contacts C1-C6, as shown in Figure 34. Due to the principles of kinematic constraint, as further explained above, each point-like contact creates a constraint in one of the six degrees of freedom, and having exactly six point-like contacts results in complete (kinematic) constraint without over-constraints. In practice, it will be appreciated that while pure point contact provides a theoretical kinematic constraint, in practice loads are spread over a small surface area rather than concentrated at a single point. However, the basic principles of kinematic constraints still apply. To constrain six degrees of freedom using six constraints, one half of the joint remains in a fixed spatial relationship relative to the other half of the joint. For example, referring to FIG. 4, the measurement portion 22a of the movable structure 22 remains in a fixed spatial relationship relative to the drive portion 22b of the movable structure 22.

[0212] As mentioned above, when applied to the lower joint 38b of FIG. 4, the second part 82 of FIG. 30 corresponds to the driving part 24b of FIG. 4, which has a higher coefficient of thermal expansion than the measuring part 24a (their CTEs are not matched). As a result, when exposed to the same environmental temperature during machine use, the driving part 24b (including the second joint 82) expands more than the measuring part 24a (including the first joint 81) when the environmental temperature increases. The kinematic design of the joint in this case helps to avoid the occurrence of distortions that would normally occur in the joint (i.e., at the interface between two parts with different CTEs) if the CTEs of the parts on both sides of a fixed joint are different. This is because the triangle formed by the three spheres 84a-84c (see FIG. 30) simply expands outward to the opposite side of the joint, and each pair of contact points C1 / C2, C3 / C4, C5 / C6 moves linearly along the corresponding V-groove 83a, 83b, 83c. This prevents unnecessary strain from being generated at the joint. A similar analysis applies when joint 80 of FIG. 30 is applied to upper joint 38a of FIG. 4. The effectiveness of the kinematic joint in this context can be explained by the fact that each of the point constraints C1-C6 only constrains a single degree of freedom, allowing movement in other degrees of freedom as needed (e.g., if one part expands or contracts or distorts differently than the other parts), thereby preventing unnecessary strain from being generated at the joint.

[0213] It can be understood that other forms of kinematic coupling are possible in which the six contact points C1-C6 are arranged differently. For example, Fig. 36 shows a 3-2-1 arrangement of contact points, where a group of three contact points C1, C2, C3 are created by the contact of spheres 84a in the pyramidal recesses, a pair of two contact points C4, C5 are created by the contact of spheres 84b in the V-groove recesses, and only one point C6 is created by the contact of sphere 84c on a flat surface. Such a kinematic coupling also helps to prevent strain on the joint due to different thermal expansions and contractions of parts 81 and 82: when part 82 expands relative to part 81, contact points C4, C5 move outward along the V-groove and contact point C6 moves along the flat surface. As a result, there may be some relative rotation between parts 81, 82, but this effect is controlled and can be compensated for, for example, by the same amount of rotation at the other end of metrology device 26 (i.e., rotation at joint 38a in FIG. 4 is the same as at joint 38b, and the entire metrology device 26 rotates slightly without changing the overall configuration).

[0214] Other types of non-kinematic couplings are also suitable for other applications, such as the elastic spacers or pads mentioned above, because they prevent at least a portion of the excess thermal expansion and contraction of the drive frame from being transferred to the metrology.

[0215] It will be appreciated that the operation of the coordinate measuring machine 21 may be controlled by a program operating on the machine 21, and in particular by a program operating 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 operating on the controller C. Such an operating program may be stored on a computer readable medium or may be embodied in a signal, for example a downloadable data signal provided on an internet website. The appended claims are to be understood as covering an operating program in itself, or as a record on a carrier, or as a signal, or in any other form.

[0216] Although the above embodiments have been described primarily in the context of coordinate measuring machines, the concepts are more generally applicable to any type of coordinate positioning machine, such as comparators, scanning machines, machine tools, positioning devices (e.g. for optical components), prototype manufacturing machines and a variety of other applications.

Claims

1. 1. A coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising drives for moving a moveable structure in a working volume of the machine, and the metrology frame comprising metrology devices for measuring a position of the moveable structure within the working volume, the metrology devices being hexapod metrology devices and the drives being non-hexapod drives, the metrology frame having a lower coefficient of thermal expansion than a coefficient of thermal expansion of the drive frame, thereby causing excess thermal expansion or contraction of the drive frame compared to the metrology frame, and the drive frame is coupled to the metrology frame via a coupling device configured to prevent at least a portion of the excess thermal expansion or contraction of the drive frame, or at least a portion of distortions associated with the excess thermal expansion and contraction of the drive frame, from being transferred to the metrology frame.

2. 1. A coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising drives for moving a moveable structure in a working volume of the machine with fewer than six degrees of freedom, and the metrology frame comprising metrology for measuring the position of a moveable structure in the working volume with more degrees of freedom than the drives, the metrology frame having a lower coefficient of thermal expansion than a coefficient of thermal expansion of the drive frame, thereby causing excess thermal expansion or contraction of the drive frame compared to the metrology frame, and the drive frame is coupled to the metrology frame via a coupling arrangement configured to prevent at least a portion of the excess thermal expansion or contraction of the drive frame, or at least a portion of distortion associated with the excess thermal expansion or contraction of the drive frame, from being transferred to the metrology frame.

3. 1. A coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising a drive arrangement for moving a movable structure in a working volume of the machine, and the metrology frame comprising a metrology arrangement for measuring a position of the movable structure within the working volume, the metrology arrangement including a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements based on which a position of the movable structure can be determined, the drive arrangement including a plurality of mechanical linkages arranged in parallel between the movable structure and a fixed structure of the machine, each mechanical linkage being actuated by a drive mechanism acting between the fixed structure and the mechanical linkage, the metrology frame having a lower coefficient of thermal expansion than a coefficient of thermal expansion of the drive frame, thereby resulting in excess thermal expansion or contraction of the drive frame compared to the metrology frame, and the drive frame being coupled to the metrology frame via a coupling arrangement configured to prevent at least a portion of the excess thermal expansion or contraction of the drive frame, or at least a portion of distortion associated with the excess thermal expansion or contraction of the drive frame, from being transferred to the metrology frame.

4. 1. A coordinate positioning machine comprising a drive frame and a metrology frame, the drive frame comprising a drive arrangement for moving a moveable structure in a working volume of the machine, and the metrology frame comprising a metrology arrangement for measuring a position of the moveable structure within the working volume, the metrology arrangement including a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements based on which a position of the moveable structure can be determined, the drive arrangement including a plurality of actuators in a parallel arrangement, the parallel arrangement of the actuators differing from the parallel arrangement of the measurement transducers, the metrology frame having a lower coefficient of thermal expansion than a coefficient of thermal expansion of the drive frame, thereby causing excess thermal expansion or contraction of the drive frame compared to the metrology frame, and the drive frame coupled to the metrology frame via a coupling arrangement configured to prevent at least a portion of the excess thermal expansion or contraction of the drive frame, or at least a portion of distortion associated with the excess thermal expansion or contraction of the drive frame, from being transferred to the metrology frame.

5. 3. A coordinate positioning machine according to claim 1 or 2, wherein the metrology device comprises a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements based on which the position of the movable structure can be determined.

6. 6. A coordinate positioning machine according to any one of claims 3 to 5, wherein the plurality of measurement transducers are a plurality of independent measurement transducers.

7. 7. A coordinate positioning machine according to claim 3, wherein the measurement transducer is a length measuring transducer.

8. A coordinate positioning machine according to any one of claims 3 to 7, wherein the measurements relate to respective differences in separation distances between the moveable and fixed structure of the machine.

9. 9. A coordinate positioning machine according to claim 8, wherein the measurement transducer is configured to provide a direct measurement of the distance interval, or a direct measurement of a change in the distance interval as the moveable structure moves in the working volume, based on which the distance interval can be determined.

10. A coordinate positioning machine according to any one of claims 3 to 9, wherein each of the measurement transducers includes an encoder scale and an associated readhead.

11. 11. A coordinate positioning machine according to any one of claims 3 to 10, wherein the metrology device comprises a plurality of extendable legs arranged in parallel, the extendable legs corresponding in number to the number of measurement transducers, each of the plurality of measurement transducers being associated with a different respective one of the plurality of extendable legs.

12. A coordinate positioning machine according to any one of claims 3 to 11, comprising six said measurement transducers.

13. A coordinate positioning machine according to any preceding claim, wherein the drive is a non-telescopic drive.

14. A coordinate positioning machine according to any preceding claim, wherein the metrology device is a top-down metrology device.

15. A coordinate positioning machine according to any preceding claim, wherein the metrology system is a bottom-up metrology system.

16. A coordinate positioning machine according to any preceding claim, wherein the drive is a top-down drive.

17. A coordinate positioning machine according to any preceding claim, wherein the drive is a bottom-up drive.

18. 18. A coordinate positioning machine according to any preceding claim, wherein the drive is configured to maintain the movable structure in a substantially constant orientation as the movable structure moves in the working volume.

19. 19. A coordinate positioning machine according to any preceding claim, wherein the drive arrangement comprises a plurality of mechanical linkages connected in parallel between the movable structure and the fixed structure of the machine, each of the mechanical linkages being actuated by a drive mechanism acting between the fixed structure and the mechanical linkage.

20. 20. A coordinate positioning machine according to claim 19, wherein the drive mechanism is a rotary drive mechanism.

21. 20. A coordinate positioning machine according to claim 19, wherein the drive mechanism is a linear drive mechanism.

22. 22. A coordinate positioning machine according to claim 19, 20 or 21, wherein each said mechanical linkage comprises at least two substantially parallel rods for maintaining the moveable structure in a substantially constant orientation as it moves through the working volume.

23. A coordinate positioning machine according to any one of claims 19 to 22, wherein the drive arrangement includes three of said mechanical linkages.

24. A coordinate positioning machine according to any preceding claim, wherein the drive arrangement is a three-slide arrangement.

25. 25. A coordinate positioning machine according to any preceding claim, wherein the drive devices are for moving the movable structure in the working volume with fewer than six degrees of freedom, and the metrology devices are for measuring the position of the movable structure within the working volume with more degrees of freedom than the drive devices.

26. 26. A coordinate positioning machine according to any preceding claim, wherein the metrology device is a hexapod type metrology device.

27. 27. A coordinate positioning machine according to any preceding claim, wherein the drive is a non-hexapod drive.

28. A coordinate positioning machine according to any preceding claim, wherein the drive is configured to move the movable structure in a working volume in three degrees of freedom.

29. 29. A coordinate positioning machine according to claim 28, wherein said three degrees of freedom are three translational degrees of freedom.

30. 30. A coordinate positioning machine according to any preceding claim, wherein the metrology device is configured to measure the position of the moveable structure in six degrees of freedom.

31. A coordinate positioning machine according to any preceding claim, wherein the measuring device and the drive device are each located between the moveable structure and the fixed structure of the machine.

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

33. 33. The coordinate positioning machine of claim 32, wherein the metrology device includes a plurality of measurement transducers in a parallel arrangement to provide a corresponding respective plurality of measurements based on which a position of the movable structure can be determined, and wherein the parallel arrangement of the actuators differs from the parallel arrangement of the measurement transducers.

34. 34. A coordinate positioning machine according to any preceding claim, wherein the drive arrangement comprises a plurality of measurement transducers separate from those of the metrology device to provide corresponding respective measurements by which the position of the movable structure can be determined independently of a position determined based on measurements of the metrology device.

35. A coordinate positioning machine according to any preceding claim, wherein the coupling device is a kinematic or quasi-kinematic coupling device.

36. 36. A coordinate positioning machine according to any preceding claim, wherein the movable structure includes a drive section associated with the drive device and a measurement section associated with the measurement device, the drive section of the movable structure being coupled to the measurement section of the movable structure via the coupling device.

37. 37. A coordinate positioning machine according to any preceding claim, comprising a fixed structure having a drive section associated with the drive device and a measurement section associated with the measurement device, the drive section of the fixed structure being coupled to the measurement section of the fixed structure via the coupling device.

38. 38. A coordinate positioning machine according to claim 36 or 37, wherein the measuring part of the fixed structure has a lower thermal expansion coefficient than the driving part of the fixed structure.

39. A coordinate positioning machine according to any preceding claim, wherein the movable structure supports a manipulation tool.

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

41. A coordinate positioning machine according to claim 39, wherein the manipulation tool is a surface sensing device or a measurement probe.

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

43. 40. A coordinate positioning machine according to claim 39, wherein the manipulation tool is a mechanical tool for shaping or machining a material.

44. 40. A method of controlling a coordinate positioning machine according to any one of claims 1 to 39, comprising the steps of coupling a manipulation tool to the movable structure, using the drive device to move the manipulation tool within a working volume with the measuring device also coupled to the movable structure, performing an operation using the manipulation tool, and determining a position of the manipulation tool for the operation within the working volume using the measuring device.

45. 45. The method of claim 44, wherein the operation is a measurement operation.

46. ​​The method of claim 44, wherein the operation is a machining operation.

47. A computer program which, when executed by a controller for a coordinate positioning machine, causes the controller to carry out the method of claim 44, 45 or 46.

48. A computer readable medium having stored thereon computer program instructions for controlling a controller of a coordinate positioning machine to carry out the method of claim 44, 45 or 46. A measurement transducer associated with the mechanical linkage of the measurement device may be configured to produce an output that is dependent on the distance interval.

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