GB2579917A

GB2579917A - Coordinate Positioning Machine

Info

Publication number: GB2579917A
Application number: GB2000354.7A
Authority: GB
Inventors: Mark Angood Stephen
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 2017-10-13
Filing date: 2017-10-13
Publication date: 2020-07-08
Anticipated expiration: 2037-10-13
Also published as: GB202000354D0; GB2579917B

Abstract

A coordinate positioning machine is disclosed that comprises a structure (22) moveable within a working volume (34) of the machine, a metrology arrangement (26) for measuring the position of the structure (22) within the working volume (34), and a drive arrangement (28) for moving the structure (22) around the working volume (34), wherein the metrology arrangement (26) comprises a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements from which the position of the moveable structure (22) is determinable, and wherein the drive arrangement (28) comprises a plurality of actuators in a parallel arrangement of a different type to that of the metrology arrangement (26).

Description

Coordinate Positioning Machine The present invention relates to a coordinate positioning machine. Coordinate positioning machines include, for example, coordinate measuring machines (CMA4s) and machine tools.

A non-Cartesian coordinate positioning machine 1 is illustrated schematically in Figure 1 of the accompanying drawings. The coordinate positioning machine 1 generally comprises first and second structures 2, 4 that are supported and moved relative to each other by a plurality of telescopic or extendable legs 6 provided between them. The first and second structures 2, 4 are sometimes referred to as platforms or stages, and the extendable legs 6 are sometimes referred to as struts or rams. Where there are six such extendable legs 6 (as illustrated in Figure 1), the machine is commonly called a hexapod.

The extendable legs 6 are typically mounted on the structures 2, 4 via ball joints 8, with each leg 6 either having its own ball joint 8 at one or both ends thereof (as illustrated in Figure 1), or sharing a ball joint 8 with an adjacent leg 6 at one or both ends.

Various relative positions between the first and second structures 2, 4 can be achieved by extending the legs 6 by differing amounts, as illustrated in Figure 1 by arrows 13. The relative position at any instant is monitored by a plurality of length-measuring transducers 10, for example with one transducer being associated with each extendable leg 6. The length-measuring transducer may comprise an encoder scale paired with a readhead. By having six such length-measuring transducers 10, the relative position can be measured in six corresponding respective degrees of freedom (three translational degrees of freedom and three rotational degrees of freedom).

One of the structures 2, 4 is typically provided as part of a fixed structure of the coordinate positioning machine 1, with the other of the structures 4, 2 moving 12, 11 relative to the fixed structure. A tool (for example a measurement probe or a drill) can be mounted on the moving structure and a workpiece mounted on the fixed structure, or vice versa, to enable an operation to be performed on the workpiece (for example measuring, probing, or scanning in the case of a coordinate measuring machine, or machining in the case of a machine tool).

For example, as illustrated in Figure 1, the lower structure 4 is fixed and the upper structure 2 is moveable, with a workpiece 9 mounted on the lower structure 4 and a probe component 3 mounted on the upper structure 2. A working volume 14 is defined between the upper structure 2 and the lower structure 4 when at their most spaced-apart positions, with the probe component 3 being positioned in the working volume 14 by operation of the extendible legs 6. Although a vertical arrow 11 is shown to indicate movement, with appropriate control of the various legs 6 the structure 2 will also be moveable horizontally and could also be tiltable.

Alternatively, the upper structure 2 could be fixed and the lower structure 4 moveable, with a probe mounted to a lower surface of the lower structure 4 and a workpiece mounted to a part of the fixed structure below that, so that the working volume (or operating volume) of the machine is below the lower structure 4 rather than above it.

Various types of non-Cartesian coordinate positioning machine are described in more detail in WO 91/03145, WO 95/14905, WO 95/20747, WO 92/17313, WO 25 03/006837, WO 2004/063579, WO 2007/144603, WO 2007/144573, WO 2007/144585, WO 2007/144602 and WO 2007/144587.

For example, WO 91/03145 describes a hexapod machine tool comprising an upper, moveable, structure that is attached to a base by six hydraulic extendable legs, similar in principle to that illustrated in Figure 1 described above. The extendable legs are attached to the base and moveable structure via ball joints. The extendable legs are hydraulic and comprise a piston rod that is moveable within a cylinder. The amount of leg extension is measured by mounting a magnetic scale to the cylinder and a suitable readhead on the piston rod. Extension of the leg thus causes the scale to move past the readhead thereby allowing the length of the leg to be measured. A computer controller operates to set the length of each leg to provide the required movement.

Described herein is a coordinate positioning machine comprising a structure moveable within a working volume of the machine, a hexapod metrology arrangement for measuring the position of the structure within the working volume, and a non-hexapod drive arrangement for moving the structure around the working volume.

Also described herein is a coordinate positioning machine comprising a structure moveable within a working volume of the machine, a drive arrangement for moving the structure around the working volume in fewer than six degrees of freedom, and a metrology arrangement for measuring the position of the structure within the working volume in more degrees of freedom than the drive arrangement.

According to a first aspect of the present invention, there is provided a coordinate positioning machine comprising a structure moveable within a working volume of the machine, a metrology arrangement for measuring the position of the structure within the working volume, and a drive arrangement for moving the structure around the working volume, wherein the metrology arrangement comprises a plurality of measurement transducers in a parallel arrangement for providing a corresponding respective plurality of measurements from which the position of the moveable structure is determinable, and wherein the drive arrangement comprises a plurality of actuators in a parallel arrangement of a different type to that of the metrology arrangement.

The metrology arrangement may be a hexapod metrology arrangement.

The drive arrangement may be a non-hexapod drive arrangement.

The metrology arrangement may be adapted to measure the position of the structure in six degrees of freedom (three translational degrees of freedom and three rotational degrees of freedom, i.e. position and orientation).

The drive arrangement may be adapted to move the structure around the working volume in three degrees of freedom The three degrees of freedom may be three translational degrees of freedom.

The metrology arrangement may comprise six measurement transducers in a parallel arrangement for providing six corresponding respective measurements from which the position of the moveable structure is determinable.

The metrology arrangement may comprise six extendable legs arranged in parallel, with the six measurement transducers being associated respectively with the six extendable legs.

The metrology arrangement and drive arrangement may each be arranged between the moveable structure and a fixed structure of the machine.

The drive arrangement may comprise a plurality of mechanical linkages connected in parallel between the moveable structure and the fixed structure.

The drive arrangement may comprise a plurality of actuators in a parallel arrangement.

The drive arrangement may comprise fewer than six actuators in a parallel 30 arrangement.

The drive arrangement may be a parallel kinematic arrangement.

The drive arrangement may be a non-Cartesian arrangement.

The drive arrangement may comprise fewer than six actuators.

The parallel arrangement of actuators associated with the drive arrangement may be different to the parallel arrangement of measurement transducers associated with the metrology arrangement.

The drive arrangement may comprise a plurality of measurement transducers, separate to those of the metrology arrangement, for providing corresponding respective measurements from which the position of the moveable structure is determinable independently of the position determined based on the measurements from the metrology arrangement.

In other words, the drive arrangement may be encoded independently of the metrology arrangement.

The measurements may be distance measurements. The measurements may relate to different separations between the moveable structure and the fixed structure.

The metrology arrangement may provide direct measurements of these separations. This is in contrast for example to an image-based or photogrammetric metrology arrangement in which distances are inferred indirectly 25 from image data The measurement transducers may each comprise an encoder scale and associated readhead.

The measurement transducers may be mechanical measurement transducers as opposed to optical or image-based or photogrammetric measurement transducers.

Each mechanical linkage may be actuated by a drive mechanism which acts between the fixed structure and the mechanical linkage.

Also described herein is a coordinate positioning machine comprising a structure moveable within a working volume of the machine, a metrology arrangement for measuring the position of the structure within the working volume, and a drive arrangement for moving the structure around the working volume, wherein the metrology arrangement comprises a parallel arrangement of measurement transducers for providing a plurality of measurements from which the position of the moveable structure is determinable, wherein the drive arrangement comprises a plurality of mechanical linkages arranged in parallel between the moveable structure and the fixed structure, and wherein each mechanical linkage is actuated by a drive mechanism which acts between the fixed structure and the mechanical linkage.

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

Each mechanical linkage of the metrology arrangement may be connected between points on the fixed structure and the moveable structure respective and may be adapted to allow a separation between those points to be varied.

The measurement transducer associated with the mechanical linkage of the metrology arrangement may be adapted to provide an output that is dependent on the separation.

Each mechanical linkage of the metrology arrangement may be an extendable or extending leg.

A mechanical linkage may also be referred to or considered to be a kinematic chain or a mechanical assembly.

A drive mechanism may be provided separately for each mechanical linkage of the drive arrangement.

The drive mechanism associated with a mechanical linkage may be arranged to act between the fixed structure and an end of the mechanical linkage.

The drive mechanism may be a linear drive mechanism.

The linear drive mechanism may be a direct linear drive mechanism.

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

The linear drive mechanism may comprise a linear motor.

The drive mechanism may be a rotary drive mechanism.

The rotary drive mechanism may be a direct rotary drive mechanism.

Each mechanical linkage may comprise at least one rigid rod.

Each mechanical linkage may comprise at least two substantially parallel rods to maintain the moveable structure at a substantially constant orientation as it moves around the working volume.

Each of the mechanical linkages may be of substantially the same arrangement or design.

The drive arrangement may comprise three such mechanical linkages.

A drive arrangement having three such mechanical linkages each with a linear drive mechanism is known as a tri-glide arrangement.

The drive arrangement may comprise or be in the form of or provide a delta robot arrangement.

The drive arrangement may comprise or be in the form of or provide a linear delta robot arrangement.

The drive arrangement may be coupled to the metrology arrangement via a coupling arrangement which is adapted to prevent at least some distortion associated with the drive arrangement from being transferred to the metrology arrangement.

The coupling arrangement may be a kinematic or pseudo-kinematic coupling arrangement.

The coupling arrangement may comprise a plurality of balls.

The coupling arrangement may comprise a plurality of resilient spacers or pads.

The moveable structure may comprise a drive part associated with the drive arrangement and a metrology part associated with the metrology arrangement, with the drive part of the moveable structure being coupled to the metrology part of the moveable structure via the coupling arrangement.

The drive part of the moveable structure may be coupled to the drive arrangement.

The metrology part of the moveable structure may be coupled to the metrology arrangement.

The fixed structure may comprise a drive part associated with the drive arrangement and a metrology part associated with the metrology arrangement, with the drive part of the fixed structure being coupled to the metrology part of the fixed structure via the coupling arrangement.

The drive part of the fixed structure may be coupled to the drive arrangement.

The metrology part of the fixed structure may be coupled to the metrology arrangement.

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

The metrology arrangement may be coupled mechanically to the moveable 20 structure.

An extendable leg may comprise any mechanical arrangement that allows the separation between a point on the fixed structure and a point on the moveable structure to be varied.

The machine may be a coordinate measuring machine.

The moveable support is adapted to support or carry an object that is to be moved around the working volume. The object may be one that is to be picked up and/or placed within the working volume. The object may be a tool for interacting with or operating on another object, such as a workpiece, located in the working volume. The tool may be a surface sensing device. The surface sensing device may be a measurement probe. The measurement probe may be a contact probe. The contact probe may comprise a stylus which makes physical contact in use with a workpiece surface to take a measurement. The measurement probe may be a non-contact probe. The non-contact probe may be an optical probe. The tool may comprise a camera for imaging the surface of a workpiece. The tool may be a mechanical tool that is typically found in a machine tool for shaping or machining metal or other rigid materials.

The movable structure may be adapted to carry an operational tool with the metrology and drive arrangements also coupled to moveable structure. The movable structure may carry an operational tool with the metrology and drive arrangements also coupled to moveable structure.

The hexapod metrology arrangement may be coupled to the moveable structure via a different attachment than that used for attaching the operational tool to the moveable structure.

The hexapod metrology arrangement may be coupled directly to the moveable structure.

According to a second aspect of the present invention, there is provided a method of controlling a coordinate positioning machine according to the above-described first aspect, the method comprising: coupling a tool to the moveable structure, using the drive arrangement to move the tool around the working volume with the metrology arrangement also coupled to the moveable structure, and performing an operation with the tool.

The method may comprise using the metrology arrangement to determine the position of the tool within the working volume for the operation.

The method may comprise associating the determined position with the performed operation.

The tool may be a surface sensing device such as a measurement probe and the operation may be a measurement operation such as taking a touch trigger measurement of a workpiece located in the working volume.

According to a third aspect of the present invention, there is provided a controller for a coordinate positioning machine, wherein the controller is configured to perform a method according to the second aspect of the present invention.

According to a fourth aspect of the present invention, there is provided a computer program which, when run by a coordinate positioning machine controller, causes the controller to perform a method according to the second 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 third 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.

According to a fifth aspect of the present invention, there is provided a computer-readable medium having stored therein computer program instructions for controlling a coordinate positioning machine controller to perform a method according to the second aspect of the present invention.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1, discussed hereinbefore, is schematic illustration of a hexapod coordinate positioning machine, having six extendable legs; Figure 2 is a schematic side view of the hexapod coordinate positioning machine of Figure 1; Figure 3 is a schematic side view of a coordinate positioning machine embodying the present invention having a metrology arrangement and a separate drive arrangement; Figure 4 is a schematic side view of a coordinate positioning machine embodying the present invention in which the metrology arrangement is decoupled to some extent from the drive arrangement; Figure 5 shows a first perspective view of a practical embodiment of the coordinate positioning machine of Figure 4; Figure 6 shows a second perspective view of the embodiment of Figure 5; Figure 7 shows a side view of the embodiment of Figure 5; Figure 8 shows a top view of the embodiment of Figure 5; Figures 9A to 9E are schematic illustrations of the operation of an embodiment of the present invention; Figure 10 is a schematic illustration of a top-down variant of the coordinate positioning machine of Figure 4; Figure 11 is a schematic illustration of a variant of the top-down coordinate positioning machine of Figure 10; Figure 12 shows a slight variant of the coordinate positioning machine of Figures 5 to 8; Figure 13 shows a top-down variant of the coordinate positioning machine of Figure 12; Figure 14 shows a variant of the top-down coordinate positioning machine of Figure 13; Figure 15 shows another variant of the top-down coordinate positioning machine of Figure 13; Figures 16A and 16B schematically illustrate an embodiment having a different type of non-hexapod drive arrangement; Figure 17 illustrates a practical embodiment of the coordinate positioning machine of Figures 16A and 16B; Figures 18A and 18B schematically illustrate a variant of the embodiment of Figures 16A and 16B in which the metrology arrangement is decoupled to some extent from the drive arrangement; Figure 19 schematically illustrates a variant of the embodiment of Figures 16A and 16B with a bottom-up rather than top-down hexapod metrology arrangement; Figure 20 schematically illustrates a variant of the embodiment of Figures 16A and 16B in which fixed-length metrology struts are used in the hexapod metrology arrangement; Figure 21 schematically illustrates a variant of the embodiment of Figure 20 in which an offset pivot plate is used for the metrology struts; Figure 22 schematically illustrates an embodiment having a delta robot type of non-hexapod drive arrangement; Figure 23 schematically illustrates a variant of the embodiment of Figure 22, having an increased amount of decoupling between the metrology and drive arrangements; Figure 24 schematically illustrates a variant of the embodiment of Figure 22, having a decreased amount of decoupling between the metrology and drive arrangements; Figure 25 schematically illustrates an embodiment having a serial robot type of non-hexapod drive arrangement; Figure 26 schematically illustrates an embodiment having a Cartesian type of non-hexapod drive arrangement; Figure 27 illustrates the concept of providing a drive arrangement having fewer degrees of freedom than the metrology arrangement; Figure 28 illustrates a dual hexapod arrangement in which the hexapod drive arrangement has constrained movement; and Figure 29 is a flow diagram representing a method of controlling a coordinate positioning machine embodying the present invention.

A side view of the coordinate positioning machine 1 discussed above with reference to Figure 1 is illustrated schematically in Figure 2. The coordinate positioning machine 1 comprises an upper structure 2 that is moveable within a working volume 14 of the machine 1. The six extendable legs 6 form both a hexapod drive arrangement 18 (shown in solid line) for moving the upper structure 2 around the working volume 14, and also a hexapod metrology arrangement 16 (shown in dotted line) for measuring the position of the upper structure 2 within the working volume 14. The coordinate positioning machine 1 therefore has combined drive and metrology struts.

By way of comparison, a coordinate positioning machine 21 according to an embodiment of the present invention is illustrated schematically in Figure 3. Like the Figure 2 machine, the coordinate positioning machine 21 comprises an upper structure 22 that is moveable within a working volume 34 of the machine 21, a drive arrangement 28 (shown in solid line) for moving the upper structure 22 around the working volume 34, and a metrology arrangement 26 (shown in dotted line) for measuring the position of the moveable upper structure 22 within the working volume 34.

Whilst in the coordinate positioning machine I illustrated in Figure 2 the hexapod metrology arrangement 16 and the hexapod drive arrangement 18 are combined, in the coordinate positioning machine 21 embodying the present invention as shown in Figure 3, the drive arrangement 28 is different to and separate from the metrology arrangement 26. A technical advantage is achieved by separating the metrology arrangement 26 from the drive arrangement 28 in this way, because it allows these two different arrangements to be designed with very different (and sometimes conflicting) technical considerations in mind.

Separating and differentiating the metrology arrangement 26 from the drive arrangement 28 allows the drive arrangement 28 to be made relatively light-weight and fast, so that the structure 22 can be moved around the working volume 34 quickly with high accelerations and rapid changes of direction. Whilst focussing on factors like weight and speed may sacrifice some degree of positional accuracy in the drive arrangement 28, this is overcome by providing a metrology arrangement 26 that is instead designed with positional accuracy in mind.

Because the metrology arrangement 26 is passive and has no need for any drive components, which add weight and generate heat, metrology errors caused by inertial and thermal distortion of parts (including the measurement scale used to measure distance) can thereby be controlled and reduced.

Use of a metrology arrangement 26 that is separate from and different to the drive arrangement 28 provides a coordinate positioning machine 21 in which the moveable structure can be driven quickly around the working volume, yet retaining the accuracy required of demanding positioning applications.

With such a design, it also becomes possible to choose a relatively inexpensive off-the-shelf drive mechanism for the drive arrangement 28, not designed particularly with high accuracy in mind, knowing that it will be coupled by a dedicated metrology arrangement 26 to provide the required accuracy, and this therefore allows production costs to be lowered.

Mechanical metrology arrangements also benefit from having low-friction joints, while drive arrangements typically require more robust and substantial joints that inevitably have a higher degree of friction, so there is a design conflict that is overcome by separating the metrology arrangement 26 from the drive arrangement 28.

In the coordinate positioning machine 21 of Figure 3, the metrology arrangement 26 is a hexapod arrangement, while the drive arrangement 28 is a non-hexapod arrangement (i.e. something other than or different to a hexapod arrangement).

Use of a hexapod-based metrology arrangement 26 is particularly beneficial because a hexapod provides a robust mechanical system having a parallel arrangement of measurement transducers that provide direct measurements of distance from which a very accurate and reliable determination of position in six degrees of freedom can be derived.

With a hexapod drive arrangement such as that illustrated in Figure 1, each of the six struts requires a motor that must necessarily form part of the associated strut, i.e. that moves with the strut. Therefore, when the hexapod is actuated to move the moveable structure around the working volume, the weight of the relatively heavy motor parts is also being moved around. Having to move this extra mass around reduces the potential speed (or acceleration) of the drive arrangement, and creates additional heat in the machine which has a negative effect when it reaches the metrology arrangement. By providing a non-hexapod drive arrangement such as that illustrated in Figure 3, these problems can be overcome, because it allows the motor parts to be moved off the moving parts.

Furthermore, by using a non-hexapod drive arrangement that provides movement to the moveable structure 22 in fewer than six degrees of freedom, fewer actuators are required (i.e. fewer than the six actuators required in a hexapod), reducing cost and complexity and also reducing the amount of heat generated, due to the fewer number of heat-generating motor parts, and therefore improving metrology results.

The hexapod metrology arrangement 26 of Figure 3 is generally similar to the hexapod arrangement of Figures 1 and 2, but without any actuation or motor components that are required to provide drive. The drive arrangement 28 in this embodiment is a so-called "tri-glide arrangement, for example as disclosed in US 2003/0005786, having three carriages 56 moving along three corresponding respective linear tracks 51. These arrangements will be described in more detail below with reference to Figures 5 to 8.

Referring again to Figure 3, the coordinate positioning machine 21 comprises a lower structure 24 that forms part of the fixed structure of the machine 21, with a workpiece 29 mounted on the lower structure 24. A measurement probe 30 is supported on the upper structure 22 so that it can be moved around the working volume 34. The working volume 34 is defined between the upper structure 22 and the lower structure 24 when at their most spaced-apart positions, with the probe component 30 being positioned in the working volume 34 by operation of the drive arrangement 28.

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

As illustrated in Figure 4, to provide even further separation between the drive arrangement 28 and the metrology arrangement 26, the drive arrangement 28 may be coupled to the metrology arrangement 26 via a coupling arrangement 38 which prevents at least some distortion associated with the drive arrangement 28 from being transferred to the metrology arrangement 26. The coupling arrangement 38 comprises a first coupling 38a associated with the moveable structure 22 and a second coupling 38b associated with the fixed structure 24.

In the schematic embodiment illustrated in Figure 4, the moveable structure 22 comprises a metrology part 22a associated with the metrology arrangement 26 and a drive part 22b associated with the drive arrangement 28, with the metrology part 22a of the moveable structure 22 being coupled to the drive part 22b of the moveable structure 22 via the first coupling 38a. The metrology part 22a of the moveable structure 22 is coupled to the metrology arrangement 26. The drive part 22b of the moveable structure 22 is coupled to the drive arrangement 28.

Similarly, the fixed structure 24 comprises a metrology part 24a associated with the metrology arrangement 26 and a drive part 24b associated with the drive arrangement 28, with the metrology part 24a of the fixed structure 24 being coupled to the drive part 24b of the fixed structure 24 via the second coupling 38b. The metrology part 24a of the fixed structure 24 is coupled to the metrology arrangement 26. The drive part 24b of the fixed structure 24 is coupled to the drive arrangement 28.

In this example, each coupling 38a, 38b of the coupling arrangement 38 is in the form of a kinematic or pseudo-kinematic coupling. In the context of locating a body relative to another, kinematic design considerations are met by constraining the degrees of freedom of motion of the body using the minimum number of constraints, and in particular involves avoiding over constraining. Over constraining can result in multiple points of contact between two bodies enabling one body to rest in more than one position against the other. Accordingly, the body's location is not repeatable as it is not known at which of the several positions the body will come to rest. In particular, where there is over constraint, there is a conflict between the constraints that are in place, so that it is not possible to determine with any certainty which combination of constraints will determine the actual position of the body. These concepts are described in H. J. J. Braddick, "Mechanical Design of Laboratory Apparatus", Chapman & Hall, London, 1960, pages 11-30.

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

In particular, in this embodiment each coupling 38a, 38b comprises a set of three balls to provide three points of contact according to kinematic design principles (only two are shown in the schematic illustration of Figure 4). It is also of benefit to use a plurality of resilient spacers or pads instead of rigid balls, e.g. three such spacers arranged at the corners of a triangle. This provides some degree of kinematic coupling, even if the contact is not point-like but instead spread over the small area of the resilient spacer. Use of resilient spacers is beneficial since they act to absorb some vibration from the drive arrangement 28 so that it is not transferred to the metrology arrangement 26.

It will also be appreciated that such a coupling can be provided at both ends (i.e in association with the moveable structure 22 and the fixed structure 24), or at one end only (i.e. in association with only one of the moveable structure 22 and the fixed structure 24), or not at all (i.e. at neither of the moveable structure 22 and the fixed structure 24).

An embodiment will now be described in more detail with reference to Figures 5 to 8, which show more detailed representations of the machine structure than the very schematic illustrations of Figures 3 and 4.

The hexapod metrology arrangement 26 illustrated in Figures 5 to 8 comprises six extendable legs 60, generally of the same construction, arranged between the upper structure 22 and the lower structure 24. As per Figure 4, the upper structure 22 comprises a metrology part 22a associated with the metrology arrangement 26 and a drive part 22b associated with the drive arrangement 28, with the metrology part 22a of the moveable structure 22 being coupled to the drive part 22b of the moveable structure 22 via the first coupling 38a. The metrology part 22a of the moveable structure 22 is coupled to the metrology arrangement 26 via ball joints 68. The drive part 22b of the moveable structure 22 is coupled to the drive arrangement 28 via ball joints 58.

Each of the six extendable legs 60 comprises an upper tube 64 and a lower tube 62, with the lower tube 62 sliding telescopically within the upper tube 64. The extendable legs 60 are generally of a similar construction to those described in WO 201 7/021 73 3 and application no. PCT/GB2017/050909, except that there is no need in this embodiment for the extendable legs to be driven, and therefore no need for any motor-related components. However, the overall construction of the extendable legs 60 is generally similar.

With the example illustrated in Figures 5 to 8, the lower structure 24 is fixed and the upper platform 22 is moveable relative to the lower structure 24 by operation of the six extendable legs 60, with a measurement probe 30 being mounted to a lower surface of the upper structure 22. In this configuration, a workpiece (not illustrated in Figures 5 to 8) would be mounted on top of the metrology part 24a of the lower structure 24, so that the working volume of the machine 21 is between the metrology parts 22a, 24a of the upper and lower structures 22, 24 respectively.

The measurement probe 30 comprises a stylus with a workpiece-contacting tip, with the measurement probe 30 being connected to the metrology part 22a of the moving structure 22 via a quill 32.

The extendable legs 60 are for positioning (i.e. determining the position of) a component supported by the moveable structure 22 (in the illustrated example the component is the measurement probe 30), or at least part a specific part of the component (such as the tip of the measurement probe) within the working volume of the machine.

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

The drive arrangement 26 in this embodiment is a so-called "tri-glidC arrangement as described, for example, in US 2003/0005786. The tri-glide arrangement is provided by three mechanical linkages 50 of substantially the same design that are connected in parallel between the moveable structure 22 and the fixed structure 24. Each mechanical linkage 50 comprises two substantially parallel rigid rods 52, 54 of fixed length, which act to maintain the moveable structure 22 at a substantially constant orientation as it moves around the working volume 34. Each mechanical linkage 50 also comprises a carriage 56, with the rods 52, 54 being pivotally coupled at their lower end to the carriage 56 and at their upper end to the drive part 22b of the moveable structure 22 via ball joints 58.

Three linear tracks 51 are arranged substantially vertically on the drive part 24b of the fixed structure 24, with the three carriages 56 being arranged to move along (up and down) the three linear tracks 51 respectively. 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 to the fixed structure 24 (specifically, the drive part 24b of the fixed structure 24). Each carriage 56 is driven in a substantially linear manner along its corresponding respective track 51 by a linear drive mechanism. The linear drive mechanism may comprise a linear motor. The linear drive mechanism may comprise a stepper motor.

Therefore, each mechanical linkage 50 is actuated by a drive mechanism which acts between the fixed structure (linear track) 51 and the mechanical linkage 50. More particularly, the drive mechanism acts between the fixed structure (linear track) 51 and an end of the mechanical linkage 50, i.e. the carriage 56. With such a drive arrangement, the moving parts can be kept relatively light-weight, in this example being thin and light-weight rods 52, 54, and it is not the case (as it is with a typical hexapod drive arrangement having six extendable legs such as that shown in Figure 1) that each motor is moving around the weight of other motors. This allows a light-weight drive arrangement that is able to move quickly with high accelerations and rapid changes of direction.

Returning to a more schematic format, operation of the tri-glide embodiment will now be described with reference to Figures 9A to 9E. Each of Figures 9A to 9E uses a similar representation to that used in Figure 3, with the two carriages being labelled as 56a and 56b respectively and the two linear tracks being labelled 51a and 51b respectively.

Due to the constraints provided by the parallel rods 52, 54 described above with reference to Figures 5 to 8, motion of the moveable structure 22 (by operation of the tri-glide drive arrangement 28) is restricted to three translational degrees of freedom, so that the moveable structure 22 retains a substantially constant orientation as it moves around the working volume. This constraint to movement in three degrees of freedom is indicated by arrows labelled 3DOF in Figure 9A.

On the other hand, with six extendable legs 60 of the hexapod metrology arrangement 26, comprising six corresponding measurement transducers in a parallel arrangement, six corresponding respective measurements are provided from which the position of the moveable structure is determinable in all six degrees of freedom, as indicated by arrows labelled 6DOF in Figure 9A.

As illustrated in Figure 9B, by lowering carriage 56a and raising carriage Sob along their respective tracks 51a, 51b, because the rods 52, 54 of the mechanical linkages 50 are of fixed length, the moveable structure 22 (and along with it the measurement probe 30) is moved leftward and downward within the working volume 34, maintaining substantially the same orientation. 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 in the extendable legs 60. From those transducer measurements, the position of the moveable structure 22 within the working volume 34 can be determined, and because the measurement probe 30 is in a known and fixed spatial relationship to the moveable structure 22, so too can the position of the measurement probe 30 (and probe tip) be determined. Figure 9C is the same is Figure 9B, but without the movement indications for clarity, thereby showing the final position of the components after the move operation.

Similarly, as illustrated in Figure 9D, by raising carriage 56a and lowering carriage Sob along their respective tracks 51a, 51b, the moveable structure 22 (and along with it the measurement probe 30) is can be moved rightward within the working volume 34, again maintaining substantially the same orientation. This causes the extendable leg 60 closest to carriage 56a to lengthen and the extendable leg 60 closest to carriage 56b to shorten, with these changes in length being measured by measurement transducers (e.g. encoders) 10 in the extendable legs 60. From those transducer measurements, the position of the moveable structure 22 and measurement probe 30 within the working volume 34 can be determined. Figure 9E is the same is Figure 9D, but without the movement indications for clarity, thereby showing the final position of the components after the move operation.

With the above-described tri-glide embodiment, the extendable lees 60 of the hexapod metrology arrangement 26 and the rods 50 of the drive arrangement 28 extend up from the bottom, and that embodiment can therefore be described as a "bottom up" arrangement. Figure 10 shows an alternative "top down" arrangement, which is generally the same as the "bottom up" arrangement except that the extendable legs 60 of the hexapod metrology arrangement 26 and the rods 50 of the drive arrangement 28 extend down from the top (hence a "top down" arrangement). To enable this, a frame 25a is provided to support the hexapod metrology arrangement 26 so that it can effectively "hang" from the top. 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 hexapod part 24a of the fixed structure 24). As with the previous embodiment, a coupling arrangement 38a, 38b is provided to isolate the metrology arrangement 26 from the drive arrangement 28.

Yet another "top down" arrangement is illustrated schematically in Figure 11.

This differs from the Figure 10 embodiment in that the hexapod metrology arrangement 26 is supported from a frame 25 which extends around the top, and is provided inside the tri-glide drive arrangement 28. The frame 25 effectively forms part of the fixed structure of the coordinate positioning machine 21, as an extension to the fixed structure 24 along with the vertical linear tracks 51 which also become part of the frame 25. Furthermore, the Figure 11 embodiment is not provided with any coupling arrangement 38 to isolate the metrology arrangement 26 from the drive arrangement 28.

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

Figure 13 shows a practical embodiment of the "top down" arrangement illustrated schematically in Figure 11, but differs from the Figure 11 embodiment by decoupling the drive and metrology to some extent at the moveable structure.

Figure 14 is a variant of Figure 13, providing decoupling of the drive and metrology both at the moveable structure and the fixed structure. Figure 15 is a further variant, having a separate metrology frame arranged within a drive frame, with decoupling of the drive frame from the metrology frame both at the moveable structure and the fixed structure.

It will be understood that the present invention is not limited to embodiments in which the drive arrangement 28 is in the form of a tri-glide. Figure 16A schematically illustrates an embodiment in which the hexapod metrology arrangement 26 is coupled with a different type of non-hexapod drive arrangement 28. Rather than a fixed-length rod 52 one end of which is driven linearly along a track 51 by a carriage 56 as with the tri-glide embodiment, in the embodiment of Figure 16A a fixed-length extending rod is instead driven through a pivoting guide 76 by a suitable linear drive mechanism provided within the guide 76, thereby changing the separation indicated by the arrow in Figure 16A and thereby moving the structure 22.

In Figures 16A and 16B, similar to Figure 11, the metrology and drive arrangements 26, 28 are supported in a top-down manner from a frame 25, with the frame 25 forming part of the fixed structure of the coordinate positioning machine 21. From the position as illustrated in Figure 16A, when both rods are driven downwards through their respective guides 76, the structure 22 can be moved to the position as illustrated in Figure 16B. As before, the position of the structure 22 is measured by the hexapod metrology arrangement 26.

It will be appreciated that, as with the tri-glide arrangement, each mechanical linkage of the drive arrangement 28 in the Figure 16A embodiment is actuated by a drive mechanism which acts between the fixed structure and the mechanical linkage, so this embodiment shares the same advantage in terms of speed and acceleration.

Figure 17 illustrates a practical embodiment of the schematically-illustrated arrangement of Figure 16A. The Figure 17 embodiment is based closely on a non-Cartesian type of coordinate measuring machine sold by the present applicant, Renishaw plc, under the trade mark EQUATOR. The hexapod metrology arrangement 26 is generally similar to that of Figure 5, comprising six extendable legs each having an upper tube 64 and a lower tube 62, with the lower tube 62 sliding telescopically within the upper tube 64. In this embodiment, the extendable legs are supported in a top-down arrangement from a frame 25 to a metrology platform 22a (part of the moveable structure 22). The pivoting guides 76 are obscured in Figure 17 by the structure of the frame 25. Three fixed-length drive rods 72 pass through the three pivoting drive guides 76 respectively and are coupled at their lower end to a drive plate 22b (part of the moveable structure 22).

In this embodiment the two parts 22a, 22b of the moveable structure 22 are separated spatially by rigid column 23. Three sets of parallel rod pairs 72, 74 are arranged to constrain motion in three degrees of freedom, similarly to the rods 52, 54 of Figure 5.

Returning to a more schematic representation, Figures 18A and 18B show a variant of the machine of Figures 16A and 16B, in which the metrology arrangement 26 is isolated further from the drive arrangement 28. This is analogous to the tri-glide embodiment described above with reference to Figure 10, so a further description is not necessary. Figure 19 shows an alternative to the Figure 16A arrangement, with a bottom-up hexapod metrology arrangement 26 instead of a top-down arrangement.

Figure 20 schematically illustrates a variant of the embodiment of Figure 16A and 16B in which fixed-length metrology struts are used in the hexapod metrology arrangement 26, similar to the fixed-length struts of the drive arrangement 28 of that embodiment. The six fixed-length extending struts as illustrated in Figure 20 is considered to functionally equivalent to the six extendable struts of previous embodiments, with the variable-length part of the strut being indicated by the arrow in Figure 20; that part is equivalent to the extendable strut of previous embodiments. The term "extendable leg" and "extending leg" are therefore to be understood herein as being equivalent, meaning any type of mechanical arrangement or linkage between two points that allows the separation between those points to be varied. The drive arrangement 28 is still a non-hexapod drive arrangement because it only has three extending struts, as shown in more detail in Figure 17. Figure 21 schematically illustrates a variant of the embodiment of Figure 20 in which a fixed support (pivot plate) 25a is used for the metrology struts that is offset spatially from the fixed support (pivot plate) 25b used for the drive struts.

Embodiments have been described above in which two different types of non-hexapod drive arrangement have been employed: a tri-glide linear drive arrangement (e.g. Figure 5) and a pivoting linear drive arrangement (e.g. Figure 17). There are many other possibilities for the drive arrangement, and just a few of these will be described briefly now; others will be apparent to the skilled person.

Figure 22 schematically illustrates an embodiment having a delta robot type of non-hexapod drive arrangement. A delta robot is a type of parallel robot, and an example is described in detail in US4976582. Figure 23 schematically illustrates a variant of the embodiment of Figure 22, having an increased amount of decoupling between the metrology and drive arrangements. Figure 24 schematically illustrates a variant of the embodiment of Figure 22, having a decreased amount of decoupling between the metrology and drive arrangements. It will be appreciated that, as with the tri-glide arrangement, with a delta robot arrangement each mechanical linkage is actuated by a drive mechanism which acts between the fixed structure and the mechanical linkage, so these delta robot embodiments share the same advantage in terms of speed and acceleration. Furthermore, with appropriate constraints (such as described in US4976582) the delta robot drive arrangement 28 can be adapted to provide movement to the structure 22 in three degrees of freedom, i.e. in fewer degrees of freedom than is being measured by the hexapod metrology arrangement 26.

More conventional non-hexapod drive arrangements are also envisaged. For example, Figure 25 schematically illustrates an embodiment having a serial robot type of non-hexapod drive arrangement, having a plurality of segments or links connected in series by rotational joints, with one end of the robot being attached to ground and the other end being free to move in space. Figure 26 schematically illustrates an embodiment having a Cartesian type of non-hexapod drive arrangement, having three parts connected in series that are moveable respectively along Cartesian axes x, y and z (as marked in Figure 26). These types of drive arrangement are well known and no further explanation of them is required here.

As explained above particularly with reference to Figure 9A, the drive arrangement 28 provides three translational degrees of freedom to the moveable structure 22, while the hexapod metrology arrangement 26 is adapted to measure in six degrees of freedom. According to one aspect of the present invention, a coordinate positioning machine is proposed which comprises a structure moveable within a working volume of the machine, a drive arrangement for moving the structure around the working volume in fewer than six degrees of freedom, and a metrology arrangement for measuring the position of the structure within the working volume in more degrees of freedom than the drive arrangement. This is illustrated schematically in Figure 27. One or both of the drive and metrology arrangements can be a parallel kinematic arrangement, such as a hexapod arrangement, tri-glide arrangement or a delta robot arrangement. In particular, it is to be noted that in this aspect the metrology arrangement need not be a hexapod metrology arrangement.

It is not normal to provide measurement, particularly direct measurement, in more degrees of freedom than movement. Typically, there would be N drive parts (rotary or linear) with each drive part being encoded separately to give N corresponding measurements. For example, for a three-axis CMIVI there are three driven linear axes, each with a position encoder, and therefore three corresponding measurements (i.e. driving and measuring both in three degrees of freedom). For a hexapod there are six variable-length struts, each with a position encoder, and six corresponding measurements (i.e. driving and measuring both in six degrees of freedom) However, the present applicant has appreciated the desirability and advantage of being able to provide a drive that is relatively inaccurate and constrained to move in a limited number of degrees of freedom (e.g. three) coupled with a separate metrology arrangement that is highly accurate and capable of measuring on all six degrees of freedom, and hence which is capable of compensating for any inaccuracies in the mechanically-constrained drive arrangement. It is even possible to apply the scheme of Figure 27 to a dual hexapod arrangement as illustrated schematically in Figure 28, in which the drive hexapod is constrained to movement in less than six degrees of freedom by an appropriate mechanical constraint.

There are many other forms of non-hexapod drive arrangement, or drive arrangements that are constrained to fewer than six degrees of freedom, as will be apparent to the skilled person. For example, there are many possible variants of the tri-glide arrangement shown. One variant is to provide an arrangement having more than three drives and associated mechanical linkages. And, instead of vertical tracks 51 as illustrated in Figure 3, the tracks may instead be arranged horizontally, e.g. radially outward from a point, so that movement of the structure 22 is also effected by movement of the carriages 56 along the horizontal tracks.

Many other such possibilities exist.

Although embodiments of the present invention have been described mainly in relation to the use of a contact probe, in which a stylus of the contact probe makes physical contact with the workpiece surface to take a measurement, it will be appreciated that the invention is not limited to contact probes. The same concepts are applicable equally to non-contact probes, such as optical probes, in which a surface is sensed without making physical contact. The invention is generally applicable to any surface sensing device that is adapted to sense a surface, whether by contact or not. The invention can also be applied to the positioning of a component other than a surface sensing device, for example for orienting a component part of an article during manufacture of the article. Or, the component could be a tool, or a part thereof, such as a tool typically found in a machine tool for shaping or machining metal or other rigid materials. The component could be the moveable structure itself. The component may comprise a camera for imaging the surface of the workpiece. The component may comprise an eddy current probe for detecting and/or measuring eddy current at or near the surface of the workpiece. Many other possibilities would be apparent to the skilled person.

It is to be noted that in an embodiment of the present invention the hexapod metrology arrangement 26 is not provided purely for calibration purposes, to be coupled temporarily to the moveable structure to perform calibration of a combined drive and metrology arrangement, and then removed for operational use of the machine. Rather, the hexapod metrology arrangement is intended to remain coupled to the movable structure to provide position measurements relating to the moveable structure during operational use. To distinguish from a calibration-only metrology arrangement, the movable structure may be adapted to carry an operational tool with the metrology and drive arrangements also coupled to moveable structure. The hexapod metrology arrangement may be coupled to the moveable structure via a different attachment than that used for attaching the operational tool to the moveable structure. The hexapod metrology arrangement may be coupled directly to the moveable structure (e.g. rather than via an attachment intended primarily for the operational tool).

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

It will be appreciated that operation of the coordinate measuring machine 21 can 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 illustrated schematically in Figure 3. It will be appreciated that control of the extendable legs can be provided by a program operating on the controller C. Such an operating program can be stored on a computer-readable medium, or could, for example, be embodied in a signal such as a downloadable data signal provided from an Internet website. The appended claims are to be understood as covering an operating program by itself, or as a record on a carrier, or as a signal, or in any other form.

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