Coordinate positioning machine

Abstract

To provide an improved balancing structure of a coordinate positioning machine which is a non-orthogonal coordinate positioning machine.SOLUTION: The coordinate positioning machine includes: a plurality of driving shafts for positioning a movable structure in a movable range of a machine; a plurality of balancing shafts for balancing the movable structure; and a force generator for generating a balance force which acts on at least one balancing shaft. The balancing shaft and the force generator are configured for each other not to substantially move the force generator and / or not to perform substantial work on the force detector, unless the movable structure is moved in a direction perpendicular to the gravity due to the rotation of the force detector around one rotary balancing shaft at a maximum.SELECTED DRAWING: Figure 10

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] FIG. 1 of the accompanying drawings shows a known coordinate measuring machine 1, which has three linear axes x, y and z arranged successively and perpendicular to one another, with the z-axis aligned with gravity g. A measuring probe 3 is mounted on a vertical post 8 which is slidable in the z-direction within a carriage 2, this relative movement defining the z-axis. The carriage 2 is itself supported on a horizontal beam 7 and is slidable in the y-direction along the beam 7, this relative movement defining the y-axis. Similarly, the beam 7 is slidable in the x-direction on a pair of rails 6, this relative movement defining the x-axis. A computer controller 5 operates to drive each component (post 8, carriage 2, beam 7) to the appropriate position along its corresponding respective axis, so as to position the measuring probe 3 at a desired position within the machine's range of motion and into sensing relationship with a workpiece 9 supported on a fixed platform 4. The measurement probe 3 may be attached to the vertical column 8 in a fixed manner as shown, or may be attached to the vertical column 8 via an articulating probe head as shown in the inset of FIG. 1, which provides two rotational axes a, b in addition to the three linear axes x, y, z.

[0003] Each axis x, y, z is driven independently of the others by a corresponding respective motor (not shown). Each axis x, y, z is also encoded or sensed independently of the others by a corresponding respective sensor, the output from which is used to determine the position of the measurement probe 3 (or a tool mounted on the pillar 8). Each axis is provided with a length measurement transducer having an encoder scale (shown diagrammatically as a series of parallel lines along each axis in Figure 1) paired with a read head (not shown). To measure relative movement between two parts, the encoder scale is suitably mounted on one part and the read head is suitably mounted on the other part.

[0004] The coordinate measuring machine 1 of FIG. 1 can be referred to as a Cartesian coordinate measuring machine since it has three linear axes x, y and z arranged perpendicular to each other. In a Cartesian or orthogonal axis configuration as shown in FIG. 1, in the absence of drive from motors (or other damping forces) associated with the x, y and z axes respectively, the z axis is aligned with gravity and the x and y axes are orthogonal to gravity, so that the system is stable or self-supporting along the x and y axes, but along the z axis the vertical column 8 will naturally fall by gravity to its lowest possible position. To stabilize the system in the Z direction, it is known for the z axis to provide a counterweight to counter the weight of the vertical column 8 (and anything attached to it, such as the measuring probe 3). Such z-axis counterweight configurations for Cartesian coordinate measuring or positioning machines are known, for example, from US Pat. No. 5,399,433, US Pat. No. 5,499,433 and US Pat. No. 5,499,433.

[0005] Another type of coordinate measuring machine 10 is shown diagrammatically in Figure 2 of the accompanying drawings. The coordinate measuring machine 10 generally comprises a movable platform 12 and a fixed platform 14 which are supported and moved relative to each other by a plurality of extendable and variable length legs 16 disposed therebetween. The fixed platform 14 forms part of the fixed structure of the machine 10. The movable and fixed platforms 12, 14 may also be referred to as stages (or structures or parts) and the extendable legs 16 may also be referred to as struts (or actuators). When there are six such extendable legs 16 (as shown in Figure 2), the machine 10 is commonly referred to as a hexapod.

[0006] The extendable legs 16 are typically attached to the platforms 12, 14 via ball joints 18, with each leg 16 either having its own ball joint 18 at one or both ends (as shown in FIG. 2) or sharing a ball joint 18 with an adjacent leg 16 at one or both ends. Each extendable leg 16 is typically formed as a pair of tubes, one of which is telescopically driven within the other tube by a drive mechanism (e.g., a linear motor) to cause extension and contraction of the extendable leg 16, as indicated by the arrows in each extendable leg 16 and as described in more detail in U.S. Pat. No. 5,399,633. As described in U.S. Pat. No. 5,399,633, some separation can be provided between the drive and metrology sections of each strut. It is also possible to provide a drive configuration that is not only separate from the metrology configuration, but also different from the metrology configuration, as described in U.S. Pat. No. 5,399,633.

[0007] Various relative positions between the movable platform 12 and the fixed platform 14 can be achieved by extending the legs 16 by different amounts. The relative position at any time is monitored by a number of length measurement transducers 17, one such transducer for each extendable leg 16. Each length measurement transducer 17 may include an encoder scale paired with a read head, the encoder scale suitably mounted on one of a pair of telescopic tubes and the read head suitably mounted on the other tube. In this way, as the legs 16 are extended, the encoder scale moves past the read head, thereby allowing the length of the extendable legs 16 to be measured (or derived from a measurement). The computer controller 15 operates to set the length of each extendable leg 16 to give the required relative movement between the platforms 12, 14. By having six such length measurement transducers 17, it is possible to measure the relative position in six corresponding respective degrees of freedom (three translational degrees of freedom and three rotational degrees of freedom).

[0008] A workpiece 19 is mounted to a lower (fixed) platform 14 and a measurement probe 13 is mounted to an upper (movable) platform 12. A range of motion (or operating volume) 11 is defined between the upper (movable) platform 12 and the lower (fixed) platform 14 and the measurement probe 13 is positioned within the range of motion 11 (i.e. moved to a desired position) by movement of the extendable legs 16. The configuration of Figure 2 may be referred to as a "bottom-up" configuration because the extendable legs 16 extend upwardly from the fixed platform 14 to the moveable platform 12. This configuration is shown diagrammatically in Figure 3 of the accompanying drawings.

[0009] Alternatively, as shown diagrammatically in Figure 4 of the accompanying drawings, a "top-down" configuration sees the extendable legs 16 extending downwardly from the fixed structure 14 onto the movable platform 12, with the measurement probe 13 mounted to the underside of the movable platform 12 and the workpiece mounted below on another part of the fixed structure 14. These types of configurations are described in more detail in US Pat. No. 5,399,323, which also describes the use of a non-hexapod drive configuration in combination with a hexapod measurement configuration.

[0010] The measuring probe 13 is only one example of a manipulation tool that can be mounted on the movable platform 12 to enable performing operations on the workpiece 19. Depending on the intended application, the manipulation tool can be adapted for measuring, probing or scanning in the case of a coordinate measuring machine, or for machining or drilling in the case of a machine tool. It is also possible to mount the workpiece 19 on the movable platform 12 and the measuring probe 13 (or other manipulation tool) on the fixed platform 14.

[0011] The coordinate measuring machine 10 of Figure 2 can be referred to as a non-Cartesian coordinate measuring machine since its axes are not arranged orthogonally according to a Cartesian coordinate system, in contrast to a Cartesian machine as illustrated in Figure 1. The coordinate measuring machine 10 of Figure 2 can be considered as having six axes of movement (or six drive axes) corresponding to (and defined by) the six extendable legs 16. In this sense, the axes of a coordinate positioning machine can be considered as relating to the degrees of freedom that are sensed (e.g. by a transducer or encoder), keeping in mind that the axes can be linear or rotary, and that a coordinate positioning machine can have a combination of linear and rotary axes.

[0012] The coordinate measuring machine 10 of Figure 2 can also be referred to as a "parallel kinematic" coordinate measuring machine since its axes of movement are defined in parallel. This is in contrast to the coordinate measuring machine 1 of Figure 1, which can be referred to as a "sequential kinematic" coordinate measuring machine since its axes of movement are defined in sequence. Another type of sequential kinematic machine is an inspection robot or manual articulated arm, in which multiple articulated arm members are connected in sequence by multiple rotary joints.

[0013] The moveable platform 12 and measurement probe 13 of the coordinate measuring machine 10 of Figure 2 have an associated weight that needs to be supported by the extendable legs 16. Thus, the motors provided to extend and retract the extendable legs 16 must also operate to maintain the moveable platform 12 in a stable vertical position, which uses energy and generates heat. Energy efficiency is particularly important in metrology applications, as the heat generated by the motors can lead to uncontrolled and unknown thermal expansion of components, affecting measurement results in ways that cannot be calibrated.

[0014] For this reason, such a machine can be provided with a counterbalancing arrangement to support at least a part of the weight of the mobile platform 12. An exemplary counterbalancing arrangement for a hexapod coordinate positioning machine is disclosed in US Pat. No. 5,399,323 and is shown in FIG. 5 of the accompanying drawings. The hexapod coordinate positioning machine of FIG. 5 has a top-down configuration, similar to that of FIG. 4, and comprises a counterbalancing arrangement 90 having a first arm 92 and a second arm 94 connected to each other via a hinge 93. The first arm 92 is connected to the mobile platform 12 via a ball 91 inserted in a holding element 26 on the top of the mobile platform 12 to form a universal ball joint, while the second arm 94 is connected to a fixed part of the machine 10 (not shown) via a revolute joint 95 and to the hinge 93 via another revolute joint 97. The hinge 93 generates a counterbalancing force by means of a spring 96 visible through an opening in the sheet material forming housing of the hinge 93, the spring 96 being more evident in the reverse view of the counterbalancing arrangement 90, as shown enclosed in the inset of FIG. 5. These springs 96 generate a force acting between the first arm 92 and the second arm 94, in particular exerting a moment of force (or torque) on the first arm 92 about the revolute joint 98, which in turn exerts a counterbalance force on the platform 12 via the ball 91 and the retaining element 26. As the platform 12 moves in the horizontal plane, the second arm 94 swings about the revolute joint 95, thereby causing the hinge 93 to move in a similarly wide arc in the horizontal plane. The hinge 93 and spring 96 also rotate about the revolute joint 97 as the platform 12 moves in the horizontal plane, so that the separation between the spring 96 and the revolute joint 95 changes depending on where the platform 12 is in its range of motion. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] U.S. Patent No. 6,058,618 [Patent Document 2] U.S. Pat. No. 5,257,461 [Patent Document 3] U.S. Pat. No. 4,964,221 [Patent Document 4] U.S. Patent No. 4,229,136 [Patent Document 5] International Publication No. 2017 / 174966 [Patent Document 6] International Publication No. 2007 / 144573 [Patent Document 7] International Publication No. 2019 / 073246 Summary of the Invention

[0016] Applicant has realised that a balancing arrangement for a non-Cartesian machine such as that illustrated in FIG. 5 faces one or more of the following problems:

[0017] First, the movable arms in such a counterbalanced configuration would extend outside the normal range of motion at some point on the movable platform 12, thereby increasing the overall size of the machine and potentially creating a hazard for the operator. For example, this occurs in the machine of FIG. 5 when the platform 12 is moved laterally toward the revolute joint 95, causing the upper portion of the L-shaped second arm 94 to swing outside the normal range of motion.

[0018] Second, such counterbalance configurations may suffer from non-ideal behavior, particularly in certain portions of their range of motion, such that the weight of the movable platform 12 is not uniformly supported around the range of motion 11. For example, in the balance of Figure 5, the counterbalance force provided by spring 96 varies slightly depending on the z position of platform 12 because of the different spring extensions involved.

[0019] Third, in such a counterbalancing arrangement, the arms 92, 94 and the springs 96 adopt different positions and orientations relative to the movable platform 12 depending on where the movable platform 12 is in the range of motion 11. The springs 96 must be relatively bulky (and therefore relatively heavy) to generate sufficient counterbalancing forces to counterbalance the heavy movable platform 12 (and the tools or components coupled thereto). Even with horizontal movements of the movable platform 12, these heavy springs 96 are moving, and with certain movements of the movable platform 12 at certain positions within the range of motion, the springs 96 may be subject to very large and very non-uniform accelerations. Thus, for these reasons, the dynamic response of the system (e.g., when accelerated) may be very non-uniform. This is a result of the fact that the counterbalancing arrangement of FIG. 5 includes only rotational axes (no linear axes, i.e., no linear degrees of freedom), with at least two rotational axes between the heavy springs 96 and the fixed structure of the machine, resulting in complex movements of the springs 96. While in the balancing arrangement of Figure 5 there are two rotational balancing axes 95, 97 between the force generator (i.e. the spring 96) and the ground (i.e. the fixed structure of the machine), in balancing arrangements of similar known types there can effectively be three rotational balancing axes between the force generator and the ground. This is explained in more detail below with reference to Figures 11 and 44 of the accompanying drawings.

[0020] Finally, z-axis balancing arrangements of the type described above in relation to Cartesian machines are simply not suitable for non-Cartesian machines, since they do not have a fixed z-axis that can be directly balanced. For example, the hexapod coordinate positioning machine 10 shown in Figure 2 effectively has six linear axes (i.e., six drive axes associated with each of the six extendable legs 16), but none of these drive axes are always aligned with gravity, and none of these drive axes are independent of the other drive axes. Rather, movement of the platform 12 in the z direction involves a change in all six drive axes of the hexapod coordinate positioning machine 10 (i.e., a change in the length of all six extendable legs 16), and therefore cannot provide a z-axis balancing arrangement as used with Cartesian machines.

[0021] It is therefore desirable to provide an improved balancing arrangement for coordinate positioning machines, particularly, but not exclusively, for non-Cartesian coordinate positioning machines.

[0022] The coordinate positioning machine that forms the basis of some embodiments of the present invention includes a number of drive axes for positioning a moveable structure within the machine's range of motion, a number of counterbalance axes for balancing the moveable structure, and a force generator for generating a counterbalance force acting on at least one of the counterbalance axes.

[0023] According to one aspect of the present invention, the balancing axis includes a linear balancing axis, and the balancing force acts on the linear balancing axis.

[0024] According to another aspect of the invention, the balance axis and the force generator are mutually configured such that movement of the movable structure in a direction perpendicular to gravity does not result in substantially any (net) movement of the force generator other than that resulting from rotation of the force generator about at most one rotational balance axis.

[0025] According to another aspect of the invention, the balance axis and the force generators are mutually configured such that movement of the movable structure in a direction perpendicular to gravity imparts substantially no (total) work (net) to the force generators other than that resulting from rotation of the force generators about at most one rotational balance axis.

[0026] According to another aspect of the invention, the balance axes are arranged in a row, the balance axes including at least one rotational balance axis, and the force generator is arranged rearward of a first (or only) rotational balance axis in (or of) the row (when viewed in a direction towards the movable structure).

[0027] According to another aspect of the invention, the balance axes are arranged in a row, the balance axes including at least one rotational balance axis, and the force generator is arranged (or positioned) at a predetermined horizontal distance (or separated or offset) from a first (or only) rotational balance axis in (or of) the row (when viewed in a direction towards the movable structure).

[0028] According to another aspect of the invention, the balance shaft is arranged in line with at most one rotating balance shaft (in the balance shafts of the line) between the force generator and a fixed structure of the machine.

[0029] According to another aspect of the present invention, there is provided a coordinate positioning machine including a plurality of drive axes (each of which is either a rotary or linear drive axis) for positioning a moveable structure within a range of motion of the machine, and a linear balancing axis (separate from any of the drive axes) for balancing said moveable structure.

[0030] According to another aspect of the present invention there is provided a method of balancing a coordinate positioning machine having a plurality of drive axes (each of which may be either rotary or linear) for positioning a moveable structure within a range of motion of the machine, the method comprising providing a linear balancing axis (separate from any of said drive axes) and balancing said moveable structure via said linear balancing axis.

[0031] It will be appreciated that the above-described aspects can be used independently of one another or in any suitable combination.

[0032] The embodiment of the invention in which linear balancing axes are employed is particularly advantageous for non-Cartesian coordinate positioning machines in which none of the machine's drive axes have a fixed orientation, and therefore none of the drive axes can be directly balanced. By providing a separate linear balancing axis that can be directly balanced, the machine can take advantage of a simple (but highly effective) linear balancing arrangement, as described below with reference to Figure 6 of the accompanying drawings. Such a balancing arrangement, unlike known balancing arrangements such as those described above with reference to Figure 5, can provide ideal (or nearly ideal) balancing behavior regardless of the position (particularly the z-position) of the movable structure within the range of motion. This is possible in one embodiment of the invention because the orientation of the linear balancing axis is substantially invariant to changes in the orientation of the drive axes, thereby providing a stable and constant reference for the balancing arrangement.

[0033] In embodiments of the invention where the force generator is not moved (or no net work is done on the force generator) when the movable structure is moved horizontally (other than possibly causing rotation of the force generator about at most one rotational balance axis), the drive arrangement thereby does less work (using less energy and generating less heat) compared to, for example, the balance arrangement described with reference to FIG. 5. In this respect, the advantage of moving the force generator as little as possible can be significant, since the force generator is usually relatively heavy (e.g. large counterweights or bulky springs). Even if there is movement of the force generator about at most one rotational axis, the dynamic response can be more controlled and / or less extreme, without abrupt changes in the acceleration of the force generator, compared to a balance arrangement of the type shown in FIG. 5.

[0034] In embodiments where the force generator is located behind the first rotational balance axis in the line of balance axes, there is no rotation of the force generator with any movement of the movable structure, which further reduces the work done by the drive arrangement. Even if it is not located exactly behind the first rotational axis in line, it is still advantageous to locate the force generator close to (preferably along) the rotational balance axis to reduce the amount of work done in rotating the force generator.

[0035] Reducing the amount of work done by the drive is advantageous as it reduces the amount of heat generated by the drive, thereby reducing the adverse effects that thermal expansion and contraction of machine parts have on the measurement results. In the case of a hexapod coordinate positioning machine, the drive struts are stronger in the vertical direction than in the horizontal direction due to their predominantly vertical orientation, so it is beneficial to avoid making the drive arrangement do extra work, especially for horizontal movements of the platform.

[0036] Unless already so stated, the balance axes may be arranged in rows.

[0037] Unless already so stated, a coordinate positioning machine may comprise at most one rotating balance axis (in the array of balance axes) between the force generator and the fixed structure of the machine.

[0038] Unless already so stated, the balance axis can include at least one linear balance axis.

[0039] The balance axes include at least two linear balance axes.

[0040] The balance axis includes two linear balance axes.

[0041] The or each linear balance axis may have a substantially constant orientation relative to gravity.

[0042] The coordinate positioning machine may include a linkage for transmitting the counterbalancing force to the moveable structure.

[0043] The linkage may be a rigid linkage.

[0044] The linkage may be a mechanical linkage.

[0045] The linkage may include a number of rigid interconnected links connected by a number of joints.

[0046] The linkage may be adapted to impart movement of the moveable structure in a direction perpendicular to gravity while causing the counterbalancing forces to do substantially no work.

[0047] The linkage and the force generator may be mutually configured to maintain a counterbalancing force in a substantially constant orientation relative to gravity as the moveable structure moves through a range of motion.

[0048] The counterbalancing force transmitted by the linkage to the moveable structure may be substantially aligned with gravity.

[0049] The linkage may be separate and / or independent of any of the drive shafts.

[0050] The force generator and the linkage may be disposed substantially in a plane relative to one another and remain so as the moveable structure moves through the range of motion.

[0051] The plane may be substantially aligned with gravity.

[0052] The linkage and the force generator may be mutually configured such that there is substantially no change in direction of the counterbalanced forces as the moveable structure moves through the range of motion.

[0053] The linkage may remain substantially configurationally unchanged during vertical movement of the moveable structure.The linkage may move as a whole during vertical movement of the moveable structure.

[0054] The balance axes may include first and second linear balance axes disposed substantially perpendicular to one another.

[0055] The second linear balance axis may be provided by the linkage.

[0056] The linkage may be adapted to effect a change in substantially vertical motion from linear to linear.

[0057] The linkage may be a Scott-Russell linkage. The linkage may be a Scott-Russell type linkage.

[0058] The balancing force may act on a predetermined linear balancing axis of the plurality of balancing axes.

[0059] The orientation of the predetermined linear balance axis may be substantially invariant regardless of any changes in the orientation of the drive shaft.

[0060] The orientation of the predetermined linear balance axis may be substantially invariant to operation of the drive shaft.

[0061] The predetermined linear balance axis may have a substantially constant orientation relative to gravity.

[0062] The predetermined linear balance axis may be substantially aligned with gravity.

[0063] The predetermined linear balance axis may be the first linear balance axis.

[0064] The predetermined linear balance axis may be rearward of any rotational balance axis in the row of balance axes (viewed in the direction towards the movable structure).

[0065] The predetermined linear balance axis may be forward of at least one rotational balance axis in a row of balance axes (seen in the direction towards the movable structure).

[0066] The linkage may be adapted to absorb horizontal movement of the movable structure and convert vertical movement of the movable structure into corresponding vertical movement of the linkage along the predetermined linear balance axis, which may be vertical movement of the entire linkage along the predetermined linear balance axis without substantially changing its configuration.

[0067] The linkage may be adapted to maintain a counterbalance force in a direction substantially parallel to the predetermined counterbalance axis.

[0068] The moveable structure may be balanced along the predetermined linear balance axis via the linkage.

[0069] At least a portion of the weight of the moveable structure may be supported through the linkage by a substantially constant balancing force (or a component thereof) acting on the linkage along the predetermined linear balance axis.

[0070] The coordinate positioning machine may include at least one support member coupled (e.g. slidably) to the predetermined linear balancing axis. A balancing force may be arranged to act on the at least one support member, and the linkage may be connected (or coupled) between the moveable structure and the at least one support member (for transferring the balancing force from the at least one support member to the moveable structure).

[0071] The at least one support member may be configured to slidably move along the predetermined linear balance axis (eg, along a linear trajectory).

[0072] The or each support member may provide a balance axis of rotation for the plurality of balance axes.

[0073] Vertical movement of the movable structure by the drive arrangement can be translated by the linkage into corresponding vertical movement of the at least one support member such that a balancing force applied to the at least one support member causes it to perform work substantially equal but opposite to the work done by gravity acting on the movable structure.

[0074] A first value can be considered substantially equal to a second value if the first value is less than or equal to 20% of the second value, or more preferably less than or equal to 10% of the second value, or more preferably less than or equal to 5% of the second value, or more preferably less than or equal to 1% of the second value.

[0075] Horizontal movement of the movable structure by the drive arrangement is translated by the linkage into maximum vertical movement of the at least one support member such that the force(s) applied to the at least one support member substantially does no work overall.

[0076] The coordinate positioning machine may include first and second support members each configured for slidable movement along a track substantially aligned with gravity. Movement of the movable structure in a direction perpendicular to the or each track may be translated by the linkage into movement of the second support member along a track with substantially no movement of the first support member along the track. Movement of the movable structure in a direction parallel to the or each track may be translated by the linkage into movement of at least the first support member along the track in the same direction and by the same amount.

[0077] A balancing force may be applied to the first support member and not to the second support member.

[0078] The force generator generates a force Fi is applied to each support member i in a direction substantially opposite to gravity, i F i 〜mg, and the link mechanism converts the displacements dx, dy, dz in the movable structure into the displacements dx i ,dy i ,dz i and thereby change or convert it into Σ i F i dz i ~mg.dz,dx i ~0 and dy i 0, where m is the mass of the movable structure, g is the gravitational acceleration, i is an integer equal to or greater than 1, and at least one F i is zero.

[0079] A counterbalancing force generated by the force generator may act between the at least one support member and a fixed structure of the machine.

[0080] Unless already so stated, the balance axis may include at least one rotating balance axis.

[0081] The balance shaft may include a single rotating shaft (e.g., excluding any that may be provided as part of a universal joint) or four rotating balance shafts (e.g., three of which may be provided as part of a universal joint).

[0082] Unless already so stated, the force generator may be positioned in the row (when viewed in a direction towards the movable structure) at a predetermined horizontal distance behind (or separate or offset from) a first rotational balance axis of the balance axis (or of the balance axis of the row).

[0083] The force generator may be disposed close to the first rotational balance axis, i.e. the predetermined horizontal distance may be small, the predetermined horizontal distance may be substantially zero.

[0084] The predetermined horizontal distance may be less than 50% (or more preferably less than 25%, or more preferably less than 10%, or more preferably less than 5%) of the typical horizontal distance (or separation or offset) between the first rotational balance axis and the movable structure, which may be any (normal or practical) position of the movable structure within its range of motion.

[0085] The representative horizontal distance (between the first rotational balance axis and the moveable structure) may be determined relative to a representative point of the moveable structure, such as the geometric centre of the moveable structure or the centre of gravity of the moveable structure.

[0086] Alternatively, the representative horizontal distance may be the distance between the first rotational balance axis and the centre of the range of motion, or may be a representative width of the range of motion.

[0087] The predetermined horizontal distance (between the first rotational balance axis and the force generator) may be determined relative to a representative point of the force generator, such as the geometric centre of the force generator or the centre of gravity of the force generator.

[0088] The predetermined horizontal distance (between the force generator and the first rotational balance axis) may be substantially constant, and this distance may be maintained as the movable structure moves through a range of motion.

[0089] Unless already so stated, the balance axis and the force generators may be mutually configured such that movement of the movable structure in a direction perpendicular to gravity causes substantially no net movement other than rotation of the force generators about at most one rotational balance axis.

[0090] Unless already so stated, the balance axis and the force generator may be mutually configured such that movement of the movable structure in a direction perpendicular to gravity does substantially no work on the force generator other than rotation of the force generator about at most one rotational balance axis.

[0091] Said at most one rotational balance axis may be the first (or only) rotational balance axis among the balance axes of said row (when viewed in the direction towards the mobile structure).

[0092] Each of the multiple drive shafts may be either a rotary drive shaft or a linear drive shaft.

[0093] Each of the multiple balance axes can be either a rotational balance axis or a linear balance axis.

[0094] The balance shaft may be separate from the drive shaft.

[0095] Each balancing shaft may be separate and / or independent and / or distinct from each of said drive shafts.

[0096] The counterbalancing force generated by the force generator may be substantially aligned with gravity.

[0097] The or each balancing axis about which the balancing force acts may have a substantially constant orientation relative to gravity.

[0098] The or each balancing axis about which the balancing force acts may be substantially aligned with gravity (eg in the case of a linear balancing axis) or may be substantially perpendicular to gravity (eg in the case of a rotational balancing axis).

[0099] The counterbalance shaft and the force generator may be mutually configured such that movement of the moveable structure in a direction parallel to gravity causes work to be done on the force generator substantially equal to work done by gravity acting on the moveable structure.

[0100] As discussed above, a first value can be considered substantially equal to a second value if the first value is less than or equal to 20% of the second value, or more preferably less than or equal to 10% of the second value, or more preferably less than or equal to 5% of the second value, or more preferably less than or equal to 1% of the second value.

[0101] The force acting on the balance axis may be at least a component of the actual balance force generated by the force generator, for example, the force acting on the balance axis may be a component of a balance force relative to the balance axis (e.g., along a linear balance axis or about a rotational balance axis).

[0102] Applying a balancing force to a balance axis can include applying a balancing force along or about the balance axis (eg, along a linear balance axis or about a rotational balance axis).

[0103] The counterbalance shaft may be passive and / or non-active and / or undriven and / or unpowered.

[0104] Balancing the moveable structure can include providing a counterbalance arrangement for the moveable structure. Balancing the moveable structure can include supporting at least a portion of the weight of the moveable structure.

[0105] A balancing force acting on a balancing shaft can be transmitted to the movable structure via at least one of the other balancing shafts.

[0106] The counterbalancing force may be transmitted through a linkage.

[0107] The counterbalance force may be a substantially constant counterbalance force.

[0108] The force generator may be adapted to generate a substantially constant counterbalancing force.

[0109] The counterbalance force (or at least its magnitude) may be substantially equal to the force of gravity (or at least its magnitude) acting on the moveable structure. The force generator may be adapted to generate a counterbalance force (of a magnitude) substantially equal to the force of gravity acting on the moveable structure.

[0110] The force generator may include a counterweight that generates tension in a string or cord or cable or wire or the like coupled thereto.

[0111] The force generator may comprise a spring that generates a spring force. The spring may be a constant force spring, such as a fusee spring.

[0112] Each balancing axis (except those that form part of a universal joint) can have (or maintain) a substantially constant orientation relative to gravity (as the movable structure moves through its range of motion).

[0113] None of the drive axes may have (or maintain) a substantially constant orientation relative to gravity (as the moveable structure moves through its range of motion).

[0114] A balancing shaft may be located between the movable structure and a fixed structure of the machine.

[0115] A drive shaft may be located between the moveable structure and a fixed structure of the machine.

[0116] The machine may include a stationary structure.

[0117] Relative motion about a rotational (drive or balance) axis can be sensed by a corresponding rotational transducer.

[0118] Relative motion along a linear (drive or balance) axis can be sensed by a corresponding linear transducer.

[0119] A drive shaft may form part of a drive arrangement for moving the movable structure through a range of motion, and a counterbalance shaft forms part of a counterbalance arrangement for supporting at least part of the weight of the movable structure.

[0120] The machine includes a fixed structure, and the drive arrangement and the counterbalance arrangement may be connected separately and / or in parallel between the movable structure and the fixed structure.

[0121] The drive arrangement and the counterbalance arrangement may be separately connected to the moveable structure.

[0122] The balancing arrangement may be configured such that at least 80% of its total mass (or the mass of the force generator) is within 10% of the horizontal spacing between the (linear and / or first rotational) balancing axis (of the row balancing axis) and the center of the range of motion.

[0123] The drive arrangement may include a number of extendable legs connected between the movable structure and a fixed structure of the machine.

[0124] The extendable legs may be oriented (on average) closer to vertical rather than horizontal (at least with respect to the central position of the movable structure within the range of motion).

[0125] The coordinate positioning machine may be equipped with six such extendable legs.

[0126] A linear (drive or counterbalance) axis may be an axis that provides a linear degree of freedom (for a drive or counterbalance configuration, as the case may be).

[0127] A linear (drive or counterbalance) axis may be an axis that provides constrained relative linear motion between two machine elements (in a drive or counterbalance configuration, as the case may be).

[0128] A rotational (drive or counterbalance) axis may be an axis that provides a rotational degree of freedom (for a drive or counterbalance configuration, as the case may be).

[0129] A rotational (drive or counterbalance) axis may be an axis that provides constrained relative rotational motion between two machine elements (in a drive or counterbalance configuration, as the case may be).

[0130] A counterbalancing force may be generated along an axis that maintains a substantially constant alignment with or relative to gravity as the moveable structure moves through its range of motion.

[0131] The coordinate positioning machine may include a brake to hold the moveable structure in the same position within the range of motion when the drive axes are not controlled to move the moveable structure within the range of motion.

[0132] Each of the drive shafts may be a linear drive shaft.

[0133] The drive shafts may be associated with respective extendable legs.

[0134] The coordinate positioning machine may be equipped with a metrology arrangement for measuring the position of the moveable structure within the range of motion.

[0135] The measurement arrangement may be adversely affected by heat generated by the drive arrangement.

[0136] The drive arrangement and the measurement arrangement may be separately connected between the movable structure and the fixed structure.

[0137] The measurement arrangement may include a number of length measurement transducers.

[0138] Each length measurement transducer may include an encoder scale paired with a readhead.

[0139] The transducers may each be associated with an extendable leg.

[0140] The coordinate positioning machine may be a non-Cartesian coordinate positioning machine, the drive axes of which may not be orthogonal to each other.

[0141] The coordinate positioning machine may be a parallel kinematic coordinate positioning machine. The drive axes of the coordinate positioning machine may be arranged in parallel.

[0142] The coordinate positioning machine may be a hexapod coordinate positioning machine.

[0143] The moveable structure may include a moveable platform and at least one tool supported on the moveable platform.

[0144] The tool may be or may include a measurement probe.

[0145] The coordinate positioning machine may be a coordinate measuring machine.

[0146] According to another aspect of the present invention, there is provided a coordinate positioning machine including a moveable structure, a drive arrangement for moving the moveable structure through a range of motion of the machine, and a counterbalance arrangement for supporting at least a part of the weight of the moveable structure (otherwise supported by the drive arrangement).

[0147] The counterbalancing arrangement may be rotatable about a counterbalancing axis, which may be a fixed counterbalancing axis. The counterbalancing arrangement may comprise a generator for generating a counterbalancing force and a linkage for transmitting the counterbalancing force to the movable structure.

[0148] A counterbalanced configuration (e.g., a linkage of a counterbalanced configuration) may be configured and / or adapted to provide (or provide, or permit, or enable) at least one of the following: (a) relative movement between the moveable structure and the balance axis in a direction perpendicular to gravity and / or perpendicular to the balance axis, without substantially (by the drive arrangement) causing the force generator to do (net) work and / or without substantially moving or being moved and / or without substantially causing (net) movement of the force generator; and (b) movement of the movable structure (e.g., relative movement between the movable structure and the balancing axis, optionally in a direction perpendicular to gravity and / or perpendicular to the balancing axis) without substantially changing (or modifying or affecting) the distance spacing (or offset or distance) and / or the relative orientation between the force generators and / or the balancing forces (generated by the force generators) and the balancing axis (or gravity, in the case of a relative orientation).

[0149] Movement of the moveable structure through a given angle about a balance axis may result in movement of the linkage and / or force generator and / or balancing force through a corresponding angle about the balance axis.

[0150] By vertically moving the movable structure a predetermined amount within its range of motion, the linkage can be caused to move (eg, translate or slide) a corresponding amount vertically.

[0151] The force generators and / or counterbalancing forces may be substantially disposed in or on or along a plane including the counterbalancing axis and the connection between the linkage and the moveable structure, and this arrangement may remain as the moveable structure moves through a range of motion.

[0152] The force generators and / or counterbalancing forces, the counterbalancing axis and the linkage may be arranged substantially in a plane, and this arrangement may remain as the moveable structure moves through a range of motion.

[0153] The balancing axis may be located outside the range of motion, for example to the side of the range of motion.

[0154] The linkage may be configured such that due to such movement of the movable structure relative to the balance axis, substantially no (net or total) work is (done or) performed by (the balance force generated by) the force generator.

[0155] The linkage may be configured such that such movement of the moveable structure relative to the balance axis results in substantially no (net) movement of the force generator in a direction perpendicular to gravity.

[0156] The linkage may be configured such that for movement of the movable structure in a direction aligned with gravity, the work done by (the balancing force generated by) the force generator is substantially equal but opposite to the work done by gravity acting on the movable structure.

[0157] The linkage may be adapted to decouple movement of the moveable structure relative to the balance axis from the force generator.

[0158] The force generator may be located on or adjacent to the balance shaft.

[0159] The force generator may be (closely) associated with the balance shaft.

[0160] The force generator may be coupled to the counterbalance shaft.

[0161] At least a portion of the force generator may be disposed on or along the balance axis.

[0162] The counterbalance arrangement or force generator may have at least 80% of its total mass within 10% of the horizontal separation between the counterbalance axis and the centre of range of motion.

[0163] The (net) distance traveled by the force generator (e.g. under the action of forces resulting from, resulting from or caused by the drive arrangement) may be less than 10% of the distance traveled by the movable structure during such movement relative to the balance axis.

[0164] The linkage may be adapted to decouple or separate movement of the movable structure relative to the balance axis from at least one support member to enable it to maintain a substantially constant orientation relative to gravity.

[0165] The distance between the force generator and the balance axis may be determined based on a representative point of the force generator, such as its geometric center or center of gravity. In this regard, the force generator may include multiple parts that move relative to one another, such that a movement of one part causes a corresponding movement of another part in an opposite direction such that the center point does not move.

[0166] The linkage may be configured and / or adapted to enable (or impart or permit) movement of the movable structure relative to the balance axis in a direction perpendicular to gravity without any (net or total) work being done (or performed) on the force generator (by the drive arrangement).

[0167] The linkage may be configured and / or adapted to enable (or cause or permit) the movable structure to move or be moved (by the drive arrangement) relative to the balance axis in a direction perpendicular to gravity without causing (by the drive arrangement) any (net or total) work to be done (or performed) on the force generator.

[0168] The linkage may be configured and / or adapted such that, due to movement of the movable structure relative to the balance axis in a direction perpendicular to gravity, substantially no work (net or total) is done (or is caused to be done or absorbed) on the force generator (by the drive arrangement).

[0169] The linkage may be configured and / or adapted such that for movement of the movable structure in a direction perpendicular to gravity, there is substantially no movement (or at least no net movement) of the force generator relative to the balancing axis and substantially no work is done (or caused to be done) by the force generator (or balancing force).

[0170] The linkage may be configured and / or adapted such that movement of the movable structure in a direction perpendicular to gravity results in substantially no work being done on or by the force generator, at least when movement is also in a direction towards / away from the balance axis (or along a line intersecting the balance axis).

[0171] The linkage may be configured and / or adapted such that for movement of the movable structure in a direction perpendicular to gravity, substantially no work is done on the force generator by movement relative to the balance axis and substantially no work is done (or caused to be done) by the force generator.

[0172] The linkage may be configured and / or adapted such that for movement of the movable structure in a direction perpendicular to gravity, there is no substantial net movement of the force generator relative to the balance axis and substantially no work is performed (or caused to be performed) by the force generator.

[0173] The linkage may be configured and / or adapted such that for movement of the moveable structure in a direction perpendicular to gravity, substantially no work is done (or caused to be done) on or by the force generator.

[0174] The linkage may be configured and / or adapted to (at least partially) isolate such movement of the moveable structure from the force generators such that no work is performed or carried out on the force generators.

[0175] The linkage may be configured and / or adapted to maintain a balancing force in a direction substantially along or parallel to the balance axis when the movable structure is moved through a range of motion by the drive arrangement.

[0176] The force generator may alternatively be referred to as a (balanced) force source or a (balanced) force origin.

[0177] Movement of the movable structure relative to something else (e.g. a balance axis) can be considered to relate to movement of a particular part of the movable structure or a point on the movable structure, for example a connection between the movable structure and the linkage (the part or point or location where the linkage connects or joins to the movable structure), or the centre of gravity of the movable structure, or the centre of a platform forming part of the movable structure.

[0178] Movement relative to the balancing axis may be considered to be movement (directly) towards or away from the balancing axis. Movement relative to the balancing axis may be considered to be movement in a direction (or along a line) that intersects the balancing axis. Movement relative to the balancing axis may be considered to be radial movement relative to or with respect to the balancing axis. Movement relative to the balancing axis may be considered to be movement along a radius extending from the balancing axis. Movement relative to the balancing axis may be considered to be movement along a line extending radially from the balancing axis.

[0179] Work done in a force generator is distinguished from work done by the force generator. Work done by a force generator is work done by a force generated internally by the force generator itself (note that the work done can be negative); if the generated force is applied to an object, the work done by the force generator (or the force generated by the force generator) is equal to the generated force multiplied by the distance traveled by the object with the force applied. On the other hand, work done in a force generator is work done by a force generated external to the force generator and applied to the force generator; work done in a force generator is equal to the applied force multiplied by the distance traveled by the force generator with the force applied. If the force generator does not move at all due to a specific movement of the movable structure, because otherwise the force generator is being moved (accelerated), no work is being done in the force generator. As will become clear from the detailed description below, there may be relative internal movements of parts of the force generator, as long as there is no overall net movement (so that no net work is done). For the known counterbalanced arrangement described above with reference to FIG. 5, since there is no change in spring length, no work is done by the spring (force) for horizontal platform movement, but as the spring moves (accelerates), work needs to be done on the spring (i.e., force generator) by the drive strut.

[0180] The term "position" as used herein may be taken to mean a position and / or an orientation. The term "vertical" as used herein may be taken to mean a direction substantially parallel to the force of gravity. The terms "horizontal" or "lateral" as used herein may be taken to mean a direction substantially perpendicular to the force of gravity.

[0181] To distinguish from cases where the stated characteristics are shown only to a small portion of the artificially limited movements of the movable structure, i.e., not to the full range of types and ranges of movements that would typically occur in normal work operations, the stated characteristics of the balanced configuration may be considered to apply to all possible movements of the movable structure (or at least movements subject to any stated limitations, such as movements perpendicular to gravity), such that at least one continuous portion of the range of motion is less than 25% of the range of motion, more preferably less than 50% of the range of motion, more preferably less than 75%, and more preferably the entire range of motion.

[0182] Where a force generator includes multiple parts (e.g., multiple counterweights) that are capable of moving relative to one another, reference to no movement of the force generator may be taken to mean no total or net movement of the parts that form the force generator. For example, if one part (e.g., a counterweight) moves up and the other moves down, it may be considered that there is no total or net movement. It will also be understood that mere rotation of a part (e.g., a pulley) does not amount to movement of the part in this context, since there is no movement of the center of the part.

[0183] Alternatively, the force generator can be considered to have a representative point based on which the position of the force generator (and therefore the movement of the force generator) can be determined. For example, the representative point can be the geometric center or center of gravity of the force generator, or a weighted average position of multiple parts taking into account the respective forces acting on the parts (e.g., a force generator can include one counterweight supported by a single pulley arrangement and another counterweight of twice the mass supported by a dual pulley arrangement).

[0184] Where there is reference to substantially no movement of a first part caused by a particular movement of a second part (optionally relative to a third part), this may be understood to mean that the movement of the first part is less than 10% of the movement of the second part (optionally relative to the third part), more preferably less than 5%, more preferably less than 1%.

[0185] Similarly, where there is reference to substantially no work being done in a first part due to a particular movement of a second part, this may be understood to mean that the work done in the first part is less than 10%, more preferably less than 5%, more preferably less than 1% of the work done in the second part by such movement. [Brief description of the drawings]

[0186] Reference is now made, by way of example, to the accompanying drawings in which: [Figure 1] FIG. 1 is a schematic diagram of a Cartesian coordinate positioning machine, previously described. [Diagram 2] FIG. 2 is a schematic diagram of the non-Cartesian coordinate positioning machine previously described. [Diagram 3] FIG. 3 is a schematic diagram of a bottom-up configuration for a non-Cartesian coordinate positioning machine, corresponding to the configuration of FIG. 2, previously described. [Figure 4] FIG. 4 is a schematic diagram of a top-down configuration, alternative to the bottom-up configuration of FIG. 3, previously described. [Diagram 5] FIG. 5 illustrates a known type of balancing arrangement for a non-Cartesian coordinate positioning machine, as previously mentioned. [Figure 6] FIG. 6 shows diagrammatically a balancing arrangement for a non-Cartesian machine that is stable in the z direction but not in the x and y directions, an arrangement which does not form an embodiment of the invention. [Figure 7] FIG. 7 shows diagrammatically a balancing arrangement for a non-Cartesian machine that is stable in the z direction but not in the x and y directions, an arrangement which does not form an embodiment of the present invention. [Figure 8] FIG. 8 shows a more schematic representation of the counterbalanced arrangement of FIGS. [Figure 9] FIG. 9 shows a general force generator that is an alternative to the pulley and counterweight arrangement shown in FIG. 8 and provides a more generalized representation of FIG. [Figure 10]FIG. 10 is a schematic diagram of a coordinate positioning machine according to an embodiment of the invention which is a development of the scheme shown in FIG. [Figure 11] FIG. 11 is a schematic diagram of the known counterbalancing arrangement of FIG. 5 shown in a similar format to that of FIG. 10 for ease of comparison. [Figure 12] FIG. 12 is a schematic diagram, in a similar format to that of FIG. 10 for ease of comparison, of a known counterbalancing arrangement for the z-axis of a Cartesian coordinate positioning machine such as that shown in FIG. [Figure 13] FIG. 13 is a schematic diagram of a known balancing arrangement for the z-axis of a Cartesian coordinate positioning machine, in a similar format to FIG. 10 for ease of comparison, but having a different x-, y- and z-axis arrangement than FIG. 12. [Figure 14] FIG. 14 is a side view of a non-Cartesian coordinate positioning machine with a counterbalanced arrangement embodying the present invention. [Figure 15] FIG. 15 is a partial top view of the non-Cartesian coordinate positioning machine shown in FIG. [Figure 16] FIG. 16 shows a balancing arrangement with linear and rotational balancing axes arranged differently than in FIGS. [Figure 17] FIG. 17 is a more schematic representation of the machine of FIGS. 14, 15 and 16, which forms the basis of an analysis of the inherent stability of the balanced configuration. [Figure 18] FIG. 18 is a diagram mainly showing the link mechanism and platform of FIG. 17 for considering the forces acting on the parts. [Figure 19] FIG. 19 illustrates how lateral movement of the platform results in only vertical movement of the upper support, but not the lower support. [Figure 20] FIG. 20 is based on the schematic representation shown in FIG. 10 and illustrates that the support member in FIG. 10 is a set of one or more support members in FIG. [Figure 21]FIG. 21 corresponds to FIG. 18 and illustrates an alternative linkage configuration that is not suitable for use in embodiments of the present invention. [Figure 22] FIG. 22 corresponds to FIG. 19 and illustrates an alternative linkage configuration that is not suitable for use in embodiments of the present invention. [Figure 23] FIG. 23 corresponds to FIG. 18 and illustrates an alternative linkage configuration that is not suitable for use in embodiments of the present invention. [Figure 24] FIG. 24 corresponds to FIG. 19 and illustrates an alternative linkage configuration that is not suitable for use in embodiments of the present invention. [Diagram 25] FIG. 25 is a view similar to FIG. 19 illustrating how the platform moves when the upper support member is moved vertically while the lower support member is fixed in place. [Figure 26] FIG. 26 is a view similar to FIG. 22 illustrating how the platform moves when the upper support member is moved vertically while the lower support member is fixed in place. [Figure 27] FIG. 27 is a view similar to FIG. 24 showing how the platform moves when the upper support member is moved vertically while the lower support member is fixed in place. [Figure 28] FIG. 28 shows an alternative "upside-down" configuration to that of FIG. 17, based on the same principles of operation. [Figure 29] FIG. 29 shows an alternative "longer range" configuration to that of FIG. 28, based on the same principles of operation. [Diagram 30] FIG. 30 illustrates an embodiment similar to FIG. 29, but using a slightly different linkage mechanism. [Diagram 31] FIG. 31 shows a "double pulley" alternative to the arrangement of FIG. 17, based on the same operating principle. [Diagram 32] FIG. 32 is a diagram showing how the links forming the linkage arrangement of FIG. 18 are actually shaped. [Diagram 33]FIG. 33 is a diagram showing how the links forming the linkage arrangement of FIG. 30 may actually be shaped. [Diagram 34] FIG. 34 illustrates how the platform can be supported by two counterbalancing configurations, each providing only a portion of the counterbalancing force, in the two types of examples shown in FIG. [Diagram 35] FIG. 35 shows an alternative counterbalanced configuration in which forces are applied to both supports, and both supports move vertically but in opposite directions as the platform moves laterally. [Diagram 36] FIG. 36 is a diagram showing a modification of the configuration of FIG. [Figure 37] FIG. 37 shows an alternative counterbalanced arrangement to that shown in FIG. 35, based on the same operating principle, which also includes a double pulley arrangement associated with each support. [Figure 38] FIG. 38 illustrates, in an example related to the embodiment of FIG. 37, how a constant force spring, such as a fussy spring, can be used in any of the previous embodiments. [Figure 39] FIG. 39 illustrates the use of a constant force spring in the embodiment of FIG. [Diagram 40] FIG. 40 illustrates a different form of linkage to that shown in the previous embodiment, having a telescoping tube and also including a double pulley arrangement. [Diagram 41] FIG. 41 illustrates how a balancing arrangement embodying the principles of operation shown in the previous embodiments may also be applied to balancing Cartesian coordinate positioning machines. [Diagram 42] FIG. 42 illustrates the balancing arrangement of FIG. 16 used to balance a serial kinematic non-cartesian robot arm. [Diagram 43] FIG. 43 illustrates a Cartesian balancing arrangement embodying the invention which may be used to balance Cartesian or non-Cartesian coordinate positioning machines. [Diagram 44]FIG. 44 shows an embodiment based on FIG. 5, but modified so that the origin of the balancing forces coincides with the first rotational balancing axis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0187] Figure 6 is a schematic diagram of a simplified balancing arrangement for a non-Cartesian coordinate positioning machine as described above with reference to Figures 2 to 5. Balancing arrangements embodying the present invention may be considered to be inspired by that shown in Figure 6, even though the balancing arrangement of Figure 6 does not itself constitute an embodiment of the present invention. The example of Figure 6 is limited to movement of the platform 12 in two dimensions (x and z) in the plane of the page, i.e. the balancing arrangement shown cannot accommodate a platform 12 moving throughout its full three-dimensional range of motion. Indeed, as will be explained in more detail below, the balancing arrangement of Figure 6 is not even suitable for balancing movement of the platform 12 in the x-direction, but rather it is the response of the balancing arrangement to movement of the platform 12 in the z-direction that is of interest.

[0188] Unlike the known counterbalance arrangement described above with reference to Figure 5, which suffers from non-ideal counterbalance performance for different z positions of the moveable platform 12, Applicant has realised that a simple counterbalance arrangement as shown in Figure 6 is desirable as it provides a constant counterbalance response regardless of the z position of the moveable platform 12. In this regard, a counterweight 30 is provided having a mass that matches exactly (or matches as closely as substantially possible) that of the moveable structure (i.e. the moveable platform 12 and measurement probe 13) being moved by the drive arrangement. The drive arrangement includes a plurality of extendable legs 16, each having an associated linear drive shaft 61.

[0189] Because the range of motion within which the platform 12 operates must be substantially free of obstructions, the counterweight 30 is conveniently positioned off to the side of the range of motion, and the platform 12 is connected to the counterweight 30 via a string 32 extending over two pulleys 46 with sufficient lateral spacing to provide an adequate lateral spacing between the platform 12 and the counterweight 30.

[0190] The force F applied to an object to achieve a given acceleration a must be equal to its mass m multiplied by the acceleration a. Thus, when an object falls under the action of gravity, the force F acting on the object is equal to its mass m multiplied by the acceleration due to gravity, g, or F = mg, where g is the gravitational constant.

[0191] Thus, assuming no support from the extendable legs 16, the force of gravity mg acts downward on both the platform 12 and the counterweight 30, creating a corresponding tension in the string 32 which results in a similar upward force mg, thereby causing the system to be perfectly balanced in the z-direction.

[0192] Furthermore, unlike the known counterbalancing arrangement shown in Figure 5, the exact same counterbalancing force acts regardless of the z position of the platform 12, i.e. the system is not subject to the changes dz shown in Figure 6. Also, the mass of the counterweight 30 can be conveniently adjusted by adding or removing mass (e.g. in the manner of counterweights used in classic weighing scales) as the mass of the moveable structure changes (e.g. when tools of different mass are attached to the platform 12).

[0193] However, as illustrated in Figure 7, this counterbalancing arrangement is certainly not stable against motion in the x direction towards and away from the counterweight. If there is a change dx in the x position of the platform 12, then because the string 32 is of fixed length, the counterweight 30 will have to rise, and in so doing work will be involved. The concept of work as applied to counterweight systems will be considered in more detail below when considering the operation of embodiments of the present invention.

[0194] Thus, following a movement such as -dx illustrated in FIG. 7, when the platform 12 is released, the counterweight 30 will naturally tend to return to its previous (lower energy) state, as shown in FIG. 6, and the platform will swing inward (in the x direction) back to center (directly beneath the left pulley 46).

[0195] Thus, despite some useful properties, the counterweight arrangements shown in Figures 6 and 7 are not particularly suitable for use in stabilising a moveable structure in a coordinate positioning machine, because the moveable structure in such a machine would ideally be stable in x, y and z in the absence of drive from the drive arrangement, and not just along a single axis in the z direction.

[0196] FIG. 8 gives a more schematic representation of the counterbalance arrangement of FIGS. 6 and 7, showing a movable platform P and counterweight C connected via a pulley arrangement supported (connected) by a fixed structure of the machine, which is itself conveniently referred to as connected to the “ground” G. The movable platform P is separately connected to the ground G via a drive arrangement D and a measurement arrangement M. The drive arrangement D is responsible for moving or driving the platform P to the range of motion, and the measurement arrangement is responsible for measuring the position of the platform P in the range of motion. The multiple drive axes of the drive arrangement D are represented diagrammatically as multiple lines (corresponding, for example, to the six drive struts or extendable legs 16 of the machine of FIG. 2, or the x, y, z drive axes of the machine of FIG. 1), and similarly the measurement axes of the measurement arrangement M, which can be parallel or column movement axes at orthogonal or non-orthogonal.

[0197] As mentioned above, the measurement arrangement M may be formed integrally with the drive arrangement, for example as shown in FIG. 2, or may be at least somewhat decoupled from the drive arrangement D, for example as described in US Pat. No. 5,399,323 and US Pat. No. 5,499,633. Thus, although the measurement arrangement M and the drive arrangement D are shown as separate in the representation of FIG. 8, each separately connected to the ground G, in practice they may be one and the same (i.e., a combination of the drive arrangement and the measurement arrangement D / M). It will also be understood that the platform P represents not only the platform 12, but also anything supported by the platform 12 (e.g., a tool, e.g., a measurement probe, etc.). With this in mind, the term "movable platform" in this specification can be considered to be the same as and interchangeable with "movable structure".

[0198] FIG. 9 gives a more generalized representation of FIG. 8, where the pulley and counterbalance arrangement shown in FIG. 8 is replaced (or represented) by a force generator Q that generates a counterbalance force mg acting between the platform P and the ground G. The representation in FIG. 9 is completely equivalent to that in FIG. 8, where the counterbalance force mg is the tension in the string on the left side of the pulley arrangement in FIG. 8. The pulley arrangement in FIG. 8 can be considered to be provided simply to reverse the direction of the counterbalance force generated by the counterweight C (which acts downwards, whereas the counterbalance force must act upwards on the platform P). Note that the force acting on the ground G is 2 mg (taking into account also the force required to counteract the tension mg in the string on the right side of FIG. 8 supporting the counterweight C, which, although not shown in the figure, acts upwards on the platform P and counterweight C, the tension in the string acts downwards on the pulley and upwards on the platform P and counterweight C).

[0199] One embodiment of the present invention is shown diagrammatically in Figure 10. Applicants have devised a new machine structure which borrows beneficial aspects of the simple balancing scheme of Figure 9 in terms of perfect (or near perfect) balancing stability in the z (vertical) direction, but which overcomes the problems mentioned above and modifies the scheme of Figure 9 to provide balancing stability in the x and y (horizontal) directions as well.

[0200] 9, a constant counterbalancing force mg is maintained, but in the arrangement of FIG. 10, this force mg does not act directly between the platform P and the ground G, but rather between an intermediate support S and the ground G, with the support S being movable (e.g., slidable) along a vertically disposed linear balancing axis L. The platform P is then supported by the support S via a linkage K. With this arrangement, the platform P is still vertically balanced by the counterbalancing force mg acting along the linear balancing axis L, but not directly, but via the linkage K. Such a machine structure thus provides multiple drive axes that can be used to position the platform P within its range of motion, and also provides a separate linear balancing axis L that can be used to balance the platform P.

[0201] The linkage K transfers the counterbalance force mg generated by the force generator Q to the platform P (or this can be viewed as the linkage K transferring the weight mg of the platform P to the support S). As will be explained in more detail below, the linkage K provides or allows horizontal movement (and counterbalance force mg) of the platform P relative to the support S, without affecting the vertical counterbalance provided by the force mg acting between the ground G and the support S. The linkage K can be viewed as isolating the horizontal movement of the platform P from the support S. These properties of the linkage K allow the simple (and highly effective) vertical counterbalance arrangement of FIG. 6 to be employed even for a platform P that moves both horizontally and vertically within its range of motion.

[0202] Because a constant counterbalancing force mg can be generated (e.g., via a simple counterbalancing arrangement) that counterbalances the weight of the platform P (and any supported components), and this constant counterbalancing force F acts in a constant direction regardless of where the platform P is in its range of motion, the counterbalancing arrangement according to embodiments of the present invention has the advantage of perfect (or near perfect) counterbalancing throughout the entire range of motion, unlike the known counterbalancing arrangement of FIG. 5 .

[0203] In FIG. 10 , it is appropriate to simply refer to the counterbalance force (generated by force generator Q) as F, rather than recognizing that it is not strictly necessary for the counterbalance force F to exactly match the weight mg of the moveable platform P as mg, but rather that it is sufficient to support at least some of the weight mg that would have to be supported by the drive arrangement (thereby reducing the amount of work and heat performed by the drive arrangement).

[0204] To allow full movement of the platform P in all three dimensions within the range of motion, including into and out of the plane of the drawing, the balancing arrangement includes an additional degree of freedom, shown in FIG. 10 as a rotational degree of freedom about the rotational balance axis R. The additional degree of freedom may alternatively be a translational degree of freedom. The additional degree of freedom may be included within or as part of the linkage K itself. It should be noted that with respect to the rotational balance axis R shown in FIG. 10, the linear balance axis L (and the balancing force F) may be offset from (and parallel to) the rotational balance axis R, with the linear balance axis L and the rotational balance axis R being substantially aligned, as will become apparent from the particular embodiment described below with reference to FIG. 14. The linear balance axis L may also be coaxial with the rotational balance axis R, as in the embodiment of FIG. 16.

[0205] The rotational balance axis R can also be in front (rear of axis L) or behind (fore of axis L) the linear balance axis L in the row arrangement (in the direction from the ground G to the platform P). The linear balance axis L can be considered as the first linear balance axis L1, and the linkage K can be considered as providing the second linear balance axis L2 for the balancing configuration. The rotational balance axis R can also be considered as the first rotational balance axis R1, and three additional rotational balance axes R2 / 3 / 4 are provided by the connections between the linkage K and the platform P. Two possible configurations of these axes are shown diagrammatically in the right-hand insert of FIG. 10 and will be described in more detail below with reference to FIGS. 14 and 16, respectively. In both configurations, the balancing force F acts (along) the linear balance axis L1.

[0206] The linkage K and the force generator Q are advantageously configured relative to one another to ensure that, for movement of the platform P in a direction perpendicular to gravity g, substantially no net work is done on the force generator Q (and no net movement of the force generator Q) other than perhaps due to the force generator rotating about at most one rotational balancing axis (which may be located through or at least proximate to the force generator Q). This is advantageous because, to generate a balancing force large enough to support a heavy platform P, the force generator Q needs to be relatively large, with a large mass. To move (accelerate) such a large mass requires a large force to be applied to it (since force equals mass multiplied by acceleration), which means that a significant amount of work is done by the drive arrangement in doing so (since work done on an object equals force applied to an object multiplied by the distance it has moved).

[0207] In the context of coordinate measuring machines which require high accuracy and which are adversely affected by unknown and / or uncontrolled and / or non-uniform heat sources (associated with the thermal expansion of the main components of the machine), it is desirable to reduce the amount of work (heat) done by the drive arrangement. In a hexapod coordinate positioning machine of the type shown in Figure 2, it is particularly advantageous to avoid having the drive arrangement do extra work for the horizontal movement of the platform, since the drive struts 16, due to their physical configuration, are stronger vertically than horizontally (and therefore it is easier to accelerate the platform and associated mass vertically than horizontally).

[0208] In the balanced configuration of Figure 5, even when the platform 12 is moving horizontally, there is movement of the heavy force generator (i.e., spring 96, supported by hinge 93) which requires work to be done by the drive strut 16 on the hinge 93 (to accelerate it). Furthermore, the acceleration of the hinge 93 is not uniform for all positions of the platform 12 in its range of motion, with some movements of the platform 12 causing very large accelerations of the hinge 93 and other movements causing smaller accelerations, which will adversely affect the dynamic response of the coordinate positioning machine. In an embodiment of the invention, there is substantially no work done in the force generator for radial movements of the platform towards and away from the balance axis, and the acceleration is uniform for circumferential movements of the platform about the balance axis, resulting in a more controlled dynamic response.

[0209] Linkage K and force generator Q also advantageously cooperate to maintain counterbalance F in a substantially constant orientation with respect to gravity g (and with respect to counterbalance axis L or R) as platform P is moved through a range of motion by drive arrangement D. This greatly simplifies the technical requirements of force generator Q that allows it to generate a constant counterbalance force F. For example, in the counterbalance arrangement of FIG. 5, for vertical movement of platform 12, arms 92, 94 change angle with respect to each other and the direction of the spring force generated by spring 96 changes. This in turn results in a changing spring force from spring 96 and a changing counterbalance force for platform 12 that will not be constant throughout the range of motion unless complex, large (heavy) constant force springs are used.

[0210] As a comparison with the scheme of FIG. 10, a schematic representation of the known non-orthogonal balancing arrangement of FIG. 5 is shown in FIG. 11. The platform P is connected to the ground G via a combined drive and measurement arrangement D / M, and on the other side, a hinge H is provided on a balancing arm A which moves around a rotation axis R1 as shown, the hinge H also rotates around a rotation axis R2, and the arm B rotates around a rotation axis R3 (plus three additional rotation axes R4-R6 given by universal joints connecting the arm B to the platform P). A force generator Q (e.g. spring 96 in FIG. 5) generates a balancing force F. The arrangement of the balancing axes is shown diagrammatically in the lower box section of FIG. 11, which also shows that the balancing force F acts on (around) the rotation balancing axis R3. It should be noted that the balancing arm A must move around the marked rotation axis R1, and the hinge H must move around the rotation axis R2, even in the case of a movement of the platform P in the plane of the drawing. Such a movement is not possible with a fixed linkage. Unlike the embodiment shown in FIG. 10, in the known counterbalancing arrangement shown in FIG. 11 the counterbalancing force F does not act in a fixed direction and is not directly related to the ground G. Moreover, the force generator Q moves (accelerates) even if there is a movement of the platform P towards or away from the arm A. This means that work is being done on the force generator Q by the drive arrangement D, and such a movement is also not uniform in the range of motion as mentioned above. This is a consequence of the fact that the counterbalancing arrangement in FIG. 11 includes only a rotation axis, and the force generator Q has at least two rotation axes (R1 and R2) between it and the ground G.

[0211] FIG. 12 shows in a similar format a schematic diagram of a known balancing arrangement for an orthogonal machine, such as that shown in FIG. 1. As mentioned above, an orthogonal machine has a simple x, y, z arrangement of orthogonal axes, so that the z axis (i.e., column 8 in FIG. 1) can be separately balanced via a balancing force F acting effectively between the z axis and the adjacent y axis. For an orthogonal machine with a different column arrangement of axes, such as that disclosed in US Pat. No. 5,399,633, the balancing arrangement would be more similar to the schematic representation of FIG. 13, where a balancing force F acts between the z axis and the ground. However, as mentioned above, it is not clear how such a balancing arrangement applies to a non-orthogonal machine, since in the arrangement of FIG. 13 the z drive axis is directly balanced, whereas a non-orthogonal machine does not have a z drive axis to balance.

[0212] A more specific embodiment of the general arrangement of Figure 10 will now be described with reference to Figures 14 and 15. As with the non-Cartesian coordinate positioning machines described above, a movable platform 12 supports a measurement probe 13 and is driven through a range of motion by extendable legs 16, and is controlled (by a controller such as controller 15 shown in Figure 2) to in particular move the measurement probe 13 into sensing relationship with a workpiece 19 supported on a fixed structure 14 of the machine. The fixed structure 14 comprises a lower portion on which the workpiece 19 is supported, and an upper portion which supports the extendable legs, in a "top-down" arrangement as described above with reference to Figure 4. Each of the extendable legs 16 is linearly extended and retracted as described above, such that the drive arrangement of Figure 14 can be considered to include a number of linear drive axes 61 associated with a respective one of the multiple extendable legs 16. The orientation of each of these linear drive axes 61 changes as the platform 12 moves through its range of motion.

[0213] The machine of FIG. 14 comprises a counterbalance arrangement 50 operating according to the principles described above with reference to FIG. 10, with a counterweight 30 connected via a pulley arrangement to the lower of the two support members 24. The support members 24 (including the upper and lower support members 24) are collectively referred to as supports. The pulley arrangement has a string (or cord or wire) 32 connected at one end to the counterweight 30 and at the other end to the lower support member 24, the string 32 passing over two pulleys 46 which are themselves supported by the housing 40. The counterweight 30 can be considered as a force generator (with associated mass) for the counterbalance arrangement 50, and is connected (via the pulley 46 and the string 32) to the lower support member 24 so as to generate a counterforce mg in the form of tension on the string 32, which exerts a tension (counterforce) on the lower support member 24. Because the counterweight 30 generates a counterforce that acts downward (in the direction of gravity) and the counterforce acting on the lower support member 24 must act upward (opposite gravity), the pulley 46 and string 32 can be viewed as providing the function of reversing the direction of gravity mg acting downward on the counterweight 30 and providing an upward counterforce mg on the lower support member 24. A single pulley 46 can be used instead of two pulleys, so long as the pulley 46 is wide enough to provide the necessary lateral separation between the counterweight 30 and the track 44.

[0214] The support member 24 is slidably movable along a vertically disposed track or rail 44 fixed to the housing 40, thereby defining a linear balance axis L of the balancing arrangement 50 (equivalent to that shown in FIG. 10). The linear balance axis L coincides with a balancing force mg acting upwards on the lower support member 24 (although these can be offset from each other and / or the balancing force can act on the lower support member 24 at an angle to the balance axis L, with a component of the balancing force acting along the balance axis L). The housing 40 itself is rotatably mounted to the fixed structure 14 of the machine via a revolute joint 41, allowing the housing 40 (together with the counterweight 30) to rotate about a vertical axis of rotation R. This rotation axis R defines a rotational balance axis R of the balancing arrangement 50 (equivalent to that shown in FIG. 10).

[0215] It should be noted that although (in this embodiment) the rotational balance axis R is offset from the linear balance axis L (i.e., offset from the line of the balance force mg acting on the lower support member 24), in other embodiments these two axes may be coincident or coaxial (see alternative embodiment shown in FIG. 16). Despite the offset, in this embodiment the counterweight 30 itself is located on (or along) the balance axis R (i.e., with zero offset from the balance axis R), which is advantageous because as the balancing arrangement 50 rotates about the rotational balance axis R, it is desirable to have the rotating mass as close as possible to the rotational balance axis R (work is done by moving the mass) in order to minimize the work done by such movement due to circumferential movement of the platform 12 about the balance axis R.

[0216] It is therefore desirable to at least locate the counterweight 30 as close as possible to the axis R, if not directly on the axis R. In this regard, the mass of the other parts of the balancing arrangement 50 (such as the support member 24, the track 44, the pulley 46, the string 32, and even the housing 40) can be considered to be insignificant compared to the mass of the counterweight 30 itself. In any event, it is desirable to have as much of the mass of the balancing arrangement 50 as possible close to the axis R of rotation.

[0217] FIG. 14, using terminology borrowed from FIG. 10, shows a row arrangement in which the elements are in the following order when viewed in the direction from the ground G towards the platform P: [G → R → L → K → P]. This is shown diagrammatically in the inset (A) of FIG. 14. The axis of rotation R can be considered to be behind (or in front of) the linear axis L (when viewed in the order from G to P). Everything after R in the row rotates around R, and since the force generator Q (counterweight 30) is associated (coupled) with the linear axis L that is behind the axis of rotation R in the row, the force generator Q (counterweight 30) also rotates around the axis of rotation R. Therefore, when the machine elements are arranged in this order, it is advantageous for the force generator Q (counterweight 30) to be arranged along the axis of rotation R or at least close to it, if the axis of rotation R is behind the linear axis L.

[0218] It should be noted that while linkage K effectively provides a second linear axis (i.e., a second linear degree of freedom) for balancing arrangement 50, universal joint 26 effectively provides three rotational axes (three rotational degrees of freedom) so that the total axes are substantially as follows: [G→R1→L1→L2→R2→R3→R4→P]. This is shown diagrammatically in inset (B) of FIG. 14. Apart from the rotational balance axes (R2-R4) provided by universal joint 26, balancing arrangement 50 includes a single rotational balance axis (R1) and two linear balance axes (L1 and L2).

[0219] The counterbalancing force F generated by the force generator Q acts (along) a linear counterbalancing axis L1. The force generator Q itself is housed in a housing 40 and is forward of the rotation axis R1 in the array arrangement of the axes. Being forward of the rotation axis R1 in the array arrangement, the force generator Q rotates about R1. However, as shown in insert (B), the force generator Q, despite being forward of the rotation axis R1 in the array, is physically located substantially along the rotation axis R1, so that the mass of the force generator Q is as close as possible to the rotation axis R1. It is recalled that since the mass of the force generator Q, in this embodiment, substantially matches the mass of the platform 12 and its attached objects, it is beneficial to locate the force generator Q as close as possible to the rotation axis R1. However, it is not essential to locate the force generator Q directly on the rotation axis R1, since some advantage can be obtained by simply moving the force generator away from the linear axis L1 and towards the rotation axis R1.

[0220] As just described, the counterbalanced configuration 50 would effectively have two linear counterbalanced axes (i.e., two linear degrees of freedom) and four rotational counterbalanced axes (i.e., four rotational degrees of freedom), or a total of six counterbalanced axes (i.e., six degrees of freedom). Generally, six degrees of freedom are required to provide a full relative motion between two parts (e.g., three linear and three rotational, or six linear, or some other suitable combination of linear and rotational), so the counterbalanced configuration 50 of FIG. 14 would have a full complement of six degrees of freedom. The two linear axes (L1, L2) of FIG. 14 are substantially perpendicular to each other, with the first linear axis (L1) being substantially aligned with gravity and the second linear axis (L2) being substantially perpendicular to gravity, while the first rotational axis (R1) is substantially parallel to the first linear axis (L1). The three rotation axes of the universal joint (R2-R4) can be considered to be arranged in a row, even though they act about the same point as if they were parallel, since two different rotation axes cannot act in parallel between two rigid structures without overlapping each other (which is not the case here, but flexure of the connected structures may be necessary to avoid such overlap).

[0221] The platform 12 is connected to a support member 24 via a linkage 22 which effectively provides a second linear balance axis (L2). The linkage 22 used in this embodiment is referred to as a Scott-Russell linkage, which includes two rigid bars, one twice the length of the other, with the shorter bar pivotally connected at one end to the centre of the longer bar. The shorter bar is pivotally connected at its other end to a lower support member 24. The longer bar is pivotally connected at one end to an upper support member 24 and at its other end to the platform 12 via a universal joint 26. The linkage 22 is coupled to the platform 12 via the universal joint 26, which is shown enlarged in insert (C).

[0222] As will be described in more detail below with reference to FIG. 17, the linkage 22 and force generator (counterweight 30) are mutually configured to maintain the counterforce mg in a substantially constant orientation relative to gravity g as the platform 12 is moved through a range of motion by the drive arrangement (extendable legs 16). In this regard, the linkage 22 is adapted to isolate or decouple lateral movement of the platform 12 from the lower support member 24 and enable this constant orientation to be maintained, which in this embodiment allows the platform 12 to move freely in x, y, and z while the lower support member 24 is constrained to move linearly along the track 44, thereby enabling the constant orientation of the counterforce mg to be maintained throughout such platform movement. This will become more apparent in the following description with reference to FIG. 17.

[0223] It will be apparent that the force generator (counterweight 30) is disposed substantially in a plane defined by the counterweight axis R and the connection 26 between the linkage 22 and the platform 12 as the platform 12 moves through its range of motion. It will also be apparent that the force generator (counterweight 30), the counterweight axis R and the linkage 22 are disposed substantially in a plane as the platform 12 moves through its range of motion.

[0224] As will become clear, the linear balance axis L (also denoted L1) is arranged in line with the rotational balance axis R (also denoted R1). When viewed in a direction from the fixed structure 14 towards the mobile structure 12, in this embodiment the rotational axis R comes before the linear axis L, but they can be arranged in another order, for example by a pivotable connection between the linkage 22 and the support member 24, which allows rotation about a vertical axis. In such an embodiment, when viewed in a direction from the fixed structure 14 towards the mobile structure 12, the rotational axis R comes after the linear axis L. Such an embodiment is described below with reference to FIG. 16.

[0225] Figure 15 shows a top view of the machine of Figure 14, with like parts being represented by like reference numerals. This top view more clearly shows how the linkage 22 provides radial movement of the platform 12 towards or away from the counterweight 30, while the revolute joint 42 provides movement of the platform 12 through an angle φ about the axis of rotation R, thereby allowing movement of the platform 12 in both x and y, or more specifically, both r and φ. The platform 12 is capable of movement in the z direction via movement of the support member 24 along a vertical track 44, which is transferred to the support member 24 via the linkage 22.

[0226] With reference to the known counterbalanced arrangement shown in FIG. 11, it was mentioned above that the counterbalanced arm A needs to swing about the axis of rotation R1 even for the movement of the platform P in the plane of the drawing, i.e. towards and away from the axis of rotation. In the embodiment of FIG. 14, the axis of rotation R is also present, but unlike the counterbalanced arrangement of FIG. 11 (and FIG. 5), this axis of rotation R is not needed for the movement of the platform 12 in the plane of the drawing, i.e. towards and away from the axis of rotation R. It is inevitable that an additional degree of freedom is needed in the counterbalanced arrangement to allow for movements out of the plane of the drawing, and the axis of rotation R in this embodiment is a result of its necessity, not of the core counterbalanced design. In other embodiments, the axis of rotation R is not needed (the additional degree of freedom can be given by a linear axis, as in the case of FIG. 43). When limited to two-dimensional movements in the plane of the drawing, the axis of rotation R is not needed in the embodiment of FIG. 14. Furthermore, in this embodiment, the counterbalancing force acting on the support member 24 is always aligned with gravity, regardless of where the platform 12 is located in its range of motion, and acts between the fixed structure 14 of the machine and the lower support member 24. In this regard, the counterbalancing force is provided by the tension in the string 32, which is supported by the fixed structure 14 (through the joint 42), similar to the representation in FIG.

[0227] In the embodiment shown in Figs. 14 and 15, the rotational balance axis R is offset from the linear balance axis L and precedes the linear balance axis L in the row (as viewed in a direction from the fixed platform 14 towards the movable platform 12). Fig. 16 shows an alternative embodiment in which the rotational balance axis R coincides with the linear balance axis L and follows the linear balance axis L in the row. In the embodiment shown in Fig. 16, the housing 40 is fixedly attached to the stationary structure 14 of the machine, rather than via the rotary joint 41 of Fig. 14. Instead, as shown most clearly in insert (C) of Fig. 16, the rotary joint is incorporated in the support member 24. Each support member 24 comprises an inner portion 24b rotatably supported by an outer portion 24a about a rotation axis R. The outer portion 24a of the support member 24 slides along the linear axis L and the inner portion 24b rotates relative to the outer portion 24a about the rotation axis R, so that the rotation axis R follows the linear axis L in the row.

[0228] Using the terminology used in Figure 10, Figure 16 shows a row arrangement in which the elements, when viewed in a direction from the ground G towards the platform P, are in the following order: [G→L→R→K→P]. This is shown diagrammatically in inset (A) of Figure 16. The axis of rotation R can be considered to be in front of (or after) the linear axis L, so that everything after R in the row rotates about R and nothing before R rotates. Thus, since the force generator Q (counterweight 30) is associated with the linear axis L, the force generator Q (counterweight 30) advantageously does not rotate about the axis of rotation R in this embodiment.

[0229] Note that, as noted above, linkage K effectively provides a second linear axis (second linear degree of freedom) for balancing arrangement 50, while universal joint 26 effectively provides three rotational axes (three rotational degrees of freedom), so that all axes are effectively: [G→L1→R1→L2→R2→R3→R4→P]. This is shown diagrammatically in inset (B) of FIG. 16. In this arrangement, the balancing axis (L1) along which the balancing force acts is rearward of any of the rotational balancing axes (R1-R4) when viewed in the direction from G to P along the row of axes.

[0230] Also, insert (B) shows the position of force generator Q in the balance axis arrangement. Unlike in FIG. 14, in FIG. 16 force generator Q is located behind rotation axis R1 in the balance axis array arrangement, and the balance force still acts (along) linear balance axis L1. Being behind rotation axis R1 in the array, force generator Q does not rotate with R1, which is beneficial because not only is there no work available to move force generator Q for horizontal movements of platform 12 in the plane of the page (towards and away from balance axis L1 and / or R1), but also for horizontal movements of platform 12 out of the plane of the page (in a circle around balance axis R1).

[0231] The arrangements of Figures 14 and 16 are shown diagrammatically in Figure 17, which is used to explain the working principle of these embodiments, in particular the properties of a Scott-Russell linkage 22 suitable for use as the linkage K described with reference to Figure 10. The platform 12 and a tool such as a measuring probe 13 supported by the platform 12 are diagrammatically represented in Figure 17 as a single platform P, the connections between the platform P and the linkage being simplified in this two-dimensional representation. This is because an appropriate joint such as the joint 26 of Figures 14 and 15 is of course required to allow small movements in the y direction (into and out of the plane of the paper), and this applies to most of the schematic representations described below which are intended to show the working principle in two dimensions, the third dimension (into and out of the plane of the paper) being taken care of, for example, by a rotational equilibrium axis R.

[0232] When a force is applied to an object, the work W done on the object (or energy imparted to the object) is equal to the force F multiplied by the distance distance moved by the object under the action of force F.

[0233]

number

[0234] Work is the transfer of energy from one place to another, or one form to another, e.g., a force applied to an object by a spring transfers the energy stored in the spring in the form of kinetic energy and / or gravitational potential energy to the object (e.g., to lift the object in the z-direction). If the object moves against the applied force, the work can be negative, e.g., the work done by gravity when an object is lifted vertically is negative because gravity is in the opposite direction to the displacement (but the work done by the lifting force is positive).

[0235] Now consider the configuration shown in Figure 17, where the platform P and counterweight C are considered as moving bodies, and for simplicity the linkage is considered to have negligible mass. The change dE in gravitational potential energy E of the two-part system is given by the total work done on the two parts by the force of gravity (F = mg).

[0236]

number

[0237] where m is the mass of the platform P and counterweight C, g is the gravitational constant, dh is the change in h, and dz is the change in z. The change dE is positive for positive dh and dz because a positive value of either of these lifts the mass against gravity, thereby giving the mass gravitational potential energy.

[0238] The following is evident from Figure 17:

[0239]

number

[0240] where l is the fixed length of the string (to the centre of the counterweight). In the above equation a, l and c are constant, while b varies with the overall height of the linkage K in the machine. It is also clear that:

[0241]

number

[0242] Substituting these into the above equation for dE, we get the following equation for dE:

[0243]

number

[0244] As a result, a change dθ in the variable θ does not result in a change dE in the overall potential energy E of the system, and therefore the system is stable and does not tend to increase or decrease in θ since doing so would not result in a lower energy state, and as a result, the system is stable and the balanced configuration performs its intended function of keeping the system in a stable configuration in the absence of any externally acting forces.

[0245] In particular, platform P is stable in both the z-direction (aligned with gravity) and the x- and y-directions (perpendicular to gravity) so that when platform P is moved to a desired position in the machine's range of motion by drive arrangement D, and drive arrangement D is then deactivated, platform P will remain in the same place in any event of deactivation. Moreover, this is a substantially "pure" or "perfect" form of stability compared to known counterbalanced arrangements, since the forces involved do not change at all for different positions of platform P in the range of motion.

[0246] Rather than considering the work done by gravity on the platform P and counterweight C, as represented by the downward arrows in Figure 17 emanating from those objects, we consider only the forces shown in the slightly simplified version shown in Figure 18. In particular, we consider the work done on the supports by the tension in the strings of the pulley system, and the work done on the platform by gravity. Since the system is (theoretically) perfectly balanced, the tension in the strings is equal to mg, and this force acts upwards on the lower support member. The upper support member is not attached to the strings, so the force acting vertically on that support member is zero. The force of gravity acting downwards on the platform is of course equal to mg.

[0247] Generalizing, the total work done on the support members by tension is given by:

[0248]

number

[0249] In the formula, F i is the force acting on the support member i, and dz i is the distance that support member i moves in the z direction. This concept is shown diagrammatically in FIG. 20, which is based on the schematic representation of FIG. 10. In general, it is considered that there are N support members i=0...(N-1), where N is an integer equal to or greater than 1, and support member i is connected to a linear balance axis L i Force F acting along i It has.

[0250] As illustrated in Figure 19, a basic property of the Scott-Russell linkage is that when the platform P moves horizontally in a direction perpendicular to gravity by an amount dx, this horizontal movement is converted or changed by the linkage into only a vertical movement dz1 of the upper support member (where there is no counterweight force), no vertical movement dz0 of the lower support member (where there is a counterweight force), and no horizontal movement dx, dy of any of the support members.

[0251] Therefore the total work done on the support member by the tension in the string is given by:

[0252]

number

[0253] Above, the lower support member is assigned index i=0 and the upper support member is assigned index i=1, and it is also the case that dz0=0 (the lower support member does not move in z) and F1=0 (the upper support member is not attached to the pulley string).

[0254] Therefore, for horizontal motion of the platform, the overall work done by the balancing forces acting on the support members is zero. In other words, since horizontal motion of the platform does not result in a change in the overall energy state, there is no incentive or movement of the platform to move laterally or horizontally, and therefore the system is stable perpendicular to the force of gravity.

[0255] When the platform P moves vertically in a direction aligned with gravity by an amount dz, this vertical movement is translating into a corresponding vertical movement dz of each support member i by the linkage. i Therefore, the force acting on the support member F i The total work done by is:

[0256]

number

[0257] In the example of FIG. 18, the above equation becomes:

[0258]

number

[0259] Therefore, the force F i The work done by is exactly equal to the work done by gravity acting on platform P, i.e., both are equal to mg × dz. As a result, there is no change in the overall energy state with a change dz in the z position of the platform, so there is no incentive or motion for platform P to move vertically, and therefore the system is stable even in the direction aligned with gravity.

[0260] Again, due to the nature of the counterbalancing arrangement, these forces do not change at all for different positions of platform P within the range of motion, so that the counterbalancing arrangement can be considered to provide perfect or near-perfect balance throughout, assuming that the weight of counterweight C matches, or nearly matches, the weight of platform P. Of course, in reality, counterweight C may not perfectly match the weight of platform P (or, rather, the movable structure, including that attached to the platform), but if there was perfect correspondence, the balance can also be considered to be perfect, or at least near-perfect.

[0261] It also becomes apparent from the above analysis that the linkage design is such that no support member moves horizontally for any movement of the platform. This allows the support members to travel along their respective tracks, and therefore allows the counterbalancing forces to be maintained in a substantially constant orientation with respect to gravity as the platform moves through its range of motion. In particular, any lateral movement of the lower support member leads to certain problems, discussed above with reference to Figures 6 and 7, where the counterbalancing forces are not maintained in a substantially constant orientation with respect to gravity as the platform moves through its range of motion. Being able to maintain the counterbalancing forces in a substantially constant orientation with respect to gravity, while still allowing the platform to move through its range of motion, provides a highly effective counterbalancing arrangement. This is particularly true for non-cartesian machines, which by their nature do not have drive axes that maintain a fixed orientation with respect to gravity.

[0262] Figure 21 shows an incomplete version of the equation, where the arm length is the longer 3q rather than 2q. The equation for h remains the same as above, but for z it becomes:

[0263]

number

[0264] Therefore,

[0265]

number

[0266] where θ ranges from 0 to 90° (as in the configuration of FIG. 21) and cosθ ranges from 0 to 1 (i.e., cosθ is positive), so that for a small positive change dθ in θ, the gravitational potential energy of the system decreases since dE is negative. Thus, in the absence of other acting forces, the configuration of FIG. 21 breaks down in itself since θ would tend to decrease rather than remain stable. The configuration of FIG. 21 is therefore not suitable for use as a counterweight and does not constitute an embodiment of the present invention. FIG. 22 corresponds to FIG. 19 and shows how horizontal movement of the platform results in z-movement of both supports.

[0267] Figure 23 shows an incomplete version of the equation, where the arm length is the shorter 1.5q rather than 2q. The expressions for dE and h remain the same, but for z we get:

[0268]

number

[0269] Therefore,

[0270]

number

[0271] In view of the above, it is clear that the configuration of Figure 23 is also not stable and the natural tendency for θ to increase. Thus, as above, the configuration of Figure 23 is also not suitable for use as a counterbalance and does not constitute an embodiment of the present invention. Figure 24 corresponds to Figure 19 and shows how horizontal movement of the platform results in z movement of both support members.

[0272] Figures 25-27 are similar to Figures 19, 22 and 24, respectively, but instead show how the platform moves when the upper support member is moved vertically to a constant z position along with the lower support member (i.e., with a balanced force applied). In balanced configurations that do not embody the invention (Figures 26 and 27), the z position of the platform changes with its motion, resulting in an overall change in the energy of the system, thereby resulting in an inherent instability in these configurations.

[0273] Furthermore, if the linear axis L of Figs. 14 and 16 is shown as the first linear axis L1 of Fig. 25, it becomes clear that the linkage of Fig. 25 effectively provides a second linear axis L2 that is substantially perpendicular to the first linear axis L1. Since the first linear axis L1 is substantially aligned (substantially vertical) with gravity, it follows that the second linear axis L2 is substantially perpendicular (substantially horizontal) to gravity. Because the nature of the Scott-Russell linkage is that linear to linear movements produce changes that are at right angles. Thus, when the linkage is coupled to the first linear axis L1, it effectively creates a second linear axis L2, thereby providing two (substantially orthogonal) linear degrees of freedom for the balanced configuration. On the other hand, the linkages shown in Figs. 26 and 27 do not, and in both cases the linkages create an apparently nonlinear axis NL coupled to the linear (vertical) axis L.

[0274] Figure 28 shows an alternative counterbalanced configuration that is an "upside down" version of that described above with reference to Figure 17. In the configuration of Figure 28, an upper support member is attached to the pulley string rather than a lower support member. A similar analysis to that described above with respect to Figure 17 applies.

[0275]

number

[0276] In the above equation, a, l and c are constant, while b varies with the overall height of the linkage K within the machine. Therefore,

[0277]

number

[0278] As a result, a change dθ in the variable θ does not result in a change dE in the overall potential energy E of the system, and therefore the system is stable and does not tend to increase or decrease in θ since doing so would not result in a lower energy state, and as a result, the system is stable and the balance configuration serves its intended function of keeping the system in a stable configuration in the absence of any externally acting forces. The linkage of Figure 28 also effectively creates two, orthogonally disposed, linear balance axes, as described above with reference to Figure 25.

[0279] Further embodiments of the present invention will now be described based on the same principles as above, and therefore the description of these further embodiments will not be as detailed as the above description.

[0280] Figure 29 shows a "longer reach" alternative to the configuration of Figure 28, with additional links and joints. The linkage of Figure 29 has the same properties as the linkage of Figure 28 in that the upper support member does not move when the platform moves only horizontally, and does not attempt to move horizontally for any movement of the platform, thereby maintaining the counterbalance force in a substantially constant orientation relative to gravity. The linkage of Figure 29 also effectively creates two orthogonally disposed linear balance axes, as described above with reference to Figure 25.

[0281] Figure 30 shows an embodiment similar to Figure 29 but using a slightly different linkage configuration, with three links of length 1.5q and one link of length 0.5q. Again, the linkage of Figure 30 has the same properties as the other linkages described above in that the support members attached to the counterforces do not move when the platform moves only horizontally and do not attempt to move horizontally for any movement of the platform, thereby maintaining the counterforces in a substantially constant orientation relative to gravity. The linkage of Figure 30 also effectively creates two orthogonally disposed linear balance axes as described above with reference to Figure 25.

[0282] Figure 31 shows a "dual pulley" configuration associated with the force generator, as opposed to the single pulley configuration of Figure 17. A counterweight twice the weight of the platform is used because the counterweight moves half the vertical distance of the platform, so that the overall potential energy of the system remains stable and the tension in the pulley strings supporting the lower support member is equal to half the gravity acting on the counterweight, i.e., the gravity acting on the platform. The linkage of Figure 31 also effectively creates two, orthogonally disposed, linear balance axes, as described above with reference to Figure 25.

[0283] Figure 32 provides an illustration of how the links forming the linkage of Figure 18 may actually be shaped to provide structural rigidity to the linkage using only sheet or plate material, such as aluminum or steel material. Similarly, Figure 33 is a diagram illustrating how the links forming the linkage of Figure 30 may actually be shaped.

[0284] FIG. 34 shows how the platform is supported by two counterbalancing arrangements, in this example two counterbalancing arrangements of the type shown in FIG. 17, each providing only half the counterbalancing force to the platform so that the overall counterbalancing force is as in the previous embodiments.

[0285] 35 shows an alternative counterbalancing arrangement in which the linkage is adapted so that the force acts on both support members, rather than just one of them as in the previous embodiments, and both support members move vertically, but in opposite directions when the platform moves horizontally. Each support member is acted upon by a force generator (via a respective pulley arrangement) with a ½ mg force acting upwards, so that as one moves upwards and the other moves downwards, the work done by one counterbalancing force is positive and the work done by the other counterbalancing force is negative, and the total work is then zero, i.e., Σ i F i dz i =0, so it is balanced and stable in the horizontal direction, and the sum of the forces is F i corresponds to the platform weight, i.e., Σ i F i = mg and is vertically balanced and stable. The linkage of Figure 35 also effectively creates two, orthogonally disposed, linear balance axes, as discussed above with reference to Figure 25.

[0286] Figure 36 shows a variation of the arrangement of Figure 35, where the counterweights are each arranged to slide on angled supports, i.e. in a direction not aligned with gravity, however the forces acting on the support members remain the same.

[0287] Figure 37 shows an alternative arrangement to the counterbalancing arrangement shown in Figure 35 in which force generators are used to apply counterbalancing forces separately to both support members and the linkage is adapted so that as the platform moves horizontally, both support members move vertically in opposite directions, translating vertical movement of the platform into corresponding vertical movement of the support members. The counterbalancing arrangement of Figure 37 includes a dual pulley arrangement associated with each support member. The linkage of Figure 37 also effectively creates two, orthogonally disposed, linear counterbalancing axes as described above with reference to Figure 25.

[0288] In each of the specific embodiments described above, a counterweight is used to generate a counterforce (in the form of tension in the strings) that acts on one or more support members of the counterbalancing arrangement. The use of a counterweight that exactly matches the weight of the structure (platform plus support tooling) is a very convenient way to implement the invention and is a simple way to provide accurate balance (with a constant counterforce that accurately balances the weight of the movable structure). However, it is not essential to use a counterweight system as a force generator to provide a constant counterforce of precise magnitude; instead, a pneumatic element or a constant force spring can be used as the force generator. A constant force spring is a variation of an extension spring, characterized by exerting a substantially constant force regardless of the degree of extension. An example of a constant spring is the well-known fuse spring arrangement, which uses a conical pulley with a helical groove around which a cord is wound, the cord being attached to a coiled mainspring that provides a decreasing force as it stretches, but the helical nature of the pulley compensates for the decreasing force provided by the mainspring as the cord unwinds, thereby providing a substantially constant force. As with a counterweight, to provide sufficient force to act as a counterbalance, the spring will have a mass that is not slight, but such that a significant amount of work must be done against it to move it around.

[0289] An embodiment using a constant force spring as a force generator is shown diagrammatically in FIG. 38, which is based on the embodiment of FIG. 37. Similarly, FIG. 39 shows the use of a constant force spring as a force generator in the embodiment of FIG. 18. The use of such a constant force spring as a force generator can be considered to have an advantage over the use of a physical counterweight 30 in some applications, since the latter requires the counterweight 30 to be accelerated when the platform 12 is moved vertically. In practice, however, the physical counterweight 30 provides a very convenient solution due to its ease of use and the ability to fine-tune the weight of the counterweight 30 simply by adding or removing additional mass elements. Furthermore, in a non-orthogonal machine such as that shown in FIG. 2, the extendable legs 16 can provide significant driving forces in the z-direction by the extension and retraction of the legs 16, so the need to accelerate the additional weight of the counterweight 30 is not particularly problematic. On the other hand, in such machines, having to drive additional mass horizontally is more of a problem, but this is not the case in embodiments of the present invention because no work is done on (or by) the counterweight 30 with the horizontal movement of the platform 12; for example, in the embodiments of Figures 14 and 16, no movement of the counterweight 30 occurs at all as the platform 12 moves horizontally.

[0290] Figure 40 shows a different form of linkage from that shown in the previous embodiment, having a horizontally disposed telescoping tube (or relatively slidable bar) 72 rigidly connected by a rigid joint 76 to a vertically slidable (balancing force acting) carriage 74, and also including a dual pulley arrangement as part of the force generator. The linkage of Figure 40 also effectively creates two orthogonally disposed linear balancing axes as described above with reference to Figure 25, as in the previous embodiment.

[0291] Furthermore, although the balancing arrangement has been described above as providing effective balancing for non-Cartesian machines, it can also provide balancing for Cartesian machines. Figure 41 illustrates how a balancing arrangement embodying the principles of operation shown in the previous embodiment may also be applied to balancing Cartesian coordinate positioning machines. Such a balancing arrangement may also be used to balance non-Cartesian continuous motion coordinate positioning machines, such as a robotic arm having multiple rotary drive axes.

[0292] For example, FIG. 42 illustrates the use of the same counterbalanced arrangement 50 as shown in FIG. 16, but now a robotic arm 60 is used to move the platform 12 instead of the hexapod drive arrangement. The robotic arm 60 comprises multiple segments 62 connected by a mixture of transverse rotary joints (or "revolute joints") 64 and in-line rotary joints (or "torsion joints") 66 forming a mechanical linkage from one end to the other. As a result, the drive arrangement of FIG. 42 has multiple rotary drive shafts 64, 66 compared to the multiple linear drive shafts 16 of FIG. 16. The counterbalanced arrangement is equally applicable to coordinate positioning machines having a mixture of linear and rotary drive shafts.

[0293] With reference to Figures 14 and 15, it has been described above that the balancing arrangement 50 rotates about a rotation axis R to enable movement of the platform 12 in the three dimensions x, y, z (or r, φ, z in radial coordinates), but this is not required. Instead, rather than being based on a radial coordinate system as shown in Figures 14 and 15, the balancing arrangement itself can be based on a Cartesian coordinate system having orthogonal axes x, y, z. In such an arrangement, the z axis is balanced, while the x, y axes provide x, y movement of the platform 12 without interfering with the balancing forces provided by the vertical balance. The balancing arrangement is provided between the fixed structure 14 of the machine and the movable platform 12, independent of the drive arrangement of the machine. Such an arrangement is shown in Figure 43, where a Cartesian balancing arrangement embodying the invention may be used to balance either Cartesian or non-Cartesian coordinate positioning machines.

[0294] In the above analysis based on the configuration shown in FIG. 17, points S0 and S1 are constrained to move along a vertical track or rail, and point P is free to move within a range of motion defined by a rigid bar. However, other configurations are possible in which points S0 and S1 are less constrained or are constrained in a different manner. It is also understood that while a linkage (e.g., a Scott-Russell linkage) has been described as being linear in nature (thereby providing a linear balance axis), a row of rotating balance axes (e.g., with each rotating balance axis aligned with gravity) can take the place of this second linear balance axis, such as the links in a bicycle chain, so that there is only a single linear balance axis (the axis along which the balance force acts).

[0295] The counterbalancing arrangement embodying the invention has proven to be extremely effective in practice, as described above, in that it provides substantially perfect counterbalance regardless of where the platform 12 is in the range of motion 11, thereby eliminating the need for a braking system altogether, since in the absence of supports separate from the drive arrangement, the platform 12 is stable without the need for brakes to prevent it from moving away from its position.

[0296] FIG. 44 shows an embodiment based more closely on the balancing arrangement of FIG. 5. The arrangement of the balancing axes remains unchanged, so that there are effectively six rotational balancing axes R1-R6 as shown in FIG. 44 (and as described above with reference to FIGS. 5 and 11). Note that the rotational balancing axis R3 is substantially perpendicular to gravity, so that when a balancing force acts on (about) the rotational axis R3, the resulting force acting on the connected arm 92 is in a substantially vertical plane, while the rotational balancing axes R1 and R2 are substantially aligned with gravity. The rotational axis R3 is substantially perpendicular to the rotational axes R1 and R2. The rotational axes R4-R6 are provided by a universal joint connecting the arm 92 to the platform 12.

[0297] An important change compared to Figure 5 is that in Figure 44 the mass generating the counterforce is moved back to coincide with the first rotational counterforce axis R1, rather than being located after the rotational axis R2. In this respect, in the embodiment of Figure 44 the force generator is a simple counterweight C, and a string 99 transfers the generated counterforce through an arm 94 to a hinge 93, which can act on (about) the rotational axis R3 in a similar manner to the configuration of Figure 5. The difference is that in this embodiment the counterforce is applied to the hinge 93 via tension in the string 99, and the source of the force (counterweight C) is located away from the hinge 93, whereas in the configuration of Figure 5 the source of the force (spring 96) is located on the hinge 93 itself.

[0298] An advantage of the configuration of Figure 44 over the configuration of Figure 5 is that by having the mass of force generator C located along (or beside, or at least near) the first balance axis R1 in the row of balance axes R1-R6 (when viewed in a direction towards platform 12), the mass does not move (or at least does not move inconsistently) as platform 12 moves through its range of motion other than due to rotation of the mass about a single rotational balance axis R1 via twisting / rotation of string 99 (rather than rotation about two or more rotational balance axes as in known balancing configurations). The force-generating mass is still connected to hinge 93 via string 99 to transmit the balance force where it is needed (at rotational balance axis R3), but the force-generating mass itself is located close to (e.g., along) the first rotational axis R1.

[0299] This can be explained in a broad sense with reference to inserts (A)-(C) of FIG. 44, which are similar to those of FIG. 14 and FIG. 16 for comparison. Insert (A) is for the known balancing arrangement of FIG. 5, showing the arrangement of balancing axes R1-R6, and the position of force generator Q (counterweight C) in the arrangement of balancing axes. Force generator Q is located forward (in hinge 93) of rotation axes R1 and R2, and a balancing force F acts (about) rotational balancing axis R3. Because force generator Q is forward of the two rotation axes R1 and R2 in the row, it rotates about R1 and R2 in the complex manner described above.

[0300] The arrangement shown in insert (B) represents a similar type of counterbalancing arrangement used in the DS10 contact scanner sold by Renishaw plc (formerly also sold under the trade name Incise), which is also a parallel kinematic machine. The counterbalancing force F again acts about (about) the rotational counterbalancing axis R3, and the force generator Q is again located forward of the rotational axes R1 and R2 in a row arrangement. However, the force generator Q (which is actually a spring) is physically located some way forward of the rotational axis R3, and is in fact connected to a point along the relevant arm (between R3 and R4 / 5 / 6) some way removed from the axis R3, similar to that shown diagrammatically in FIG. 11. As a result, the force generator Q effectively rotates about the three rotational axes R1, R2 and R3, but rotation about the rotational axis R3 only occurs if the motion of the platform 12 has a vertical component, rather than a purely horizontal motion. However, even with purely horizontal movement of platform 12, the effect of rotation about axis of rotation R2 is potentially more significant because force generator Q is not located proximate to axis of rotation R2.

[0301] Contrast this with the arrangement shown in insert (C), which represents the embodiment shown in Figure 44. The balancing force F still acts (about) rotational balancing axis R3, but force generator Q is physically located rearward of rotational axes R2 and R3 and substantially coincident with rotational axis R1. Thus, force generator Q is effectively shielded or isolated from rotational motion about rotational axes R2 and R3, and only experiences rotational motion about rotational axis R1, providing similar benefits to the embodiment of Figure 14.

[0302] 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 devices, 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 component may include a camera for imaging the surface of the workpiece. The component may include an eddy current probe for detecting and / or measuring eddy currents at or near the surface of the workpiece. Many other possibilities may be apparent to those skilled in the art.

[0303] It will be appreciated that the operation of the coordinate measuring machine may be controlled by a program running on the machine, in particular by a program running on a coordinate measuring machine controller, such as controllers 5 and 15 shown diagrammatically in Figures 1 and 2. It will be appreciated that the control of the extendable legs may be performed by a program running on the controller 5 or 15. 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 from 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.

[0304] Although the above embodiments have been described primarily in the context of a coordinate measuring machine, the concepts are more generally applicable to any type of coordinate positioning 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 plurality of drive shafts for positioning a movable structure within a movable range of the machine; A plurality of balancing shafts for balancing the movable structure; a force generator for generating a balancing force acting on at least one of said balancing shafts; Equipped with 1. A coordinate positioning machine, wherein the balance axis and the force generator are mutually configured such that movement of the moveable structure in a direction perpendicular to gravity results in substantially no net movement of the force generator and / or substantially no work on the force generator other than that resulting from rotation of the force generator about at most one rotational balance axis.

2. 1. A coordinate positioning machine, comprising: a plurality of drive shafts for positioning a movable structure within a movable range of the machine; A plurality of balancing shafts for balancing the movable structure; a force generator for generating a balancing force acting on at least one of said balancing shafts; Equipped with 1. A coordinate positioning machine comprising: a coordinate axis system including a coordinate positioning mechanism for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system including a coordinate system for detecting a position of a coordinate system

3. 1. A coordinate positioning machine, comprising: a plurality of drive shafts for positioning a movable structure within a movable range of the machine; A plurality of balancing shafts for balancing the movable structure; a force generator for generating a balancing force acting on at least one of said balancing shafts; Equipped with 4. A coordinate positioning machine, wherein the balance axis is arranged in alignment with at most one rotating balance axis between the force generator and a fixed structure of the machine.

4. 4. A coordinate positioning machine according to claim 1, further comprising a linkage for transmitting a counterbalancing force to the moveable structure.

5. 5. The coordinate positioning machine of claim 4, wherein the linkage is adapted to effect movement of the moveable structure in a direction perpendicular to gravity while causing a counterbalancing force to do substantially no work.

6. A coordinate positioning machine according to any preceding claim, wherein the balancing shafts are arranged in a row.

7. 7. A coordinate positioning machine according to claim 6, comprising at most one rotational balance axis between the force generator and a fixed structure of the machine.

8. A coordinate positioning machine according to any preceding claim, wherein the balance axes include at least one linear balance axis.

9. 9. A coordinate positioning machine according to claim 8, wherein the or each linear balance axis has a substantially constant orientation relative to gravity.

10. A coordinate positioning machine according to any preceding claim, wherein the balance axes include first and second linear balance axes arranged substantially perpendicular to each other.

11. A coordinate positioning machine according to claim 10 when dependent on claim 4, wherein the second linear balance axis is provided by the linkage.

12. 12. A coordinate positioning machine according to claim 11, wherein the linkage is a Scott-Russell linkage, and / or the linkage is adapted to effect changes in substantially vertical movement from straight line to straight line.

13. A coordinate positioning machine according to any preceding claim, wherein a balancing force acts on a predetermined linear balancing axis of the plurality of balancing axes.

14. 14. A coordinate positioning machine as claimed in claim 13, wherein the orientation of the predetermined linear balance axis is substantially invariant to changes in orientation of the drive shafts and / or actuation of the drive shafts, e.g. the predetermined linear balance axis has a substantially constant orientation with respect to gravity such that it is substantially aligned with gravity.

15. A coordinate positioning machine according to claim 13 or 14 when dependent on claim 10, wherein the predetermined linear balance axis is the first linear balance axis.

16. 16. A coordinate positioning machine according to claim 13, 14 or 15 when dependent on claim 4, wherein the linkage is adapted to absorb horizontal movement of the moveable structure and to translate vertical movement of the moveable structure into corresponding vertical movement of the linkage along the predetermined linear balance axis.

17. A coordinate positioning machine as claimed in any one of claims 13 to 16 when dependent on claim 4, wherein the movable structure is balanced via the linkage along the predetermined linear balance axis, e.g. at least a part of the weight of the movable structure is supported via the linkage by a substantially constant balancing force acting on the linkage along the predetermined linear balance axis.

18. 18. A coordinate positioning machine according to any of claims 13 to 17 when dependent on claim 4, including at least one support member coupled to the predetermined linear balance axis, the at least one support member configured for slidable movement along the predetermined linear balance axis, a balancing force being arranged to act on the at least one support member, and the linkage being connected between the moveable structure and the at least one support member.

19. 20. The coordinate positioning machine of claim 18, wherein vertical movement of the moveable structure by the drive arrangement is translated by the linkage into corresponding vertical movement of the at least one support member such that a balancing force acting on the at least one support member causes it to do work that is substantially equal but opposite to the work done by gravity acting on the moveable structure.

20. 20. A coordinate positioning machine according to claim 18 or 19, wherein horizontal movement of the moveable structure by a drive arrangement is translated by the linkage into at most vertical movement of the at least one support member such that the force(s) acting on the at least one support member does substantially no net work.

21. 21. A coordinate positioning machine according to claim 18, 19 or 20, comprising first and second support members each configured for slidable movement along a track substantially aligned with gravity, movement of the moveable structure in a direction perpendicular to the or each track is translated by the linkage into movement of the second support member along the track with substantially no movement of the first support member along the track, and movement of the moveable structure in a direction parallel to the or each track is translated by the linkage into movement of at least the first support member along the track in the same direction and by the same amount, optionally with a counterbalancing force acting on the first support member and not on the second support member.

22. The force generator generates a force F i is applied to each support member i in a direction substantially opposite to gravity, i F i 〜mg, and the link mechanism converts the displacements dx, dy, dz in the movable structure into the displacements dx i , dy i , dz i and thereby change or convert it into Σ i F i DZ i ~mg.dz,dx i ~0 and dy i 0, where m is the mass of the movable structure, g is the gravitational acceleration, i is an integer equal to or greater than 1, and at least one F i 22. A coordinate positioning machine according to any one of claims 18 to 21, wherein: is zero.

23. 23. A coordinate positioning machine according to any preceding claim, wherein the balance axes include at least one rotational balance axis.

24. 24. A coordinate positioning machine according to claim 23 when dependent on claim 6, wherein the force generators are arranged in the row rearward of or at a predetermined horizontal distance from a first rotational balance axis.

25. 25. The coordinate positioning machine of claim 24, wherein the predetermined horizontal distance is substantially constant and / or the force generator is positioned proximate to the first rotational balance axis, and the predetermined horizontal distance is less than 50% of the horizontal distance between the first rotational balance axis and the moveable structure.

26. 26. A coordinate positioning machine according to any preceding claim, wherein the balance axes and the force generators are mutually configured such that movement of the moveable structure in a direction perpendicular to gravity does not substantially cause a net movement of the force generators other than by cause of rotation of the force generators about at most one rotational balance axis.

27. A coordinate positioning machine according to claim 26 when dependent on claim 6, wherein said at most one rotational balance axis is the first or only rotational balance axis in said row.

28. 28. A coordinate positioning machine according to any preceding claim, wherein the balancing axes are separate from the drive axes, and / or each balancing axis is separate from and / or independent and / or different from each of the drive axes.

29. 29. A coordinate positioning machine according to any preceding claim, wherein the or each balance axis about which a balancing force acts has a substantially constant orientation relative to gravity, for example substantially aligned with gravity in the case of a linear balance axis, or substantially perpendicular to gravity in the case of a rotational balance axis.

30. 30. A coordinate positioning machine according to any preceding claim, wherein the counterbalance force is a substantially constant counterbalance force.

31. A coordinate positioning machine according to any preceding claim, wherein the force generator comprises a counterweight which generates tension in a string or cord or cable or wire or the like which is coupled to the force generator.

32. 32. A coordinate positioning machine according to any preceding claim, wherein each of the drive axes is a linear drive axis, and / or none of the drive axes has or maintains a substantially constant orientation relative to gravity as the moveable structure moves through its range of motion.

33. 33. A coordinate positioning machine according to any preceding claim, wherein the coordinate positioning machine is a non-Cartesian coordinate positioning machine, and / or a parallel kinematic coordinate positioning machine, and / or a hexapod coordinate positioning machine, and the coordinate positioning machine may be a coordinate measuring machine.

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