MEASURING APPARATUS COUNTERBALANCE

Abstract

A positioning apparatus including a quill on which a probe apparatus can be mounted, at least one motor for positioning the quill in a substantially vertical dimension, and a pneumatic counterbalance mechanism for the quill. The positioning apparatus is configured, based on at least one factor relating to the quill, to automatically effect a change in the pneumatic counterbalance mechanism's pressure so as to thereby adapt the counterbalance force on the quill provided by the pneumatic counterbalance mechanism.

Claims

1. A positioning apparatus comprising a quill on which a probe apparatus can be mounted, at least one motor for positioning the quill in a substantially vertical dimension, and a pneumatic counterbalance mechanism for the quill, in which the positioning apparatus is configured, based on at least one factor relating to the quill, to automatically effect a change in the pneumatic counterbalance mechanism's pressure so as to thereby adapt the counterbalance force on the quill provided by the pneumatic counterbalance mechanism.

2. A positioning apparatus as claimed in claim 1, in which the at least one motor comprises a direct drive motor, optionally a linear motor.

3. A positioning apparatus as claimed in claim 1, in which the at least one factor relates to the actual or expected vertical position of the quill.

4. A positioning apparatus as claimed in claim 1, further comprising one or more position encoders for measuring the vertical position of the quill, and in which the at least one factor comprises the output from at least one of said one or more position encoders.

5. A positioning apparatus as claimed in claim 1, in which the at least one factor relates to the actual or expected power requirement of the at least one motor.

6. A positioning apparatus as claimed in claim 1, in which the at least one factor relates to the actual or expected direction of travel of the quill.

7. A positioning apparatus as claimed in claim 6, configured to adapt the pneumatic counterbalance mechanism's pressure so as to assist the motor in moving the quill in the direction of travel.

8. A positioning apparatus as claimed in claim 1, in which the at least one factor relates to at least one module loaded or to be loaded on the quill.

9. A positioning apparatus as claimed in claim 8, in which the at least one factor relates to the weight of the at least one module.

10. A positioning apparatus as claimed in claim 8, in which the at least one factor is determined by assessing the effect the module loaded on the quill has on the coordinate positioning apparatus.

11. A positioning apparatus as claimed in claim 1, comprising a pressure regulator which is configured to maintain the pneumatic counterbalance mechanism's pressure at a set pressure, and in which the apparatus is configured to alter the set pressure based on the at least one factor.

12. A positioning apparatus as claimed in claim 1, in which the change in the pneumatic counterbalance mechanism's pressure is determined from a look-up table and/or function.

13. A positioning apparatus as claimed in claim 1, configured to automatically adapt the pneumatic counterbalance mechanism's pressure in response to an expected or measured change in the load on the quill.

14. A positioning apparatus as claimed in claim 1, in which the positioning apparatus comprises a Cartesian coordinate positioning apparatus.

15. A method of operating a positioning apparatus comprising a quill on which a probe apparatus can be mounted, at least one motor for moving the quill in a substantially vertical dimension, a pneumatic counterbalance mechanism for the quill, the method comprising automatically effecting a change in the pneumatic counterbalance mechanism's pressure so as to thereby adapt the counterbalance force on the quill provided by the pneumatic counterbalance mechanism, based on the quill's status.

16. A positioning apparatus, comprising: first and second members relatively moveable in a substantially vertical degree of freedom, in which an energy conduit is connected to at least one of the first and second members, wherein the load, in the degree of freedom of the first and second members, imparted by the energy conduit on at least one of the members it is connected to varies dependent on the relative position of the first and second members, and further comprising a pneumatic counterbalance configured to apply a load, in the degree of freedom of the first and second members, that varies, dependent on the relative position of the first and second members, inversely to the load applied by the energy conduit, such variation in load being achieved by varying the air pressure of the pneumatic counterbalance.

Description

[0039] Embodiments of the invention will now be described, by way of example only, with reference to the following drawings, in which:

[0040] FIG. 1 is a schematic isometric view of the front of a gantry-type CMM according to a first embodiment of the present invention;

[0041] FIG. 2 is an enlarged view of the top of the quill of the CMM of FIG. 1;

[0042] FIG. 3 schematically shows the quill of the CMM of FIG. 1 in cross-section, and also the possible units involved in controlling the quill's counterbalance mechanism;

[0043] FIG. 4 is an enlarged view of the top of the quill of the CMM of FIG. 1 according to another embodiment of the invention; and

[0044] FIG. 5a is a graph illustrating how the counterbalance mechanism's pneumatic pressure might vary over time during a downward quill motion in accordance with one aspect of the invention and FIG. 5b is a graph illustrating how the counterbalance mechanism's pneumatic pressure might vary over time during the same downward quill motion in a system of the art.

[0045] An overview of an example embodiment of how the invention can be implemented will be described below. In this case, the invention is implemented as part of a CMM 100. FIG. 1 shows a CMM 100 with its protective housings/covers (e.g. main covers/hard covers) removed so that the relevant components of the CMM 100 can be seen. FIG. 2 shows an enlarged view of the top end of the quill 110 of CMM 100.

[0046] As shown, a tool, for example an inspection device such as a probe 102 for inspecting a workpiece, can be mounted on the CMM 100. In the embodiment shown, the probe 102 is a contact probe, in particular a contact analogue scanning probe, for measuring the workpiece by a stylus of the probe contacting the workpiece. However, as will be understood the CMM 100 could carry any sort of inspection device, including touch-trigger probes, non-contact (e.g. optical) probes, or another type of instrument if desired.

[0047] In the embodiment shown, the CMM 100 is a gantry-style Cartesian CMM and comprises a platform 105 on which an artefact to be inspected can be placed, and a movement system which provides for repeatable and accurate control of the position of the probe 102 relative to the platform 105 in three orthogonal degrees of freedom X, Y and Z.

[0048] In particular, the movement system comprises a cross-beam 106, a carriage 108, and a quill 110. The cross-beam 106 extends between first 112 and second 114 raised rail members and is configured to move along the rails along a Y axis via a bearing arrangement (in this embodiment an air bearing arrangementnot shown), and powered by a motor, such as a linear motor (not shown). The carriage 108 sits on and is carried by the cross-beam 106, and is moveable along the cross-beam along an X axis via a bearing arrangement (in this embodiment an air bearing arrangementnot shown) and powered by a motor, such as a linear motor (not shown). The quill 110 is held by the carriage 108, and is moveable relative to the carriage 108 along a Z axis via a bearing arrangement (again, in this embodiment via an air bearing arrangementnot shown), and powered by a motor, such as a linear motor. The stator 400 of the quill's linear motor is visible in FIGS. 1 and 2, but the armature 402 (see FIG. 3) is not visible in FIGS. 1 and 2. As will be understood, the stator 400 is fixed relative to the carriage 108 (e.g. it can be anchored at a lower end to the carriage 108 and at an upper end to a tower 194 which is mounted to the carriage 108 so as to move therewith), and the armature 402 can be fixed and mounted to the quill 110 so as to move therewith.

[0049] A pneumatic counterbalance mechanism for the quill is provided for counterbalancing the weight of the quill 110 so as to reduce the work required of the quill's motor. In particular, the pneumatic counterbalance is configured to provide an opposing force substantially equal to the weight of the quill 110 (and the articulated head 116 and probe 102) such that substantially zero force is required by the quill's motor to keep it at a stationary position. The quill 110 is hollow and the pneumatic counterbalance comprises a piston 300 within a counterbalance cylinder 302 located inside the quill 110 (see FIG. 3). The piston 300 is anchored to a tower 194 (in this case a carbon-fibre tube) via a cable 196. The tower 194 is mounted to the carriage 108 so as to move therewith. As described in more detail below, in accordance with the invention, the apparatus is configured to adapt the pneumatic counterbalance automatically in response to certain circumstances so as to alter the counterbalance force provided.

[0050] As will be understood, motors, for example direct drive motors such as linear motors, can be provided for effecting the relative motion of the various members along their axis (of which the stator 400 of the quill's linear motor is shown in FIGS. 1 and 2). Also, position encoders can be provided for reporting the position of the cross-beam 106, carriage 108 and/or quill 110. The scale 404 and readhead 406 of the quill's linear encoder are visible in FIGS. 1 and 2. The linear motor and encoder arrangement for driving and monitoring the position of the quill 110 is also schematically shown and described in more detail below in connection with FIG. 3.

[0051] In the particular example shown, an articulated head 116 is provided on the lower free end of the quill 110 for carrying the probe 102. In this case, the articulated head 116 comprises two orthogonal rotational axes. Accordingly, in addition to the three orthogonal linear degrees of freedom X, Y and Z, the probe 102 can be moved about two orthogonal rotational axes (e.g. A and B axes). A machine configured with such an articulated head is commonly known as a 5-axis machine.

[0052] Articulated heads for tools and inspection devices are well known, and for example described in WO2007/093789. As will be understood, an articulated head need not necessarily be provided, and for example the probe 102 could be mounted to the quill assembly 110 via a fixed head which does not provide any rotational degrees of freedom. Optionally, the probe itself can comprise an articulated member so as to facilitate rotation about at least one axis.

[0053] An energy conduit 502 is provided between the quill 110 and the carriage 108. The energy conduit 502 comprises one or more electrical wires and/or pipes for providing power, communications, and/or gas, to and/or from the quill 110, the articulated probe head 116, and the probe 102. For example, the pipe(s) could supply gas for the quill's air bearings (not shown) and/or for the pneumatic counterbalance. For the sake of clarity, most of the wires and pipes are not shown in the Figures; only the pipe 420 for supplying air to the inside of the quill 110 for the pneumatic counterbalance mechanism is shown in FIGS. 2 and 3. In any case, in this embodiment, in addition to the wires and pipes, the energy conduit 502 comprises a support track 503 which flexes with relative movement of the quill 110 and carriage 108. The support track 503 is configured to keep the wires and pipes associated with it tidy and to control how they flex with the relative movement of the quill 110 and carriage 108. A first end of the support track 503 of the energy conduit 502 is connected to the carriage 108 (in this case to the carriage's tower 194, via bracket 195), and a second end of the support track 503 of the energy conduit 502 is connected to the quill 110 (in this case via a bracket 198).

[0054] As is standard with measuring apparatus, a controller 118 can be provided which is in communication with the CMM's motors and position encoders, the articulated head 116 (if present) and the probe 102 so as to send and/or receive signals to and/or from them so as to control the motion of the relatively moveable members as well as receive feedback and measurement data. A computer 120, e.g. a personal computer (which can be separate to or integrated with the controller 118) can be provided which is in communication with the controller 118. The computer 120 can provide a user-friendly interface for an operator to, for example, program and initiate measurement routines. Suitable computers and associated control/programming software is widely available and well known. Furthermore, a joystick 122 or other suitable input device can be provided which enables an operator to manually control the motion of the probe 102. Again, such joysticks are well known and widely available.

[0055] A variety of tools/inspection devices could be stored in a rack 450 located within the CMM's working volume. Furthermore, the probe 102 mounted on the CMM 100 could be automatically changed to/from the rack 450 in a known manner.

[0056] Referring now to FIG. 3, a cross-section of the CMM's quill 110 is schematically shown. As shown, a linear motor and encoder apparatus are provided for effecting and monitoring movement of the quill 110 along the z-axis relative to the carriage 108 (which is not shown in FIG. 3). In particular, in this embodiment, the linear motor comprises an elongate stator 400 fixed relative to the carriage 108 (not shown) (in particular, the linear motor is fixed at a bottom end to the carriage's box structure, and at a top end to the carriage's tower 194neither shown in FIG. 3) and an armature 402 fixed to the quill 110. In this embodiment the encoder comprises a scale 404 fixed to the quill 110 and a readhead 406 fixed to the carriage 108 for reading the scale. As will be understood, the stator 400 and armature 402 could be mounted the other way around (i.e. the armature could be mounted to the carriage, and the stator to the quill), and likewise for the scale 404 and readhead 406.

[0057] FIG. 3 also schematically shows various parts of an example controller 118. In particular, in this example, there is shown a main processing unit (e.g. main processing board) 410, an encoder interface 412, a motor power amplifier 414 and a counterbalance controller 416. As will be understood, these various units/interfaces/amplifiers could be provided as separate or as combined components (e.g. on the same or different boards and/or via the same or different processors/circuitry) and need not all be provided in or by the controller 118 (e.g. the counterbalance controller could be located separate from the controller 118. As will also be understood, the circuitry for such units/interfaces/amplifiers could comprise bespoke or generic processor units, e.g. microprocessors, central processing units (CPU), Field-Programmable Gate Arrays (FPGAs), or the like.

[0058] As also shown, a pressurised gas supply 418 such as a pneumatic pump or, for example, a compressed gas supply is provided which provides supply of pressurised gas (e.g. air) to the inside of the quill 110, e.g. via pipe 420 (which is supported by the support track of the energy conduit 502). The pressurised gas inside the counterbalance cylinder 302 acts against the piston 300 and the inside walls of the counterbalance cylinder 302 so as to provide an upwards force along the z-axis, thereby supporting at least some (and preferably substantially all) of the weight of the quill 110 and any components mounted thereon, such as the articulated probe head 116 and the probe 102. The pressurised gas supply 418 could be configured to try to maintain a set pressure within the counterbalance cylinder 302, e.g. the apparatus, for example the pressurised gas supply, could comprise a pressure regulator, such as a digital pressure regulator.

[0059] According to an example embodiment, during normal use, the controller 118 is configured to control the x, y and z axes of the CMM 100, and for example the rotational positions of the articulated head's 116 rotational axes. For example, this could be in response to signals received from an input device, such as the joystick 122 and/or computer 120. Optionally, the controller 118 (e.g. the main processing unit 410) could execute a program comprising a predefined course of motion, and control the axes of the CMM 100 and articulated head 116 accordingly. As shown in FIG. 3 in connection with the z-axis, the controller 118 can effect movement of the z-axis by way of the main processing unit 410 instructing commands to the motor power amplifier 414 which in turn powers the armature 402 so as to operate the linear motor. The controller 118, by way of the main processing unit 410, readhead 406, encoder interface 412 and the motor power amplifier 414, can operate to implement a servo loop, so as to ensure that the quill 110 is moving toward or at the desired position.

[0060] As briefly mentioned above, in accordance with the invention, the apparatus is configured to adapt the pneumatic counterbalance automatically in response to certain circumstances so as to alter the counterbalance force it provides. This has been found to be particularly beneficial to CMMs where a direct drive, such as a linear motor, is used to control the z-axis position of the quill 110. This is because the effect of the heat generated by such motors on the metrological performance of the CMM can be more pronounced due to direct drive, and in particular linear motors, typically being mounted closer to the CMM's metrology structure. Also, in contrast to other types of motors, such as ball-screw or geared systems, which cannot be back-driven, it is often necessary to power a direct drive/linear motor, to hold a given position should the counterbalance mechanism not counterbalance the load on the quill, thereby producing heat even when stationary. If the power required to hold position is different for different positions, then the amount of heat generated may be different for different positions, thereby affecting metrology differently in different positions. Accordingly, avoiding significant changes in the power requirement of a direct drive/linear motor (and hence avoiding significant changes heat generated by the motor) can be more important to the metrological performance of the CMM compared to belt driven, ball-screw, or geared DC motors.

[0061] Accordingly, in one embodiment, the apparatus is configured to adapt the pneumatic counterbalance's pneumatic pressure automatically depending on the z-axis position of the quill 110. The mass, and hence weight, carried by the quill 110 can vary depending on the z-axis position, for example due to the proportion of the energy conduit 502 that is carried by the quill 110. For example, in this embodiment, this could be due to the proportion of the energy conduit 502 that is carried by the quill 110 being greater in a relatively raised/higher position compared to which it is in a relatively lowered position. Accordingly, the linear motor will have to work harder just to hold position at a first vertical/z-axis position compared to a second vertical/z-axis position. This means that the linear motor will generate more heat at the first vertical/z-axis position compared to when it is the second vertical/z-axis position. Such variation in heat output can adversely affect metrology. As will be understood, the first vertical/z-axis position could be higher than the second vertical/z-axis position, or the first vertical/z-axis position could be lower than the second vertical/z-axis position.

[0062] It might also be that irrespective of the energy conduit 502 (e.g. even if the energy conduit 502 is substantially balanced by a corresponding second energy conduit 504 as in FIG. 4), the power requirement of the motor could vary depending on the z-axis position, due to, for example, hysteresis in the energy conduit, varying cable tension, etc. Accordingly, such variations could be determined (e.g. mapped) during a calibration stage and subsequently used by the counterbalance controller 416 to control the pressurised gas supply 418 accordingly. For example, a function or map could be determined during the calibration stage. Such a function/map could be configured so as to try to ensure that substantially the same motor power (which could be substantially zero motor power) is required to hold the quill 110 at all z-axis position, or at least for a significant proportion of the quill's z-axis position (e.g. for at least 50%, preferably at least 75% of the quill's z-axis travel range).

[0063] According to one aspect of the invention, the counterbalance controller 416 receives an input from the encoder interface 412 which indicates the current z-axis position of the quill 110. The counterbalance controller 416 can then use this input to determine how to control the pressurised gas supply 418 so as to vary the pressure of the gas within the counterbalance cylinder 302 accordingly. For example, based on the input from the encoder interface 412, the counterbalance controller 416 can be configured such that it controls the pressurised gas supply 418 to ensure a relatively greater pressure inside the counterbalance cylinder 302 for relatively higher positions of the quill 110, and to control the pressurised gas supply 418 to ensure a relatively lower pressure inside the counterbalance cylinder 302 for relatively lower positions of the quill 110, or vice versa. If the pressurised gas supply 418 comprises a pressure regulator, the counterbalance controller 416 could be configured to change the pressure which the pressure regulator is set to try to maintain.

[0064] The counterbalance controller 416 could be configured to control the pressurised gas supply 418 (e.g. via changing the pressure a pressure regulator is set to achieve) so as to at least reduce any variation in the linear motor power required for holding a stationary position along the z-axis. If desired, the counterbalance controller 416 could be configured to control the pressurised gas supply 418 so as to ensure that the linear motor power required for holding a stationary position is substantially constant for a significant proportion of the z-axis. Either way, this could be achieved, for example, by the counterbalance controller 416 using the input from the encoder interface 412 to, for example, look up in a pre-calibrated table, or determine via a predetermined function, a particular setting (e.g. a particular gas pressure), which is then used to automatically determine how to control the pressurised gas supply 418.

[0065] As will be understood, in an alternative embodiment, the counterbalance controller 416 could automatically determine how to control the pressurised gas supply 418 based on an input from the main processing unit 410 (or the motor power amplifier 414). The input from the main processing unit could indicate the actual and/or demanded position of the quill 110. This could be rather than, or in addition to, an input of the position of the quill 110 from the encoder interface 412.

[0066] In another embodiment, the apparatus is configured to adapt the pneumatic counterbalance automatically depending on what is mounted on the end of the quill 110 (e.g. depending on the tool/probe mounted on the quill 110). This could be so as to adjust the pressure of the gas inside the counterbalance cylinder 302 automatically so as to adjust for any change in weight caused by the change in probe loaded on the quill 110, and therefore avoid a change in the work required of the linear motor (e.g. to ensure that counterbalance continues to substantially counterbalance the total weight of the quill and what it carries). This can be particularly significant when an optical probe, such as a camera or video probe is loaded on the quill 110, because such probes can have relatively heavy imaging optics.

[0067] For example, when a new probe is mounted on the articulated head 116 from the rack 450, the counterbalance controller 416 can receive an input indicative of the change. For instance, the counterbalance controller 416 could receive an input from the main processing unit 410 which indicates what probe is now (or is about to be) loaded on the quill 116. The counterbalance controller 416 could then use this input to look up in a preconfigured table, one or more parameters related to the weight of the probe, and then instruct the pressurised gas supply 418 to change the pressure of the gas inside the counterbalance cylinder 302 accordingly. In an alternative embodiment, the main processing unit 410 could provide the counterbalance controller 416 one or more parameters related to the weight of the probe (rather than the counterbalance controller 416 having to look it up).

[0068] In another alternative embodiment, in response to the counterbalance controller 416 receiving an input which indicates that a probe change has taken place, the counterbalance controller 416 could determine the effect of the module on the coordinate positioning apparatus and thereby determine how to control the pressurised gas supply 418 so as to change the pressure of the gas inside the quill 110 accordingly adjust the pressure accordingly. For example, the counterbalance controller 416 could initiate a weighing operation to determine a parameter relative to the weight of the probe loaded on the quill 110. For example, this could comprise the main processing unit 410 controlling the quill's z-axis position in a particular predetermined way (such as keeping the quill 110 in a stationary position, moving the quill up and/or down, and/or for accelerating the quill 110 over a known distance) and determining from the motor power amplifier 414 what the power requirement is for performing such control. The power requirement (or one or more parameters indicative of the power requirement) can be input to the counterbalance controller 416 (e.g. optionally via the main processing unit 410) to determine a parameter related to weight of the probe. As another example, the main processing unit 410 could control the motor power amplifier 414 to move the quill 110 with a particular force. The acceleration of the quill 110 could be measured. This would enable the mass/load of the quill (or a parameter related thereto) to be determined. In any case, the counterbalance controller 416 could then use the information from any or a combination of such above described routines to determine how to control the pressurised gas supply 418 so as to change the pressure of the gas inside the counterbalance cylinder 302 accordingly.

[0069] The above described embodiment requires determining a parameter related to the weight of the probe. Such a parameter could be the weight of the probe (e.g. in a unit of weight, such as grams). However, as will be understood, this need not necessarily be the case. For example, a parameter which is dependent on/indicative of the weight could be identified instead. For example, rather than using a look-up table to determine the weight of a probe loaded (or to be loaded) on the quill 110, the look-up table could be used to determine what setting (e.g. what air pressure) the pressurised gas supply 418 should be set to depending on the probe loaded (or to be loaded) on the quill 110. As another example, the weighing operation could merely determine a particular setting (e.g. what air pressure) the pressurised gas supply 418 should be set to, rather than actually determining the weight (in a unit of weight) of the probe.

[0070] In another embodiment, the counterbalance controller 416 could be configured to automatically control the pressurised gas supply 418 so as to adjust the pressure inside the counterbalance cylinder 302 depending on the direction motion of the quill 110. For instance, in the described embodiment when the quill 110 is moving upward gas the pressure within the counterbalance cylinder 302 could be increased, and when the quill 110 is moving downward the pressure within the counterbalance cylinder 302 could be reduced, thereby assisting the linear motor. Accordingly, the counterbalance controller 416 could receive an input from the controller 410 indicating a direction in which the quill 110 is moving, and the counterbalance controller 416 could control the pressurised gas supply 418 so as to adjust the supply of pressurised gas accordingly (e.g. by adjusting the pressure a pressure regulator of the pressurised gas supply 418 is set to maintain). For instance, if the counterbalance controller 416 receives an input from the controller 410 indicating that the quill 110 is moving, or is about to move, upwards, the counterbalance controller 416 can control the pressurised gas supply 418 so as to increase the pressure inside the counterbalance cylinder 302, whereas if the counterbalance controller 416 receives an input from the controller 410 indicating that the quill 110 is moving downwards, the counterbalance controller 416 can control the pressurised gas supply 418 so as to decrease the pressure inside the counterbalance cylinder 302. As will be understood, rather than waiting for the movement to occur before adjusting the pressure, the counterbalance controller 416 could pre-empt a future motion by adjusting the pressure shortly before the motion is due to occur.

[0071] An example of how pressure within the counterbalance cylinder 302 might vary over time when implementing this aspect of the invention is illustrated in FIG. 5a. As shown, when the quill 110 is stationary at a notional axial position of Z=100, the pressure is maintained at a first constant set (i.e. target) pressure (e.g. 5 bar). However, when the quill is about to be moved downwards, the counterbalance controller 416 (on the basis of an input from the controller 410) can know in advance that such motion is to take place and thereby control the pressurised gas supply 418 so as to decrease the set (i.e. target) pressure inside the counterbalance cylinder 302 during such motion (e.g. 4.5 bar) so as to assist the linear motor (e.g. to reduce the motor power requirement, or to increase the travel speed for a given motor power/current). On returning to a stationary position, the counterbalance controller 416 (on the basis of an input from the controller 410) can bring the pressurised gas supply 418 back up to a level where it substantially counterbalances the total load on the quill. As shown, in this case (and in accordance with the other above described aspect of the invention), the counterbalance controller 416 does not return the pressure back to the same set (i.e. target) pressure, but instead to a slightly different set (i.e. target) pressure (e.g. 4.9 bar) because the total load on the quill is different at this different z-axis position (e.g. because the quill 100 is carrying less of the energy track at this new z-axis position). As also shown, during the motion there might be some small fluctuation in the actual pressure within the counterbalance cylinder 302 due to lag/hysteresis in the pressure regulator which is trying to maintain the pressure at the new set level of 4.5 bar.

[0072] The graph of FIG. 5a is to be contrasted with the graph of FIG. 5b which illustrates the how pressure within the counterbalance cylinder 302 might vary over time when using a known pressure regulator which is configured to maintain a set (i.e. target) pressure of a pneumatic counterbalance system. As shown, in this case, the system is configured such that the at all times (i.e. when stationary and during motion), the same pressure is maintained within the counterbalance cylinder 302. As shown, there might be some initial fluctuations during the motion due to lag/hysteresis in the pressure regulator, but nominally the pressure is the same, and in contrast to the present invention, as illustrated in FIG. 5a, the system is configured such that the pressure is maintained constant in all circumstances.

[0073] In the above described embodiments, the counterbalance controller 416 is shown as having dedicated inputs from the various other units/interfaces/amplifiers in the controller 118. However, as will be understood, this need not necessarily be the case and other configurations are possible. For example, the counterbalance controller 416 could receive a single input from the main processing unit 410 (e.g. which could relay signals from the encoder interface 412 and/or amplifier 412). Optionally, the counterbalance controller is provided by the main processor unit 410 (i.e. in which case the unit 416 as a separate entity need not exist).

[0074] Optionally, the main processing unit 410 communicates with, and controls, the pressurised gas supply 418 directly (this could be the case when the unit 416 as a separate entity does or does not exist).