GB2272769A - Surface following device - Google Patents

Abstract

A surface following device has a workpiece contacting probe 26 rigidly connected to a head or movable arm 24. The arm 24 is supported for vertical movement on a carriage 18 which is in turn movable along a track 14 attached to support pillars 16. A belt is attached at its two ends E1, E2 at the base of the arm 24, and is coupled to motors M1, M2. Guide rollers R1, R2, R3, R4 and further guide roller 34 co-operate with the motors M1, M2 and the mass of the arm 24 to enable the probe to be moved in any direction in the plane of the track 14. The direct coupling of each of the drive motors M1, M2 to the arm 24, and the mass of the arm 24 enable the probe 26 to be driven over the contour of a surface without the need of any compliance in the machine or any complex control system to operate the motors. <IMAGE>

Description

SURFACE FOLLOWING DEVICE The present invention relates to a surface following device for tracing the contour of the surface of a workpiece.

According to the present invention there is provided a surface following device comprising: a bed; a probe supported for at least two-dimensional motion relative to the bed; means for providing a biasing force on the probe, to bias the probe in a first direction; a first motor coupled to the probe to produce a first drive force on the probe having a first component acting in opposition to the biasing force, and a second component acting perpendicular to the biasing force; a second motor coupled to the probe, to produce a second drive force on the probe, having a first component acting in opposition to the biasing force, and a second component acting in opposition to said first component of the first driving force and perpendicular to the biasing force; wherein said first and second motors are each coupled to the probe by means of a flexible drive member acting between a said motor and the probe.

Thus, the first and second motors may act antagonistically in a direction perpendicular to the biasing force, and in unison to produce a resultant force which opposes the biasing force thereby enabling the probe to be driven in any direction in said two dimensions.

Preferably, the support comprises a track fixedly supported relative to the bed, a carriage mounted for movement along the track, and a head for supporting the probe, mounted on and for movement relative to the carriage in a direction non-parallel to the length of the track.

In a preferred embodiment the direct couplings between the first and second motors and the probe are provided by a belt having two free ends attached to the probe, and guided by a pair of rollers at either end of the track, and a further roller on the head. Preferably each motor drives one roller of the pair of rollers at either end of the track. In one example of the present invention, further belt guiding rollers are provided on the carriage, to constrain the belt to extend only (a) in the direction of the length of the track and (b) in the direction of movement of the head relative to the carriage.

Because the motors are mechanically coupled it is desirable to control the motors in a manner in which they are electrically coupled. A control system for controlling a single motor (e.g. with a velocity servo) may thus be provided to control both motors.

kn embodiment of the present invention will now be described, by way of example, and with reference to the accompanying drawings in which; Fig 1 shows a surface following device according to a first embodiment of the present invention; Fig 2 shows a section on II-II of the surface following device shown in Fig 1; Figs 3a-3f show schematic representations of the operation of the surface following device in Figs 1 and 2; Fig 4 shows a control system for controlling the motors of the surface following device in Figs 1 to 4; Fig 5 shows a scanning operation using the surface following device of Figs 1,2 and 3a to 3f Fig 6 shows a detail of the surface following device in Figs 1 and 2; Fig 7 shows a surface following device according to a second embodiment of the present invention; Fig 8 shows a detail of Fig 7; and Fig 9 shows a representation of a surface following device according to a third embodiment of the present invention.

Referring now to Figs 1 and 2, a surface following device 10 comprises a bed 12 and a track 14 extending substantially parallel to the surface of the bed 12 and supported at its ends by pillars 16. A carriage 18 is supported for movement along the length of the track (defined as the X direction) by four guide rollers 20.

The carriage 18 supports, by means of four further guide rollers 22, a head of the machine 24 for motion of the head perpendicular both to the surface of the bed 12, and the X direction (the direction of movement of the head 24 is defined as the Z direction). The head 24 carries at its lower (i.e. adjacent the bed 12) end a hard probe 26 having a semi-spherical free end for contacting a workpiece.

A belt 28 is attached to the head 24 adjacent the probe 26 by its two free ends El and E2. The belt 28 passes around a pair of driving rollers 32, situated at either end of the track 14 on pillars 16, and a belt-guiding roller 34 provided on the head at its upper end. The belt 28 is constrained to extend (a) along the length of the head 24 between its free ends El and E2 and the belt-guiding roller 34, and (b) along the length of the track 14 between the driving rollers 32, by four further belt-guiding rollers R1 to R4 situated on the carriage 18.

Driving rollers 32 are each driven by a motor; motor M1 and motor M2. Thus, the probe 26 is supported by the head 24, carriage 18 and track 14 for motion in a two-dimensional plane (i.e. the X-Z plane) relative to the bed 12. The probe 26 may be driven in any direction in the two-dimensional plane by suitable rotation of the motors M1 and/or M2, causing the drive rollers 32 to feed the belt 28 through the various guide rollers in a number of different ways. The weight of the head 24 and probe 26 provide a biasing force on the probe 26, constantly urging it toward the surface of the bed 12 of the machine. The biasing force may be provided by other means, such as a spring, or by an electrical biasing voltage on the motors.

Movement of the carriage 18 relative to the track 14 in the X direction, and movment of the head 24 relative to the carriage 18 in the Z direction are each measured by a scale and readhead system (not shown). Operation of the motors M1 and M2 to cause the probe 26 to travel in various ways will now be described with reference to Figs 3a-3f.

Referring now to Figs 3a and 3b, the operation of the motors M1 and M2 (to drive the rollers 32) for driving the probe 26 in the X direction along the bed 12 of the machine from left to right will now be described. If the motor M2 is rotated anti-clockwise, the point D on the belt 28 will be reeled towards the motor M2. Free end E2 of the belt 28 will thus experience a force upward.

However, because the downward biasing force on the probe 26 (by virtue of its mass) is greater than the frictional resistance to movement of the carriage 18 along the track 14, the probe 26 remains in contact with the bed 12 of the machine and the length of the belt between free end E2 and the roller R4 does not shorten. Thus, the result of anti-clockwise rotation of the motor M2 is that the carriage 18 is pulled toward the motor X2 uniquely in the X direction. Movement of the carriage 18 from left to right causes (by virtue of the free end El of the belt 28 being fixed to the probe 26 and thus effectively fixed to the end E2) the length of belt 28 between the motor M1 and roller R1 to lengthen; the belt 28 must thus feed around the guide roller 34 and motor M1 in an anti-clockwise direction at an equal rate to the rotation of the motor M2. It can be seen from Figs 3a and 3b, that movement of the carriage 18 from left to right results in point A moving from between motor M1 and roller R2 to between the motor M1 and the roller R1; the movements of the points B to D are also shown, and show that each point on the belt 28 has moved an equal distance to every other point on the belt (since the belt is of fixed length). Thus, to move the probe 26 from left to right in the X direction one of the motors must be driven, and the other of the motors must be left idle in order to allow the belt 28 to feed around it, and enable the biasing force of the probe 26 to keep the probe in contact with the surface of the bed 12.

Operation of the motors M1 and M2 to cause the probe 26 to move vertically upwards will now be described with reference to Figs 3c and 3d. As can be seen from Fig 3c, the probe 26 is lying against the surface of the bed 12 at the start of the operation, and the motors M1 and M2 are applying no force to the probe via the belt 28. The object of the operation is to move the probe 26 upwards and uniquely in the Z direction. To move the probe 26 upwards it is necessary to apply an upwards force to the probe, and this can be done by rotating motor M1 (for example) in the clockwise direction, to cause an upward force to act on the free end El of the belt 28. However, as was shown in the description of Figs 3a and 3b, this results (if motor M2 is left to rotate freely) merely in a sideways movement of the probe 26. Thus, to counteract the force in the X direction on the probe 26, motor M2 must also apply an upward and lateral force to the free end E2 of the belt 28; this can be done by rotating the motor M2 anti-clockwise. Motor M1 and motor M2 must thus be driven an the same angular speed but in different directions in order to drive the probe 26 uniquely in the Z direction. It can be seen from Figs 3c and 3d, that points A and D on the belt 28 are reeled toward motors N1 and M2 respectively. The "slack" of the belt 28 between motor M1 and roller R2, and motor M2 and roller R3, is taken up by the upward movement of guide roller 34, which occurs as a result of the upward movement of the probe 26.

It has thus been shown that: a) In order to produce movement of the probe 26 uniquely in the X direction the motors M1 and M2 must be rotated at the same speed, and in the same direction b) rotation of the motors M1 and M2 at the same speed and in different directions produces movement of the probe 26 uniquely in the Z direction.

The third "boundary" control condition of movement of the probe 26 is if one motor remains locked stationary, while the other motor is driven; this will now be described with reference to Figs 3e and 3f. It can be seen from Fig 3e that the probe 26 is initially resting against the surface of the bed 12, and the carriage 18 is situated at the left end of the track 14. In the example illustrated, motor M1 is locked and cannot move, and motor M2 is rotated in the anti-clockwise direction. As illustrated previously, rotation of the motor M2 in an anti-clockwise direction causes an upward force on the free end E2 of belt 28, and a sideways movement of the carriage 18. However, on this occassion motor M1 is restrained from rotating and so the length of the belt 28 between motor M2 and free end El, must remain constant throughout the operation. The result of this condition is that as rotation of motor M2 reduces the length of the belt 28 between the roller R4 and the motor M2, the probe 26 moves both sideways and upwards in a 45' movement relative to the bed 12. As before, the "slack" part of the belt 28 fed round between motor M2 and roller R3 is taken up by the upward movement of guide roller 34 consequent to the corresponding upward movement of the probe 26. Thus, locking one of the motors, and driving the other one results in a movement of the probe 26 at an angle of 45 to both the X and Z directions.

One of the consequences of directly mechanically coupling both the motor M1 and the motor M2 to the probe 26, is that the motors M1 and M2 are also directly mechanically coupled to each other. This has the advantage of greatly simplifying the control of the motors M1 and M2. An example of a typical control system for controlling the motors M1 and M2 will now be described with reference to Fig 4.

Referring now to Fig 4, a voltage Vs, output from amplifier 40 is applied to motor M2, a voltage drop from node N1 to N2 producing an anti-clockwise torque on the motor M2. The voltage Vs is also applied, via a diode bridge 42 to motor M1; a voltage drop from node N3 to node N4 across motor M1 produces a clockwise torque of the motor. An off-set voltage V0 is also applied across the diode bridge 42 to motor M1, and has an opposite polarity to the voltage Vs. The voltage V0 has a similar magnitude to the typical value of voltage Vs applied to the motor M2 to drive the motor (this will be discussed in detail later). Each of the motors M1 and M2 drive a mechanically coupled tachometer TM1 and TM2 respectively, which generate the voltages VM1 and VM2. The voltages VMl and VM2 are combined to produce a feedback voltage Vfb which is compared with a reference voltage Vref, at amplifier 40. The nature of the feedback signal Vfb depends upon the control regime with which it is desired to drive the probe 26. For example, in order to achieve a constant surface speed of the probe 26 relative to a workpiece, the motors must be driven such that: 'oXl2 + X22 = K Where: xM1 is the angular speed of the motor M1, ZM2 is the angular speed of the motor M2, and K is a constant.

Thus, if it is desired to control the motors to produce a constant surface speed, the feed-back signal is generated in the following way: Vfb = EVM12 + VM2 ] The feedback signal need not be generated from tachometers. A feedback signal can alternatively be generated from the scales and readheads or by any other suitable means.

A scanning operation according to the present invention will now be described with reference to Fig 5. Referring now to Fig 5 the probe 26 is driven initially uniquely in the X direction along the bed 12 of the machine. To achieve this, a positive voltage is applied to motor M2, causing it to apply a driving force to the belt 28 in an anti-clockwise direction; motor M1 is driven with a voltage (VS-V0), causing it to apply a driving force to the belt in a clockwise direction. Because the offset voltage Vg is similar to the voltage Vs, the driving voltage of motor M1 will be small compared with the driving voltage of motor M2. Furthermore the voltage (V5-V0) is chosen such that it is insufficiently large to cause motor M1 to overcome the downward biasing force on the probe 26. Thus motor M2 rotates anticlockwise to reel in the free end E2 of the belt 28, and because M1 is not being driven with a high enough voltage to overcome the biasing force on the probe, the probe 26 stays in contact with the bed of the machine and motor M1 also rotates anticlockwise. The effect of applying voltage (Vs-VO) to motor M1 is simply to produce clockwise torque, and this provides an upward force on the free end El, to reduce the force with which the probe 26 bears against the surface of the bed 12, (and thus the lateral force on head 24 due to friction between the probe 26 and the bed 12 of the machine). The probe 26 is therefore driven from left to right, uniquely in the X direction (as shown in Fig 5).

When the probe 26 comes into contact with the vertical surface S1 of workpiece W, the resistance to movement of motor M2 increases, and rotation of motor M1 instantaneously ceases (since the probe is no longer being driven from left to right in the X direction). The voltage VM1 from tachometer TM1, and the voltage VM2 from tachometer TM2 thus fall, resulting in a much lower feedback voltage Vfb. The differential output Vs of the amplifier 40 thus increases, and this has two effects.

Firstly, a greater voltage is applied to motor M2 (to apply a torque in an anti-clockwise direction) and this higher voltage acts to reel in the free end E2 of belt 28 with an even greater force, causing the belt 28 to pull the probe 26 vertically upwards along the surface S1. The second effect of the increase in Vs, is a corresponding increase in (Vs-V) and thus a greater voltage is applied to motor M1 (to apply a torque in a clockwise direction).

As has been illustrated in Figs 3c and 3d upward movement of the probe 26 in the Z direction results in an anticlockwise rotation of motor M2 and a clockwise rotation of motor M1. Both the motors nl and M2 are therefore now acting to pull the probe 26 upwards along the surface S1 of the workpiece W. The offset voltage V0 performs the function of maintaining the voltage on motor M1 at a fractionally lower level than the voltage on motor M2 so that the probe will stil be urged fractionally against the workpiece in the X direction as it mounts the surface S1; this will make sure that the probe 26 maintains contact with the surface S1.

As the probe reaches surface 82, which is inclined at 45 to the bed 12 of the machine, the sideways bias (in the X direction) causes the probe 26 to start to slide over the edge (joining S1 and S2) in a curved path (caused by the curvature of the contact end of the probe 26). This slight sideways movement of the probe 26 causes a small reduction in the resistance to rotation of the motor M2 and motor M2 starts to rotate faster, since for a given driving voltage Vs, it is having to perform less work.

Thus, the output VX2 of tachometer TX2 increases and the value of Vfb increases, thus causing a reduction in the output voltage Vs of amplifier 40. Motors M1 and M2 will thus be driven with a lower voltage in their respective directions. However, as the probe 26 continues to travel over the edge between surfaces S1 and S2, motor M2 will experience progressively less resistance to its driving action, (and therefore for a given input voltage will be driving faster), and motor M1 will be slowing down since the probe 26 is now moving slower in the Z direction. As explained above, this results in a further increase in the feedback voltage Vfb, and a further reduction in the output Vs of amplifier 40. This feedback process is reiterated many times until the probe 26 begins to travel along the straight surface 52 at 45' to the bed 12 of the machine. As illustrated with reference to Figs 3e and 3f, to drive the probe 26 at 45' to the X and Z directions motor M2 rotates anticlockwise, and motor M1 remains stationary. Because the control system is acting to maintain a constant surface velocity of probe 26, motor M2 is now rotating at an angular speed 2 times it previous angular speed. Motor M1 is once again driven by a voltage (VS-V0) in a clockwise direction, and this once again acts to reduce the force with which the probe 26 bears against the surface S2, and thus reduce the frictional resistance to movement of the probe 26 and head 24.

When the probe 26 reaches the edge of adjoining surfaces S2 and S3, the same phenomenon in the control system as was described before the transition of the probe 26 between surface S1 and S2 occurs. The equilibrium conditions for driving the probe 26 along the surface 53 are identical to those described for driving the probe along the surface of the bed 12.

When the probe 26 starts to move over the edge between surfaces S3 and S4, the bias on the probe 26 (by virtue of its mass) causes the probe to start to run down the slope 84. This results in an increase in rotation of the motor M1, and a decrease in the rate of rotation of motor M2.

Since less work is required to drive the probe 26 down the slope S4 the voltage Vfb increases, and thus the output of amplifier 40 decreases. This process continues until motor M2 ceases to rotate (as was explained with reference to Figs 3e and 3f) and motor M1 rotates at a speed times its rotational speed for movement of the probe 26 uniquely in the X direction. The voltage on motor M1 is now acting to provide a breaking force against the movement of the probe 26 down surface S4. The voltage Vs is now smaller than the voltage V0, but due to the diode bridge 42 the motor M1 is still driven with a voltage (Vs-Vo) in the clockwise direction to slow the probe 26 down. The voltage Vs causes the application of a small driving force on motor M2, acting to lift the probe 26 off the surface S4, and thus reduce the frictional force between the probe 26 and the surface S4.

Movement of the probe 26 over the edge between the surface S4 and S5 result in a similar phenomenon to movement of the probe 26 over the edge between the surfaces S3 and S4.

The motors M1 and M2 break the motion of the probe 26 down the surface S5 by driving against the direction of motion.

Motor M1 applies a greater driving force than motor M2, to provide biasing of the probe 26 into the surface S5 in order to maintain contact.

Because the probe 26 in the illustrated embodiment of the present invention is a hard probe, it is important that the probe 26 is not driven into a workpiece with too great a force. In such an event, the force would be transmitted directly via the head 24 of the machine to carriage 18, and the track 14, causing very high loads on each of these members, and also due to distortion of the apparatus, inaccurate data on the workpiece profile. It is therefore desirable to provide some form of mechanical fuse which breaks when, for example, the force between the probe 26 and the head 24 exceeds a predetermined threshold. The breaking of the fuse may be used either to stop the machine completely, or to disregard any subsequently generated data until such a time as the fuse has been reset. Referring now to Fig 6, the probe 26 is kinematically supported on the head 24 by a support mechanism comprising three pairs of radially extending equispaced rollers 50, which are provided on the upper face of the probe 26, and three correspondingly placed balls 52 provided on the lower face of the head 24. When the probe 26 and head 24 are connected together each of the balls 52 seats in the cleft provided by the convergent surfaces of an adjacent pair of rollers 50. The balls 52 are retained in contact with the rollers 50 by a spring 54, whose stifness determines the magnitude of the force required on the probe 26 to trip the fuse. The rollers 50 and balls 52 are wired in series with each other, and an interface 56 is provided to detect the resistance of the circuit and emit a pulse signal when it exceeds a predetermined threshold (i.e. when one or more of the roller 50 / ball 52 contacts has, or is just about to be broken).

An alternative to the mechanical fuse described above, will be to provide a one-dimensional analogue probe on the head 26, which outputs an analogue signal corresponding to the deflection of a stylus from some null position. Such an output could be summed with the outputs of the scale and readheads to provide the true position of the stylus within the reference frame of the machine. When the analogue output from the probe exceeds a predetermined threshold, the machine control system is instructed to reduce the driving forces on one or more of the motors, thereby reducing the loads on the machine. The advantage of using such an analogue probe to provide the fuse signal is that valid data may still be obtained over the period during which excess loads are present on the machine, due to the resilience in the probe.

A second embodiment of the present invention, which provides scanning of a workpiece in two orthogonal directions, but which however still only requires two motors will now be described with reference to Fig 7.

A machine 100 comprises a first track 114, extending in the X direction, and a carriage 118 mounted for movement along the first track. The carriage supports a head 124 for movement relative to the carriage in the Z direction, and the head supports a probe 126 having a semi-spherical free end for contacting a workpiece. The carriage 114 is mounted at each end on a further pair of carriages 160A and 160B, which in turn are each mounted for movement along a further track 162A and 162B respectively. The two tracks 162A and 162B extend parallel to each other and are supported at their ends by four pillars 164A to 164D. The carriage 118 is thus supported for two dimensional movement in the X-Y plane, while the head 124 is supported for three dimensional movement. Referring now to Fig 8, a belt 128 is attached to the head 124 adjacent the probe 126 by its two free ends E10 and E20. The belt passes around a pair of motors M10 and X20, situated at the ends of the tracks 162A and 162B adjacent the pillars 164A and 164D respectively. The belt is constrained to extend along the length of the head 124, the track 114, and the tracks 162A and 162B by ten belt-guiding rollers R11 to R20, and a further belt-guiding roller 134. This embodiment of the present invention thus provides means for scanning in either an X-Y, or an X-Z plane by locking movement of the carriage 118 in the X or Y directions respectively. It is thus possible to scan the profile of a workpiece along two perpendicular paths in the X and Y directions, or alternatively along two parallel paths e.g.

in the X direction separated by a small distance in the Y direction.

When one of the X or Y movements of the carriage 118 is locked, the machine behaves in exactly the same way as the machine described in the first embodiment of the present invention. For example, with movement of the carriage 118 in the X direction locked, and the motor M10 left to rotate freely, a clockwise rotation (when viewed from above) of motor M20 will produce an upward force on the free end E20 of the belt 128. However, by virtue of the mass of the probe 126 and head 124, the downward biasing force on the probe 126 is greater than the frictional resistance to movement of the carriage 118 in the Y direction (provided in this embodiment by movement of the entire track 114 and carriages 160A and 160B along the tracks 162A and 162B respectively). The action of the motor M20 will thus act to reel in the free end E20 of the belt 128, and thus the point A on the belt will move from between roller R18 and motor M20 to between motor M20 and roller R20; the corresponding movement of the point B will occur from between roller R19 and motor M10 to between motor M10 and roller R15. Thus, the rotation of one of the motors M10 and M20 in conjuction with the free movement of the other of the motors at the same speed of rotation will provide motion uniquely in the Y direction if the movement of the carriage 118 in the X direction is locked. This is also true for locking of the motion of the carriage 118 in the Y direction.

Similarly, if motor M10 is locked stationary the x-axis is locked, and motor M20 is rotated clockwise then the probe 126 will move at 45e upwards in the negative Y direction.

Clockwise rotation of motor M10, and anticlockwise rotation of motor M20 will result in movement of the probe 126 in the positive Z direction.

By introducing a second belt, and a third driving motor it is possible to provide for scanning of a workpiece in the X-Y plane as well as the X-Z and Y-Z planes. Referring now to Fig 9, a further belt 228 is provided, and this belt extends around a motor M30, and rollers R115 to R210, situated in the XY plane directly below roller M10 or M20 may be used to bias the probe 126 in the positive x, negative y (or negative x, positive y) direction. For example driving M10 clockwise biases probe 126 in -ve x and +ve y. The embodiment of Fig 9 thus provides for the possibility of scanning in three orthogonal planes.

Claims (1)

1. A surface following device comprising: a bed; a probe supported for at least two-dimensional motion relative to the bed; means for providing a biasing force on the probe, to bias the probe in a first direction; a first motor coupled to the probe to produce a first drive force on the probe having a first component acting in opposition to the biasing force, and a second component acting perpendicular to the biasing force; a second motor coupled to the probe, to produce a second drive force on the probe, having a first component acting in opposition to the biasing force, and a second component acting in opposition to said first component of the first driving force and perpendicular to the biasing force; wherein said first and second motors are each coupled to the probe by means of a flexible drive member acting between a said motor and the probe.