PROBE FOR MEASURING WORKPIECES

Abstract: A probe for measuring the dimensions of a workpiece has a spherical sensing element supported on a base for three-dimensional displacement relative thereto in response to engagement between the sensing element and a workpiece. The probe comprises transducers (26) connected to a circuit (30) for determining the displacement of the sensing element in terms of the coordinate components value (X1, Y1, Z1) of the displacement. To make possible the use of the probe as a trigger probe, there is a circuit (40) for re-combining the component values thereby to determine the original displacement in terms of a resultant value (41) and a comparator (32) is provided to generate a step signal (17) when the resultant value (41) exceeds a given threshold value (r). Since the threshold value is constant, the step signal (17) occurs in response to the same displacement regardless of the direction of such displacement.

Description: PROBE FOR MEASURING ORKPIECES This invention relates to a probe for measuring the dimensions of workpieces. Background to the Invention: It is known to measure the dimensions of workpieces in coordinate positioning apparatus having measuring devices for determining the coordinate position of a probe relative to the workpiece. The apparatus is operated to move the probe towards a surface of the workpiece whose coordinate position is to be measured and the probe is adapted to the output a signal responsive to a stylus, forming part of the probe, engaging the surface. In so-called "trigger probes" the probe signal is a step signal produced responsive to engagement between the stylus and the surface, and the position of the surface is measured in terms of the reading of the measuring devices of the machine at an instant following said engagement. In so-called "proportional probes" the measuring devices of the machine are used to move the probe to a known demanded position relative to the n r .surface to be measured, and the probe itself has a short- range measuring device whereby to determine the amount by which the 'actual position of the surface differs from the demanded position. ' χ_ - <-* A known type of proportional probe (British Patent No. 1,573,447) has a base, a stylus, resilient means for supporting the stylus on the base for displacement relative thereto between a rest position and a displaced position, 0 means defining a pair of orthogonal coordinate axes, and means for measuring said displacement in terms of component outputs defining respective coordinate components of the displacement. 5 It is one of the objects of this invention to adapt a measuring probe of the kind described so that it can be used as a trigger probe. This has the advantage that the same probe can be used for either purpose. Also, inasmuch as-the measuring prob-e can be constructed to have high 0 accuracy, such accuracy would also be available in -the. trigger mode for determining the instant at which the step signal is generated. Alternatively, it is an object of this invention to provide 5 a trigger probe of improved accuracy. In known trigger probes there is a difficulty in accurately relating the instant at which the stylus engages the workpiece to the instant at which the step signal is produced. The unavoidable " displacement of the stylus between these two 0 events (the "pre-travel" of the stylus) is not always uniform for all conditions of operation. In particular, the pre-travel may be different for different directions of displacement of the stylus relative to the base. Since the measurement by the measuring devices of the machine is 5 carried out while the probe is in motion, then, for a given speed of that motion, any variation in pre-travel results in a variation of the measurement. An attempt to overcome the difficulty was made in Britj. h 5 Patent No. 1,593,050 which shows a probe comprising a base, a plate supported on the base on a generally annular seat provided in a fixed position thereon, bias means urging the plate into engagement with the seat, a stylus secured to the plate and having a free end remote from the seat, the 0 stylus being moveable in opposition to the bias means between a rest position in which the plate engages the seat at least three locations and a displaced position in which the plate is tilted on or axially removed from the seat, a transducer arranged at the axis of the seat annulus, the 5 transducer having a moveable part connected to the plate and a fixed part connected to the base so as to produce an output corresponding to the displacement of the stylus, and means for generating a step signal when the transducer - output crosses a predetermined magnitude. !0 The improvement made by the last-mentioned trigger probe • was not sufficient for increasing demands for accuracy. The problem was that it is difficult to make the generally annular seat such as to provide the same seating accuracy all round the annulus, i.e. in some places the seat surface was higher than in others. Also there is a difficulty in ensuring that the transducer is positioned at the centre of the annulus. As a result, when the stylus was tilted, the pre-travel differed for different direction of tilt. 0 Even if the seat was made in the form of a circular knife edge (British Patent No. 2,094,478) it was difficult to ensure with sufficient accuracy that the height of this knife edge was uniform all round its circumference and that the transducer was at the centre thereof. 5 The problem may be stated as follows. In the known trigger probes the step signal occurs when the moveable part of the transducer has a given position relative to the fixed part. Due to manu acturing tolerances of mechanical components there arises variation in pre-travel. This is equivalent to the rest position of the stylus being " different for different direction of displacement, a situation which would obviously falsify the measurement. Brief Description of the Invent inn According to the present invention there is provided a probe for measuring the dimensions of workpieces; comprising a base, a stylus having a free end whereby, in use, to engage a workpiece, support means for supporting the stylus on the base for displacement relative thereto between a rest position and a displaced position, means defining a pair of orthogonal coordinate axes, and means for determining a said displacement in terms of coordinate outputs defining respective coordinate components of the displacement: charaterised by combining means for combi . ning said outputs into a resultant o.utput being a function of said displacement, and threshold means for generating a step signal when said resultant output crosses a given value during a said displar me . The step signal occurs at a value of said resultant output which is constant for all directions of said displacement and is the radius ( the "trigger radius") of a circle or, if three components are used, of a sphere whose centre can be used to define the rest position of the stylus. Said given value υf the resultant output may, for example, be 1 volt calibrated to correspond to a displacement of 0.01 millimeter. In use, the probe is mounted in coordinate positioning apparatus (the "machine"} such that the coordinate axes of 4 BAD ORIGINAL t- z> the probe are parallel to the corresponding axes of the machine. The stylus is provided at the free end thereof with a spherical element or ball whereby to engage the workpiece. The probe is calibrated by setting the component outputs thereof to zero, and thereafter opera-ting the machine to move the ball into engagement with respective datum surfaces provided at fixed positions in the machine, the direction of such engagement being substantially normal to the datum surfaces concerned. Each such engagement produces a set of said component values which are fed into a computer for computation of a said resultant output and generation of a said step signal. In each case the step signal occurs at a given pre-travel following the instant of engagement. Since the trigger radius is the same for all directions of engagement, i.e. for any orientation of the datum surfaces, the pre-travels are similarly uniform. The readings of the measuring devices of the apparatus (the "machine sςales" i at the instants of the respective step signals are recorded as datum values and are fed to- th.e computer. The computer is programmed to compute the position of the centre of the ball, making allowance for the radius of the ball and of the trigger radius. It will be understood that the centre of the trigger radius and the centre of the ball coincide. The position of that common centre may be taken to define the rest position of the stylus. Measurement of the workpiece then proceeds by moving the stylus ball into engagement with the respective surfaces of the workpiece, recording the respective outputs of the machine scales and computing the actual positions of the surfaces by adding or subtracting, as the case may require, the ball and the trigger radii. It may occur that the system looses calibration due to electronic drift but so long as such drift does not exceed 5 BAD ORlGiN AL the trigger radius the performance need not be significantly affected. In any case, this difficulty can be covercome by the provision of means for biasing the component outputs to zero when the stylus is free. _.r Brief description of the Drawings In the accompanying drawings: Fig.l is an elevation of a coordinate measuring machine. Fig.2 is an elevation of a probe used in the machine. Fig.3 is a section on the line III-III in Fig.l Fig. is a circuit diagram. Fig.5 is an enlarged section on the line V—V In Fig.2. Description of an Example of the Invention An example of the invention will now be described", with reference to the above " drawings . Referring the Fig.l, the . robe, denoted P, comprises a sensor 13 mounted in a coordinate measuring machine M comprising a combination of carriages 10,11,12 adapted to support the sensor 13, for movement relative to a workpiece 14 in the X,Y,Z directions of the rectangular coordinate system. The movement of the sensor is measured by a combination of opto-electronic scales, e.g. 15, and digital counters 16 constituting the measuring system of the machine or the "machine scales". The probe P is adapted to produce a step signal 17 when the machine is operated to move the sensor into engagement with a selected surface of the workpiece. The signal 17 is connected to the counters 16 and the latter are adapted for the content thereof, at the instant of the signal 17, to be output as signals 18. The latter signals define the coordinate position of the sensor 13 in terms of the outputs of the measuring system of the machine. Referring to Figs 2,3 the sensor 13 comprises a stylus 20 having at one end a spherical sensing element or ball 21 whereby to engage the workpiece 14 as shown. The other end of the stylus is secured to one end 23, the free end, of a coiled spring 22. The other end, 24, of the spring is secured to a base 25 itself secured to the machine. The base 25 has an axis 22A extending in the Z-direction. The sensing element 21 is said to be at its rest position, and to be free, when no force acts thereon and it is said to be displaced when the machine is operated to bring the element 21 into engagement with the workpiece 14 and the force of such engagement moves the stylus away from the rest position. The displacement takes place in opposition to the reaction force of the spring 22 which returns ' the element 21 to the rest position when the machine is. operated to withdraw the probe from the workpiece and the displacing force ceases. Displacement of the element 21 in an X-Y plane causes the spring to bend approximately about a point 22B at the mid-length thereof. Displacement of the element 21 in the Z-direction causes compression or extension of the spring 22. Distinct from the measuring system of the machine M, the probe P itself has a short-range measuring system comprising four transducers 26 (Figs.2, 3) arranged symmetrically about the axis 22A and at the outside of the spring 22. The arrangement is such that two diametrally opposite ones of the transducers lie in the X-Y plane while the other two transducers lie in the Y-Z plane. In this way the position of the transducers defines a pair of coordinate axes X,Y (Fig.3). Each transducer 26 comprises 7 an armature 27 (Fig.2) secured to the base 25 and a core 28 secured to the free end 23 of the spring through structure 29. Each armature 27 comprises coils centred on an ax±s 26A parallel to the axis 22A and the associated core is correspondingly free to move in the Z-direction. The transducers 26 are arranged so that, when the stylus 20 is at rest, the cores are situated at an X-Y plane approximately through the point 22B. Therefore, if the element 21 is displaced transversely to the Z-direction, the resulting arcuate movement of the cores about the point 22B takes place predominately In the Z-direction and an acceptably small radial clearance between each core and the associated coils is sufficient to accommodate the inevitable small X-Y component of the movement. Referring to Fig.4, the transducers are connected to a discriminating circuit 30 for discriminating between X,Y and Z displacements of the element 21 and to -produce corresponding analogue voltage signals XI,Yl and Zl- which define the coordinate position of any displacement of the' centre 21A of the element 21 from the rest position (Figs.4,5). The circuit 30 is known from our British Patent No. 1,573,447. The signals X1,Y1,Z1 are taken through respective rectifiers 30A and calibrating potentiometers 30B to a zeroing or stabilising circuit 31 provided for stabilising the signals XI,Yl Zl against drift e.g. slow voltage changes due to temperature changes. The stabilised signals are denoted X2,Y2,Z2. Further, a combining circuit 40 is provided for combining the signals X2,Y2,Z2 into a single signal 41 which, as will be seen, is a function of any displacement of the element 21 regardless of the direction of such displacement. The step signal 17 is generated by a comparator 32 which changes the signal 17 from low to high when the signal 41 exceeds a given magnitude. The circuits 31 and 40 will now be described in 8 det ai 1 . The circuit 31 comprises in respect of the output XI aja/ amplifier 35 producing said output X2, and the latter output is connected to a transconductance amplifier 36 adapted to produce a signal 37 showing excursions of the signal X2 from zero in a direction opposite to that of corresponding excursions of the signal XI. The signal 37 is connected to the amplifier 35 whereby the signal X2 is biased to zero. The signal 37 is also connected to earth through a capacitor 38 to produce a time constant by which the action of the amplifier 36 is delayed so that for rapid changes of the signal XI, such as would occur when the element 21 is engaged with the workpiece 14 during a measuring operation, the signal X2 is not affected and remains proportional to the signal XI. Thus, when the proportional signal X2 appears in due course at the output of the circuit 40 as the signal 41, the signal 17 can be generated as required. However, for relatively slow changes in the signal X2, e.g. thermal drift, the time constant is such that the signal X2 remains zero. In this way, when the element 21 is at rest between measuring operations, the signal X2 always has the zero value required as a preliminary to a measuring operation. Further, the signal 17 is connected to the amplifier 36 by a line 39 and such that when the signal 17 is high the amplifier 36 is inhibited. Thereby, if it should occur that the element 21 is held in engagement with the workpiece 14 for any length of time (and provided the displacement of the element 21 was sufficient to trigger the comparator 32), the signal X2 will not be affected and the signal 17 will remain high to signify that the is system still in the triggered condition. Circuit elements corresponding to the amplifiers 35,36, the capacitor 38 and the line 39 are also provided in respect of each of the signals Y1,Z1 to ensure that the signaLs Y2, Z2 are biased to zero in circumstances like those described in respect of the signal X2. Regarding the combining circuit 40, as indicated above, this circuit combines the signal X2,Y2,Z2 into the single signal 41 Intended to be a function of displacement of the element 21 regardless of the direction of .such displacement, i.e. regardless of whether or not the direction of displacement is aligned with the X,Y,Z directions. This is now explained with additional reference to Fig.5 which shows the element 21 in engagement with a surface 14A of the workpiece 14 at a point 14B, the surface 14A lying at an angle to the X-Y axes of the sensor 13 and of the machine M, and the sensor having been moved toward the surface " 14Δ in the direction o.f a line V normal to that surface . Before the engagement at the point 14B the axis 22A coincided with the centre 21A. Following the engagement, continued travel of the probe caused the axis 22A to move along the line V relative to the centre 21A to a point 22A1 while the circuit 30 produced the signals X2,Y2 which correspond to displacements x,y of the axis 22A. The displacements x,y have a resultant displacement r such that r 2 = x 2 + y 2 (1) or, in three dimensions, r 2 = x 2 + y 2 + z 2 (2) wherein r is the radius of a sphere s and x,y,z are the coordinates of any given point on this sphere. Accordingly, in the circuit 40 (Fig.4), the signals X2,Y2,Z2 are fed to respective squaring circuits 42 whose outputs 43 are connected to a summing circuit 44 whose output Is the signal 41. The signal 41 is therefore 10 proportional to the sum of the suares of the signals X2,Y2,Z2 in accordance with the formula (2) above. The comparator 32 is connected to a bias feed 32A to produce the signal 17 when the value of the signal 41 reaches a given bias value or "trigger value" which, is this example is 1 volt, and the circuit 40 is calibrated for this value to correspond to a displacement equal to 0.01 millimeter. It will be clear that since the trigger value is 1 volt, i.e. unity, the square of r can be disregarded. A square root-forming circuit 44A may be introduced if a trigger value other than unity is to be used. It may be said that the sensor 13 has to travel 0.01 millimeter beyond the point of engagement between stylus and workpiece before the trigger signal occurs. The distance of 0.01 millimeter is therefore the pr.e-travel of the probe and it will be clear that this pre-travel. is_ not affected by the direction of the relative displacement between the stylus 20 and the base 25 of the probe. The circuits 30,31,40 and the comparator 32, which constitute an analogue computer, are regarded as a part of the probe P because the signal 17 is the essential probe output as far as the trigger mode of operation is concerned. However, the circuits 30,31,40 and the comparator 32 need not be in the same housing as the sensor 13 and may be embodied in an interface unit 50 (Fig.l) connected to the transducers 26 by a cable 51. The counters 16 (Fig.l) may form part of a digital computer 52 adapted to drive the carriages 10,11,12 and to store the signals 18 and to receive a trigger signal such as 17 all in a manner known per se in the context of known trigger 11 probes For the purpose of using the probe P as a proportionals- probe, the outputs of the rectifiers 30A are connected by lines 30G directly to the computer 52 for use of the probe in a manner known per se for proportional probes. Referring further to Fig.l, the machine M, the probe 13 including the unit 50, and the computer 52 constitute a system for measuring workpieces. The machine M includes a datum unit 53 (see also Fig.2, 3) for calibrating the system. The unit 53 has a cube-shaped head 54 defining datum surfaces 55. To calibrate the system the outputs X2,Y2,Z2 are allowed to attain zero and the sensor 13 is driven for the element 21 to engage the respective surfaces 55 to produce respective said signals 17. The values of the corresponding signals 18 are then the datum values of the system. Subsequent engagement of the element 21 with surfaces of the workpiece produces data relating those surfaces to the datum value as a basis for computation of- the dimensions of the workpiece itself. In such computation, the radius of the element 21 and the pre-travel value r of the trigger sphere are taken into account again as known in connection with known trigger probes where, however, there existed the problem of having to cope with varying pre-travel values. 12

Claims: CLAIMS : 1. A probe for measuring the dimensions of workpieces; comprising a base (25), a stylus (20) having a free end^ (21) whereby, in use, to engage a workpiece (14), support means (2-2) for supporting the stylus (20) on the base (25) for displacement relative thereto between a rest position and a displaced position, means (26) defining a pair of orthogonal coordinate axes (X,Y), and means (30) for determining a said displacement in terms of coordinate outputs (XI, X2) defining respective coordinate components (x,y) of the displacement; characterised by combining means (40) for combining said outputs (XI, Yl) into a resultant output (41) being a function of said displacement, and threshold means (32) for generating a step signal (17) when said resultant output (41) crosses a given value (r) during a said displacement. 2.- Probe according to claim 1 wherein said combining means (40) comprise, in respect of each said component output (XI, Yl) a squaring means (42) for determining the square .of the output, and the probe further comprising summing means (44) for summing the outputs of the squaring means (42), the output (41) of the summing means (42) defining said resultant output. 3. Probe according to claim 2 wherein there is provided, between said summing and threshold means (44,32), a means (44A) for forming the square root of the output (41) of the summing means (44) . 4. Probe according to claim 1 comprising setting means (31) for setting each said component output to a predetermined level (zero). 13 5. Probe according to claim 4, said setting means comprising bias means (35,36) adapted to bias each said component to zero and said setting means including delay means (38) for delaying the effect of the bias means (35,36). 6. Probe according to claim 1 wherein the said support means comprise resilient means (22) connected between the base and the stylus, the resilient means, when free, determining the rest position of the stylus. 7. A system for measuring workpieces comprising: a) a probe (P) including a base (25), a stylus (20) having a free end (21) whereby, in use, to engage a workpiece (14) to be measured, support means (22) for supporting the stylus on the base (25) for displacement relative thereto between a rest position and a displaced position, means (26) defining a pair of orthogonal coordinate axes (X,Y), and means (30) for determining a said displacement in terms of outputs (XI, Yl) defining respective components (x,y) of said displacement; b) a coordinate positioning apparatus (M) for moving the stylus (20), in operation, relative to a workpiece (14) and the apparatus including means (15,16) for measuring the position of said free end (21) relative to a datum (53); characterised by c) combining means (40) for combining said outputs (XI,Yl) in a resultant output (41) being a function of said displacement; 14 d) threshold means (32) for generating a step signal (17) given value (r) during a said displacement; and e) means (52) for determining the output of the measuring means (15,16) of the positioning apparatus (M) responsive to the occurrance of said step signal (17). 15

Inventors: ROGERS MICHAEL JOHN, ROGERS, MICHAEL, JOHN
Applicants: RENISHAW PLC
Application year: 1985
Main classification: G01B7
IPC codes: G01B7, G01B21