Mounting for Surface-Sensing Stylus and a Method of Using Said Mounting

This invention relates to a mounting for a surface-sensing stylus and to a machine for and a method of using the mounting for determining the profile of a workpiece. The scope of the invention is specified in the claims hereto.

The invention as claimed makes it possible to determine said profile in a coordinate positioning machine by means of a stylus which is moved over the profile in such a way that the engagement between the stylus and the profile is maintained continually during this movement and that the contact pressure between the stylus and the workpiece does not vary or does not vary excessively. Also, the invention makes it possible for the mounting to be of relatively simple construction, and for the stylus to follow relatively large excursions of said profile without sacrificing adaptability to a variety of operating conditions. The invention may be used for measuring the profile or for providing data for copying the profile in a profile-copying system.

An example of apparatus according to this invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 is an elevation of a co-ordinate measuring machine including a stylus mounting.

FIG. 2 is a section on the line II--II in FIG. 1.

FIG. 3 is a section on the line III--III in FIG. 1 and shows details of the stylus mounting.

FIG. 4 is a diagram of the overall operating system of the machine shown in FIG. 1 and shows a computer, motors and feedback transducers of this system.

FIG. 5 is a detail of FIG. 4 and shows a diagram of control systems for motors contained in the stylus mounting.

FIGS. 6a and 6b are a flow diagram for operation of the computer shown in FIG. 4.

FIG. 7 is a detail of FIG. 1 and illustrates a modified mode of operating the stylus mounting.

FIG. 8 is a plan view of FIG. 7.

FIG. 9 shows a modification of the control systems shown in FIG. 5.

Referring to FIGS. 1 and 2, a co-ordinate measuring machine CMM comprises an operating member 10 supported on a table 11 for translatory movement in the three directions X,Y,Z of the orthogonal co-ordinate system. The member 10 is drivable in the directions X,Y,Z by respective motors MX,MY,MZ in a manner known per se. The operating member 10 has secured thereto a probe head PH comprising (FIG. 3) a base or housing 20 supporting a shaft 22 rotatable by a motor M1 relative to the housing 20 about an axis A1. The shaft 22 is secured to a member or further housing 23 supporting a shaft 24 rotatable by a motor M2 relative to the housing 23 about an axis A2 perpendicular to the axis A1. The shafts 22,24 are supported for rotation in respective bearings 22A,24A in the respective housings 20,23. The shaft 24 has secured thereto a frame or stylus holder 25 to which is secured a stylus 26 having an axis A3 perpendicular to the axis A2. The stylus 26 has a free end defined by a spherical sensing element 27 and a supported end defined essentially by the axis A2. The axes A1,A2,A3 preferably have a common point of intersection 28 (FIGS. 1,2).

The arrangement is such that the motors M1,M2 can position the sensing element 27 angularly about the axes A1 or A2, and the motors MX,MY,MZ can position the head PH or, more specifically, the point 28 linearly anywhere within the operating field of the machine CMM. Linear position transducers TX,TY,TZ are provided for measuring linear displacement LX,LY,LZ of the member 10 in the respective directions X,Y,Z. Angular position transducers T1,T2 are provided for measuring angular displacements W1,W2, of the stylus 26 about the respective axes A1,A2, i.e. relative to the member 10. The motors MX,MY,MZ,M1,M2 are also collectively referred to by the letter M, and the transducers are collectively referred to by the letter T.

A computer 50 (FIG. 4) is programmed to output drive signals 32X, 32Y, 32Z, 32Z, 322, collectively denoted 32 for driving the respective motors M thereby to position the sensing element 27 relative to a workpiece 12 supported on the table 11, position feedback signals 33X, 33Y, 33Z, 331, 332, collectively denoted 33, being provided from the respective transducers T. However, the motors M1 and M2 are also adapted to be constant torque devices. In other words the motors M1,M2 are operable to move their respective armatures to a given angular position, and hold the armature in that position, or to apply to their armatures a substantially constant torque. The motors M1,M2 are operable in two modes, viz the positioning mode and the constant torque or bias mode. The arrangement is such that in the bias mode a given force FB (FIG. 2) of engagement between the sensing element 27 and the workpiece 12 can be maintained by, in this case, the motor M1. If the device PH is moved relative to the workpiece 12 in the sense of reducing the force FB, the motor M1 moves the sensing element 27 to maintain the engagement, and if the device PH is moved in the sense of increasing the force FB, the motor M1 accommodates that movement while maintaining the force FB. In other words, the stylus is pivotal about the axis A1 both in response to and in opposition to the bias torque of the motor M1.

Referring to FIG. 5, the computer 50 is arranged to control the motors M1, M2, which are DC motors, through respective control systems C1,C2 to which the drive signals 321,322 are connected as demand signals and wherein the feedback signals 331,322 are connectable to the signals 321,322 through summing junctions 521,522. This connection of the feedback signals 331,332 is effected through respective mode control switches 541,542. The computer 50 generates respective mode control signals 511A, 511B and 512A, 512B for operating the respective switches 541, 542. The arrangement is such that if either of the signals 511A, 512A is output, the respective switch 541, 542 is closed and the respective motor M1, M2 is operated in the positioning mode, but if either of the signals 511B, 512B is output the respective switch 541, 542 is opened whereby the respective motor M1, M2 is deprived of the position feedback signal 321, 322 and therefore operates in the bias mode. The systems C1,C2 have preset gain amplifiers 551, 552 for the feed forward signal so as to limit the torque of the motors to a preset value.

The machine CMM and the probe head PH may be used for a scanning operation to automatically determine a profile 40 of a workpiece 12 relative to a given workpiece datum DX,DY. By way of example, the profile 40 is assumed to lie in a plane 41 extending in the directions X,Y. A computer programmer determines position data 550 (FIG. 6) for a path 42 (FIG.2) which lies in the plane 41 and which is spaced from the profile 40 by a distance 43 smaller than the radial distance R between the center of the sensing element 27 and the point 28. Thus if the probe head PH is moved for the point 28 to describe the path 42, the sensing element 27 can maintain engagement with the profile 40. In the present example the workpiece 12 is a rotary cam and the path 42 is conveniently a circle surrounding the cam profile, i.e. the profile 40.

The path 42 is so pre-programmed that the axis A3 maintains an approximately constant angle W3 relative to a tangent 44 through the point of contact between the sensing element 27 and the profile. In this way the pressure between the sensing element 27 and the workpiece remains within a given tolerance and excessive variation of bending forces on the stylus 26, and consequent variations of measuring accuracy, are avoided. However, the stylus 26 may embody a means for sensing bending load thereon, e.g. a strain gauge 26A, whose output is a function of the angle W3 and which may be used to serve the motors MX,MY,MZ to positions such that the angle W3 remains substantially constant. A stylus provided with such a strain gauge and a servo system for responding to the strain gauge output are shown in our British Patent Application No. 8803847 filed 18th Feb. 1988.

The determination of position data for a path such as the path 42 is well known per se. Also known is the organization of a machine such as the machine CMM for driving an operating member through such a path. Therefore, these aspects are not specifically described other than saying that, in the present example, the data 550 comprise the orthogonal positions of the point 28 relative to a datum DX,DY at given spatial intervals between points i=o and i=n (FIG. 2) on the path 42. Further, the programmer determines position data 560 (FIG.6) being values for the signals 321, 322 whereby the stylus 26 is moved into a neutral position in which it is clear of the workpiece 12 even when the member 10 is at its starting position i=o for the scanning operation. This ensures that the member 10 can be moved into the starting position i=o without danger of collision between the stylus and the workpiece. Next, the programmer determines position data 570 (FIG.6) being values for the signals 321,322 whereby the stylus 26 is moved into a starting condition for the scanning operation, that is, the condition in which the sensing element 27 can engage the workpiece. The data 560,570 include the signals 511A,511B and 512A,512B as will become apparent later herein. The programmer also determines the radius R of the stylus. The data 550,560,570 and R are transferred into the computer store in the usual way.

Regarding the execution of the scanning operation, the point 28 is assumed, initially, to have a position in which the member 10 well clear of the workpiece. For the operation itself the computer contains a program 600 written to perform the following operations:

The computation at step 638 is based on the formulae:

wherein PX,PY,PZ are the coordinates of the position P of the center 27A of the sensing element 27. In the example described formula (3) is not applicable but it is given here for the general case.

The radius of the sensing element 27 has not been taken account of because this need not be done in the case where the cam 12 is a model for copy-machining and the cutter used for the machining process has the same radius as the sensing element 27. However, the above formulae may be adapted to allow for the radius of the sensing element 27 if required.

Referring to FIGS. 7 and 8, the apparatus may be used for scanning a workpiece essentially simultaneously at several planes e.g. 41A,41B. To this end one of the motors M1,M2, in this case the motor M2, has an oscillatory motion applied thereto to drive the sensing element 27 alternately between positions Z1,Z2 while the other motor, M1, is in the bias mode. In this example the workpiece, 12A, is a cone and the positions Z1,Z2, lie at different diameters of the cone. The motors MX,MY are driven to move the point 28 through an appropriate path 42A such that the sensing element 27 can maintain contact with the cone at each plane 41A,41B. The steps 624 to 634 of the program (FIG.6) are executed each time when the sensing element 27 is at the position Z1 or Z2 so that the profile, 40A, of the cone is determined at each of the planes 14A,14B. The oscillatory motion of the motor M2 is generated by data 580 stored in the computer 50 and defining the respective positions Z1,Z2. In the program 600 the data 580 are applied to the motor M2 to move the sensing element 27 from the one position Z1,Z2 to the next, prior to the execution of the steps 624 to 634. The application of the data 580 is not illustrated in FIG. 6 but it will be readily understood that this merely requires the outputting of signals 322 of appropriate values defining the respective positions Z1,Z2 and the holding of the motor M2 in that position for the execution of one cycle of the steps 624 to 634.

The stylus 26 may be balanced by a balance mass 25A secured to the stylus holder 25 (FIGS.7,8) thereby to reduce the power requirement of the motor M2.

In a modification, not illustrated, the constant torque is applied to the stylus 26 by a motor defined by a coiled spring so that the electric motor for applying torque is not required. However, an electric motor, e.g. M1, may still be used for positioning the stylus irrespective of the force of the spring.

In a modification of the control systems C1,C2, FIG. 9 shows a control system C1A wherein the switch 541 is used to switch the drive signal 321 between a position demand line 321A and a maximum current demand line 321B. The line 321A is led through the summing junction 521 for the position feedback signal 331 to a summing junction 543 for a velocity feedback signal 544 from a tachometer 548 and from there to a summing junction 549 for a current feedback signal 545 from a current sensor 546. The line 321B is connected to close a switch 547 controlling the signal 545 such that the current feedback is established only if the current demand signal 321B is selected at the switch 541. In this way a constant torque can be reliably established at the motor M1. The amplifier 551 may be of relatively high gain so that, in the position control mode, a high response rate is available for the motor, the velocity of the motor M1 being limited by the signal 544. A similar control system (not shown) is of course provided for the drive signal 322.