METHOD OF AUTOMATICALLY ADJUSTING TRACER HEAD

Abstract: A method of automatically adjusting a tracer head makes it possible to eliminate from a profiling control sys­ tem the need to manually effect zero adjustment of the tracer head (TR) and to adjust differential amplifier circuits for the tracer head (TR). For this purpose, each of the following steps, is carried out by the control effected by a processor (CPU): a step of accumulating respective outputs of differential amplifier circuits (AMPX, AMPY, AMPZ) as zero-point offset amounts while maintaining the tracer head (TR) in a non-contact state: a step of successively maintain­ ing the tracer head (TR) in a first contact state and a second contact state, and while doing so, accumulating coordinates respectively representing first and second positions of the tracer head (TR) at the time when the tracer head (TR) is in the first contact state and in the second contact state, togeth­ er with first and second displacement amounts which have been compensated for by the zero-point offset amounts: a step of calculating and accumulating respective gains of the differential amplifier circuits (AMPX, AMPY, AMPZ) on the basis of the first and second positional coordinates and the first and second displacement amounts compensated for by the zero-point offset amounts: and a step of effecting profil­ ing by the use of the accumulated zero-point offset amounts and gains.

Description: The present invention relates to an automatic compensation method of a tracer head employed in a tracing control system. Heretofore, initial adjustment or compensation of a tracer head in a tracing control system has been made manually. That is, zero adjustment of the tracer head, held in a non-contact state or out of contact with the surface of a model, has been effected manually so that the output of each differential amplifier circuit for amplifying the output of a differential transformer for detecting stylus deflection is reduced to zero. After this operation the tracer head is displaced as predetermined and the gain of each differential amplifier circuit is adjusted so that its output may take a predetermined value at this time. However, this conventional method has the defects of not only involving long-time, complex adjustment but also variations in cutting accuracy since the results of manual adjustment or compensation vary according to different operators. An object of the present invention is to provide an automatic tracer head adjustment method which involves no manual operations, and hence increases the cutting speed and accuracy. JP-A-57-194857 discloses an automatic tracer head compensation method according to the precharacterising part of claim 1. The method of the present invention is characterised by the features of claim 1. For a better understanding of the present invention and to show how it may be put into effect reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 is a block diagram illustrating the arrangement of a tracing control system to which a method of the present invention is applied; and Figs. 2 and 3 are flowcharts explanatory of the method of the present invention. Fig. 1 illustrates in block form the arrangement of a tracing control system to which a method of the present invention is applied. Reference character TR indicates a tracer head, ST a stylus, CT a cutter, MAC a tracing machine, MX, MY and MZ servo motors in the X-, Y-and Z-axis directions respectively, PCX, PCY and PCZ position sensors, CNTX, CNTY and CNTZ reversible counters for counting feedback pulses from respective position sensors to indicate the current position of the tracer head, D/AX, D/AY and D/AZ digital-to-analog converters for converting drive signals for the servo motors from digital to analog form, AMPX, AMPY and AMPZ differential amplifier circuits for differentially amplifying stylus deflection signals from differential transformers of the tracer head, MPX a multiplexer for selectively switching the outputs of the differential amplifier circuits, A/D an analog-to-digital converter for converting the output of the multiplexer from analog to digital form, CPU a processor, MEM a memory comprised of a data memory part M1 and a control program part M2, I/O a data input/output device, KB a keyboard and OPP an operator panel. The operation of the embodiment of the present invention in Fig. 1 will be described with reference to the flowcharts shown in Figs. 2 and 3. The CPU starts its operation at step 1, in which it reads the X-coordinate X of the tracer head at the start of operation from the reversible counter CNTX via a bus BUS and stores it as an X-coordinate XST at the start of operation in the data memory part M1 of the memory MEM. Incidentally, it is the tracing machine MAC that is actually driven by the servo motors and the tracer head stands still, but its coordinates are represented as coordinates relative to the tracing machine MAC. Next, in step 2 the absolute value of deflection EX in the X-axis direction at the start of operation is compared with a predetermined value EXOC. Since the stylus ST mounted on the tip of the tracer head TR is not in contact with a block BL at the start of operation, the deflection EX at this time naturally ought to be zero, but the deflection EX read out by the CPU via the multiplexer MPX, the A/D converter A/D and the bus BUS does not become zero in general because of inequality between secondary winding outputs DX1 and DX2 of the transformer provided in the stylus which results from positional deviations of iron cores of the transformer, or because of imbalance present in the differential amplifier circuit AMPX. When the deflection EX at the start of operation (when the tracer head is in the non-contact state) is larger than the predetermined value EXOC, since predetermined accuracy on linearity cannot be expected, the operation ends at step 3 raising an alarm. On the other hand, when the deflection EX in the non-contact state is smaller than the predetermined value EXOC. the operation proceeds to step 4 in which the deflection EX is stored as a zero-point offset amount EXO of the differential amplifier circuit AMPX in the data memory part M1. In the next step 5 the stylus ST is fed at a velocity -VXFT in the X-axis direction (precisely speaking, the tracing machine MAC is fed at a velocity VXFT in the X-axis direction), and in the next step 6 a check is made to determine if the difference between the amount of current deflection EX and the aforementioned offset amount EXO has exceeded a predetermined amount of deflection EXTC. If not, the abovesaid feed and check are repeated until the abovementioned difference exceeds the predetermined amount of deflection. When the stylus moves into contact with the block BL and the difference between the amount of current deflection EX and the offset amount EXO exceeds the predetermined value EXTC, the feed rate VX is changed to zero in step 7, after which it is checked in step 8 whether the movement of the stylus in the X-axis direction has stopped or not. That is, in step 8 a check is made to determine if the content ERRX of an error counter (not shown) for counting the difference between command pulses and feedback pulses from the position sensor PCX has become smaller than a predetermined residual error ESL, and if the result of this checking is affirmative, then it will be decided that the stylus has come to a standstill. When the stylus has thus been stopped at a first standstill point, the difference between the amount of current deflection EX and the offset amount EXO, that is, the amount of deflection of the stylus at the first standstill position compensated for by the offset amount (the amount of first detection) is stored as EXG1 in the data memory part M1, and in step 10 the X-coordinate X of the first standstill point is stored as XG1 in the data memory part. Upon completion of this storage, the operation proceeds to steps 11 and 12, in which the stylus is fed toward the block BL at a predetermined velocity -VXFL until the X-coordinate X of the stylus becomes equal to XG1-XD, that is, until the stylus moves further toward the block BL than the abovesaid XG1 by XD. When this condition is fulfilled, the movement of the stylus in the X-axis direction is stopped in steps 13 and 14. When the stylus has thus been stopped at a second standstill point, the processing by the CPU proceeds to step 15 shown in Fig. 3, in which the difference between the amount of second current deflection EX and the offset amount EXO, that is, the amount of deflection of the stylus at the second standstill point compensated for by the offset amount (the amount of second detection) is stored as EXG2 in the data memory part M1, and in step 16 the X-coordinate X of the second standstill point is stored as XG2 in the data memory part. Upon completion of this storage, the processing by the CPU proceeds to step 17, in which is calculated the ratio of the difference between the X-coordinates of the first and second standstill points, (XG2 - XG1), to the difference between the amounts of deflection compensated for by the offset amount, (EXG2 - EXG1), and this value with an inverse sign is stored as a gain KX of the differential amplifier circuit AMPX in the data menory part M1. After the gain KX has been calculated and stored as described above, the stylus is moved toward the operation starting point at a predetermined velocity VXFT in step 18 and, after being returned to the starting point in steps 19, 20 and 21, it is stopped there. After the zero-point offset amount EXO and the gain KX of the differential amplifier circuit AMPX for detecting deflection of the stylus in the X-axis direction have thus been stored, zero-point offset amounts EYO and EZO and gains KY and KZ of the differential amplifier circuits AMPY and AMPZ for detecting deflection of the stylus in the Y-and Z-axis directions are stored respectively in blocks 30 and 40 having the same contents as the above-described steps 1 to 21, completing the entire process of the CPU. The zero-point offset amount and gain of the differential amplifier circuit for detecting stylus deflection in each axis, thus stored in the data memory part M1, will be used for zero-point compensation of the amount of stylus deflection in each axis and correction of the gain in the subsequent tracing operation. That is, when starting the subsequent tracing control, the CPU compensates for and corrects output data of the unadjusted differential amplifier circuits AMPX, AMPY and AMPZ on the basis of the zero-point offset amounts and gains stored in the data memory part M1, calculating the amounts of deflection. Letting the zero-point offset amounts and the gains of the respective differential amplifier circuits be represented by EXO, EYO, EZO and KX, KY, KZ, the following calculations are conducted for input data EXi, EYi and EZi converted from analog to digital form, thereby obtaining corrected amounts displacement EX, EY and EZ. EX=KX (EXi-EXO), EY=KY (EYi-EYO), EZ=KZ (EZi-EZO). While in the above embodiment the zero-point offset amounts and the gains are stored for the three axes while feeding the stylus in the three axes one after another, it is also possible to store the zero-point offset amounts and the gains simultaneously for the three axes while feeding the stylus in a direction including any of the three-axis components. Further, although in the above embodiment the first standstill point for providing the first contact state is set as the coordinate position where a predetermined amount of stylus deflection is obtained, it is also possible to set the first standstill point by providing predetermined coordinates on the basis of the preknown position and shape of the block BL. Similarly, the second standstill point can also be set as a point where the amount of deflection is larger than that at the first standstill point by a predetermined amount, instead of setting the second standstill point at a predetermined distance from the first standstill point for providing the second contact state. Moreover, the first and second contact states are obtained by first and second standstill points in the above, but it is also possible to obtain the first and second contact states through utilization of the position of the stylus which is moving at a feed rate sufficiently lower than the data read rate and processing throughput of the processor. As described above in detail, a step of storing the output of each differential amplifier circuit as a zero-point offset amount while maintaining the tracer head in the non-contact state, a step of successively maintaining the tracer head in first and second contact states, and while doing so, storing first and second coordinates of the tracer head in the first and second contact states, together with first and second amounts of stylus deflection compensated for by the zero-point offset amount, a step of calculating and storing the gain of the differential amplifier circuit on the basis of the first and second coordinates and the first and second amounts of stylus deflection compensated for by the zero-point offset, and a step of performing tracing control using the stored zero-point offset and gain, are carried out under control of a processor without involving any manual operation, thus providing the advantage of increasing the cutting speed and accuracy.

Claims: An automatic tracer head compensation method for a tracing control system which is provided with a tracer head (TR) holding a stylus (ST), differential transformers mounted in the tracer head (TR) to detect stylus deflection, an analog-to-digital converter (A/D) for converting detected stylus deflection signals from the differential transformers in the tracer head (TR) from analog to digital form, a processor (CPU) for processing the amount of stylus deflection sensed by the tracer head (TR) converted to a digital quantity, a memory (M1) for storing the processed output of the processor (CPU), and means (D/AX,D/AY,D/AZ) for converting a digital control quantity from the processor (CPU) to control the feed to which the tracer head (TR) is sensitive, the method including a step of storing as a zero-point offset amount (EXO,EYO,EZO) the outputs derived from said differential transformers when the tracer head (TR) is in a non-contact state; characterised in that differential amplifier circuits (AMPX,AMPY,AMPZ) are provided for amplifying the difference between secondary winding outputs of the respective differential transformers thereby to detect the amount of stylus deflection sensed by the tracer head (TR), and coordinate detecting means (PCX,PCY,PCZ,CNTX,CNTY,CNTZ) are provided for detecting coordinates associated with the tracer head (TR), and in that the compensation method comprises: a step of putting the tracer head (TR) into a first contact state and therein storing a first coordinate (XG1,YG1,ZG1) of the tracer head (TR) and a first amount of detected stylus deflection (EXG1,EYG1,EZG1) including compensation by the zero-point offset amount (EXO,EYO,EZO); a step of putting the tracer head (TR) into a second contact state different from the first contact state and therein storing a second coordinate (XG2,YG2,ZG2) of the tracer head (TR) and a second amount of detected stylus deflection (EXG2,EYG2,EZG2) including compensation by the zero-point offset amount (EXO,EYO,EZO); a step of calculating using said processor, and storing, the gain (KX,KY,KZ) of circuits each including a respective differential amplifier circuit (AMPX,AMPY,AMPZ) and differential transformer on the basis of said first and second coordinates (XG1,XG2,YG1,YG2,ZG1,ZG2) and said first and second amounts of detected stylus deflection (EXG1,EXG2,EYG1,EYG2,EZG1,EZG2) including compensation by the zero-point offset amount (EXO,EYO,EZO); and a step of calculating under control of the processor (CPU), compensated amounts of stylus deflection (EX,EY,EZ) sensed by the tracer head (TR) from an amount of stylus deflection (EXi,EYi,EZi) obtained by A/D converting a signal from the tracer head (TR) when a tracing operation is in progress, the calculating step involving use of the stored zero-point offset amounts (EXO,EYO,EZO) and gains (EX,EY,EZ), wherein the gains (KX,KY,KZ) are calculated from the expressions: KX = - (XG2-XG1)/(EXG2-EXG1) KY = - (YG2-YG1)/(EYG2-EYG1) KZ = - (ZG2-ZG1)/(EZG2-EZG1) An automatic tracer head adjustment method according to claim 1, wherein the compensated amounts of deflection (EX,EY,EZ) are calculated from the expressions: EX = KX (EXi-EXO) EY = KY (EYi-EYO) EZ = KZ (EZi-EZO)

Inventors: YAMAGUCHI, MASASHI, YAMAZAKI, ETUO
Applicants: FANUC LTD.
Application year: 1984
Main classification: B23Q35
IPC codes: B23Q35