Position Pickup for Numerically Controlled Machine Tools

(a) Field of the Invention

This invention relates to a position pickup device for use in numerically controlled machine tools in which the moving machine part is coupled to a resolver (synchro transmitter), or the like, which is fed on the primary side with two voltages. The voltages, which are electrically shifted 90.degree. relative to each other, are generated from a clock frequency by means of a sine-cosine generator constructed from divider stages. The phase of the voltage on the secondary side of the transmitter relative to the voltage on the primary side is a measure for the value of position. More particularly it relates to an arrangement for improving accuracy in such a device.

(b) Discussion of the Prior Art

Reference is made to the arrangement described in U.S. Pat. No. 3,173,001 where voltages on the secondary side of a synchro transmitter are converted into square wave voltages derived from a clock frequency and their phase is compared with reference values in the form of square wave voltages. The control commands for the drives on the individual axes of the machine are derived from this comparison.

In computer controlled machine tools, it is necessary to form a digital signal for the actual position.

In accordance with co-pending application Ser. No. 14,431 assigned to the same assignee as the present invention, this problem is solved by generating a value proportional to the angular position of a resolver excited by a sine-cosine generator as determined at the time the output of the secondary side of the resolver crosses through zero. A dividing chain driven by a clock generator times the output of the sine-cosine generator and a second time chain, also driven by the clock generator, drives a register which counts over an interval initiated by the sine-cosine generator and ended by the zero crossing of the resolver output. The zero crossover is detected by a comparator coupled through a filter to the resolver secondary. In this way with essentially no additional costs it is possible to obtain a true position value suitable for digital processing in the computer. However, the filters and comparators used in this system, even where high grade filters are used, have a thermal phase drift which can effect the accuracy of such a device.

It is the object of the present invention to increase the accuracy of measurement in such a device while compensating the phase drift. In accordance with the present invention this problem is solved by switching the filter at fixed intervals to a voltage derived from the primary side voltage and by detecting a change of the signal state of the sine-cosine generator at the zero crossing and using this value, in successive switching operations to correct the value determined as the actual position. In this manner, a correction value for the compensation of the phase drift can be obtained with the measuring equipment available anyway for acquisition of the measured value. This leads to a further advantage in that relatively cheap filters may also be used since even the phase drift of these filters is detectable and hence can be compensated.

FIG. 1 is a block diagram of an embodiment of the invention which is shown in conjunction with a numerical machine tool control;

FIG. 2 is a schematic circuit diagram of the measurement system used in the machine tool control of FIG. 1; and

FIG. 3 is a chart showing timing relationships in the circuit of FIG. 2.

The carriage 10 of a machine tool is shown in FIG. 1 which can be moved in the direction of the X and Y axes by drive motors 11 and 12. For this purpose, the motors 11 and 12 are appropriately addressed via thyristor elements 9 by a numerical (CNC) control 1 which includes a microprocessor. For forming the control commands, the actual position of the carriage 10 must be determined. To this end, drive motors 11 and 12 are coupled to resolvers 3 and 4, respectively, the rotors of which are rotated in proportion to the displacements on the respective axes. The stator windings of resolvers 3 and 4 are fed with two voltages of equal amplitude, but electrically shifted by 90.degree. relative to each other, which are generated from a clock frequency T by means of digital sine-cosine generator 2. The phase of the voltage induced in the rotor winding relative to the voltage in the stator windings therefore depends on the position in space of the rotor winding relative to the stator windings.

The position of the carriage 10 can therefore be established from the phase of the voltage on the secondary side. This is accomplished when the voltage induced in the rotor winding crosses zero. At that instant a value corresponding to the state of the outputs of sine-cosine generator 2 is transferred into registers 5 or 6, as is appropriate under the circumstances. The values so stored are transferred from the registers into control 1 at fixed times via the data lines 51 and 61.

The measuring system will be explained in detail with reference to FIG. 2.

Sine-cosine generator 2 is fed from a clock generator, not shown, having an output clock frequency T; it comprises three series connected divider or counter stages 21 having decimal division ratios of 1:10. Divider stages 23 and 24, having division ratios of 1:2, are connected to the output Q and the inverting output Q, respectively, of the last cascaded stage 22, which also has a division ratio of 1:2, so that between the two outputs of divider stages 23 and 24, square wave voltages with a frequency of 4 kHz, can be taken off via amplifiers 25. In this illustrative embodiment, the square wave voltages have a resolution of 5 .mu..

The square wave voltages, shifted electrically relative to each other by 90.degree. at the outputs of the amplifiers 25, are fed to primary stator windings 31 and 32, respectively, of transmitter 3. These waveforms can be seen on the lines 31 and 32 of FIG. 3. The voltage taken off at rotor winding 33 of mitter 3 is fed to an evaluator stage 8, which consists of low pass filter 81 followed by comparator 82. Comparator 82 switches at each zero crossing of the rotor voltage. The waveform of the output voltages from filter 81 and from comparator 82 are shown in lines 81 and 82 of FIG. 3. There, the times t.sub.1 and t.sub.2 when the output voltage of filter 81 crosses zero can be seen as depending on the phase of the rotor voltage relative to the stator voltages (shown in lines 31 and 32, respectively). A change of position in time of the zero crossings relative to the input of the stator windings is therefore an index for a turn of the rotor and thus an index for displacement of the machine part. In order to obtain a signal corresponding to the position of the rotor, the signal state of the sine-cosine generator 2 present at the instant in question must be established, i.e. its instantaneous division and counting state, as the latter is a measure for the phase of the stator voltages relative to the rotor voltage. To this end, the clock frequency T is also fed to three series-connected divider stages 26, each of which has a division ratio of 1:16. In this case a signal in binary form is generated as it will be needed in this form in further processing. In the illustrative embodiment, the three dividers 26 provide for a total count of 4096. Divider stages 26 are reset to zero by clearing stage or driver 27 triggered by the output Q of divider 23, every time a period of 4000 alternations of the sine-cosine generator 2 has been swept through. The parallel count then begins again, running to the time of zero crossing of the rotor voltage and generating a new representation of the state of sine-cosine generator 2. This value is transferred into register 5 via lines 28 at the time of zero crossing, i.e., the switching of comparator 82. From there it is called up by control 1 via data line 51 at defined time intervals of, say, 4 milliseconds.

As indicated in the drawing, the same generator 2 and the same divider stages 26 can be used to supply the same square wave voltages to the stator windings of the second transmitter 4. In this case, the voltage taken off from the rotor of resolver 4 is picked up by an evaluator stage 7 consisting of filter 71 and comparator 72 to effect the transfer of the respective state of divider stages 26 at the time of zero crossing of the rotor voltage into register 6 via lines 29. This value is then likewise fed to control 1 via data lines 61 at the proper time.

As is evident, the same system can also be used for still other synchro transmitters or for linear measuring systems of similar design.

As can be seen from FIG. 2, filter 81 can be selectively switched between the secondary winding 33 and a level converter 16 by means of a switch 13. Switching is done at fixed intervals within the measurement pauses by means of a stage 15 energized by the CNC control 1. If the filter is connected to the level converter 16, then, upon zero crossing of the voltage in comparator 82, a certain measured value will be transferred to register 5. This can then be transferred into a memory position. If the measured value changes, for example, due to thermal phase drift of the filter 81, the change, i.e., the difference between the new and old measured values can then be used as a correction value for the measured values obtained in the detection of actual position. In this way, the phase drift of the filter and comparator is largely compensated.

The same correction can also be carried out with respect to filter 71. For this purpose a switch 14, also controlled by stage 15 is provided. Switch 14 selectively connects the filter 71 to the secondary winding of synchro 4 or to the level converter 16.