SPEED CORRECTING DEVICE FOR USE IN SPEED COMMAND SYSTEM

Abstract: In a speed correcting device for use in a speed com­ mand system of a numerical controller, tracer controller or like apparatus in which a movable machine part is driven at a speed corresponding to a speed command value (V), an error (ΔV) in the moving speed of the movable machine part is removed by detecting the moving speed in a state of the speed command value (V) being zero and by adding to the speed command value (V) such a moving speed that can­ cels the detected moving speed.

Description: The present invention relates to a speed correcting device for use in a speed command system for an apparatus having a movable machine part driven at a speed corresponding to a speed command value and, more particularly, to a speed correcting device which permits driving of the movable machine part at a speed accurately corresponding to the speed command value. In a machine tool or the like, a movable machine part is driven at a speed corresponding to a speed command value fed from for example, a tracer controller, numerical controller or the like, but this method may be defective in that the driving speed of the movable machine part differs from the speed command value under the influence of an offset voltage which occurs in the speed command system owing to a secular change of an element used. To avoid this, it is a general practice in the prior art to provide a variable resistor for speed control in the speed command system and manipulate the variable resistor by an operator, thereby bringing the actual moving speed of the movable machine part into agreement with the speed command value. However this method not only increases the work of the operator but also lacks accuracy because of difficulties associated with the manual method of adjustment of the variable resistor. Fig. 1 of US-A-4 099 113 discloses a numerical control servo system according to the precharacterising part of claim 1, in which iron plate is cut to preset lengths. It is essentially a displacement control system, in which displacement errors must be compensated by additional circuitry not required in the speed-control adaptation provided by the present invention. A correction value is put into a deviation memory before the servo control starts. EP-A-0 044 967 describes a pure position control system wherein a position compensation circuit is activated when movement has stopped, the output of a reference position value/actual position value subtraction unit being stored as a compensation signal. Then the drive is moved to a corrected position. According to the present invention there is provided a movable machine servo-control system comprising a speed control loop including a speed detector coupled to the movable machine and providing the actual value of the speed to a first adder arranged to supply a difference quantity between a speed command value and the feedback signal from the speed detector, which difference quantity commands the input of a speed control unit, and comprising a position feedback loop including a position detector, and a counter counting pulses from said position detector, and a second adder coupled to receive at a first input the speed command value, and at a second input a signal dependent on the content of the counter, and comprising an offset compensating circuit coupled to an input of the second adder, characterised in that the offset compensating circuit is realised by inserting an arithmetic circuit between the counter and the second input of said second adder, which, upon activation of the arithmetic circuit in a state of the speed command signal at the first input of the second adder being zero, or being cancelled by also being fed to the second input of the second adder, divides the content of the counter by a corresponding time interval, thus producing a compensating feedback signal which is fed to the second input of said second adder. The speed command value may be fed from, for example, a numerical controller, tracer controller or the like. An embodiment of the present invention may provide a speed correcting device which is free from the above-said defects of the prior art and permits driving of a movable machine part at a speed exactly corresponding to a speed command value. An embodiment of the present invention may provide a speed correcting device which automatically brings the moving speed of a movable machine part to a speed command value. An embodiment of the present invention may provide a speed correcting device with which one may achieve automatic correction of the moving speed of a movable machine part into agreement with a given speed command value even while the latter is being yielded at a value other than zero. For a better understanding of the 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: Figs. 1 to 4 are block diagrams illustrating embodiments of the present invention; Fig. 5 is a block diagram illustrating specific arrangements of a counter 8, an arithmetic circuit 9, a signal generator 10 and an adder 1 in the embodiment of Fig. 1; Fig. 6 is a flowchart explanatory of an operation of a microprocessor for obtaining a moving speed Δv of a movable machine part; and Figs. 7 and 8 are flowcharts explanatory of different examples of an operation of the microprocessor 30 for controlling a speed command value for the movable machine part. Fig. 1 illustrates in block form an embodiment of the present invention. In Fig. 1, reference numeral 1 indicates an adder; 2 designates a D/A converter; 3 identifies an adder; 4 denotes a speed control unit; 5 represents a motor; 6 shows a speed detector; 7 refers to a position detector; 8 signifies a counter; 9 indicates an arithmetic circuit; and 10 designates a signal generator which yields an output "1" for a certain period of time Δt when a push button 11 is pressed. The adder 1 is supplied with a speed command value v (a digital quantity) from a tracer controller, numerical controller or the like (not shown); the D/A converter 2 applies the output of the adder 1 to the adder 3; and the speed control unit 4 drives the motor 5 on the basis of the output of the adder 3, i. e. a difference between the output of the D/A converter 2 and the detection result by the speed detector 6. Since such operations are already well-known, no detailed description will be given of them. With the device shown in Fig. 1, the speed correcting operation starts with reducing the speed command value V to zero which is provided to the adder 1. At this time, if an offset voltage is present in the speed command system from the D/A converter 2 to the motor 5 owing to a secular change of an element used, the motor 5 is driven at a speed corresponding to the offset voltage in spite of the speed command value V being zero, and pulses corresponding to the amount of movement of a movable machine part (not shown) are applied from the position detector 7 to the counter 8. The counter 8 counts the number of pulses which are applied thereto from the position detector 7 when the signal generator 10 yields an output "1" upon depression of the push button 11 by the operator. The count value of the counter 8 corresponds to the amount of movement Δℓ of the movable machine part in the time Δt. Accordingly, the amount of movement Δℓ of the movable machine part in the time interval Δt can be obtained by depressing the push button 11 of the signal generator 10. When the output signal of the signal generator 10 goes to a "1", the arithmetic circuit 9 performs an operation of the following Eq. (1) on the basis of the amount of movement Δℓ of the movable machine part applied from the counter 8 and the predetermined time Δt, by which operation the moving speed Δv of the movable machine part at this time is calculated and is applied to the adder 1. Δv = Δℓ ÷ Δt (1) The moving speed Δv thus obtained is generated by an offset voltage of the speed command system, so that by providing the speed Δv as an amount of correction to the adder 1, the offset voltage is cancelled, reducing the moving speed of the movable machine part to zero. Accordingly, by correcting the speed command value V by the moving speed Δv, i. e. (V - Δv), the movable machine part moves at a speed corresponding to the speed command value V. Fig. 5 is a block diagram illustrating, by way of example, specific arrangements of the counter 8, the arithmetic circuit 9, the signal generator 10 and the adder 1 used in the embodiment of Fig. 1. The parts corresponding to those in Fig. 1 are identified by the same reference numerals. Reference numeral 30 indicates a microprocessor; 31 designates its bus; 32 identifies a memory; 33 denotes a data input device; 34 represents an A/D converter; 35 shows a clock oscillator; 36 refers to a frequency divider; and 37 signifies a receiver. The microprocessor 30 is connected via the bus 31 to the memory 32, the A/D converter 34, the D/A converter 2, the counter 8 and the data input device 33. The microprocessor 30 operates in synchronism with output pulses of the clock oscillator 35 and sevices an interrupt by the output of the frequency divider 36 which frequency divides the output pulses of the clock oscillator 35. Fig. 6 is a flowchart explanatory of an operation of the microprocessor 30 for obtaining the moving speed Δv. Upon turning ON the switch 11, the microprocessor 30 resets the counter 8 (S1) and reads out the value of the counter 8 after the time Δt has elapsed (S2, S3). Then the microprocessor 30 computes ΔL/Δt (S4) and stores the result Δv in the memory 32 (S5). Fig. 7 is a flowchart showing an example of an operation of the microprocessor 30 for controlling the speed command for the movable machine part. When supplied with an interrupt signal INT from the frequency divider 36, the microprocessor 30 reads out the output of the A/D converter 34 to which is applied a displacement signal of a tracer head in a tracer controller, and computes the speed command value V by a known method (S6, S7) and then adds the computed speed command value v with -Δv stored in the memory 32 (S8). After this, the microprocessor 30 outputs the value V - Δv to the D/A converter 2 (S9). Fig. 2 illustrates in block form another embodiment of the present invention, which differs from the embodiment of Fig. 1 in that an adder 12 is provided for applying a difference between the speed command value V and the speed Δv from the arithmetic circuit 9, i. e. (Δv' = Δv - V), to the adder 1. Incidentally, the same reference numerals are used as those in connection with Figs. 1 and 2, where the components are the same. When the operator depresses the push button of the signal generator 10 during the application of a predetermined arbitrary speed command value V (including zero) from a tracer controller or numerical controller, the counter 8 and the arithmetic circuit 9 perform the same operations as described above. Consequently, the speed Δv (the actual moving speed of the movable machine part) which is derived from the arithmetic circuit 9 at this time is the sum of the speed command value V and a speed Δv s caused by the offset voltage of the speed command system as indicated by the following Eq. (2): Δv = V + Δv s (2) Since the adder 12 obtains the difference between the speed Δv output from the arithmetic circuit 9 and the speed command value V as described above, the output Δv' of the adder 12 is equal to the speed Δv s caused by the offset voltage. Therefore, the influence of the offset voltage of the speed command system can be removed by adding the output Δv' of the adder 12 as an amount of correction to the adder 1, so that the movable machine part can be moved at a speed equal to the speed command value V. Incidentally, the arithmetic circuit 9 continues outputting the speed Δv obtained as described above until the output of the signal generator 10 goes to a "1" again. In this way, this embodiment is capable of obtaining an amount of correction during application of an arbitrary speed command value, and hence it is simple in operation as compared with the embodiment of Fig. 1 which requires the reduction of the speed command value V to zero when obtaining the amount of correction. The counter 8, the arithmetic circuit 9, the signal generator 10 and the adders 1 and 12 in Fig. 2 can also be realized through utilization of the circuit arrangements depicted in Fig. 5. Also it is possible to compute the moving speed Δv in a similar manner. Fig. 8 is a flowchart explanatory of an example of an operation of the microprocessor 30 for controlling the speed command for the movable machine part in the case where the operation of the embodiment of Fig. 2 is placed under the control of the microprocessor 30. When supplied with the interrupt signal INT from the frequency divider 36, the microprocessor 30 reads out the output of the A/D converter 34 to which is applied the displacement signal of the tracer head, and computes the speed command value V in a known method (S10, S11) and then subtracts Δv stored in the memory 32 from the speed command value V to obtain Δv' (S12). After this, the microprocessor 30 adds -Δv' to the speed command value V (S13) and outputs the result to the D/A converter 2 (S14). Fig. 3 illustrates in block form another embodiment of the present invention, which is a modified form of the embodiment of Fig. 1 and in which the speed command value V is provided in an analog form. This embodiment differs from the embodiment of Fig. 1 in that the output of the arithmetic circuit 9 is applied via a D/A converter 14 to an adder 13 which carries out an addition of analog quantities. With such an arrangement, it is possible to eliminate the influence of the offset voltage as in the embodiment of Fig. 1 even if the speed command value V is given as an analog quantity. The same reference numerals are used as those in connection with Figs. 1 and 3, where the components are the same. Fig. 4 illustrates in block form still another embodiment of the present invention, which is a modified form of the embodiment of Fig. 2 and in which the speed command value V is provided in an analog form. This embodiment differs from the embodiment of Fig. 2 in that the speed command value V is applied via an A/D converter 15 to the adder 12 and in that the output of the adder 12 is applied via a D/A converter 14 to the adder 13 which performs an addition of analog quantities. With such an arrangement, it is possible to avoid the influence of the offset voltage as in the embodiment of Fig. 2 even if the speed command value V is given as an analog quantity. In Fig. 4, the same components as those in Fig. 2 are identified by the same reference numerals. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims: A movable machine (5) servo-control system comprising a speed control loop including a speed detector (6) coupled to the movable machine (5) and providing the actual value of the speed to a first adder (3) arranged to supply a difference quantity between a speed command value (V) and the feedback signal from the speed detector (6), which difference quantity commands the input of a speed control unit (4), and comprising a position feedback loop including a position detector (7) and a counter (8) counting pulses from said position detector (7), and a second adder (1,13) coupled to receive at a first input the speed command value (V), and at a second input a signal dependent on the content of the counter (8), and comprising an offset compensating circuit (8,9,10) coupled to an input of the second adder (1,13), characterised in that the offset compensating circuit is realised by inserting an arithmetic circuit (9) between the counter (8) and the second input of said second adder (1,13), which, upon activation of the arithmetic circuit (9) in a state of the speed command signal (V) at the first input of the second adder (1,13) being zero, or being cancelled by also being fed to the second input of the second adder (1,13), divides the content of the counter (Δ1) by a corresponding time interval (Δt), thus producing a compensating feedback signal (ΔV,ΔV') which is fed to the second input of said second adder (1,13). A servo-control system according to claim 1, and comprising another adder (12) arranged to receive the commanded speed value (V) and the divided quantity (Δ1/Δt) and to provide the difference therebetween as said feedback signal (ΔV') to the second input of the second adder (1,13), thus cancelling as aforesaid the speed command signal (V) at the first input of the second adder (1,13). A servo-control system according to claim 1 or 2, wherein the divided quantity (Δ1 / Δt) is calculated for a time Δt for which a manually operable device is actuated by an operator.

Inventors: YAMAZAKI, ETUO
Applicants: FANUC LTD.
Application year: 1983
Main classification: G05B19
IPC codes: G05B19, G05D13