SPEED CONTROL DEVICE FOR ELECTRIC MOTOR

Abstract: A speed control device for a motor having a speed feedback loop and a current feedback loop in which parallel capacitors (C1, C2) are connected to resistors (R4, R5) for specifying the limiting amplitude of an amplitude limiting circuit (33) when a speed deviation signal is used as a command signal for controlling a current through the amplitude limiting circuit (33) to limit the through rate dv/dt of the amplitude limiting circuit (33), thereby suppressing the occurrence of overshoot in the signal response characteristics and improving the stability in the transient response characteristics.

Description: DESCRIPTION TITLE OF THE INVENTION A SPEED CONTROL APPARATUS FOR MOTORS TECHNICAL FIELD The present invention relates to a speed control apparatus for motors. BACKGROUND ART A schematic block diagram of a conventional speed, control apparatus for motor driving using a transistorized sevo-controL-system is shown in Fig. I. A speed command input 10 is added to a speed feedback input 25 at a- first adder circuit 11. The added signal is compensated for phase characteristics at a phase compensating circuit 12r passes. through an amplitude limiting: circuit 13r and is added to a current feedback input 24 at a second adder circuit 14 The added signal is amplified at amplifier 15r is compared with a signal from reference signal. generator 19 at comparator circuit 16, and is divided into signals which control each transistorized control element in a motor control device 20. The signals. control. a motor 21 through insulating amplifiers 17. For the above-mentioned amplitude limiting circuit 13 and second adder circuit 14, the circuit shown in Fig. 2 is used This circuit is a circuit limited by the maximum input amplitude of the current loop, consequently when the gain of the current loop (load inertia etc.) changes, there is a problem in that the current limiting operation becomes unstable. Although there is another on-off control method which does not use the above -mentioned system, when the rotary speed and the characteristics of the motor change, there is another problem in that the frequency of the chopper and the ripple of the electric current are changed. The present invention is proposed in order to solve the abovementioned problems. DISCLOSURE OF TRE. INVENTION An object of the present invention, in consideration of the above-mentioned problems of conventional apparatus, is to obtain a- speed control apparatus enabling the suppression of the occurrence of overshoot in the signal response characteristic and the improvement of the stability of the transient response characteristic, based on the idea of limiting the through rate (ddtV) of the command voltage in the amplitude limiting system of the electric current loop. According to the present- invention, there is provided a speed. control apparatus for motors having a current feedback loop, comprising a first adder circuit at which the speed command input is added to the speed feedback input, a phase compensating circuit for compensating the phase of the output from said first adder circuit, an amplitude limiting circuit having an operational amplifier for receiving the output of said phase compensating circuit and sending the current command input signal, a second adder circuit at which said' current command input signal is added to the current feedback input signal, a reference waveform generating circuit, a comparator circuit for comparing the output of said second adder circuit with the output of said reference waveform generating circuit, a motor control device for receiving the output of said comparator circuit, a motor controlled with said motor control device, and a tacho-generator coupled with said motor; said speed control apparatus further comprising a capacitor connected in parallel with the resistor which defines the amplitude limit of said amplitude limiting circuit, whereby the value of the through rate dV of said amplitude limiting circuit is limited. BRIEF EXPLANATION OF DRAWINGS Figure 1 is a block diagram of a conventional speed control apparatus for motors, Fig. 2 is a circuit diagram of an amplitude limiting circuit and a second adder circuit used for the apparatus in Fig. 1, Fig. 3 is a block circuit diagram of a speed control apparatus for motors according to an embodiment of the present invention, Fig. 4 is a circuit diagram of the amplitude limiting circuit used for the apparatus in Fig. 3, Fig. 5 is a graph showing the relation between input and output of the circuit. in Fig 4, Fig 6 is a waveform chart of the output signal when the step waveform is applied to the: circuit in Fig 4 BEST MODE FOR CARRYING OUT THE INVENTION A speed control apparatus for motors according to an embodiment of the present invention is explained below in references to Fig. 3' This apparatus is identical with the conventional speed control apparatus in Fig. 1 except for an amplitude limiting circuit The speed command input LQ. is added to the speed feedback input 25 from a tacho-generator 22 at a firs.t adder circuit 11 and the sum. of the two inputs is applied to a phase compensating circuit 12 After the phase compensation, the sum is supplied to the amplitude limiting circuit 33. The circuit diagram of the amplitude limiting circuit 33 is shown. in Fig. 4. At one input terminal of an operational amplifier 41, resistors R1 and R2 and diodes Dl and D2 are connected to one another. The other input terminal of the operational amplifier 41 is grounded through a resistor. At the output terminal of the operational amplifier 41, the other terminal of resistor R2, a resistor R4,. a capacitor CI, a resistor R5, and a capacitor C2 are connected to. one another. The other terminal. of the diode Dl, the other terminal of the resistor R4, and the other terminal of the capacitor C1 are coupled to one another and connected to the positive terminal of the power source through a resistor R3. The other terminal of the diode D2, the other terminal of the resistor R5, and the other terminal of the capacitor C2 are coupled to one another' and connected to the negative terminal of the power source through a resistor R6. The input terminal 42 of the amplitude limiting circuit 33 is connected to the other terminal of resistor RI, and an output terminal 43 is connected with the output terminal of the operational amplifier 41. The output of the amplitude limiting circuit 33, i.e., the current command input 23 is added to the current feedback input 24 at a second adder circuit 14. The current feedback input 24 is a control voltage which is proportional to the armature current of the motor and is supplied through an insulating amplifier 18. The output of the second- adder circuit 14 is amplified through an amplifier 1S- and applied to a comparator circuit 16 At the comparator circuit 16t the amplified output is compared with the reference signal from a reference waveform generator r for example, a triangular waveform generator. The output of the comparator circuit 16 is divided into- four outputs and the four outputs are supplied to- the base circuits of each transistor switch in a motor control device 20 through the insulating amplifier 17. The transistor switches form a bridge circuit, and a circuit consisting of a motor 21 and a resistor- RO connected in series . is connected with the bridge circuit. The motor 21 is coupled to the tacho-generator 22 mechanically and the output of the tacho-generator 22 is fed back as the speed feedback input 25. Below the operation of the amplitude limiting circuit 33 is explained in reference to Figs. 4 through 6. In. the circuit of Fig. 4, if the capacitors CI and C2 are not connected, when the diode D1 is conducting, the gain of the operational amplifier 41 R2.R4 is Rp/Rl, where Rp is a equivalent resistance R2+R4 in the parallel connection of the resistors R2 and R4. If the capacitor Cl is connected, application of a small input signal forms an integrating circuit with the capacitor Cl and the resistor R2 connected in parallel and creates an input to output characteristic in the circuit of Fig. 4 as shown in the graph of Fig 5. Therefore, application of a step waveform signal to the input will result in an output of the amplitude limiting circuit 33 of the waveform shown in Fig. 6. The incline in the graph of the input to output characteristic in Fig. 5 is R2/Rl and the output value of the flat line at the right upper portion is Rp/Rl.- When the diode D2 is conducting, the same operation is carried. out using the capacitor C2 and the resistor R5, instead. of the capacitor C1 and the resistor R4, respectively.. As. mentioned above, the signal of the waveform having a limited through rate dV instead of the step dt waveform,. can control the speed of the motor to reduce overshoot and other bad influences and enable good control of the transient characteristic.

Claims: CLAIMS A speed control apparatus for motors having a current feedback loop, comprising a first adder circuit at which the speed command input is added to the speed feedback input, a phase compensating circuit for compensating the phase of the output from said first adder circuit, an amplitude limiting circuit having an operational amplifier for receiving the output of said phase compensating circuit and sending the current command input signal, a second adder circuit at which said current command input- signal is added to the current feedback input signal, a reference waveform generating circuit, a comparator circuit for comparing the output of said second adder circuit with the output of said reference waveform generating circuit, a motor control device for receiving the output of said comparator circuit, a motor controlled with said motor control device, and a tacho-generator coupled with said motor; said speed control apparatus further comprising a capacitor connected in parallel with a resistor which defines the amplitude limit of said amplitude limiting circuit, whereby the value of the through rate (avt) of of said amplitude limiting circuit is limited

Inventors: KOBARI, KATSUO, KOZAI, YOSHINORI, SAKAMOTO, KEIJI
Applicants: FANUC LTD
Application year: 1981
Main classification: H02P5
IPC codes: H02P7