SPEED CONTROLLER

Abstract: A speed controller which controls the speed of a servo motor (l2) by the PWM-control of the inverter, wherein the power supply voltage of the inverter is detected to maintain constant the gain (K) of the amplifier (ll) in the current loop.

Description: This invention relates to a velocity control apparatus and method, and to a PWM control device, for controlling the velocity of a servomotor by PWM (pulse-width modulation) control of an inverter. In recent years, microprocessors have come to be widely used in velocity control apparatus for controlling motor drive. With a motor drive circuit composed of digital circuitry using a microprocessor, table information such as amplitude characteristics decided in conformity with motor characteristics is created, and the velocity of a servomotor is controlled by PWM control of an inverter based on a set velocity signal. In a conventional velocity control apparatus of this kind, the range of fluctuation allowed with regard to power supply voltage is set to be within a prescribed limit. The apparatus is provided with a function which protects the motor from overcurrents when the aforementioned limit is exceeded, and with a protective function which shuts down the system automatically when the voltage is too low. However, no particular consideration is given to fluctuations within the prescribed limit. More specifically, a fixed current loop constant K is provided. Therefore, when power supply voltage Vdc becomes high, current loop gain K′ (= K·Vdc/Fa, where Pa represents carrier wave amplitude) rises in proportion to the voltage value and there is the danger that the motor current will oscillate. Conversely, when the power supply voltage becomes low, current loop gain diminishes and the response of the velocity loop to a commanded velocity signal slows down. The result is a deterioration in the quick response of the control system. The present invention has been devised to solve the foregoing problems and its object is to provide a velocity control apparatus and method, and a PWM control device, which make stable velocity control possible by compensating for fluctuation in power supply voltage and obtaining a constant current loop gain. According to a first aspect of the present invention there is provided a velocity control apparatus, with a current control loop, for controlling drive current of a servomotor by a PWM-controlled inverter circuit, thereby to control the velocity of the servomotor, comprising a DC power supply for said inverter circuit and being characterised by: voltage detecting means for detecting an output voltage value of the DC power supply; and gain control means for holding constant the current loop gain in dependence upon the detected output voltage value of the DC power supply. According to a second aspect of the present invention there is provided a PWM control device for PWM-controlling an inverter circuit in a velocity control apparatus, with a current control loop, for controlling drive current of a servomotor thereby to control the velocity of the servomotor, the PWM control device being characterised by: constant computing means for computing a current loop constant to be inversely proportional to DC power supply voltage applied to the inverter circuit when it is in use; and signal generating means for forming a PWM control signal in dependence upon said current loop constant and a command current value in such a manner that the current loop gain becomes constant. According to a third aspect of the present invention there is provided a velocity control method in a velocity control apparatus connected to a servomotor as a controlled system, the velocity control apparatus having a current control loop and comprising an inverter circuit controlling drive current of the servomotor thereby to control the velocity of the servomotor, the velocity control apparatus further comprising command means forming a current command and a velocity command for the servomotor, and arithmetic means computing a PWM command voltage in dependence upon a differential signal between the current command from the command means and a current feedback signal from the servomotor, the velocity control method being characterised by: computing, by a PWM control device comprising constant computing means, a current loop constant inversely proportional to DC power supply voltage applied to the inverter circuit; forming, by signal generating means, a PWM control signal in dependence upon said current loop constant and a current command value in such a manner that the current loop gain is constant; executing computation of the current loop constant in the PWM control device within one sampling period of a current value fed back from the servomotor; computing the PWM command voltage; and deciding and outputting motor current. Thus, in accordance with the present invention, when the servomotor is PWM-controlled, the power supply voltage is detected at all times and the current loop gain is held constant to enhance the stability of the control system. Further, in accordance the PWM control device of the invention, it is possible to construct a control system stabilized with respect to fluctuations in power supply voltage. Brief Description of the Drawings Fig. 1 is a block diagram illustrating an embodiment of the present invention, Fig. 2 is a view illustrating an example of the construction of a motor control system, Fig. 3 is a time chart for describing PWM control, Fig. 4 is a view for describing computation with regard to current loop gain K′, and Fig. 5 is a flowchart illustrating a processing procedure for setting gain. An embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 2 is a view illustrating an example of the construction of a PWM-controlled motor control system. An inverter section 2 supplied with commercial AC voltage is connected to a converter section l as a DC power supply having a predetermined voltage value. The inverter section 2 is a transistorized inverter comprising a plurality of switching transistors and is for driving a three-phase AC motor 3. The inverter section 2 is connected to a PWM control circuit 4 and controls a current supplied to the primary side of the motor 3 in response to a PWM control signal applied to the base of each transistor from the PWM control circuit 4. The motor can thus be controlled to vary its velocity. Fig. 3 is a time chart for describing PWM control of the transistors in the inverter section 2 by means of the PWM control circuit 4. A triangular wave having a period of 2T is supplied as a carrier wave, and on time T on of the transistors in the inverter section 2 is decided by comparing the triangular wave with the level of a PWM command voltage E. Note that ±Pa represents the amplitude of the carrier wave. Here the command voltage E is inputted as K(Ic-If) with respect to a current loop constant K, and on time of the transistors, namely the value of drive current applied to the motor 3, is decided in dependence upon the magnitude of the command voltage E. The aforementioned Ic is a command current value decided by a torque command value, and If is a current feedback signal from the motor 3. Fig. 1 is a block diagram illustrating the current loop of the motor current controlled by the command current value I c . The command current Ic is compared with the current feedback signal lf from the motor 3 by an adder l which computes a current error. The value of the error current is outputted by a current feedback loop which includes an amplifier 11 having a current loop gain K′ capable of variably setting the current loop gain constant K, and a motor section 12 having an electrical constant K″. The current loop constant K is set in such a manner that the current loop gain K′ will be constant irrespective of a fluctuation in power supply voltage. The current loop gain K′ in the current loop of Fig. 1 will now be described with reference to Fig. 4. In Fig. 4, two triangles Δoa₁b₁, Δoa₂b₂ are similar since the three angles of one are equal to the three corresponding angles of the other. Therefore the following equation holds: a₁b₁ / a₂b₂ = oa₁ / oa₂ The following substitutions are made: a₁b₁ = (T - Ton)/2 a₂b₂ = T/4 oa₁ = Pa - K (lc - lf) oa₂ = Pa Arranging, we have T on /T = l - {Pa - K(Ic - If)}/(2Pa) The applied voltage of the motor is as follows: {Ton·Vdc + Toff·(-Vdc)}/T = {(2Ton/T) - l}Vdc If the right side of Eq. (1) is inserted into Eq. (2) and the expression is arranged, the applied voltage of the motor will be K·Vdc(Ic - If)/Pa. Accordingly, current loop gain K′ becomes as follows: K′ = K·Vdc/Pa Therefore, since Pa is constant, the current loop gain K′ will be rendered constant, irrespective of a fluctuation in the power supply voltage Vdc, by deciding the current loop constant K so as to be inversely proportional to the power supply voltage Vdc. Fig. 5 is a flowchart illustrating a processing procedure for setting gain in the abovementioned motor current control loop. The motor current is sampled and detected as a digital value in a prescribed interval (step a). The current error Ic - If is calculated based on the current value If (step b). At the same time, the power supply voltage Vdc supplied to the inverter section 2 also is detected (step c). The current loop constant K is calculated using this detected power supply voltage Vc (step d), and the PWM control signal is calculated from the current error and current loop constant in such a manner that the current loop gain will be rendered constant (step e). The motor current is decided by the PWM control signal thus outputted, and velocity control is carried out (step f). Though an embodiment of the present invention has been described, the invention it not limited thereto but can be modified in various ways without departing from the scope of the claims. The velocity control apparatus and the PWM control device of the present invention are capable of rendering current loop gain constant by setting a current loop constant that is inversely proportional to a fluctuation in power supply voltage. This makes possible stabilized velocity control of a servomotor.

Claims: A velocity control apparatus, with a current control loop, for controlling drive current of a servomotor (3) by a PWM-controlled inverter circuit (2), thereby to control the velocity of the servomotor (3), comprising a DC power supply (1) for said inverter circuit (2) and being characterised by: voltage detecting means for detecting an output voltage value (Vdc) of the DC power supply (1); and gain control means (11) for holding constant the current loop gain (K′) in dependence upon the detected output voltage value (Vdc) of the DC power supply (1). A PWM control device for PWM-controlling an inverter circuit (2) in a velocity control apparatus, with a current control loop, for controlling drive current of a servomotor (3) thereby to control the velocity of the servomotor (3), the PWM control device being characterised by: constant computing means for computing a current loop constant (K) to be inversely proportional to DC power supply voltage (Vdc) applied to the inverter circuit (2) when it is in use; and signal generating means for forming a PWM control signal in dependence upon said current loop constant (K) and a command current value (Ic) in such a manner that the current loop gain (K′) becomes constant. A velocity control apparatus according to claim 1, having an inverter circuit (2) connected to a servomotor (3) as a controlled system, the velocity control apparatus comprising: command means for forming a current command (Ic) and a velocity command for the servomotor (3); and arithmetic means for computing a PWM command voltage in dependence upon a differential signal between a current command (Ic) from the command means, and a current feedback signal (If) from the servomotor (3). A velocity control method in a velocity control apparatus connected to a servomotor (3) as a controlled system, the velocity control apparatus having a current control loop and comprising an inverter circuit (2) controlling drive current of the servomotor (3) thereby to control the velocity of the servomotor (3), the velocity control apparatus further comprising command means forming a current command (Ic) and a velocity command for the servomotor (3), and arithmetic means computing a PWM command voltage in dependence upon a differential signal between the current command (Ic) from the command means and a current feedback signal (If) from the servomotor (3), the velocity control method being characterised by: computing, by a PWM control device comprising constant computing means, a current loop constant (K) inversely proportional to DC power supply voltage (Vdc) applied to the inverter circuit (2); forming, by signal generating means, a PWM control signal in dependence upon said current loop constant (K) and a current command value (Ic) in such a manner that the current loop gain (K′) is constant; executing computation of the current loop constant (K) in the PWM control device within one sampling period of a current value fed back from the servomotor (3); computing the PWM command voltage; and deciding and outputting motor current.

Inventors: KONO SHINICHI FANUC MANSION HA, TAKAHASHI HIRONOBU FANUC DAI-, KONO, SHINICHI, FANUC MANSION HARIMONI 7-203, TAKAHASHI, HIRONOBU, FANUC DAI-3 VILLA-KARAMATSU
Applicants: FANUC LTD, FANUC LTD.
Application year: 1989
Main classification: H02P7
IPC codes: H02P25, H02P27, H02M5, G05B11