Servo-motor driving method

Abstract: A conventional method for driving a servo-motor performed control by means of a current feedback loop, using a current sensor and an A/D converter. In consequence, the number of parts was great, cost reduction was difficult and it was difficult to avoid control errors caused by errors in the precision of the current sensor. The present invention comprises a method for driving a servo-motor by detecting a power supply voltage (V) of a servo-motor (4), controlling a voltage command (Vcmd) to a power converter (3) by numeric calculation using a detected value (Vcc) of this power voltage (V) thereby controlling power of a drive current (Iu, Iv, Iw).

Description: The present invention relates to a method and apparatus for driving a servo-motor, and more particularly to a new improvement for detecting power supply voltage without using a current feedback loop, comprising a current sensor and an A/D converter, which is conventionally used for motor current detection, and controlling current using this detected value, thereby improving control properties and lowering cost. Fig. 1 shows a conventional method of this type which has generally been used as a method for driving a servo-motor. That is, a current command Icmd is input to a calculator 1. The output 1a of the calculator 1 is sent via a proportional and/or integral controller 2 and is input as a voltage command 2a to a power converter 3, comprising a known power element configuration. The power converter 3 supplies a three-phase drive current Iu, Iv, Iw to a servo-motor 4. Two current sensors 5 detect one part of the three-phase drive current Iu, Iv, Iw, and the detected current value Ivcc is input to the calculator 1 via an A/D converter 6, thereby forming a current feedback control. Since the conventional servo-motor driving method has the configuration described above, it has problems such as the following. That is, a current sensor and an expensive A/D converter are required in order to form the current feedback system, constituting a considerable obstacle to lowering the cost of the control system. EP- 0419656 A1 describes a speed controller which controls the speed of a servo motor by the PWM-control of the inverter, wherein the power supply voltage of the inverter is detected to maintain constant the gain of the amplifier in the current loop. "Current Sensorless Field Oriented Control of Synchronous Reluctance Motor" by Matsuo et. al . from the conference record of the industry applications conference IAS annual meeting, Toronto October 3 - 6, 1993, vol. Part 1, Nr. Meeting 28, pages 672 - 678 describes a control strategy for a synchronous reluctance motor which eliminates the need for a current sensor. The control scheme includes a voltage reference calculator which generates the required voltage references from the torque command and the motor speed. According to the present invention there is provided a method for driving a servo-motor as set out in accompanying claim 1 and an apparatus for driving a servo-motor as set out in accompanying claim 4. Preferred features of the invention are set out in Claims 2, 3, 5 and 6. The present invention has been realized in order to solve the above problems and particularly aims to provide a method for driving a servo-motor wherein, by detecting power supply voltage without using a conventional current feedback loop for motor current detection and controlling current using this detected value, control properties can be improved and cost lowered. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing a conventional control method; Fig. 2 is a block diagram showing a servo-motor driving method according to the present invention; Fig. 3 is a control block diagram showing calculation contents of the current controller of Fig. 2; Fig. 4 is a control block diagram showing the normal control state of Fig. 3; and Fig. 5 is a block diagram for calculating a voltage command of the current controller of Fig. 2. There will be detailed below the preferred embodiments of the servo-motor driving method of the present invention with reference to the accompanying drawings. Like and similar members to the conventional example are explained using like reference characters. As shown in Fig. 2, current command Icmd, which comprises a current command value, is input to a current controller 2A and a voltage command Vcmd from the current controller 2A is applied to a known power converter 3. A drive power source 6, for driving power elements not shown in the diagram, is connected to the power converter 3. The power converter 3 supplies a three-phase drive current Iu, Iv, Iw to a servo-motor 4. Furthermore, a detected value Vcc of a power supply voltage V of the drive power source 6 and a detected value Ivcc of a power current I of the drive power source 6 are captured by the current controller 2A. The control system shown in Fig. 2 differs from the conventional control system of Fig. 1 in respect of the fact that the current controller 2A applies a voltage command 2a, which is controlled using the detected value Vcc of the power supply voltage V and the detected value Ivcc of the power current I of the drive source 6, to the power converter 3. Then, the servo-motor 4 is drive-controlled by the three-phase drive current Iu, Iv, Iw obtained from the power converter 3. Next, the calculation control of the current controller 2A will be explained more specifically. Fig. 3 shows the calculation contents of the servo-motor 4 and the current controller 2A as blocks. The current command Icmd is input to a first calculator 10 via a reverse calculation system (Ra + Las) which is reverse with respect to the calculation system (1/Ra + Las) of the servo-motor 4. The output 10a from the first calculator 10 is input to a second calculator 11. The output 11a of the second calculator 11 passes via the calculation system (1/Ra + Las), a torque constant kt and an inertia 1/Js, whereby a motor speed ω is obtained. Here, the abovementioned Ra represents motor resistance, La represents motor inductance and s represents a Laplace operator. Each induced voltage constant KE obtained from the motor speed ω is input to the calculators 10 and 11. Next, when calculation control with the configuration of Fig. 3 is functioning normally, it reaches the calculation control state shown in Fig. 4, where the response of the current response loop of the servo-motor 4 is 1, and the current control system accurately follows the current target value. However, in the current control system shown in Fig. 4, in order to directly receive fluctuations in the power supply voltage V, a detected value Vcc of the power supply voltage V must be detected and the voltage command Vcmd must be corrected accordingly. In order to perform the above correction, control is carried out by means of a numeric calculation using a calculation control system comprising the calculation control blocks shown in Fig. 5. Firstly, the current command Icmd is input to the second calculator 10 from the first calculator 10A via a first loop 20, wherein the response of the current response loop from the motor resistance R is 1, and a second-loop 21 (τes, a product of the electrical time-constant τe and the Laplace operator s, where τe is equal to La [motor inductance] / Ra [motor resistance]) in the same way as already depicted in Fig. 3. A voltage calculator 21A controls the output 11a of the second calculator 11 by numeric calculation of Vcom/Vcc (where Vcom is a projected value of the power supply voltage V and Vcc is the detected value of the power supply voltage V). Then, the voltage command Vcmd, which has now been corrected in accordance with the fluctuation in the power voltage V, is applied to the power converter 3. The program sequence for calculating the voltage command Vcmd in Fig. 5 is expressed by equation (1) in the following expression: [Exp. 1] Vcmd = {Ra (Ik - τe / T [Ik - Ik-1]) + kE • ωk} Vcom/Vcc Ik current command when sampling k Ik-1 current command when sampling (k-1) ωk motor speed when sampling k T sampling time Ra motor resistance τe electrical time-constant = La/Ra La motor inductance kE Induced voltage constant Vcom projected value of power voltage Vcc detected value of power voltage Vcmd voltage command Furthermore, during normal calculation control, an equality is achieved wherein K•Icmd × kt × ω = Vcc × Ivcc (where k is a proportional constant, Icmd is a motor current command, kt is the torque constant, w is the motor speed, Vcc is the detected value of power supply voltage and Ivcc is the detected value of power supply current). Therefore a case where the above equality is not achieved is determined to be a state of motor overcurrent, namely a control irregularity. The servo-motor driving method of the present invention has the configuration described above and therefore obtains the following advantageous effects. That is, since the entire system can be controlled with a numeric calculation open loop, there is no need for the current feedback system using a current sensor and an A/D converter which has conventionally been used. The consequent reduction of parts enables cost to be reduced. While there have been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.

Claims: A method for driving a servo-motor (4) based on a current command, comprising the steps of: detecting a value of a power supply voltage for driving said servo-motor (4); and current-controlling a drive current of said servo-motor (4) using the said detected value of the power supply voltage; the said method being characterised by determining a case where the equality K·Icmd x kt x ω = Vcc x Ivcc is not achieved to be a state of motor overcurrent, where K is a proportional constant, Icmd is a motor current command, kt is the torque constant, ω is the motor speed, Vcc is the detected value of power supply voltage and Ivcc is the detected value of power supply current. A method for driving a servo-motor (4) according to Claim 1, wherein said current-controlling comprises performing a numeric calculation using motor resistance (Ra), motor inductance (La), an induced voltage constant (kE) and a torque constant (kt) of said servo-motor (4). A method for driving a servo-motor (4) according to Claim 1 or 2, wherein said current-controlling is performed using a reverse calculation system Ra + Las, which is reverse with respect to the calculation system of said servo-motor (4) where Ra is motor resistance, La is motor inductance and s is a Laplace operator. Apparatus for driving a servo-motor (4) in response to a current command, comprising: means for detecting a power supply voltage for driving said servo-motor (4); and means for current-controlling a drive current of said servo-motor (4) in response to a detected value of said power supply voltage; characterised in that the apparatus comprises means for determining a case where the equality K·Icmd x kt x ω = Vcc x Ivcc is not achieved to be a state of motor overcurrent where K is a proportional constant, Icmd is a motor current command, kt is the torque constant, ω is the motor speed, Vcc is the detected value of power supply voltage and Ivcc is the detected value of power supply current. Apparatus for driving a servo-motor (4) according to Claim 4, wherein said means for current-controlling comprises means for performing a numeric calculation using motor resistance (Ra), motor inductance (La), an induced voltage constant (kE) and a torque constant (kt) of said servo-motor (4). Apparatus for driving a servo-motor (4) according to any one of Claims 4 or 5, wherein said current-controlling is performed using a reverse calculation system Ra + Las, which is reverse with respect to the calculation system of said servo-motor (4) where Ra is motor resistance, La is motor inductance and s is a Laplace operator.

Inventors: YASUI TAKAJI, YASUI, TAKAJI, Yasui, Takaji, c/o Tamagawa Seiki K.K.
Applicants: TAMAGAWA SEIKI CO LTD, TAMAGAWA SEIKI KABUSHIKI KAISHA
Application year: 1998
Main classification: H02P5
IPC codes: H02P5, H02P7, H02P27, H02K17, H02P29