JPH0217892A - Cancellation of offset current - Google Patents

Abstract

PURPOSE:To detect offset current in a real time even if a motor is in operation by detecting the offset current intaking advantage of distorted waveform in the reversal of the polarity of the detected current. CONSTITUTION:An offset current detection section 51 monitors the polarity of a current command ICMD and a current deviation I. When the polarity changes, the current command is read to let it be an offset current value IOFS. Let ICP+IOFS be a new offset current corrected value ICP. An offset cancellation section 52 outputs true U-phase detected current Iu' which cancelled the offset current by detecting the offset current corrected value ICP from the detected current Iu. A phase voltage generation section 9 outputs U-phase command voltage Vu to a PWM signal generation circuit 10 in accordance with the current deviation I between the current command ICMD and the detected current Iu.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an offset current canceling method in a servo control device, and particularly to a method of offset canceling using a distorted waveform of a detected current.

<Prior art> In an AC motor control device that controls an AC motor using digital control, a current command for each phase is generated based on the difference between the speed command and the actual speed, and the current command and the detected current for each phase are A PWM signal is output by comparing the difference with a reference carrier wave, and the AC motor is controlled by the PWM signal.

In such AC motor control, if an offset occurs in the current detection section and the offset current is superimposed on the detected current,
The control performance of the AC motor deteriorates, and highly accurate speed control becomes impossible.

Therefore, conventionally, in order to cancel the offset current, (i) stop the AC motor and cut off the current command, and (11) take in the detected current (offset current) 'OFl+ when the current command is zero in this state. (2) During motor operation, the offset current is canceled by subtracting it from the detected current.

FIGS. 5 and 6 are explanatory diagrams of servo control of a synchronous motor, which is one type of AC motor. In Figure 1, E is a three-phase power supply, M is a synchronous motor with a permanent magnet as a rotor, and IDU
, IDW is a phase current detector for U and W phases, 1 is a PWM control circuit, 2 is a rotor position detector such as a pulse encoder for detecting the rotor position and speed of the synchronous motor M, 3 is a rectifier circuit, and 4 is a rectifier circuit. It is a transistor inverter.

PWM @control circuit 1 is the actual speed detected by the rotor position detector 2 and the speed command■. The speed is controlled by comparing the currents of the windings of the U, V, and W phases of the synchronous motor by turning on/off each transistor TA to TF of the transistor inverter 4.

The PWM control circuit 1 is configured as shown in FIG. 6 (only the U phase is shown). In the figure, 5 is a signal processing circuit which counts the pulses S output from the rotor position detector 2 and outputs an actual speed signal V and a rotor position signal P. 6 is a sine wave generating circuit having a ROM configuration that outputs a sine wave signal sinωt of a phase corresponding to the current position P of the rotor, and 7 is a voltage V indicating a speed command. A speed error generation circuit 8 outputs a speed error signal VE which is the difference between the actual speed v5 outputted from the signal processing circuit 5, and a speed error generation circuit 8 which multiplies the U-phase sine wave signal and the speed error signal vE to generate the U-phase phase. Current command' cMo
9 is a phase current command I. M.O.
and U-phase current I after offset cancellation.

A phase voltage generation circuit outputs the U-phase command voltage (phase voltage) ■8 based on the difference between the phase voltage vU and the reference rush wave (10).
The PWM signals PA, P, (transistor TA in Fig. 5) are compared according to their magnitudes.
, T,), and 11 is an offset cancel circuit that generates an offset current I detected and stored in advance. PS is subtracted from the detection current Iu and output.

<Problems to be Solved by the Invention> In such conventional offset canceling methods, the motor is always stopped and the offset current is detected.
Moreover, once the - degree detection setting is made, the - degree is maintained at - constant from then on.

Therefore, in the conventional method, it is not possible to accurately cancel the offset when the offset current value fluctuates. In particular, the offset current is easily affected by temperature, and its value fluctuates during continuous operation. Conventional methods have had the problem of not being able to deal with such cases and not being able to accurately cancel the offset.

From the above, an object of the present invention is to provide offset cancellation in a servo control device that can detect offset current in real time even during motor operation by using the distorted waveform when the polarity of the detection current is reversed, and can therefore perform accurate offset cancellation. The purpose is to provide a method.

<Means for Solving the Problems> FIG. 1 is a block diagram of main parts of a servo control device according to the present invention.

51 is an offset current detection section, 52 is an offset cancellation section, and 53 is a calculation section.

Further, M is an AC motor, IDU is a current detector, IC0° is a current command, ■1 is a detected current, and 'CP is an offset current correction value.

<Operation> The offset current detection unit 51 detects an offset current value using a distorted waveform that occurs when the polarity of the detection current IU is reversed, and calculates an offset current correction value (■) based on the offset current value. P, and the offset canceling unit 52 generates the correction value l. p is the detection current! . Subtract from to cancel the offset.

<Embodiment> FIG. 1 is a block diagram of the main parts of a servo device that implements the present invention, and only the U phase is shown. The same parts as in the conventional device shown in Fig. 5 are given the same reference numerals, and M is an AC motor;
IDU and IDW are U-phase and W-phase current detectors, 4 is a transistor inverter, 9 is a phase voltage generator that generates a phase voltage vU based on the difference between the command current and the detected current, and 10 is a PWM signal generator. be.

51 is an offset current detection unit which detects the offset current value I by using the distorted waveform that occurs when the polarity of detection current 1 is reversed.
. In addition to detecting FS in real time, offset current correction value ■. Calculate and output P.

52 is an offset canceling section, which detects the current 11,
Offset current correction value from l! . , the true U-phase detection current I with the offset current canceled by subtracting
Output U.

53 is a current command I. ,,. This is a calculation section that calculates the difference between the current IU' and the detected current IU' and inputs the result to the phase voltage generation section 9 and the offset current detection section 51, respectively.

The offset cancellation method according to the present invention will be explained below.

Transistor TA in transistor inverter 4 (
To, T,) is turned off from on and transistor T, (To, T,) is turned on from off, or vice versa, when both transistors are turned on and off at the same time, both transistors are turned on at the same time. There is a risk that (
(top and bottom short circuit). Therefore, in controlling the transistor inverter, on/off control is performed to ensure that the on-transistor TA (T, , T,) is turned off and then the off-transistor T, (To, T,) is turned on.

Therefore, the relationship between the detected current IU and the current command ICMO is the detected current I as shown in FIG. A dead zone appears when the polarity is reversed. Therefore, the sine wave current command 'C11° (3rd
(solid line), the detection current IU becomes as shown by the dotted line in Figure 3, generating a distorted waveform when the polarity is reversed, and when the detection current ILl is positive, it becomes I. ,,lo>■,,l, and when the detected current is negative, ■. M. <IU.

Here, when the detection current (2) and the offset current (IoFs) are superimposed, the detection current IU is shifted upward by the offset, as shown by the dashed line in FIG. As is clear from this Figure 4, I. If we can detect the point in time when the magnitude relationship between +1° and Iu is reversed, then! . ,,. is the offset current I. It becomes FS.

From the above, the offset current detection section 51 receives the current command I output from the calculation section 53. □. and detected current! Difference Δ of 0
(2) Monitor the polarity, read the current command when the polarity changes, and set the current value as the offset current I. FS.

Next, the offset current detection unit 51 determines the offset current correction value I using the following equation 1 (initial value of ICP is zero). p is calculated and input to the offset canceling section 52.

The offset canceling unit 52 detects the detected current ■. offset current correction value I. True U-phase detection current with offset current canceled by subtracting P1. Output J'.

The calculation unit 53 will thereafter issue a current command (■). ,. and offset-cancelled true detection current 1. / is calculated and inputted to the phase voltage generation section 9 and the offset current detection section 51. As a result, speed control without the influence of the post-a offset becomes possible.

Note that if the offset in the current detection unit IDU does not change, 1oFs=0 from then on, and the offset current correction value IcP obtained from equation tl) is kept constant.

Also, even if the offset changes in the current detection unit IDtJ during operation, the offset can always be canceled correctly using equation [1]. For example, if the offset is current Δl0
When FG increases, the detection current ■. Since Δ'OF* is superimposed on ', the offset current detection unit 51 detects Δl0F9 as the offset current and updates the offset current correction value 'cp using equation 11. Therefore, the detection current! . is subsequently canceled by a total of (IOFS+Δl0FS).

<Effects of the Invention> According to the present invention, the offset current value is detected using the distorted waveform of the detection current that occurs at the time of polarity reversal, and the offset current is subtracted from the detection current to cancel the offset. The offset current can be detected in real time even when the motor is operating, and therefore accurate offset cancellation is possible even when the offset current fluctuates.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the main parts of the servo control device according to the present invention, Figs. 2 to 4 are relational diagrams and waveform diagrams of current command and detected current to explain offset current detection, Figs. FIG. 6 is a block diagram of a conventional servo control device. 51...Offset current detection unit, 52...Offset cancellation unit, 53...Calculation unit IDU...Current detector, 'ch.・・Current command, ILl・・Detection current, 'cp・
・Offset current correction value patent applicant: Fanuc Co., Ltd. agent Patent attorney Chiki Saito Figure 2 Tυ Figure 3 Figure 4

Claims (2)

[Claims] (1) In an offset current cancellation method in a servo control device that outputs a PWM signal based on the difference between the current command of each phase and the detected current of each phase and controls an AC motor using the PWM signal, when the polarity of the detected current is reversed. An offset current canceling method comprising detecting an offset current superimposed on the detected current using a generated distorted waveform, and subtracting the offset current from the detected current to cancel the offset. (2) The offset current canceling method according to claim 1, wherein the command current value when the sign of the difference is reversed is set as the offset current value.