JPH0818565B2 - Current control device, current control method, and electric power steering device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a current control device for a PWM converter that drives an electric motor, and more particularly to an electric power steering device that assists steering operation of an automobile with the electric motor. The present invention relates to a digital control type current control device and a current control method suitable for use in a device which responds at high speed and requires less current pulsation, and an electric power steering device equipped with the current control device.

[Conventional technology]

Generally, in a device that drives a motor by controlling the switching of a DC voltage such as a PWM converter,
Since the current flowing through the electric motor causes pulsation due to the switching operation, there is known a method of removing the pulsating component by a low pass filter as described in JP-A-61-184171.
However, if a low-pass filter is used, the detection delay occurs, so that the response of the current control system cannot be improved.

On the other hand, conventionally, as a method for obtaining a current from which a pulsating component is removed without using a low-pass filter that causes a delay in current detection, for example, JP-A-58-198165 and JP-A-61-10
As described in Japanese Patent Application No. 9469 and Japanese Patent Application No. 61-222163, there is a method of detecting the current of a PWM converter at a timing synchronized with a carrier signal which is a reference for switching operation. By this method, the current can be detected at the time when the instantaneous value of the pulsating current becomes almost the average value of the current, so that the current without the pulsating component can be instantaneously obtained without using the low-pass filter, and the response of the current control is improved. be able to.

In these known examples, the carrier wave generation circuit that generates a carrier wave signal is an analog circuit in mind, so that the carrier wave signal has a triangular waveform in any case. Therefore, the carrier wave signal may be directly used for the current detection timing and may be synchronized with the maximum value or the minimum value.

[Problems to be Solved by the Invention]

However, when the carrier wave generation circuit is configured using a digital circuit, the sawtooth carrier wave signal that can be configured by the up counter has a smaller number of gates than the triangular wave carrier signal that must use the up / down counter, and the circuit is It will be easy. In particular, since the number of gates must be reduced in a one-chip microcomputer, a carrier signal for PWM generation is generally a sawtooth waveform. For example, there is a one-chip microcomputer 8096 manufactured by Intel (US), one-chip microcomputer H8 / 532 manufactured by Hitachi, Ltd.

When such a digital circuit is used, even if the current is detected at the timing synchronized with only the carrier wave, the average value of the current cannot be obtained. That is, the above prior art does not consider the use of the sawtooth carrier signal.

Therefore, an object of the present invention is to detect an instantaneous value of a current that substantially matches an average value from among pulsating currents in a PWM converter using a sawtooth carrier wave signal suitable for a digital circuit, An object of the present invention is to provide a current control device and method capable of improving responsiveness.

Further, in the electric power steering device that requires high-speed torque control, the response of the current control system is conventionally slow, so as described in JP-B-63-19386, the differential gain of the steering torque is increased to increase the current. A method of compensating for the delay of the control system and a method of using the detected current value for overcurrent compensation instead of feedback control as described in JP-A-61-184171 are used. However, in the former case, a torque detector with less noise must be used in order to increase the differential gain, and in the latter case, the current becomes an open loop in terms of control. Therefore, compensation for the back electromotive force of the motor and fluctuation of power meter is required. There was a problem that it could not be done.

Therefore, a second object of the present invention is to provide an electric power steering apparatus capable of improving the response of the current control system and reducing the differential gain of the steering torque.

[Means for solving the problem]

In addition to the first PWM generating circuit for driving the PWM converter, the above-mentioned two purposes are that the instantaneous value of the feedback current is the average value of the instantaneous value of the feedback current as a function of the voltage command value from the voltage command value. A means for calculating a voltage value for current detection for determining substantially coincident timing and a second PWM generation circuit for obtaining a second PWM signal by comparing the sawtooth carrier wave signal and the voltage value for current detection Is provided, and the current detection circuit is configured to detect the current at a timing synchronized with the second PWM signal.

Further, the above-mentioned two purposes are at least one of an operation of taking a half value of the voltage command value from the voltage command value and an operation of taking an average value of the voltage command value and the maximum value of the carrier signal. And first means for obtaining at least one current detection voltage value, and second means for comparing the carrier wave signal and the current detection voltage value to obtain a current detection pulse signal,
This is achieved by configuring the current detection circuit to detect the current at a timing synchronized with the current detection pulse signal.

[Action]

In the present invention thus configured, in the first PWM generation circuit, the voltage command value and the sawtooth carrier wave signal are compared, and when the voltage command value is equal to or higher than the carrier wave signal, the PWM signal (first PWM signal) is output. It turns on, and otherwise operates to turn off the PWM signal. This PWM signal causes the PWM converter to perform switching operation, and the PWM
The current flowing through the converter pulsates. That is, when the PWM signal is on, the current gradually increases. Also, the second
The PWM generation circuit calculates the current detection voltage value and the sawtooth-like voltage value calculated from the voltage command value as a function of the voltage command value so as to determine the timing at which the instantaneous value of the feedback current substantially matches its average value. If the current detection voltage value is greater than or equal to the carrier signal,
The PWM signal (second PWM signal) is turned on, and when it is less than that, the PWM signal is turned off. Here, the timing at which the second PWM signal falls from ON to OFF is the momentary value of the gradually increasing current when the current detection voltage value is half the voltage command value. Is synchronized when the average value is close to the average value. Therefore, this PW
By detecting the current at the time when the M signal falls from ON to OFF, the current having almost the average value and no pulsation can be obtained instantly, and the responsiveness of the current control system can be improved.

By the way, the current flowing through the electric motor gradually increases when the first PWM signal is on, but gradually decreases when the first PWM signal is off. Therefore, the instantaneous value of the current substantially matches the average value not only during the on period of the first PWM signal but also during the off period. Therefore, as the current detection voltage value that determines the timing of the current detection, an average value of the voltage command value which is a value equal to or higher than the voltage command value and the maximum value of the carrier signal may be calculated. In this case, The timing at which the second PWM signal obtained by the second PWM generation circuit falls from on to off is synchronized when the instantaneous value of the gradually decreasing current substantially matches its average value. Therefore, also in this case, by detecting the current at the time when the PWM signal falls from ON to OFF, it is possible to instantaneously obtain a current having an almost average value and no pulsation. Also,
By doing so, the current can be detected and controlled even when the voltage command value becomes zero or the reverse direction and the off period of the first PWM signal continues.

Note that the above is the case where the current is detected in either the ON period or the OFF period of the first PWM signal, but the current is detected in both the ON period and the OFF period of the first PWM signal. Of course, in this case, current control with higher response can be performed.

Further, in the above, the PWM signal is used to instruct the current detection timing, but it is sufficient to instruct the time when the current detection voltage value becomes equal to or lower than the carrier signal. The pulse signal may be obtained.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows an embodiment of the present invention when applied to an electric power steering device. When the driver steers the steering wheel 1, the steering torque τ and the steering angle θ are detected by the torque detector 3 attached to the steering wheel shaft 2. Further, the vehicle speed v is detected by the vehicle speed sensor 5 attached to the tire 4a. The detected steering torque τ, steering angle θ, and vehicle speed v are input to the digital control circuit 6 and used to calculate the drive PWM signals Pa, Pb, Pc, Pd for generating the optimum assisting force. To be Next, the PWM bridge circuit 7 is the driving PWM
The voltage VB of the battery 8 is switched by the signals Pa, Pb, Pc and Pd to supply the motor voltage VM. The electric motor 9 is driven by the electric motor voltage VM, and the electric motor current iM flows. Torque is generated from the electric motor 9 by this electric motor current iM, and the tires 4a, 4b are passed through the steering mechanism members 10a, 10b.
It generates an assisting force to turn around. As a result, the tires 4a, 4b can be easily turned even if the steering force of the steering wheel operated by the driver is small.

Here, the current control system will be described in detail. First,
As a method to detect the motor current iM, PWM is used in Fig. 1.
As the switching elements 11a, 11b, 11c, 11d of the bridge circuit 7, ordinary FETs (field effect transistors) 11a, 11b and FETs 11c, 11d with current detection terminals are used. When the FETs 11c and 11d with current detection terminals are turned on, the same current as the motor current iM flows in both FETs. Therefore, the motor current iM can be detected by measuring the voltages vic and vid at the current detection terminals at that time. Therefore, the voltages vic and vid of the current detection terminal are input to the digital control circuit 6.

Next, the inside of the digital control circuit 6 will be described. In the current command circuit 12 which inputs the steering torque τ, the steering angle θ, and the vehicle speed v, the current command value iR for giving the optimum auxiliary torque is calculated according to these input values, and the subtraction circuit
It is output to 13. The subtraction circuit 13 calculates the deviation iε between the current command value iR and the current feedback value if described later. This deviation iε is input to the voltage command circuit 14 and the voltage command value vR is calculated. The carrier generation circuit 15 is composed of an up counter and generates a sawtooth carrier signal vc. In the first PWM generating circuit 16 which inputs the voltage command value vR and the sawtooth carrier signal vC, both are compared and vR ≧ vC
Control PWM signal P1 is turned on when, and control PWM signal when vR <vC
The signal P1 is turned off.

This time chart is shown in FIG. For example, time t1
Since the carrier wave signal vC becomes 0, the control PWM signal P
1 turns on, and P1 turns off after the time t3 when vR = vC.

Such a control PWM signal P1 is input to the PWM switching circuit 17. In the PWM switching circuit 17, the motor current i in the direction corresponding to the left / right switching signal SC output from the current command circuit 12
The driving PWM signals Pa, Pb, Pc, and Pd are switched so that M flows.
That is, one set of PWM signals of Pa, Pd or Pb, Pc is operated by the control PWM signal P1 according to the left / right switching signal SC.

Next, a method of detecting a current, which is a feature of this embodiment, will be described in detail. In the current detection switching circuit 18 which inputs the voltages vic and vid of the current detection terminal obtained from the PWM converter 7, the current detection terminal of the FET having the current detection terminal among the FETs with the current detection terminal is switched by the left / right switching signal SC. The voltage is selected and output as the detection current id. Further, the proportional circuit 19 determines, as a function of the voltage command value vR, the timing at which the instantaneous value of the feedback current if substantially coincides with the average value of the voltage command value vR. Here, the voltage command value vR is halved. It is used to output the current detection voltage vd. next,
The second PWM generation circuit 20 operates so that the current detection PWM signal P2 is turned on when the current detection voltage vd is equal to or higher than the sawtooth carrier signal vC, and is turned off when vd <vc.

The situation is shown in FIG. For example, from time t1 to time t2
Are on, and from time t2 to time t4 are off. The sample hold circuit 21 holds the value of the detected current id at the timing when the current detection PWM signal P2 changes from on to off. This value is fed back by the A / D converter 22 as the current feedback value if which is a digital value.

In this way, as can be seen from FIG. 2, the current feedback value if that substantially matches the average value of the motor current iM from the pulsating detection current id is calculated using the sawtooth carrier wave vC and the voltage command value vR. You can get it instantly. If the current feedback value if without pulsation is used, it is not necessary to use a low-pass filter, so that the responsiveness of the current control system can be improved. Therefore, it is possible to configure an electric power steering device that has a good steering feeling even if the differential gain of the steering torque is small.

FIG. 3 is another embodiment different from FIG. 1 when a one-chip microcomputer incorporating a PWM generator and an A / D converter is used.

The embodiment of FIG. 3 differs from the embodiment of FIG. 1 in that a one-chip microcomputer 23 is used in the digital control circuit 6, which will be described in detail.

The one-chip microcomputer 23 is the main arithmetic unit
24, PWM generating circuits 25a, 25b, 25c, A / D converter 26. First, as the control calculation in the main calculation unit 24, the calculation as shown in the flowchart of FIG. 4 is performed every control sampling time. When this subroutine is started, the steering torque τ, the steering angle θ, and the vehicle speed v are input in step 101. Next, at step 102, the current command value iR is calculated from these input values. Step
103 is a current feedback value if and a current command value iR described later.
From this, the voltage command value vR is calculated. Then, in step 104, the voltage command value vRa or vRb in the direction in which the PWM signal should be output is output according to the direction of the current command value iR. This step corresponds to the operation of the PWM switching circuit 17 shown in FIG. Further, in step 105, the current detection voltage vd for outputting the current detection PWM signal P2 is calculated. Basically, as in FIG. 1, vd = vR / d (1) may be set, but since the interrupt function of the one-chip microcomputer 23 is used in this embodiment, the following equation is used.

vd = vR / 2-vt (2) Here, vt is the A / D actually after the interrupt signal is activated.
This is for considering the delay time td until the conversion is started. In addition, since the interrupt signal is not generated when vd is 0 or a negative value in the equation (2), the minimum voltage value vmin that can generate the interrupt pulse must be taken into consideration. Therefore, the current detection voltage vd in step 105 is calculated by the following equation.

The above calculation is performed by the main calculation unit 24 at regular intervals.

Next, the PWM generation circuits 25a, 25b, 25c will be described. In each PWM generation circuit, the sawtooth carrier signal output from the built-in carrier generation circuit and the input voltage,
That is, vRa, vRb, vd are compared and the corresponding PWM signal P1
a, P1b and P2 are generated. The PWM signals P1a and P1b are input to the PWM drive circuits 27a and 27b, respectively, and the PWM drive circuits 27a and 27b
Drive PWM that is input to 7b and drives the PWM bridge circuit 7
It is used to obtain the signals Pa, Pb, Pc, Pd. This point is the first
The same operation as in the illustrated embodiment is performed.

A method of starting the A / D converter 26, which is a feature of this embodiment, will be described with reference to FIG.

In FIG. 3, the current detection PWM signal P2 output from the PWM generation circuit 25C is input to the interrupt terminal INT in the microcomputer 23. When the PWM signal P2 falls from ON to OFF, it is detected by the main arithmetic unit 24 as an interrupt signal, and the interrupt routine shown in FIG. 5 is executed.
In step 201, the activation signal P3 for activating the A / D converter 26 is generated from the main arithmetic unit 24 and activates the A / D converter 26. At this time, the A / D converter 26 samples and holds the voltage of the current detection terminal on the ON side, that is, the value of vic or vid, and starts A / D conversion. The delay time td from the generation of the interrupt signal to the start of A / D conversion must be taken into consideration because the interrupt function of the microcomputer is used as described above. Next, in step 202, wait until the A / D conversion ends,
After confirming the end of A / D conversion, the process proceeds to step 203. A / D converted current detection terminal voltage vic or vid is PWM signal P
Since it is proportional to the central current when 1a or P1b is on, that is, the average value of the motor current iM, this is input to the main arithmetic unit 24 as the current feedback value if. By using this value in step 103 of FIG. 4, the responsiveness of the current control calculated by the main calculation unit 24 can be improved.

As described above, by using this embodiment, the digital control circuit 6 can be configured only by the existing one-chip microcomputer including the PWM generation circuit and the A / D converter.
It is possible to reduce the size and cost. In particular, when applied to an electric power steering device as in this embodiment, a digital control circuit and a PWM bridge circuit are used as an electric motor,
Since it can be integrated into the steering mechanism member in a small size, it is possible to provide an even lower cost system.

FIG. 6 is a time chart showing another embodiment for detecting the average value of the electric motor current from the instantaneous value of the electric motor current iM. When the instantaneous value of the electric motor current iM is used, the instantaneous value of the electric motor current i matches its average not only during the ON period of the control PWM signal P1 but also during the OFF period. Therefore, in the embodiment of FIG. 6, the electric motor current iM is directly detected and output to the sample hold circuit 21, and the current detection signal is determined as follows. That is, the current detection voltage vd is an average value of the voltage command value vR and the maximum value vMAX of the carrier signal vc. The current detection voltage vd is used to determine the current detection pulse signal P'2.

FIG. 7 shows a circuit diagram for calculating the current detection pulse signal P'2 of FIG. In this figure, the current detection voltage calculation circuit 28 calculates vd = (vR + vMAX) / 2. Then, the current detection voltage vd and the carrier wave signal vc are compared, and the second PWM generation circuit 20 obtains the current detection PWM signal P2.

Although this signal can be used as it is to obtain the detection timing, in the present embodiment, a monostable multivibrator 29 is added to generate the current detection pulse signal P'2. Then, the motor current iM is detected by the fall of the current detection pulse signal P'2, and the current feedback value if is obtained.

In the present embodiment, it is possible to instantaneously detect the average current even when the control PWM signal is off, the voltage command value is zero, or in the opposite direction, even when the off period of the control PWM signal continues. The current can be detected and controlled.

Further, by using the monostable Baltic vibrator 29, when the current detection voltage vd becomes very close to vMAX and the current detection PWM signal becomes a whisker-like signal downward,
Since a pulse with a constant width can be secured, the current can be detected stably.

Although the embodiment of the present invention has been described above, the detection of the current flowing through the electric motor may be performed not only in one of the ON period and the OFF period of the control PWM signal, but also in both, and in this case, a higher response is obtained. Current control can be obtained.

Also, as the switching element, not only the FET,
A power transistor, a gate turn-off thyristor, an electrostatic induction thyristor, etc. may be used. Further, it goes without saying that a method of detecting the voltage drop of the shunt resistor instead of the FET with a current detection terminal, a method of using a Hall current transformer (CT) for the motor current, and the like can be applied as the current detection method.

Further, if there is a function capable of directly setting the start of the A / D converter by the output of the PWM generation circuit inside the microcomputer, that function may be used as a method for reducing the processing of the main arithmetic unit.

〔The invention's effect〕

According to the present invention, a sawtooth carrier signal suitable for a digital control circuit is used, and an instantaneous current value that substantially matches the average value can be detected from the pulsating current, so that the response of the current control system can be improved. it can.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a current control device according to an embodiment of the present invention applied to an electric power steering device, and FIG. 2 is a time chart showing a current detection method in the current control device. FIG. 3 is a schematic diagram showing a current control device according to another embodiment of the present invention using a one-chip microcomputer, and FIG. 4 is a procedure of current control calculation performed in the main calculation unit of the current control device. 5 is a flowchart showing a procedure of a current detection process performed in the main arithmetic unit when an interrupt signal is generated, and FIG. 6 is a current control according to still another embodiment of the present invention. FIG. 7 is a time chart showing a current detection method of the device, and FIG. 7 is a diagram showing a circuit for calculating a current detection pulse signal of the embodiment. Explanation of code 1 …… Handle 4a, 4B …… Tire 6 …… Digital control circuit 7 …… PWM bridge circuit (PWM converter) 9 …… Motor 10a, 10b …… Steering ring mechanism member 11c, 11d …… Current detection FET with terminal (current detection circuit) 12 …… current command circuit 13 …… subtraction circuit (calculation circuit) 14 …… voltage command circuit (the same) 15 …… carrier generation circuit 16 …… first PWM generation circuit 19 …… Proportional circuit (first means) 20 ...... Second PWM generation circuit (second means) 21 ...... Sample hold circuit (current detection circuit) 28 ...... Current detection voltage calculation circuit (first means) 29 ... Monostable multivibrator (second means)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuji Marumoto 4026, Kujimachi, Hitachi City, Ibaraki Prefecture Hitachi Research Laboratory, Hitachi Ltd. Factory Sawa Plant (72) Inventor Yasuo Noto 2520, Takaba, Katsuta City, Ibaraki Prefecture Hitachi, Ltd. Sawa Factory (72) Inventor Masayuki Kume 2477, Kashima Yatsu, Takaba, Katsuta City, Ibaraki 3 Hitachi Automotive Engineering Incorporated (72) Inventor Kazuo Tahara 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture Hitachi Research Laboratory, Hitachi, Ltd. (56) References JP 63-247165 (JP, A) JP 59-130781 (JP) , A)

Claims (7)

[Claims] 1. A carrier generation circuit for generating a carrier signal,
A first PWM generation circuit that obtains a first PWM signal by comparing the carrier wave signal and a voltage command value, a PWM converter that is switching-controlled by the first PWM signal, and an output voltage of the PWM converter Driven by the motor, a current detection circuit that detects a current flowing through the motor, and a voltage calculation circuit that calculates the voltage command value based on a feedback current value and a specified current value obtained from the current detection circuit. In the current control device, the carrier generating circuit is configured to generate a sawtooth carrier signal as the carrier signal, and the instantaneous value of the feedback current is a function of the voltage command value from the voltage command value. A means for calculating a current detection voltage value for determining a timing that substantially matches the average value, and a means for comparing the carrier wave signal and the current detection voltage value A second PWM generation circuit for obtaining two PWM signals is provided, and the current detection circuit is configured to detect a current at a timing synchronized with the second PWM signal. 2. The current detection voltage value is equal to the voltage command value.
The current control device according to claim 1, wherein the current control device has a value of 1/2. 3. The current control device according to claim 1, wherein the voltage value for current detection is an average value of the voltage command value and the maximum value of the carrier signal. 4. The current controller according to claim 1, wherein the first PWM generating circuit, the second PWM generating circuit and the carrier wave generating circuit are built in a one-chip microcomputer. 5. A current command circuit for calculating a current command value corresponding to a steering force from a steering wheel, a voltage command circuit for calculating a voltage command value based on the current command value and a feedback current value, and a carrier wave generation for generating a carrier wave signal. A circuit, a first PWM generating circuit that obtains a first PWM signal by comparing the carrier wave signal and the voltage command value, a PWM converter that is switching-controlled by the first PWM signal, and the PWM conversion An electric motor driven by the output voltage of the electric motor, a current detection circuit that detects a current flowing through the electric motor to obtain the feedback current value, and a steering mechanism member that applies an auxiliary force generated from the electric motor in a steering direction of a tire. In the electric power steering device configured by, the carrier generation circuit is configured to generate a sawtooth carrier signal as the carrier signal. Along with the voltage command value, as a function of the voltage command value, means for calculating a current detection voltage value for determining the timing at which the instantaneous value of the feedback current substantially matches its average value; and the carrier signal. A second PWM generation circuit that obtains a second PWM signal by comparing with the current detection voltage value is provided, and the current detection circuit detects a current at a timing synchronized with the second PWM signal. An electric power steering device characterized in that 6. A carrier generation circuit for generating a carrier signal,
A PWM generator circuit that obtains a control PWM signal by comparing the carrier wave signal and a voltage command value, a PWM converter that is switching-controlled by the control PWM signal, and a motor that is driven by the output voltage of the PWM converter. And a current control circuit configured to detect a current flowing through the electric motor, and a voltage calculation circuit that calculates the voltage command value based on a feedback current value and a current command value obtained from the current detection circuit, The carrier wave generating circuit is configured to generate a sawtooth carrier wave signal as the carrier wave signal, and from the voltage command value, an operation that takes a half of this voltage command value,
First means for performing at least one operation of obtaining an average value of the voltage command value and the maximum value of the carrier wave signal to obtain at least one current detecting voltage value; Second means for obtaining a current detection pulse signal by comparing with a voltage value is provided, and the current detection circuit is configured to detect a current at a timing synchronized with the current detection pulse signal. Current control device. 7. A current flowing through an electric motor driven by a PWM converter is detected as a feedback signal, a voltage command value is calculated from this feedback signal and a current command value, and this voltage command value and a carrier signal are compared. And control PW
In the current control method of the PWM converter that makes the M signal and controls the switching of the PWM converter by this control PWM signal, the carrier signal is a sawtooth carrier signal, and from the voltage command value, the voltage command At least one of the voltage command value and the average value of the maximum value of the carrier signal is performed to obtain at least one current detection voltage value, and A current control characterized by making a current detection pulse signal by comparing a carrier wave signal and this current detection voltage value, and detecting the current flowing through the PWM generation circuit at a timing synchronized with this current detection pulse signal. Method.