JPH10109656A - Control device for electric power steering device - Google Patents

Abstract

(57) Abstract: An electric power having improved controllability by reducing limit cycle vibration based on a quantization error such as truncation of lower-order bit data of a calculation result of a microcomputer constituting a control device. A control device for a steering device is provided. SOLUTION: A detected steering torque T is an 8-bit data.
The steering assist command value is input to the arithmetic element 101, and the 16-bit steering assist command value I is output. The output data is passed through an addition element 102 to an operation element 103.
, The lower 8-bit data is discarded, and the upper 8-bit data is output to the PWM signal processing circuit 110. Lower 8 bits data (quantization error) truncated
δ is temporarily stored in the storage element 104, is read out in the next sampling period, is fed back to the addition element 102, and is added, so that the quantization error can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a control device for an electric power steering device.

[0002]

2. Description of the Related Art An electric power steering apparatus for a vehicle detects a steering torque and a vehicle speed generated in a steering shaft by operating a steering handle, and outputs the detection signals to an electronic control circuit composed of a micro computer. A steering assist command value, which is a control target value of a motor that generates a steering assist force upon input, is calculated, and based on the calculated steering assist command value, a motor driving circuit in which a semiconductor element is connected to an H bridge. In some cases, the duty ratio of a PWM signal (pulse width signal) that determines the drive time of a semiconductor element is determined, and the motor is driven by controlling a motor drive circuit based on the determined duty ratio.

A steering assist command value, which is digital data calculated by a micro computer, is converted into analog data by an A / D converter, and a motor drive circuit is converted by the converted analog data. Some motors are controlled to drive the motor.

[0004]

The control circuit having the above-mentioned configuration is generally a low-resolution circuit for processing an 8-bit signal. Even if the operation result of the micro-computer is a 16-bit signal, In a low-resolution circuit in which a PWM signal processing circuit or an A / D conversion circuit processes an 8-bit signal, it is not possible to determine whether to output 0 or 1 as an output value when processing a signal having a resolution lower than about 1 bit. The output signal oscillates at the bit resolution. This is called limit cycle oscillation.

[0005] Such vibrations include a vibration component in the steering assist command value, and are felt as unpleasant vibrations during steering, and deteriorate the performance of the product, which is not preferable. SUMMARY OF THE INVENTION It is an object of the present invention to reduce a limit cycle vibration generated due to such a lack of resolution of a PWM signal processing circuit and an A / D conversion circuit, and to improve a steering feeling.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and includes a steering torque detecting means for detecting at least a steering torque generated in a steering shaft, and a steering assist command value based on the detected steering torque. An electric power which includes a steering assist command value calculating means for calculating, and a motor control means for controlling a motor based on the calculated steering assist command value, and which applies a steering assist force corresponding to a steering torque to a steering mechanism. -In the control device of the steering device, the steering assist command value calculating means is constituted by digital calculating means having a larger number of bits than the motor control means;
A steering assist command value is calculated based on the steering torque data sampled every predetermined time and output to the motor control means, and the lower-order bit data of the steering assist command value truncated at that time is used next time. The steering assist command value is calculated by adding the steering torque data to the sampled steering torque data.

The electric time constant of the motor that applies a steering assisting force corresponding to the steering torque to the steering mechanism is at least の of the sampling period of the steering torque.
Let be π. The steering assist command value may be a motor current command value.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features and embodiments of the present invention will be described below. The steering assist command value is obtained by calculating the steering torque, the vehicle speed and other parameters based on a predetermined arithmetic expression. When such a steering assist command value is calculated with a finite word length of 8 bits. Includes the multiplication, the operation result is output in 16 bits. However, the PWM signal processing circuit (or A / D conversion circuit) that receives the calculated steering assist command value is generally configured to receive 8-bit data.
Only the upper 8 bits of the operation result output in 6 bits are P
The data is output to the WM signal processing circuit, and the lower 8-bit data is discarded. This is a quantization error.

Therefore, the truncated lower 8-bit data is added to the steering torque data detected in the next sampling period to calculate a steering assist command value, whereby the lower 8-bit data is truncated. It was made to reduce the conversion error.

FIG. 1 shows a quantization error reduction circuit 100 represented by a transfer function. 101 is an operation element for calculating a steering assist command value corresponding to a steering torque, 102 is an addition element, and 103 is a 16-bit data. An operation element for discarding the lower 8 bits of data, 104 is a storage element for temporarily storing the lower 8 bits of data cut off by the operation element 103,
105 is a gain adjustment element.

Reference numeral 110 denotes a PWM signal processing circuit for generating a PWM signal based on the steering assist command value I output from the computing element 103, and 111 denotes a motor control circuit for driving and controlling the motor based on the PWM signal. , A circuit in which a semiconductor element is connected to an H bridge. A motor 112 is represented by {1 / (Ls + R)} if its electric characteristics are represented by a transfer function. Here, L is the inductance of the motor, s is the Laplace operator, and R is the resistance between the terminals of the motor.

The operation of the quantization error reduction circuit will be described.
The detected steering torque T is input to the calculation element 101 as 8-bit data, and the steering assist command value is calculated to be 16 bits.
A bit steering assist command value I is output. The output data is input to the operation element 103 via the addition element 102, the lower 8-bit data is discarded, and the upper 8-bit data is output to the PWM signal processing circuit 110. On the other hand, the truncated lower 8-bit data (quantization error) δ is temporarily stored in the storage element 104, the gain is adjusted by the gain adjustment element 105, read out in the next sampling period, and read to the addition element 102. The data is output as data delayed by one sample period, added to the steering torque extracted in the next sampling period, and the steering assist command value I is calculated.

As described above, the lower 8 bits of data (quantization error) δ, which are successively discarded, are fed back to the steering assist command value I based on the steering torque extracted in the next sampling period, and are added. , The quantization error due to the truncation of the lower 8-bit data can be reduced.

The above-described quantization error reducing circuit shown in FIG. 1 is designed so that the transfer characteristic from the occurrence of the quantization error to the output of the adder 102 has the characteristics of a high-pass filter, as shown in FIG. Is set. The effect of the high-pass filter is high when the transfer characteristic is (1-Z ^-1 ) ⁿ . As the value n increases, the high-pass filter effect increases. From characteristics n
= 1 is preferred.

Here, the quantization error will be described. Since the quantization error δ can be regarded as white noise, it can be handled as data independent of the steering assist command value I which is an output. Since the steering torque and the motor current are in a proportional relationship, the effect of the quantization error can be evaluated as an effect on the motor current, and the following equation (1) holds.

| G (s) · M (s) | ≦ | G (s) | _∞ | M (s) | _∞ ≦ 1 / R (1) where R : Motor terminal resistance M (s): 1 / Ls + RL L: motor inductance s: Laplace operator G (s): transfer characteristic of quantization error reduction circuit (1-
Z ^-1 ) Transfer function obtained by converting ⁿ into a continuous time system | 〜 | _∞ : Indicates the maximum gain value.

In order to satisfy the relationship of the above equation (1), the electric time constant of the motor may be set to be equal to or lower than the crossover frequency ωc of the high-pass filter. That is, the electric time constant of the motor may be set to 1 / π or less of the Nyquist frequency ωN or 1 / 2π or less of the sampling period Ts. This is because there is a relationship ωN = １ ／ Ts between the sampling period Ts and the Nyquist frequency ωN.

As described above, the quantization error reducing circuit shown in FIG. 1 is set so that the transfer characteristic from the occurrence of the quantization error to the output of the adder 102 has the characteristics of a high-pass filter. Since the quantization error signal is an output having passed through the high-pass filter, the quantization error is distributed at a certain frequency or higher. As a countermeasure, by inserting a low-pass filter at the output side of the quantization error reduction circuit, it is possible to eliminate the influence of quantization errors distributed over a certain frequency.

In an actual circuit, the motor which generates the steering assist force has the electrical characteristics shown in FIG. 3 and functions as a low-pass filter. A pass filter is not required, and the electric time constant of the motor may be at least 1 / 2π of the sampling frequency of the steering torque.

As described above, when a low-pass filter is inserted at the output side of the quantization error reduction circuit, the overall transfer characteristics are as shown in FIG. 4, and the gain is 1 / R (R is (Resistance between motor terminals), and the quantization error can be reduced. Note that the above-described quantization error reduction circuit does not include a reduction circuit composed of specific circuit elements in the control circuit, and is a function executed inside the CPU configuring the control circuit. .

[0021]

Embodiments of the present invention will be described below.
FIG. 5 is a diagram schematically illustrating the configuration of an electric power steering apparatus suitable for carrying out the present invention. The shaft 2 of the steering handle 1 is provided with a reduction gear 4, a universal joint 5a,
5b, which is connected to a tie rod 8 of a steered wheel via a pinion rack mechanism 7. The shaft 2 is provided with a torque sensor 3 for detecting the steering torque of the steering handle 1, and a motor 10 for assisting the steering force is connected to the shaft 2 via a clutch 9 and a reduction gear 4. I have.

The electronic control circuit 13 for controlling the power steering device is supplied from the battery 14 to the ignition key-1.
Power is supplied via 1. The electronic control circuit 13 calculates a steering assist command value based on the steering torque detected by the torque sensor 3 and the vehicle speed detected by the vehicle speed sensor 12, and based on the calculated steering assist command value, the motor 10. Control the current supplied to the

The clutch 9 is controlled by an electronic control circuit 13. The clutch 9 is engaged in a normal operation state, and is disconnected when the electronic control circuit 13 determines that the power steering device has failed and when the power is off.

FIG. 6 is a block diagram of the electronic control circuit 13. In this embodiment, the electronic control circuit 13 is mainly composed of CP
U, but here the functions executed by the program inside the CPU are shown. For example, the phase compensator 21 does not indicate the phase compensator 21 as independent hardware, but indicates a phase compensation function executed by the CPU.

Hereinafter, functions and operations of the electronic control circuit 13 will be described. The steering torque signal input from the torque sensor 3 is phase-compensated by the phase compensator 21 to enhance the stability of the steering system, and is input to the steering assist command value calculator 22. The vehicle speed detected by the vehicle speed sensor 12 is also input to the steering assist command value calculator 22.

The steering assist command value calculator 22 determines a steering assist command value I which is a control target value of the current supplied to the motor 10 based on the input torque signal and vehicle speed signal.

The circuit composed of the comparator 23, the differential compensator 24, the proportional calculator 25, the integral calculator 26 and the adder 27 ensures that the actual motor current value i matches the steering assist command value I. This is a circuit for performing feedback control.

The proportional calculator 25 outputs a proportional value proportional to the difference between the steering assist command value I and the actual motor current value i. Further, the output signal of the proportional calculator 25 is integrated in the integrating calculator 26 in order to improve the characteristics of the feedback system, and the proportional value of the integrated value of the difference is output.

The differential compensator 24 increases the response speed of the motor current value i actually flowing to the motor to the steering assist command value I calculated by the steering assist command value calculator 22.
A differential value of the steering assist command value I is output.

The differential value of the steering assist command value I output from the differential compensator 24, a proportional value proportional to the difference between the steering assist command value output from the proportional calculator 25 and the actual motor current value, and The integration value output from the integration calculator 26 is added in an adder 27, and a current control value E for controlling a motor current, which is a calculation result, is output to a motor drive circuit 41.

FIG. 7 shows an example of the configuration of the motor drive circuit 41. The motor drive circuit 41 converts the current control value E input from the adder 27 into a PWM signal and a current direction signal, and converts the current control value E into a PWM signal and a current direction signal.
It comprises an FET gate drive circuit 45 for opening and closing the gates of T1 to FET4. The boost power supply 46 is connected to the FET1
, And a power source for driving the high side of FET2.

In the quantization error reduction circuit 100 described with reference to FIG. 1, the PWM is directly obtained from the steering assist command value I.
Although the description has been made so as to calculate the signal, in the embodiment, a circuit element for improving the characteristics of the feedback system is added, the current control value E is obtained from the steering assist command value I, and the current control value E is converted by the conversion unit 44 into PWM. Converting to signal The above-described quantization error reduction circuit 100 outputs a current control value E (16-bit signal).
Is input in the conversion unit 44 which converts a 16-bit signal into an 8-bit PWM signal.

In the following description, it is assumed that the PWM signal is obtained from the current control value E, and the quantization error is reduced in the process.
In the process of obtaining the PWM signal from, ie, the process of obtaining the PWM signal to the steering assist command value I, the lower 8 bit data truncated in the process of obtaining the PWM signal may be added to perform the quantization error reduction processing.

The PWM signal (pulse width modulation signal) is a signal for driving the gates of the H bridge-connected FETs (field effect transistors) switching elements FET1 and FET2, and the current control value E calculated by the adder 27. Determines the duty ratio of the PWM signal (the time ratio for turning on / off the gate of the FET).

The current direction signal is a signal indicating the direction of the current supplied to the motor, and is a signal determined by the sign (positive or negative) of the current control value E calculated by the adder 27.

FET1 and FET2 are switching elements whose gates are turned ON / OFF based on the duty ratio of the PWM signal, and are switching elements for controlling the magnitude of the current flowing to the motor. . Also, FET3
And the gate of the FET 4 is turned ON or OFF based on the above-mentioned current direction signal (when one is ON, the other is OF).
F), which is a switching element for switching the direction of the current flowing through the motor, that is, the direction of rotation of the motor.

When the FET 4 is in a conductive state, a current flows through the FET 1, the motor 10, the FET 4, and the resistor R1, and a positive current flows through the motor 10. FET3
Is in the conductive state, the current flows through the FET2 and the motor 1.
0, a current flows through the FET3 and the resistor R2, and the motor 10
Current flows in the negative direction.

The motor current detecting circuit 42 detects the magnitude of the positive current based on the voltage drop across the resistor R1, and detects the negative current based on the voltage drop across the resistor R2. Detect size. The detected actual motor current value i is fed back to the comparator 23 and input (see FIG. 6).

Next, a quantization error reduction process for obtaining a PWM signal from the current control value E, that is, a quantization error for obtaining a PWM signal by extracting upper 8 bit data from the calculated 16 bit data. The process of reducing the error will be described with reference to the flowchart of FIG.

First, the steering torque is sampled to detect the steering torque T, and a current control value E for controlling the motor current is calculated (step P1). This can be achieved by extracting the output from the torque sensor 3 at predetermined time intervals and calculating it. Next, in the process of obtaining the PWM signal from the current control value E based on the previous steering torque T, the lower 8-bit data δ (δ is zero for the first time) which has been truncated is read out from the memory, and the sampled steering torque T The current control value E is added to the current control value E (step P2).

The current control value E is calculated again by adding the truncated lower 8-bit data δ. A 16-bit operation result is obtained. The higher 8 bits of data are output as a PWM signal (step P3), and the truncated lower 8 bits of data are output to the memory as new lower 8 bits of data δ. Save (Step P4).

The process returns to step P1 for recalculation by adding the truncated lower 8 bits to the current control value E input by sampling the next steering torque.

In the embodiment described above, the electronic control circuit is constituted by an 8-bit CPU, and in the calculation of the current control value E, the upper 8 bit data of the 16-bit data obtained in the calculation process is a PWM signal. , And the lower 8 bits are added to the next calculation as an error.

However, the above description is based on the assumption that the electronic control circuit is constituted by an 8-bit CPU. Even when the electronic control circuit is constituted by a CPU other than the 8-bit CPU, the upper bit data is similarly transmitted. When outputting as a PWM signal, the lower bit data becomes an error. The processing method of the present invention for adding such an error to the next calculation is as follows.
It goes without saying that a technique for reducing the quantization error based on the digital operation can be applied to data processing by a CPU other than 8-bit.

In the above-described quantization error processing, the lower bit data is discarded when the upper bit data is output as a PWM signal, but the lower bit data is discarded.
The most significant digit of the data may be rounded off and included in the upper bit data. Also in this case, the lower bit data
The data is added and processed in the process of recalculating the current control value E by sampling the next steering torque.

[0046]

As described above, in the control device for the electric power steering device according to the present invention, the operation result of the microcomputer constituting the control device is a 16-bit signal, and the PWM signal processing circuit and the A 8 / D conversion circuits
Even in a low-resolution circuit for processing a bit signal, an error generated by truncation of lower bit data or the like is added in a process of re-calculating a steering assist command value or a current control value by sampling the next steering torque. Since processing, the error that occurs in the calculation process can be reduced,
Limit cycle vibration generated due to insufficient resolution of the PWM signal processing circuit and the A / D conversion circuit can be reduced, and the steering feeling can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a quantization error reduction circuit according to the present invention as a transfer function.

FIG. 2 is a diagram illustrating transfer characteristics of a quantization error reduction circuit.

FIG. 3 is a diagram illustrating electric characteristics of a motor.

FIG. 4 is a diagram for explaining transfer characteristics when a low-pass filter is inserted on the output side of the quantization error reduction circuit.

FIG. 5 is a diagram schematically illustrating the configuration of an electric power steering device.

FIG. 6 is a block diagram of an electronic control circuit.

FIG. 7 is a block diagram of a motor drive circuit.

FIG. 8 is a flowchart for explaining a quantization error reduction process.
G.

[Explanation of symbols]

REFERENCE SIGNS LIST 100 Quantization error reduction circuit 101 Arithmetic element for calculating steering assist command value 102 Addition element 103 Arithmetic element for truncating lower bit data 104 Storage element for temporarily storing truncated lower bit data 105 Gain adjustment element 110 PWM signal processing Circuit 111 Motor control circuit 112 Motor

Claims (3)

[Claims] 1. A steering torque detecting means for detecting at least a steering torque generated in a steering shaft; a steering assist command value calculating means for calculating a steering assist command value based on the detected steering torque; A control device for an electric power steering device, comprising: a motor control means for controlling a motor based on an auxiliary command value; Is composed of digital arithmetic means having a larger number of bits than the motor control means, calculates a steering assist command value based on steering torque data sampled at predetermined time intervals, and outputs the command to the motor control means. At the same time, the lower bit data of the truncated steering assist command value is added to the steering torque data sampled next time. Electric power, characterized in that for calculating a steering assist command value - control of the steering device. 2. The motor according to claim 1, wherein an electric time constant of the motor for applying a steering assisting force according to the steering torque to the steering mechanism is at least ππ of a sampling period of the steering torque. Control device for electric power steering device. 3. The control device for an electric power steering apparatus according to claim 1, wherein the steering assist command value is a motor current control value.