JPH10310072A - Electric power steering system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering system in which steering feeling can be improved without using an angle sensor. SOLUTION: In a system to add assist torque to steering torque by driving an assist motor 40 by a motor drive command value as a base assist current command value T1 decided by output of a torque sensor 20 to detect steering torque and output of a car speed sensor 30 to which a steering torque differentiation command value T2 decided by a first differentiator 24 is added, it is provided with a cogging frequency determination means 52 to determine a cogging frequency by detecting rotation speed of the assist motor 40, a center frequency changeable band pass filter 53 to let a required frequency coincide with a center frequency, and detect pulsation torque at the center frequency, and a second differentiator 54 to differentiate its output to advance a phase, and a compensation current command value corresponding to output of the second differentiator 54 is added to the motor drive command value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering system for improving operability by adding an assist torque to a steering torque of a steering wheel of an automobile when the steering wheel is operated.

[0002]

2. Description of the Related Art An electric power steering system for an automobile determines an assist current command value from an output of a torque sensor for detecting a steering torque of a steering wheel and an output of a vehicle speed sensor for detecting a vehicle speed of the automobile.
The motor drive command value is obtained by adding the steering torque differential command value to the command value, and the assist motor is driven via the drive circuit to add the assist torque to the steering torque, thereby improving the operability of the steering wheel.

[0003] However, the assist motor generates a so-called cogging torque in which the torque pulsates according to the rotation angle of the rotor. This cogging torque is transmitted to the steering wheel via the steering shaft and transmitted to the driver's hand as vibration, which causes a problem that the feeling at the time of steering deteriorates.

[0004]

An electric power steering apparatus disclosed in Japanese Patent Application Laid-Open No. 2-193768 is known to improve the feeling during steering. According to the present invention, a compensation current command value to be added to the motor in order to cancel the cogging torque generated according to the rotation angle of the rotor is calculated in advance. Then, the rotation angle of the rotor is detected by the angle sensor, and a compensation current command value corresponding to the rotation angle is supplied to the assist motor to smooth the torque,
This is to improve the feeling during steering. However, since angle sensors are expensive and costly, there has been a demand for a sensor that improves feeling without using an angle sensor.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electric power steering system capable of improving steering feeling without using an angle sensor.

[0006]

In order to solve the above-mentioned problem, means according to the first aspect of the present invention will be described with reference to the reference numerals used in the embodiments. The motor drive command is obtained by adding the steering torque differential command value T2 determined by the first differentiator 24 to the basic assist current command value T1 determined by the output of the sensor 20 and the output of the vehicle speed sensor 30 for detecting the vehicle speed. Value, and drives the assist motor 40 with the motor drive current determined by the motor drive command value,
In an electric power steering system for improving the operability of the steering wheel 10 by adding an assist torque to the steering torque, a cogging frequency calculating means 52 for detecting a rotational speed of the assist motor 40 and calculating a cogging frequency of the assist motor is provided. A variable center frequency band-pass filter 53 that operates so that its center frequency matches the frequency determined by the cogging frequency calculation means 52 and detects pulsating torque at the center frequency from the output of the steering torque sensor 20; A second differentiator for advancing the phase by differentiating the output of the center frequency variable band-pass filter 53, and providing a compensation current command value corresponding to the output of the second differentiator 54 to the motor drive command value. The feature is that it is added to

According to a second aspect of the present invention, the compensation current command value at the cogging frequency is stored in the cogging frequency table 56.
The characteristic is that it is changed according to the cogging frequency.

According to a third aspect of the present invention, the output of the center frequency variable band-pass filter 53 at the cogging frequency is divided into two, one of which is differentiated by a second differentiator 54 to join T31 and the other T30. It has the characteristic in the place where

The invention of claim 4 is characterized in that the compensation current command value at the cogging frequency is changed according to the vehicle speed by a vehicle speed table 57.

The invention of claim 5 is characterized in that the compensation current command value at the cogging frequency is changed according to the current of the assist motor by the assist current table 58.

It should be noted that imparting a light feeling to the steering performance can be achieved by increasing the gain of the first differentiator 24 for differentiating the steering torque, but as a result, there is a problem that the steering of the steering wheel is too light. Occur. But,
In the present invention, since the independent center frequency variable band-pass filter 53 is provided to smooth the cogging torque, it is possible to improve the feeling of the steering wheel and obtain an appropriate steering characteristic. According to the present invention, a light feeling of steering can be set to a desired feeling by the steering torque differential command value T2 from the first differentiator 24.
The cogging can be independently set to a desired feeling by the compensation current command value T3 from the second differentiator 54.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. First, in FIG. 2, when the steering wheel 10 is operated, the steering torque is transmitted to the input shaft 11a of the gear case 11 and is transmitted to the rack 12 via the pinion at the lower end of the output shaft 11b.
Drive. This rack 12 changes the direction of wheels 14 via a steering link 13.

On the other hand, a torque sensor 20 attached to the gear case 11 detects the steering torque, and a vehicle speed sensor 30
Detects the speed of the vehicle, and its electric signal is input to an electric control device (hereinafter referred to as ECU 50). ECU50
Is the output of the torque sensor 20 shown in FIG.
The basic assist current command value T1 is determined from 0 and the assist vehicle speed table 23. Further, the steering torque differential command value T2 is determined from the first differentiator 24 for differentiating the steering torque, the vehicle speed sensor 30, and the steering torque differential vehicle speed table 25. Further, the compensation torque command value T3 is determined by passing the steering torque through a center frequency variable band-pass filter 53 and a second differentiator 54 described later. The motor drive command value is the sum of these command values T1, T2, T3, and the motor drive current is supplied to the assist motor 40 via the current control means 22 to be driven.

The torque of the assist motor 40 is transmitted to the input shaft 16a of the gear case 16 via the electromagnetic clutch 15.
And drives the rack 12 via a pinion at the lower end of the output shaft 16b. Since the rack 12 is driven by the output shaft 16b of the assist motor 40 together with the output shaft 11b of the steering wheel 10, the operability of the steering wheel 10 is significantly improved.

The motor rotational speed detecting means 51 estimates the rotational speed of the assist motor 40 from the current value and the terminal voltage value of the assist motor 40 and inputs the result to the cogging frequency calculating means 52. The assist motor 40 generates a so-called cogging torque in which the motor shaft torque periodically changes according to the rotation angle due to the relationship between the internal rotor and the magnetic field (attraction force, repulsion force). Incidentally, the motor rotation speed detecting means 51
Is generally used to perform inertia compensation control of the motor rotor, viscosity of the system, and friction compensation control. The present invention has an advantage that the motor rotation speed detecting means 51 can be used as it is. is there.

Here, the cogging frequency is represented by F (HZ), the motor rotation speed is represented by N (rpm), and the motor cogging coefficient (the number of peaks or valleys of cogging generated by one rotation of the motor) is represented by Km. The frequency is obtained by the following equation. F = N · Km / 60 That is, the cogging frequency changes according to the rotation speed of the motor.

Variable center frequency bandpass filter 53
The torque inputted to the bandpass filter 53 (hereinafter simply referred to as a bandpass filter 53) includes components at various frequencies due to the steering speed and the like in addition to the components at the cogging frequency. The bandpass filter 53 (hereinafter simply referred to as the bandpass filter 53) sets the center frequency to F0, F0 shown in FIG.
By operating as F1 and F2 so as to constantly match the frequency F obtained by the cogging frequency calculating means 52, only the cogging frequency component can be extracted and sent to the second differentiator 54. Then, when this is input to the second differentiator 54, the second differentiator 54 advances the phase of the pulsating torque, further amplifies it if necessary, and outputs a compensation current command value T3 for canceling the cogging torque to the motor drive. Add to the command value (T1 + T2).

Next, the operation of the first embodiment will be described. First, when the steering wheel 10 is operated in a state where the bandpass filter 53 is not connected, the torque sensor 20 detects the steering torque, the vehicle speed sensor 30 detects the speed of the vehicle, and a motor drive command determined from these. The assist motor 40 is driven based on the value (T1 + T2), and the assist torque Ta is added to the steering torque. This assist torque pulsates above and below the center value Ta [FIG. 4 (a)].

By the way, the assist torque has its central value T
In the peak of the cogging torque larger than a, the steering torque is correspondingly small, and in the valley of the cogging torque smaller than the center value Ta, the steering torque is large and the steering torque also pulsates. That is, the output of the torque sensor 20 reverses the pulsating waveform of the cogging torque (the phase angle is-
180 degrees). In addition, a mechanical backlash or torsion of a torsion bar for detecting a steering torque acts to further delay the phase angle θ (FIG. 4B).

On the other hand, when the rotational speed of the assist motor 40 is estimated and calculated by the motor rotational speed detecting means 51 and the cogging frequency F is calculated by the cogging frequency calculating means 52, it is input to the bandpass filter 53. Then, when the output of the torque sensor 20 passes through the bandpass filter 53, the pulsating torque at the calculated cogging frequency F is extracted [(c) of FIG. 4].

The pulsating steering torque signal is differentiated by the second differentiator 54 so that the signal component (frequency) of normal steering is cut, and only the cogging torque component at the cogging frequency F is extracted. Also, the phase angle θ
[(D) in FIG. 4], and if necessary, the compensation current command value T3 amplified by the amplifier 55 is added to the motor drive command value (T1 + T2). As a result, the cogging torque is canceled [FIG. 4 (e)], the torque becomes smooth, and the feeling of the steering wheel 10 is improved.

According to the first embodiment, the pulsating torque at the cogging frequency F is extracted from the output of the torque sensor 20 by focusing on the fact that the steering torque also pulsates due to the cogging torque of the assist motor 40. Since the compensation current command value T3 for canceling this cogging torque is added to the motor drive command value, an excellent effect that the feeling of the steering wheel 10 can be improved satisfactorily without using an expensive angle sensor. Is played.

FIG. 5 shows a second embodiment of the present invention. The difference from the first embodiment is that a cogging frequency table 56 for changing the output of the second differentiator 54 in accordance with the cogging frequency. In that it has Generally, torque sensor 2
When the output of 0 is differentiated by the second differentiator 54, the gain tends to increase as the frequency increases. But,
In the second embodiment, the cogging frequency table 56
Thus, this effect can be compensated for.

Incidentally, the cogging torque tends to decrease as the cogging frequency increases. In the second embodiment, by lowering the gain at a high frequency, even when the steering torque frequency matches the cogging frequency,
The compensation current command value T3 from the second differentiator 54 is prevented from excessively increasing.

FIG. 6 shows a third embodiment of the present invention. The difference from the second embodiment is that a bandpass filter 53 is provided.
Is divided into two, and one is to combine the output T31 differentiated by the second differentiator 54 with the other T30 to adjust the phase angle. According to the third embodiment, there is an effect that a phase delay due to gear backlash, torsion bar torsion, or the like can be compensated as needed by the correlation between T31 and T30.

FIG. 7 shows a fourth embodiment of the present invention. The difference from the third embodiment is that a vehicle speed table 57 for changing the compensation current command value according to the vehicle speed is provided. It is in. In general, at high vehicle speeds, the assisting force is reduced and the cogging torque is also reduced, so that a compensation current corresponding to the vehicle speed can be obtained. Further, according to the fourth embodiment, when the vehicle speed becomes high and the frequency of the steering torque matches the cogging frequency, it is possible to prevent the sense of incongruity that occurs when the assist torque becomes relatively large.

FIG. 8 shows a fifth embodiment of the present invention. The difference from the third embodiment is that an assist current table 58 for changing a compensation current command value according to the current of the assist motor 40 is provided. It has a point. According to the fifth embodiment, the cogging torque increases as the current of the assist motor 40 increases. However, since the compensation current command value according to the motor current flows, appropriate compensation can be performed. It is possible to eliminate a feeling of strangeness at the time of steering.

[0028]

As described above in detail, the present invention relates to an electric power steering system in which an assist motor is added to the steering torque of a steering wheel to add an assist torque to the steering torque. The pulsating torque at the cogging frequency calculated by the means is extracted, and the compensation current command value for canceling the pulsating torque is added to the assist command value. To play.

[Brief description of the drawings]

FIG. 1 is a control block diagram of a first embodiment of an electric power steering system according to the present invention.

FIG. 2 is a block diagram illustrating a system according to a first embodiment.

FIG. 3 is a graph showing a distribution of a cogging frequency of a steering torque.

FIG. 4 is a graph showing a waveform of torque in each section.

FIG. 5 is a control block diagram of a second embodiment of the electric power steering system according to the present invention.

FIG. 6 is a control block diagram of a third embodiment of the electric power steering system according to the present invention.

FIG. 7 is a control block diagram of a fourth embodiment of the electric power steering system according to the present invention.

FIG. 8 is a control block diagram of a fifth embodiment of the electric power steering system according to the present invention.

[Explanation of symbols]

 Reference Signs List 10 steering wheel 20 torque sensor 21 basic assist current command value 24 first differentiator 30 vehicle speed sensor 40 assist motor 52 cogging frequency calculating means 53 center frequency variable band-pass filter 54 second differentiator 56 cogging frequency table 57 vehicle speed table 58 Assist current table

Claims (5)

[Claims] 1. A basic assist current command value determined by an output of a torque sensor for detecting a steering torque of a steering wheel and an output of a vehicle speed sensor for detecting a vehicle speed,
The motor drive command value is obtained by adding the steering torque differential command value determined by the first differentiator, and the assist motor is driven by the motor drive current determined by the motor drive command value to convert the assist torque to the steering torque. An electric power steering system for improving the operability of the steering wheel by adding a cogging frequency calculating means for detecting a rotational speed of the assist motor and calculating a cogging frequency of the assist motor; A variable center frequency band-pass filter that operates so as to match the center frequency to the detected frequency and detects a pulsating torque at the center frequency from the output of the steering torque sensor; and differentiates the output of the center frequency variable band-pass filter. And a second differentiator for advancing the phase by An electric power steering system, wherein a compensation current command value according to an output of the second differentiator is added to the motor drive command value. 2. The electric power steering system according to claim 1, wherein the compensation current command value at the cogging frequency is changed according to the cogging frequency. 3. The output of the center frequency variable band-pass filter at the cogging frequency is divided into two, one of which is differentiated by a second differentiator to join the other. Or the electric power steering system according to 2. 4. The electric power steering system according to claim 1, wherein the compensation current command value at the cogging frequency is changed according to a vehicle speed. 5. The electric power steering system according to claim 1, wherein the compensation current command value at the cogging frequency is changed according to a current of an assist motor.