WO2009128126A1

WO2009128126A1 - Electric power-steering device

Info

Publication number: WO2009128126A1
Application number: PCT/JP2008/057250
Authority: WO
Inventors: 享錢谷; 滋樹太田垣; 克哉池本; 聖二坂西; 憲一河上
Original assignee: 三菱電機株式会社
Priority date: 2008-04-14
Filing date: 2008-04-14
Publication date: 2009-10-22

Abstract

An electric power-steering device reduces abnormal noises and vibrations without degrading desirable response characteristics of a motor current to a steering input, and has a motor current tracking performance required for detection of fault in a motor current control function. The electric power-steering device comprises a steering torque sensor to detect a steering torque of a steering system, a motor (5) to exert a steering auxiliary force to the steering system, a target current setting means (4) to set up a target current of the motor in accordance with a steering torque detected by the steering torque sensor, a motor current detection means (7) to detect a motor current flowing through the motor, a deviation computation means to compute a deviation between the target current and the motor current, and a motor current control means (6) to control drive of the motor in accordance with the computed deviation. Further, between the target current setting means (4) and the deviation computation means, the device includes a frequency characteristics adjustment means (9) to adjust a response of low-frequency components of the motor current versus the target current.

Description

Electric power steering device

The present invention relates to a power steering control device for an automobile that assists a steering torque generated by steering by a driver with an electric motor.

FIG. 6 is a configuration diagram of a conventional electric power steering control device. This electric power steering control device includes steering torque detection means 1, steering wheel angle information detection means 2, vehicle speed detection means 3, target current setting means 4, motor 5, motor current control means 6, motor current detection means 7, and failure detection. Consists of means 8.

Steering torque detection means 1 detects the driver's steering force as steering torque. The steering wheel angle information detection means 2 detects the steering wheel angle, and the vehicle speed detection means 3 detects the vehicle speed. The target current setting means 4 assists the driver's steering force based on detection signals from the steering torque detection means 1, the steering wheel angle information detection means 2, the vehicle speed detection means 3, and the motor current detection means 7 described later. The target current 10 to be supplied to the motor 5 is calculated.

The motor current detection means 7 detects the current 11 of the motor 5 for assisting the driver's steering force. The motor current control means 6 calculates a motor drive signal so that the deviation between the target current 10 output from the target current setting means 4 and the motor current 11 detected by the motor current detection means 7 becomes zero, The motor current 11 is controlled. As an example, the motor current control means 6 is realized by a proportional / integral control means (PI control) or a proportional / integral / derivative control means (PID control). The failure detection means 8 performs failure detection based on the target current 10 and the motor current 11.

In addition, a pulse width modulation control signal output from the motor current control means 6 on a path connecting the motor current control means 6 and the motor current detection means 7 and a path connecting the motor current control means 6 and the target current setting means 4. A device provided with an attenuating means for reducing high-frequency components due to (PWM) or the like is known as the prior art. By providing such a configuration, an effect of reducing noise mixed in from each route is obtained.

Japanese Patent No. 2959957

However, the prior art has the following problems.
In the motor current control means 6 in FIG. 6, the frequency response characteristic of the motor current 11 with respect to the target current 10 is set so as to be the first-order lag characteristic represented by the transfer function of the following equation (1).

The frequency band of the driver's steering input operation is 10 Hz or less. In addition, the resonance frequency of the steering system that requires vibration suppression control is usually several tens of Hz or less. Therefore, in order to remove unnecessary signal components such as noise having a frequency higher than those from the target current 10, it is desirable to set the cutoff frequency f _mtr [Hz] within the range of the following expression (2).

Further, the conventional failure detection means 8 is configured such that the state where the absolute value of the difference between the target current 10 and the motor current 11 is greater than a predetermined threshold (Ierr) continues for a predetermined time (Terr) or more. Fault detection was performed. As a result, a short of the motor wire, a failure of the motor drive circuit, or the like is detected.

However, for example, when the motor current response characteristic is set to the above formula (1) and the cut-off frequency is set as f _mtr = 150 [Hz], the following problem occurs. FIG. 7 is a current response waveform of the conventional electric power steering apparatus. More specifically, in FIG. 7, the response of the motor current 11 when the target current 10 changes stepwise is shown as a response waveform 14.

FIG. 7 shows that it takes time for the motor current 11 to converge to the target current 10. For this reason, the deviation 15 between the target current 10 and the motor current 11 becomes longer in a state exceeding the threshold value (Ierr1) of the failure detection function as shown in the lower part of FIG. As a result, when the state exceeding the threshold value (Ierr1) continues for a predetermined time (Terr1) or longer, the failure detection means 8 erroneously detects as a failure.

In order to prevent such erroneous detection of a failure, a method of increasing the failure detection threshold (Ierr1) or increasing the time until failure determination (Terr1) can be considered. However, using these methods is not preferable because the behavior of the vehicle from when the failure actually occurs until the failure is confirmed and the steering assist is stopped increases.

Also, when the motor current 11 fluctuates due to disturbance or the like, it similarly takes a long time for the motor current 11 to converge to the target current 10. Even in this case, there is a possibility of erroneous detection as a failure. Furthermore, there may be cases where there is an adverse effect on noise, vibration, or steering feeling.

Therefore, in order to solve the above-described problem, it is necessary to sufficiently improve the followability of the motor current 11 with respect to the target current 10. That is, the cutoff frequency f _mtr [Hz] in the above equation (1) must be set to a value (usually 500 [Hz] or more) larger than the range of the above equation (2).

In summary, the cutoff frequency f _mtr [Hz] in the above equation (1) needs to consider the following two cases.
Case 1) In order to remove a frequency component higher than the frequency band of the steering input included in the target current 10, the cut-off frequency f _mtr [Hz] needs to satisfy the following expression (3).

Case 2) In order to satisfy the requirement of the followability of the motor current 11 with respect to the target current 10, the cutoff frequency f _mtr [Hz] needs to satisfy the following expression (4).

However, the conditions of the above formulas (3) and (4) cannot be satisfied. For this reason, in the prior art, the cut-off frequency f _mtr is set in accordance with the request of Case 2 with higher priority. As a result, the requirement of case 1 is not satisfied, which may cause a deterioration of steering feeling and the like.

Another object of the attenuation means disclosed in Patent Document 1 is to reduce high-frequency noise caused by a pulse width modulation control signal (PWM) output from the motor current control means 6. For this reason, the cut-off frequency is several tens [KHz], which acts only in a frequency band higher than the cut-off frequency (f _mtr ) of the above formulas (3) and (4). Must not.

The present invention has been made to solve the above-described problems, and suppresses abnormal noise and vibration without impairing response characteristics of a desired motor current with respect to a steering input, and detects a failure in a motor current control function. An object of the present invention is to obtain an electric power steering device having a motor current follow-up property necessary for this purpose.

An electric power steering control device according to the present invention includes a steering torque sensor that detects steering torque of a steering system, a motor that applies a steering assist force to the steering system, and a motor based on the steering torque detected by the steering torque sensor. Target current setting means for setting the target current, motor current detection means for detecting the motor current flowing in the motor, deviation calculation means for calculating the deviation between the target current and the motor current, and driving the motor based on the deviation. In the electric power steering equipped with the motor current control means for controlling, a frequency characteristic adjusting means for adjusting the responsiveness of the low frequency component of the motor current to the target current is further provided between the target current setting means and the deviation calculating means. It is to be prepared.

According to the present invention, by providing the frequency characteristic adjusting means capable of adjusting the responsiveness of the low frequency component of the motor current to the target current, the abnormal noise can be obtained without impairing the desired motor current response characteristic to the steering input. In addition, an electric power steering device having a motor current follow-up property required to suppress motor vibration and detect a failure in the motor current control function can be obtained.

1 is a configuration diagram of an electric power steering device according to Embodiment 1 of the present invention. FIG. 3 is a current response waveform of the electric power steering apparatus according to Embodiment 1 of the present invention. It is a block diagram which shows the correlation of the transfer function of the electric power steering apparatus which concerns on Embodiment 1 of this invention. It is a Bode diagram of various transfer functions of the electric power steering device concerning Embodiment 1 of the present invention. It is a block diagram of the electric power steering apparatus which concerns on Embodiment 2 of this invention. It is a block diagram of the conventional electric power steering control apparatus. It is a current response waveform of the conventional electric power steering apparatus.

Hereinafter, preferred embodiments of the electric power steering apparatus of the present invention will be described with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a configuration diagram of an electric power steering apparatus according to Embodiment 1 of the present invention. Compared with FIG. 6, which is a conventional diagram, FIG. 1 according to the first embodiment is different in that frequency characteristic adjusting means 9 is further provided after target current setting means 4. The frequency characteristic adjusting means 9 in the first embodiment is a phase lag filter.

The frequency characteristic adjusting unit 9 adjusts the frequency characteristic of the target current 10 set by the target current setting unit 4. Details of the function of the frequency characteristic adjusting means 9 will be described later.

Further, in the motor current control means 6 in FIG. 1, the frequency response characteristic of the motor current 11 with respect to the target current 12 after the frequency characteristic adjustment is set so as to be a first order lag represented by the transfer function of the following equation (5) Has been.

Further, the failure detection means 8 has continued for a predetermined time (Terr) or more in which the absolute value of the difference between the target current 12 after the frequency characteristic adjustment and the motor current 11 is larger than a predetermined threshold (Ierr). In this case, failure detection is performed. Thereby, a short circuit of the motor line, a failure of the motor drive circuit, and the like are detected, and the steering assist is stopped when the failure is confirmed. Here, both the predetermined threshold value (Ierr) and the predetermined time period (Terr) are determined from the viewpoint of appropriately performing the vehicle behavior at the time of occurrence of the failure.

Next, the possibility of erroneous detection by the failure detection means 8 will be considered. The followability of the motor current 11 with respect to the target current 12 after the frequency characteristic adjustment is expressed by the above equation (5). Here, when the followability is low (that is, when the cutoff frequency f _mtr in Equation (5) is small), it takes time for the deviation between the two to converge. Therefore, as described in the prior art, the failure detection unit 8 may erroneously detect a state that only takes time to converge as a failure.

FIG. 2 is a current response waveform of the electric power steering apparatus according to Embodiment 1 of the present invention. More specifically, in FIG. 7, the response of the motor current 11 when the target current 12 after the frequency characteristic adjustment changes stepwise is shown as response waveforms 17 and 18. Here, the response waveform 17 is a case where the value of the cutoff frequency f _mtr is 600 Hz, and the response waveform 18 is a case where the value of the cutoff frequency f _mtr is 150 Hz.

In FIG. 2, comparing the responses at two different cutoff frequencies reveals the following. First, when the cut-off frequency f _mtr is set to 150 Hz, it can be seen that it takes time for the motor current 11 to converge to the target current 12 after the frequency characteristic adjustment. For this reason, the deviation 20 between the target current 12 and the motor current 11 after the frequency characteristic adjustment is longer than the threshold (Ierr1) of the failure detection function as shown in the lower part of FIG. As a result, when the state exceeding the threshold value (Ierr1) continues for a predetermined time (Terr1) or longer, the failure detection means 8 erroneously detects as a failure.

On the other hand, when the cutoff frequency f _mtr is set to 600 Hz, it can be seen that the motor current 11 quickly converges to the target current 12 after the frequency characteristic adjustment. For this reason, the deviation 19 between the target current 12 and the motor current 11 after the frequency characteristic adjustment has a short state exceeding the threshold value (Ierr1) of the failure detection function as shown in the lower part of FIG. As a result, the state exceeding the threshold value (Ierr1) does not continue for a predetermined time (Terr1) or more, and the failure detection means 8 has no possibility of erroneous detection.

Therefore, in order to eliminate erroneous detection, the value of the following equation (6) is set in the motor current control means 6 as the cutoff frequency in the above equation (5).

Subsequently, a setting method of the frequency characteristic adjusting means 9 will be described. The frequency characteristics of the motor current 11 with respect to the target current 10 are preferably expressed by the following equations (7) and (8) in order to remove frequency components higher than the frequency band of the steering input included in the target current 10. .

Here, if the transfer function of the frequency characteristic adjusting means 9 is G _cmp (s), the transfer function from the target current 10 to the motor current 11 can be expressed by the following equation (9) using the above equation (5). Become.

FIG. 3 is a configuration diagram showing the correlation of the transfer function of the electric power steering apparatus according to Embodiment 1 of the present invention. As shown in FIG. 3, in order to make the above equations (7) and (9) coincide with each other, it is necessary to satisfy the relationships of the following equations (10) and (11).

Therefore, the frequency characteristic adjusting means 9 (G _cmp (s)) may be a phase delay filter having a coefficient as shown in the following equation (12). With this setting, the frequency characteristic adjusting unit 9 can adjust the response of the low frequency component of the motor current.

FIG. 4 is a Bode diagram of various transfer functions of the electric power steering apparatus according to Embodiment 1 of the present invention. More specifically, the frequency characteristics of the gain and phase of the three transfer functions G _mtr (s), G _T (s), and G _cmp (s) when f _T = 150 [Hz] are shown. . By setting the three transfer functions G _mtr (s), G _T (s), and G _cmp (s) as shown in equations (5), (7), and (12), respectively, desired characteristics shown below are obtained. 1 and 2 can be obtained.

Desired characteristic 1: The frequency characteristic from the target current 10 to the motor current 11 is expressed by the above equation (7). Thus, a signal component in a frequency band higher than the frequency of the driver's steering operation included in the target current 10 can be removed, and a good steering feeling can be obtained.

Desired characteristic 2: The frequency characteristics from the target current 12 to the motor current 11 after adjusting the frequency characteristics are expressed by the above equations (5) and (6). Accordingly, since the current follow-up performance is sufficient, there is no possibility of erroneous detection of a failure by the failure detection means 8. Furthermore, when a failure actually occurs, it is possible to quickly detect the failure and stop the steering assist.

As described above, in the first embodiment, the motor current control means 6 considers the suppression of vibration and abnormal noise and the failure detection by the failure detection means 8, and the motor current is compared with the target current after the frequency characteristic adjustment. The characteristic is set so that it follows sufficiently quickly.

Further, in order to satisfy the request for the steering feeling, the frequency characteristic adjusting means 9 is provided at the subsequent stage of the target current setting means 4. The characteristic of the frequency characteristic adjusting means 9 is such that signal components in a frequency band higher than the frequency of the steering operation of the driver included in the target current 10 are removed while taking into consideration the characteristics of the motor current control means 6 determined in advance. Thus, the response characteristic of the motor current 11 with respect to the target current 10 is set to be optimized.

Further, the failure detection means 8 compares the target current 12 after the frequency characteristic adjustment and the motor current 11 to detect a failure. As described above, the characteristics of the motor current control means 6 are set so that the motor current 11 follows the target current 12 after adjusting the frequency characteristics sufficiently quickly. For this reason, when a failure occurs, it is possible to quickly detect the failure, stop the steering assist, and ensure an appropriate driving state.

As described above, according to the first embodiment, by providing the frequency characteristic adjusting means, the noise and vibration are suppressed without impairing the response characteristic of the desired motor current with respect to the steering input, and the motor current control function is improved. It is possible to obtain an electric power steering device having a motor current followability necessary for detecting a failure.

In the above description, the case where the frequencies f _T and f _mtr included in the transfer function defined as Expression (12) are set as fixed values in advance has been described. However, the present invention is not limited to this. The frequency characteristic adjusting means 9 can also change the frequency defining the characteristic of the above equation (12) according to the vehicle speed detected by the vehicle speed detecting means 9. Thereby, an appropriate response characteristic according to the vehicle speed can be obtained.

Embodiment 2. FIG.
FIG. 5 is a configuration diagram of an electric power steering apparatus according to Embodiment 2 of the present invention. The configuration of FIG. 5 is different from the configuration of FIG. 1 in the first embodiment in that a frequency characteristic adjustment presence / absence changeover switch 27 is newly provided.

Normally, noise and vibration generated from the engine and tires are high when driving at high speeds. For this reason, some handle vibrations and abnormal noises do not matter. Rather, responsiveness at the time of sudden steering operation or fine steering is required. Therefore, it is conceivable to switch on / off the function of the frequency characteristic adjusting means 9 described in the first embodiment according to the vehicle speed.

When driving at high speed, the function of the frequency characteristic adjusting means 9 is bypassed to improve the follow-up performance of the motor current 11 and improve the response to the steering wheel operation. On the other hand, during normal traveling, as described in the first embodiment, by utilizing the function of the frequency characteristic adjusting means 9, both good steering feeling and sufficient current following performance can be realized.

Thus, in order to turn on / off the function of the frequency characteristic adjusting means 9 according to the vehicle speed, the frequency characteristic adjustment presence / absence changeover switch 26 performs the following operation.
When the vehicle speed <Vth (Km / h), the switch is switched to the a side (that is, the function of the frequency characteristic adjusting means 9 is utilized)
When vehicle speed ≧ Vth [Km / h], the switch is switched to the b side (that is, the function of the frequency characteristic adjusting means 9 is bypassed).

The vehicle speed threshold (Vth) used for the switching determination described above may have hysteresis. In order to simplify the description, the frequency characteristic adjustment presence / absence changeover switch 26 is shown as an independent function in the configuration of FIG. However, the frequency characteristic adjustment means 9 can also have a switching function by the frequency characteristic adjustment presence / absence changeover switch 26.

As described above, according to the second embodiment, the optimal steering feeling corresponding to the vehicle speed can be realized by switching the function of the frequency characteristic adjusting means according to the vehicle speed.

Claims

A steering torque sensor for detecting a steering torque of the steering system;
A motor for applying a steering assist force to the steering system;
Target current setting means for setting a target current of the motor based on the steering torque detected by the steering torque sensor;
Motor current detecting means for detecting a motor current flowing through the motor;
Deviation calculating means for calculating a deviation between the target current and the motor current;
In an electric power steering comprising: motor current control means for controlling the driving of the motor based on the deviation;
An electric power steering control device further comprising frequency characteristic adjusting means for adjusting a response of a low frequency component of the motor current to the target current between the target current setting means and the deviation calculating means.
In the electric power steering control device according to claim 1,
The frequency characteristic adjusting means is an electric power steering control device whose characteristic is changed according to a vehicle speed.
In the electric power steering control device according to claim 1 or 2,
An electric power steering control device further comprising failure detection means for detecting a failure based on a target current adjusted by the frequency characteristic adjustment means and a motor current detected by the motor current detection means.
In the electric power steering control device according to claim 3,
The failure detecting means obtains an absolute value of deviation between the target current adjusted by the frequency characteristic adjusting means and the motor current detected by the motor current detecting means, and the absolute value of the deviation is predetermined. An electric power steering control device that determines that a failure occurs when a state equal to or greater than a value continues for a predetermined time or more.
In the electric power steering control device according to any one of claims 1 to 4,
The electric power steering control device, wherein the frequency characteristic adjusting means is a phase delay filter.