JP2010163109A - Electric power steering control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an electric power steering control device with improved steering feeling corresponding to steering speed. <P>SOLUTION: This electric power steering control device includes: a steering torque detector for detecting steering torque; a steering speed detecting part for detecting steering speed; a compensation calculation part for outputting a current command by performing calculation of a transfer function having at least a low frequency pole being a pole and at least two intermediate zero points being zero points with respect to the steering torque detected by the steering torque detector; a transfer function changing part for changing the characteristics of the low region pole or the two intermediate zero points according to the steering speed; and a current control part for controlling a motor so that the current of the motor matches the current command. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electric power steering control device that assists a steering force with a motor.

  Generally, an electric power steering control device detects steering torque generated when a driver rotates a steering wheel with respect to a motor connected via a gear to a steering shaft that connects the steering wheel and the steering wheel. Then, a current command for the motor is calculated by the compensation calculation unit according to the steering torque. Further, the motor torque is generated so as to assist the steering by the driver by generating the torque by controlling the motor current so as to coincide with the current command.

  In the calculation of the current command in the compensation calculation unit, the current command is calculated using a gain, various filters, and the like for the purpose of not impairing the stability of the vehicle and the steering feeling. For this purpose, calculations such as changing the characteristics of the gain and filter according to the vehicle speed (vehicle speed) and the amplitude of the torque measurement value are generally performed.

JP 7-309250 A JP 2002-234454 A

  The electric power steering control device disclosed in Patent Document 1 applies a non-linear gain called an assist curve, which changes according to the magnitude of the steering torque and the vehicle speed, to the detected value of the steering torque, The result of a filter operation called an adaptive torque filter is output as a current command. In addition, by changing the pole of the adaptive filter according to the change ratio of the input / output of the assist curve, that is, according to the detected steering torque value or the vehicle speed, the pole or steady gain (frequency) in the transfer function from the steering torque to the current command is changed. 0 gain) is changed. As a result, the control system is configured to obtain a suitable steering feeling according to the vehicle speed and the magnitude of the steering torque within a stable range.

  However, since no consideration is given to the steering feeling according to the rotational speed of the motor, that is, the steering speed, such as the torque ripple of the motor, the influence of disturbances such as torque ripple being transmitted to the steering wheel cannot be sufficiently reduced. There was a problem that led to a worse feeling.

  The electric power steering control device disclosed in Patent Document 2 applies a low-pass filter to the current command or the steering torque detection value, that is, inserts the low-pass filter in series in the control calculation from the steering torque detection value to the current command. In this configuration, the influence of detection noise included in the detected steering torque value is removed from the current command. Further, the pole of this low-pass filter is changed according to the vehicle speed and the steering speed.

  However, in the technique according to Patent Document 2, since the low-pass filter has a characteristic of delaying the phase, the gain of the controller, that is, the assist gain cannot be sufficiently increased in order to maintain the stability of the control system. There was a problem that the influence of disturbances such as the above could not be reduced sufficiently. Furthermore, in such a method that cuts off high frequency components using a low-pass filter, although the control effect of the cut off frequency components is reduced, the influence of detection noise can be reduced, but at the cut off frequency There has been a problem that the influence of disturbances such as torque ripple transmitted to the steering wheel cannot be reduced.

  In other words, in the conventional power steering control device, the influence of disturbance such as torque ripple generated according to the steering speed cannot be sufficiently suppressed, so that the steering feeling according to the steering speed cannot be sufficiently improved. It was.

  The present invention has been made in view of the above, and an object of the present invention is to obtain an electric power steering control device that improves the steering feeling according to the steering speed.

  In order to solve the above-described problems and achieve the object, the present invention provides a steering torque detector that detects steering torque in an electric power steering control device that assists a steering torque applied by a driver with a motor. A steering speed detector for detecting a steering speed, a low-frequency pole that is at least one pole, and an intermediate zero that is at least two zeros, and a low frequency equal to or lower than a pole frequency corresponding to an absolute value of the low-frequency pole The gain is flat in the region, and in the middle frequency region from the pole frequency to the zero point frequency corresponding to the geometric mean of the two intermediate zeros, the gain decreases as the frequency increases, and at least the zero point frequency and the zero point frequency In the high frequency region up to 3 times, the steering torque detector calculates a transfer function having a frequency response characteristic in which the gain increases as the frequency increases. A compensation calculation unit that outputs a current command by performing on the steering torque, a transfer function changing unit that changes characteristics of the low-frequency pole or the two intermediate zeros according to the steering speed, and the current command And a current control unit for controlling the motor so that the currents of the motors coincide with each other.

  According to the present invention, since the pole or zero point of the transfer function is changed according to the steering speed, the influence of disturbances such as torque ripple that varies according to the steering speed on the steering wheel is reduced according to the steering speed. As a result, an electric power steering control device with an improved steering feeling according to the steering speed can be obtained.

  Embodiments of an electric power steering control device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing an electric power steering control apparatus according to Embodiment 1 of the present invention. The left and right steering wheels 6 are steered according to the rotation of the steering shaft 2 connected to the steering wheel 1. A steering torque detector 3 is disposed on the steering shaft 2 and detects a steering torque acting on the steering shaft 2 as a steering torque signal τs. The motor 4 is connected to the steering shaft 2 via the speed reduction mechanism 5, and the motor torque τm generated by the motor 4 can be applied to the steering shaft 2. The steering speed detector 7 detects the rotational speed of the steering shaft 2, that is, the steering speed, based on the detected values such as the motor rotation angle and the motor voltage. The control unit 8 supplies a current Im to the motor 4 as a result of controlling the current of the motor 4 based on the steering torque signal τs detected by the steering torque detector 3 and the steering speed detected by the steering speed detector 7.

  Next, the main part of the control unit 8 and the outline of its operation will be described with reference to FIG. A control unit 8, a compensation calculation unit 9, a current control unit 10, and a control constant changing unit 120 are provided. The control constant changing unit (transfer function changing unit) 120 calculates the set value of the control constant of the compensation calculating unit 9 based on the steering speed detected by the steering speed detecting unit 7. Based on the steering torque signal τs detected by the steering torque detector 3, the vehicle speed detected by the vehicle speed detector (not shown), and the control constant set by the control constant changing unit 120, the compensation calculation unit 9 performs calculation described later. As a result, the current command Ir is output. The current control unit 10 performs control so that the current Im of the motor 4 matches the current command Ir calculated by the compensation calculation unit 9. Note that the steering torque signal τs input to the actual compensation calculation unit 9 may be a signal obtained by detecting the physical steering torque by the steering torque detector 3 or a signal obtained by adding a correction thereto. An input to the calculation unit 9 is simply described as a steering torque τs.

  Next, the calculation operation of the compensation calculation unit 9 will be described. The compensation calculation unit 9 inputs the steering torque τs to the low frequency amplification unit 102. The low frequency amplifying unit 102 generally determines a steering torque τs based on a function called an assist curve that is generally determined by reading a look-up table and the like, and a control constant set by the control constant changing unit 120 as described later according to the steering speed. Outputs the amplified signal.

  The low-pass filter 101 outputs a result obtained by performing a low-pass filter operation on the output of the low-frequency amplifier 102 as a low-frequency compensation torque τl. The frequency response characteristic of the low-pass filter 101 is a flat characteristic with a gain of approximately 1 at a frequency lower than the low-pass filter frequency ωl [rad / s], which is a control constant determined as described later according to the steering speed, and low It has a characteristic that the gain decreases in a frequency region higher than the high-pass filter frequency ωl. Here, the term “flat” means that the difference from the steady gain is substantially constant at a small value (for example, 6 [dB]) or less.

Here, in the low frequency amplifying unit 102 using the assist curve as described above, the amplification ratio is a non-linear one that changes in accordance with the steering torque τs or the vehicle speed. A description will be given assuming that amplification is performed at a ratio of the area gain Kl1. The transfer function Gl1 (s) of the low frequency compensation torque τl with respect to the steering torque τs has a characteristic obtained by multiplying a gain Kl1 and a low-pass filter as in the following equation.
Gl1 (s) = Kl1 · ωl / (s + ωl) (Formula 1)

Further, the compensation calculation unit 9 inputs the steering torque τs to the proportional compensation unit 103, and the proportional compensation unit 103 responds to the input with a proportional gain Kp that is a control constant determined by the control constant changing unit 120 as described later. An amplification operation is performed, and the result is output as a proportional compensation torque τp. The transfer function Gp (s) of the proportional compensation unit 103 is as follows.
Gp (s) = Kp (Formula 2)

Further, the compensation calculation unit 9 inputs the steering torque τs to the differential compensation unit 104, performs a calculation by multiplying the differential or pseudo-differential calculation and the differential gain Kd which is a predetermined control constant, and outputs the result as the differential compensation torque τh. Assuming that pure differentiation is used in the above calculation for simplification of explanation, the transfer function Gd (s) of the differential compensation unit 104 is expressed by the following equation.
Gd (s) = Kd · s (Formula 3)

  In addition, the adder 105 inside the compensation calculation unit 9 inputs the result of adding the low-frequency compensation torque τl, the proportional compensation torque τp, and the differential compensation torque τh to the noise filter 106.

The noise filter 106 performs a low-pass filter operation represented by Fn (s) of the following equation so as to cut off a frequency component higher than the noise cutoff frequency ωn [rad / s], which is a predetermined control constant, for the input. Command Ir is output.
Fn (s) = 1 / (s / ωn + 1) (Formula 4)

As a result, the compensation calculation unit 9 performs a calculation represented by the following transfer function C (s) as a whole.
C (s) = Fn (s). {Gl1 (s) + Gp (s) + Gd (s)} (Formula 5)

The above (Formula 5) is converted into the following (Formula 6) to (Formula 9).
C (s) = {b2 · s ^ 2 + b1 · s + g0 · ωl} / {(s + ωl) · (s / ωn + 1)} (Formula 6)
g0 = Kp + Kl1 (Formula 7)
b1 = Kp + Kd · ωl (Formula 8)
b2 = Kd (Formula 9)
Here, if it is assumed that the detection noise or the like can be ignored, the noise filter 106 for the purpose of noise removal may not be used, that is, the pole ωn of the noise filter can be ignored as infinite. Is essentially a transfer function having one pole having an absolute value ωl (low-frequency pole described later) and two zeros (intermediate zero described later) that are the roots of a second-order numerator polynomial. The operation is performed.

  Next, the correspondence between the characteristics desired for the compensation calculation unit 9, in particular, the transfer function C (s) of the compensation calculation unit 9 and the closed loop characteristics of the control system as a result of using the transfer function C will be described with reference to FIG. . Here, for the sake of simplicity of explanation, the steering speed detection unit 7 and the control constant changing unit 120 will be omitted.

  First, the dynamic characteristics of the entire electric power steering control device will be described. FIG. 3 is a block diagram showing the dynamic characteristics of the electric power steering control device shown in FIGS. 1 and 2. In the following, the dynamic characteristics of the electric power steering control device will be explained in association with the configuration shown in FIG. .

  In FIG. 3, when the driver rotates a steering wheel angle θh that is an angle of the steering wheel 1, the steering shaft 2 is elastically deformed to generate a steering torque τs. That is, a relative angle θe is generated by an angle differentiator 11 representing a difference between the steering wheel angle θh and the steering angle (that is, the rotation angle of the steering wheel 6 and the motor 4) θm. The steering torque τs is generated by multiplying the elastic constant Ks of 2. The steering torque τs is transmitted to the steering wheel 1, and the steering torque τs is detected by a steering torque detector 3 (not shown) and input to the control unit 8. A motor current Im that matches the current command Ir calculated in the control unit 8 is supplied to the motor 4. Here, for the sake of simplicity, assuming that the current command Ir and the motor current Im coincide with each other, the transfer function of the control unit 8 is the transfer function C (s) of the compensation calculation unit 9. A motor torque τm is generated in the motor 4 by the motor current Im supplied from the control unit 8 to the motor 4. That is, the motor torque Im is generated by multiplying the motor current Im by the torque constant Kt in the torque constant unit 13. Next, a motor torque τm and a steering torque τs are applied to the steering mechanism including the motor 4, the speed reduction mechanism 5, the steering shaft 2, the steering wheel 6 and the like, and a road surface reaction force τd is applied in the opposite direction. That is, the torque resulting from the addition of the motor torque τm and the steering torque τs and the subtraction of the road surface reaction force τd, which is a disturbance torque, is applied to the steering mechanism inertia part 15 by the torque adder 14, and the steering mechanism inertia part 15 As a result of driving, the steering angle θm is output.

Next, characteristics from the current command Ir or the motor current Im as the control input to the steering torque τs as the detected value, that is, the characteristics of the controlled object 16 will be described using a simple model. When the transfer function of the controlled object 16 is described as P (s), a simple model of P (s) is expressed by the following equation. Note that the definition of the output of the control target P (s) in the mathematical expression is described by reversing the steering torque τs and the positive and negative signs in FIG. 3 for the convenience of explanation as a regular classic control.
P (s) = Kt · Ks / (J · s ^ 2 + D · s + Ks) (Formula 10)

  In the above (Expression 10), J is the moment of inertia of the steering mechanism inertia portion 15, and D is a constant representing the influence of deformation viscosity, friction, etc. on the steering shaft 2 and is relatively small. From (Equation 10), the control object P (s) is represented as a secondary resonance system, and in a low frequency region that is a fraction of the resonance frequency (approximately 10 [Hz]), the gain is relative to the frequency. And flat characteristics. Further, in a high frequency region several times higher than the resonance frequency, it is approximately approximated to the characteristics of a second-order integration system, the gain decreases with increasing frequency, and the phase is approximately constant at −180 [deg]. .

  Here, the main purpose of the electric power steering control device is to assist the steering torque τs generated by the steering by the driver, but considering that the force against the steering is mainly the road reaction force, The purpose of the power steering control device can be considered to reduce the force transmitted from the road surface reaction force τd to the steering torque τs in FIG. 3 with desirable characteristics. Therefore, the transfer function from the road surface reaction force τd to the steering torque τs is referred to as steering sensitivity Sd (s).

  On the other hand, the steering sensitivity Sd (s) represents not only the influence of the road surface reaction force on the steering torque, but also general disturbance torque applied to the steering shaft 2 such as torque ripple generated by the motor 4 gives the steering torque. Represents the impact. Therefore, although it is necessary to consider the influence on the steering feeling of the driver, generally, the gain of the steering sensitivity Sd (s) is desired to be reduced over as wide a frequency range as possible.

When the open-loop transfer function including the transfer function C (s) of the control unit 8, that is, the compensation calculation unit 9, and the control object P (s) is expressed by L (s) of the following equation, the steering sensitivity Sd (s) is expressed as 12).
L (s) = C (s) · P (s) (Formula 11)
Sd (s) = P (s) / (1 + L (s)) (Formula 12)

  If the gain of the open loop transfer function L (s) is smaller than 1 (that is, 0 [dB]) from the effect of the denominator in (Expression 12), the denominator approaches 1, and the gain reduction effect of the steering sensitivity Sd (s) is obtained. It turns out that you can't get enough. Therefore, the gain crossover frequency (hereinafter simply referred to as the crossover frequency) at which the gain of L (s) crosses 0 [dB] is a large index representing the performance of the control system. A frequency generally called a control band also corresponds to this crossing frequency.

  Here, as described above, the control target P (s) has a flat gain characteristic at a frequency lower than the resonance frequency, and the steady gain is the torque constant Kt. When the motor 4 assists with a motor torque larger than the steering torque generated on the steering shaft 2, that is, the gain of the transfer function C (s) of the compensation calculation unit 9 is made larger than 1 / Kt, that is, from the steering torque τs. If the gain up to the motor torque τm is larger than 1, it can be easily understood that the crossing frequency needs to be selected larger than the resonance frequency. In other words, it is necessary to select a high crossing frequency so that the resonance frequency representing the dynamic influence of the elastic constant Ks on the steering shaft is in a lower frequency region than the control band of the steering control device.

  Specifically, the resonance frequency of the control target P (s) is approximately 10 [Hz], and the crossing frequency is selected to be a value larger than approximately 30 [Hz]. As a result, the phase of the control target P (s) is close to −180 [deg] in the vicinity of the normally selected cross frequency.

  Next, characteristics desired for the control system in the vicinity of the crossing frequency, which are largely related to the stability of the control system, will be described. A frequency region near the crossing frequency will be referred to as a high frequency region. When a control object having a crossing frequency phase close to −180 [deg] as described above is controlled only by a proportional element similar to the elastic constant Ks of the steering shaft 2, it becomes oscillating with characteristics similar to those of a spring. Stability deteriorates. In practice, the control loop composed of the control unit 8 and the control target 16 has a phase delay caused by a modeling error with respect to a simple model such as a control delay of the current control unit 10 or a detection delay in the steering torque detector 3. Because of this, the control system diverges in an unstable manner. In such a feedback control system, at the crossing frequency, the phase of L (s) needs to be advanced by an amount called a phase margin as compared with -180 [deg]. This phase margin usually needs to be secured about 30 [deg] or more.

  The characteristics in the vicinity of the crossing frequency determine the characteristics in the high frequency region as compared with the low frequency region that greatly affects the driver's steering feeling described later. As described above, the phase of the open loop transfer function L (s) is determined. In order to advance from -180 [deg], since the phase of the control target P (s) is close to -180 [deg], the transfer function C (s) of the control unit 8, that is, the compensation calculation unit 9, is set to 0 [deg]. There is a need to go further. Therefore, the transfer function C (s) of the compensation calculation unit 9 needs to have a characteristic close to differentiation or pseudo differentiation in a high frequency region near the crossing frequency. As a result, in the high frequency region near the crossing frequency, the gain of the transfer function of the compensation calculation unit 9 needs to have a characteristic that increases as the frequency increases.

  However, in practice, this can be realized by the presence of a phase delay caused by a noise error 106 for removing a signal quantization error or noise excited by differentiation, or a phase delay caused by a modeling error as described above. There is an upper limit for the crossover frequency, and the limit is highly dependent on the hardware performance of the control target 16 and the control unit 8.

  Next, desirable characteristics in the low frequency region that greatly affect the driver's steering feeling will be described. In general, in the low frequency range lower than 1 Hz to several Hz, it is necessary to naturally transmit the influence of the road surface reaction force to the driver, that is, the steering wheel 1. For this purpose, the steering sensitivity Sd (s) described above needs to be constant in the low frequency range, that is, the frequency characteristics of the gain must be flat.

Here, in the low frequency region sufficiently lower than the crossing frequency, the gain of the open loop transfer function L (s) is normally sufficiently larger than 0 [dB]. That is, it is expressed by the following formula.
| L | >> 1 (Formula 13)

From (Expression 11), (Expression 12), and (Expression 13), the steering sensitivity Sd (s) in the low frequency region is approximated by the following expression.
| Sd | ≈ | P | / | L | = 1 / | C | (Formula 14)

  From (Equation 14), it can be seen that the gain characteristic of the steering sensitivity Sd (s) is approximated by the reciprocal of the gain of the transfer function C (s) of the compensation calculation unit 9 in the low-frequency region where (Equation 13) holds. . Therefore, in order to realize the gain of the steering sensitivity Sd (s) flat with respect to the frequency in the low frequency region, it is necessary to make the gain of the transfer function C (s) of the compensation calculation unit 9 flat with respect to the frequency. I can understand. In general, it is desirable to increase the assist ratio, which is the ratio of assisting by the motor torque τm with respect to the steering torque τs, particularly when the vehicle speed is low. That is, it is desired to reduce the steady gain of the steering sensitivity Sd (s) by increasing the steady gain of the transfer function C (s) and the open loop transfer function L (s).

  Next, desirable characteristics of the middle frequency region, which is between the low frequency region and the high frequency region (region near the crossing frequency), will be described. As described above, there is a limit to the crossover frequency that can be realized. Further, as described above, it is generally desirable to increase the gain of the open loop transfer function L (s). Here, when the crossing frequency is increased to several times or more of the low frequency region that greatly affects the steering feeling, in the middle frequency region, the gain of the transfer function C (s) of the compensation calculation unit 9 is set to the frequency. If the characteristic is such that the gain decreases with increasing frequency (the characteristic that the gain increases with decreasing frequency), the gain in the low frequency region can be increased even if the gain near the crossing frequency cannot be increased. be able to. In order to realize such a characteristic, the transfer function C (s) of the compensation calculation unit 9 may be set to a characteristic that approximates the integral characteristic in the middle frequency region. That is, the transfer function C (s) of the compensation calculation unit 9 can be realized by having a pole corresponding to the frequency between the low frequency region and the middle frequency region where a flat gain characteristic is desired. The frequency corresponding to the low-frequency pole and its absolute value is simply called the pole frequency.

  As described above, the characteristics approximate to the integral characteristics such that the gain decreases with increasing frequency in the middle frequency range, and the gain increases with increasing frequency in the high frequency range near the crossing frequency. It can be easily understood that two zeros are required between the medium frequency region and the high frequency region in order to realize the characteristic approximated to the differential characteristic. These two zeros are referred to as intermediate zeros, and the frequency corresponding to the intermediate zero is simply referred to as the zero frequency. The two zeros are selected as close real values or conjugate complex numbers, and the frequency called the zero frequency is the absolute value of each zero when the two zeros are conjugate complex numbers (both are the same value). ) Is the same value.

  Next, the correspondence between the characteristics desired for the above-described compensation calculation unit 9 and the configuration of the compensation calculation unit 9 according to the first embodiment will be described. The transfer function C (s) of the compensation calculation unit 9 according to the first embodiment is expressed by the above (Equation 6), and has a low-frequency pole whose absolute value is ωl and in a low-frequency region lower than the pole frequency ωl. Has a characteristic that the gain of C (s) is flat with an increase in frequency. Further, since the transfer function C (s) of the compensation calculation unit 9 has a second-order numerator polynomial, it has two zeros, that is, intermediate zeros, and the square root of (g0 · ωl) / b2 is the geometric mean of the zeros, that is, the above-described zeros. Corresponds to the frequency. In the middle frequency region from the pole frequency ωl to the zero point frequency, the gain decreases with increasing frequency, and in the high frequency region above the zero point frequency, the gain increases with increasing frequency. Here, the compensation calculation unit 9 according to the first embodiment further has a pole of the noise filter 106 having a frequency of ωn as shown in (Equation 6), and this pole has a crossing frequency, that is, a high frequency. In order to ensure a sufficient phase margin in the region, it is set to a value that is at least three times the zero frequency. FIG. 4 is a diagram for explaining the frequency response of the transfer function C (s) of the compensation calculation unit 9. The description regarding FIG. 4 will be described later in detail.

  As described above, the compensation calculation unit 9 according to the first embodiment adds the noise reduction effect by the noise filter 106, and further achieves the minimum necessary to realize a desirable form as the transfer function C (s) of the compensation calculation unit 9. It can be seen that this is an example configuration.

  Next, constants used in the calculation in the compensation calculation unit 9, that is, control constants set in the control constant changing unit 120 will be described. As described above, the setting of the differential gain Kd and the noise filter frequency ωn is largely determined by hardware conditions. Further, since it is generally required to increase the assist ratio as a characteristic of the electric power steering control device, the description will be made assuming that the low frequency gain Kl1, that is, g0 in (Expression 6) is set large.

  FIG. 4 shows the frequency response of the transfer function C (s) of the compensation calculation unit 9 and FIG. 5 shows the frequency of the open loop transfer function L (s) for three cases where different values are set as the control constants of the compensation calculation unit 9. FIG. 6 shows the frequency response of the gain of the steering sensitivity Sd (s). The controlled object P (s) used for these simulations has a resonance frequency of 8 [Hz] as an example. In addition, the crossing frequency is set to about 40 [Hz] in consideration of the phase delay included in the control object P (s). In each figure, a solid line indicates a standard setting, and is hereinafter referred to as a standard setting. Also, in each figure, the chain line indicates a setting in which only the low-pass filter frequency ωl of the low-pass filter 101, which is a control constant, is set larger than the standard setting, and is hereinafter referred to as a low-frequency emphasis setting. Further, the broken line in each figure shows a characteristic in which the characteristic of the intermediate zero is changed by setting only b1 in (Equation 6) larger than the standard setting, that is, the proportional gain of the proportional compensation unit 103 based on (Equation 8). In addition to increasing Kp, the low-frequency gain Kl1 of the low-frequency amplifying unit 102 is reduced based on (Equation 7) so that g0 in (Equation 6) is constant, and is hereinafter referred to as intermediate enhancement setting. .

  Here, in the standard setting shown by the solid line in FIGS. 4, 5, and 6, the phase margin at the crossing frequency does not become too small in the open loop transfer function L (s) shown in FIG. Each control constant is determined so that the gain of the steering sensitivity Sd (s) shown changes smoothly without becoming too large at a specific frequency.

  On the other hand, in the low frequency emphasis setting indicated by the chain line, as shown in FIG. 4, the pole frequency (open diamond) in the transfer function C (s) of the compensation calculation unit 9 is changed to the standard pole frequency (black diamond). Is bigger than. As a result, in the relatively low frequency region (about 1 [Hz] to 10 [Hz]) as shown in FIG. 4, the gain of the transfer function C (s) of the compensation calculation unit 9 is larger than the standard setting, It can be seen that the gain of the steering sensitivity Sd (s) shown in FIG. 6 is reduced in that frequency region.

  In the intermediate emphasis setting indicated by a broken line, as shown in FIG. 4, the gain of the transfer function C (s) of the compensation calculation unit 9 in the frequency region (3 [Hz] to 30 [Hz]) before and after the zero point frequency. It can be seen that (white triangle) is larger than the standard gain (black triangle) and the gain of the steering sensitivity Sd (s) is reduced as shown in FIG.

  Here, the frequency response gain of the steering sensitivity Sd (s) shown in FIG. 6 is based on the Bode sensitivity integral theorem, and the gain of the steering sensitivity Sd (s) is spread over all frequencies even if the control constant is changed. It is known that the integrated value is invariant. That is, it is known that when the steering sensitivity Sd (s) at a certain frequency is reduced, the steering sensitivity Sd (s) at another frequency is increased. In the example of FIG. 6 described above, when the low frequency emphasis setting indicated by the chain line is used, the steering sensitivity Sd (s) is reduced in the relatively low frequency region as described above, but is slightly lower than the crossing frequency. It can be seen that the frequency is higher than the standard setting around 20 [Hz], which is a low frequency. In the intermediate emphasis setting indicated by the dotted line, the steering sensitivity Sd (s) is reduced in the vicinity of the zero frequency as described above, but is larger than the standard setting in the vicinity of the crossing frequency of 40 [Hz]. I understand that. As a result, the gain of the steering sensitivity Sd (s) has the lowest low frequency emphasis setting in the frequency region lower than the first boundary frequency shown in FIG. 6, and the first boundary frequency to the second boundary frequency. It can be seen that the intermediate emphasis setting is the smallest in the region, and the standard setting is the smallest in the frequency region higher than the second boundary frequency.

  Next, the operation of the control constant changing unit 120 according to the first embodiment will be described. The control constant changing unit 120 is intended to reduce the influence of disturbance of the frequency according to the rotational speed of the motor, that is, the steering speed, such as the torque ripple of the motor, on the steering feeling. As a general rule, the torque ripple frequency (ripple frequency) of the motor is generated at a frequency proportional to the rotational speed of the motor, so that the control constant of the compensation calculation unit 9 is changed according to the ripple frequency, that is, according to the steering speed. Thus, the steering sensitivity Sd (s) at the ripple frequency is further reduced, and the influence of torque ripple transmitted to the steering torque is reduced.

  Next, details of the operation of the control constant changing unit 120 will be described. The control constant changing unit 120 is based on the steering speed detected by the steering speed detecting unit 7, and when the ripple frequency proportional to the steering speed is lower than the first boundary frequency shown in FIG. Is set as the above-mentioned low-frequency emphasis setting. That is, the pole frequency ωl of the low-pass filter 101 is set to a larger value than the standard setting.

  Next, when the steering speed increases and the ripple frequency is between the first boundary frequency and the second boundary frequency in FIG. 6, the control constant changing unit 120 sets the control constant of the compensation calculating unit 9 to the above-described control constant. Set as intermediate emphasis setting. That is, as the steering speed increases so that the ripple frequency becomes higher than the first boundary frequency, the pole frequency ωl is decreased to a standard setting value. At the same time, the control constant of the compensation calculation unit 9 is set so that b1 in (Expression 6) becomes large. Specifically, the proportional gain Kp in the proportional compensation unit 103 is increased, and the low-frequency gain Kl1 in the low-frequency amplifying unit 102 is decreased so that the sum of Kp and Kl1 is the same. Here, the sum of Kp and Kl1 is made the same so that the steady gain of the transfer function C (s) of the compensation calculation unit 9, that is, g0 in (Equation 6) does not change, and the steady assist ratio. This is to maintain the steering feeling by keeping the constant.

  Here, the operation of the control constant changing unit 120 when the ripple frequency changes more than the first boundary frequency changes the corresponding low-frequency pole so that the pole frequency ωl becomes small (Equation 6 ) Is set to a large value to change the characteristic of the root of the numerator polynomial, that is, the intermediate zero. Considering the degree of freedom in expressing the transfer function including the denominator and taking into account that the differential gain Kd, that is, b2 that is greatly related to the stability is set as large as possible, the change in the above-described intermediate zero can be expressed as 2 The ratio of the first order coefficient b1 to the second order coefficient b2 in the second order polynomial having two zeros (intermediate zeros) as roots, that is, the numerator polynomial of C (s) is increased.

  Next, when the steering speed is further increased and the ripple frequency is higher than the second boundary frequency in FIG. 5, the control constant changing unit 120 sets the control constant of the compensation calculating unit 9 as the standard setting described above. . That is, the control constant of the compensation calculation unit 9 is set so that b1 in (Expression 6) is smaller than when the ripple frequency is between the first boundary frequency and the second boundary frequency. Specifically, the proportional gain Kp in the proportional compensation unit 103 is decreased, and the low-frequency gain Kl1 in the low-frequency amplifier 102 is increased so that the sum of Kp and Kl1 is the same. In other words, as the steering speed increases so that the ripple frequency becomes higher than the second boundary frequency, the second order coefficient of the first order coefficient b1 in the second order polynomial having two zeros (intermediate zeros) corresponding to the zero frequency as roots. The characteristic of the intermediate zero is changed so as to reduce the ratio to b2.

  In the above, the operation when the ripple frequency changes before and after the first boundary frequency and the second boundary frequency may be gradually changed so that the control constants ωl, Kp, and Kl1 change continuously. Further, the control constant may be changed discontinuously when the ripple frequency exceeds the first boundary frequency and the second boundary frequency. However, when Kp is changed discontinuously, the current command Ir changes sharply. A shock will occur. In order to prevent this, the value of the state variable of the low-pass filter 101 in the compensation calculation unit 9 is discontinuously set so that the current command Ir does not change suddenly when switching Kp, in other words, the output of the adder 105 is continuous. By changing, the control constant can be switched without causing the current command Ir to change sharply and adversely affect the steering feeling.

  When the control constant changing unit 120 operates as described above, the transfer function C (s) of the compensation calculation unit 9 is set to low frequency emphasis when the ripple frequency is lower than the first boundary frequency, and the first boundary frequency. When the frequency is between the first boundary frequency and the second boundary frequency, the intermediate emphasis setting is changed. When the frequency is larger than the second boundary frequency, the standard setting is changed. The gain of the steering sensitivity Sd (s) is always changed with respect to the ripple frequency. The smallest one shown in FIG. Therefore, transmission of the torque ripple whose frequency changes according to the steering speed to the steering wheel can be reduced according to the steering speed, and the steering feeling according to the steering speed can be improved.

  In the above description, it has been described that the setting is changed to three different settings of the low-frequency emphasis setting, the intermediate emphasis setting, and the standard setting. However, the same effect can be obtained only by combining two low-frequency emphasis settings and standard settings. Can also be easily understood. In this case, the low frequency emphasis setting described above may be set at a steering speed where the ripple frequency is lower than the third boundary frequency shown in FIG. 6, and the standard setting may be set at a higher frequency. Further, the operation of the control constant changing unit 120 in this case changes the characteristic of the corresponding low-frequency pole so that the pole frequency ωl of the low-pass filter 101 becomes smaller as the steering speed increases.

  In addition, it can be easily understood that the same effect can be obtained by only two combinations of the intermediate emphasis setting and the standard setting. In this case, the above-described intermediate emphasis setting may be set at a steering speed where the ripple frequency is lower than the second boundary frequency shown in FIG. 6, and the standard setting may be set at a higher frequency. In this case, b1 is set smaller as the steering speed increases. Specifically, as the steering speed increases, the proportional gain Kp of the proportional compensator 103 is reduced, and the low-frequency gain Kl1 of the low-frequency amplifier 102 is set large so that the sum of Kp is constant. In other words, the characteristics of the intermediate zero are changed so that the ratio of the first-order coefficient b1 to the second-order coefficient b2 in the second-order polynomial having two zeros (intermediate zeros) as roots is set small.

  In the above description, only the setting of the control constant of the compensation calculation unit 9 according to the steering speed has been described. However, as described in Patent Document 1, the control constant of the compensation calculation unit 9 according to the vehicle speed and the steering torque is described. Needless to say, it may be changed.

  In the above description, the steering speed detection unit 7 is described as detecting the steering speed. However, for example, only the magnitude of the steering speed may be determined based on the threshold value.

  In the above description, the compensation calculation unit 9 has been described in accordance with the configuration described in FIG. 2, but it is obvious that various configurations can be realized as long as the transfer function shown in (Equation 6) is realized. Yes, for example, it is needless to say that (Equation 6) may be realized in the form of a delayed advance filter as in Patent Document 1. Furthermore, the configuration shown in FIG. 2 and the transfer function of (Equation 6) for the compensation operation unit 9 can be realized with a minimum and simple operation. On the other hand, even for those having a transfer function in which the same number of poles and zeros are added, as long as the added poles and zeros are close (less than 2 times), it is substantially different from that according to this embodiment. It is clear that there is no.

  As described above, according to the first embodiment of the present invention, the control constant changing unit (transfer function changing unit) 120 determines whether the transfer function C (s) of the compensation calculating unit 9 has a low-frequency pole or the Since it is configured to change the intermediate zero point, the influence of disturbances such as torque ripple that varies according to the steering speed on the steering wheel can be reduced according to the steering speed, thus improving the steering feeling according to the steering speed. It becomes possible to make it.

Embodiment 2. FIG.
Next, an electric power steering control apparatus according to Embodiment 2 of the present invention will be described. The overall configuration is the same as that shown in FIG. FIG. 7 is a block diagram showing the configuration of the control unit 8 in the second embodiment, and the same reference numerals are given to the same parts as those in the first embodiment. The difference between the second embodiment and the first embodiment is that the setting of the low-frequency amplification unit 202 is different from that of the low-frequency amplification unit 102 in the first embodiment, and the low-pass filter 201 is different from the first embodiment. The input is different from the low-pass filter 101 in the first embodiment. Further, the operation of the control constant changing unit 220 is changed accordingly.

  The control unit 8 includes a compensation calculation unit 9, a control constant changing unit 220, and a current control unit 10. The compensation calculation unit 9 determines the steering torque τs detected by the steering torque detector 3 and the steering speed detected by the steering speed detection unit 7. To enter. The compensation calculation unit 9 outputs a current command Ir as a result of the calculation described below. The current control unit 10 performs control so that the current Im of the motor 4 matches the current command Ir calculated by the compensation calculation unit 9.

  Next, the calculation operation of the compensation calculation unit 9 will be described. The compensation calculation unit 9 inputs the steering torque τs to the low frequency amplification unit 202. The low frequency amplifying unit 202 outputs an amplified signal based on a function called an assist curve that is generally determined by reading a lookup table based on the steering torque τs. This amplification ratio is set as a low-frequency gain Kl2.

  Further, the compensation calculation unit 9 inputs the steering torque τs to the proportional compensation unit 103, and the proportional compensation unit 103 uses the proportional gain Kp, which is a control constant set by the control constant changing unit 220, as in the first embodiment. An operation for amplifying τs is performed, and the result is output as a proportional compensation torque τp. The transfer function Gp (s) of the proportional compensation unit 103 is as shown in (Expression 2).

The compensation calculation unit 9 inputs a signal obtained by subtracting the output of the proportional compensation unit 103 from the output of the low-frequency amplification unit 202 to the low-pass filter 201, and the low-pass filter 201 calculates a low-pass filter on the input value. The result of performing is output as the low-frequency compensation torque τl. The transfer function of the low-pass filter 201 is a low-pass filter that is exactly the same as the low-pass filter 101 in the first embodiment. As a result of the above calculation, the transfer function Gl2 (s) from the steering torque τs to the low-pass compensation torque τl is The following formula.
Gl2 (s) = (Kl2-Kp) ωl / (s + ωl) (Formula 15)

  The compensation calculation unit 9 inputs the steering torque τs to the differential compensation unit 104, and the differential compensation unit 104 outputs the differential compensation torque τh by the same calculation as in the first embodiment. The transfer function Gd (s) of the differential compensation unit 104 is as shown in (Equation 3).

Further, the noise filter 106 performs the calculation shown in (Equation 4) as in the first embodiment. As a result, the compensation calculation unit 9 performs a calculation represented by the following transfer function C (s) as a whole.
C (s) = Fn (s). {Gl2 (s) + Gp (s) + Gd (s)} (Formula 16)

(Expression 16) is converted into the following (Expression 17) to (Expression 20).
C (s) = {b2 · s ^ 2 + b1 · s + g0 · ωl} / {(s + ωl) · (s / ωn + 1)} (Expression 17)
g0 = Kl2 (Formula 18)
b1 = Kp + Kd · ωl (Equation 19)
b2 = Kd (Formula 20)

  In the above, (Equation 17) has exactly the same form as (Equation 6) in the first embodiment, and b1 and b2 shown in (Equation 19) and (Equation 20) are the same as those in the first embodiment. It can be seen that (Expression 8) and (Expression 9) in FIG. Further, from the comparison between (Equation 7) and (Equation 18), the sum of the low-frequency gain Kl1 of the low-frequency amplification unit 102 and the proportional gain Kp of the proportional compensation unit 103 in the first exemplary embodiment is calculated. 2 is replaced with only the low-frequency gain Kl2 of the low-frequency amplifier 202, it can be seen that the second embodiment operates in exactly the same manner as the first embodiment. Therefore, if the replacement is performed in this manner, the control constants in the compensation calculation unit 9 are set as the standard setting, the low frequency emphasis setting, and the intermediate emphasis setting shown in FIGS. 4, 5, and 6 just as in the first embodiment. Settings can be made.

  Next, the operation of the control constant changing unit 220 will be described. The control constant changing unit 220 is based on the steering speed detected by the steering speed detecting unit 7, and when the ripple frequency proportional to the steering speed is lower than the first boundary frequency shown in FIG. Is set as the above-mentioned low-frequency emphasis setting. That is, the pole frequency ωl of the low-pass filter 201 is set to a larger value than the standard setting.

  Next, when the steering speed increases and the ripple frequency is between the first boundary frequency and the second boundary frequency in FIG. 6, the control constant changing unit 220 sets the control constant of the compensation calculating unit 9 to the above-described control constant. Set as intermediate emphasis setting. That is, as the steering speed increases so that the ripple frequency becomes higher than the first boundary frequency, the pole frequency ωl is decreased to a standard setting value. At the same time, the control constant of the compensation calculation unit 9 is set so that b1 in (Equation 17) increases. Specifically, the proportional gain Kp in the proportional compensation unit 103 is increased. That is, the ratio of the first-order coefficient b1 to the second-order coefficient b2 in the second-order polynomial having two zeros (intermediate zeros) corresponding to the zero-point frequency as roots is increased. Here, in the first embodiment, the low-frequency gain Kl1 in the low-frequency amplification unit 102 is changed at the same time so that the steady gain of the transfer function C (s) of the compensation calculation unit 9 does not change. 2, as shown in (Equation 17) and (Equation 18), the steady gain g0 of the transfer function C (s) of the compensation calculation unit 9 is determined only by the low-frequency gain Kl2 in the low-frequency amplification unit 202. Therefore, even if the proportional gain Kp is changed, it is not necessary to change the low-frequency gain Kl2 of the low-frequency amplification unit 202.

Next, when the steering speed is further increased and the ripple frequency is higher than the second boundary frequency in FIG. 5, the control constant changing unit 220 sets the control constant of the compensation calculating unit 9 as the above-described standard setting. . That is, the control constant of the compensation calculation unit 9 is set so that b1 in (Expression 17) is smaller than when the ripple frequency is between the first boundary frequency and the second boundary frequency. Specifically, the proportional gain Kp in the proportional compensation unit 103 is reduced.
In other words, the zero characteristic is changed so as to reduce the ratio of the first-order coefficient b1 to the second-order coefficient b2 in the second-order polynomial having two zeros (intermediate zeros) corresponding to the zero-point frequency as roots.

  In the second embodiment, as in the first embodiment, the control constants ωl and Kp are continuously applied when the ripple frequency changes before and after the first boundary frequency and the second boundary frequency. You may change gradually so that it may change. Further, the control constant may be changed discontinuously when the ripple frequency exceeds the first boundary frequency and the second boundary frequency. However, when Kp is changed discontinuously, the current command Ir changes sharply. A shock will occur. In order to prevent this, when the value of the state command of the low-pass filter 201 in the compensation calculation unit 9 is changed discontinuously so that the current command Ir does not change suddenly at the time of switching Kp, the current command Ir changes sharply. The control constant can be switched without adversely affecting the steering feeling.

  Since the second embodiment is configured as described above, as in the first embodiment, the influence of disturbances such as torque ripple that varies according to the steering speed on the steering wheel is reduced according to the steering speed. Therefore, it is possible to improve the steering feeling according to the steering speed.

  As described above, the electric power steering control device according to the present invention is suitable for application to an electric power steering control device that assists the steering force by the motor.

It is a figure which shows the structure of the electric power steering control apparatus by Embodiment 1. FIG. FIG. 3 is a block diagram showing a configuration of a control unit in the electric power steering control device according to Embodiment 1. FIG. 3 is a block diagram illustrating dynamic characteristics of the electric power steering control device according to the first embodiment. It is a figure showing the frequency response of the control unit in the electric power steering control device by Embodiment 1. It is a figure showing the frequency response of the open loop transfer function using the electric power steering control apparatus by Embodiment 1. FIG. It is a figure showing the frequency response gain of the steering sensitivity using the electric power steering control device by Embodiment 1. FIG. 6 is a block diagram showing a configuration of a control unit in an electric power steering control device according to Embodiment 2.

DESCRIPTION OF SYMBOLS 1 Steering wheel 2 Steering shaft 3 Steering torque detector 4 Motor 5 Deceleration mechanism 6 Steering wheel 7 Steering speed detection part 8 Control unit 9 Compensation calculating part 10 Current control part 11 Angle difference part 12 Elastic constant part 13 Torque constant part 14 Torque addition 15 Steering mechanism inertia part 16 Control object 101 Low-pass filter 102 Low-frequency amplifier 103 Proportional compensation part 104 Differential compensation part 105 Adder 106 Noise filter 120 Control constant changing part 201 Low-pass filter 202 Low-frequency amplifying part 220 Control constant change Part

Claims (6)

In the electric power steering control device that assists the steering torque applied by the driver with the motor,
A steering torque detector for detecting steering torque;
A steering speed detector for detecting the steering speed;
A low-frequency pole that is at least one pole and an intermediate zero that is at least two zeros, the gain is flat in a low-frequency region below a pole frequency corresponding to the absolute value of the low-frequency pole, The gain decreases as the frequency increases in the middle frequency range up to the zero frequency corresponding to the geometric mean of the two intermediate zeros, and the frequency increases in the high frequency range up to at least three times the zero frequency and the zero frequency. A compensation calculation unit that outputs a current command by performing a calculation of a transfer function having a frequency response characteristic in which the gain increases with respect to the steering torque detected by the steering torque detector;
A transfer function changing unit that changes the characteristics of the low-frequency pole or the two intermediate zeros according to the steering speed;
A current control unit that controls the motor so that the current of the motor matches the current command;
An electric power steering control device comprising:   The electric power steering control device according to claim 1, wherein the transfer function changing unit changes the low-frequency pole so that the pole frequency decreases as the steering speed increases.   The transfer function changing unit changes the two intermediate points by changing the ratio of the first order coefficient to the second order coefficient in a second order polynomial rooted at the two intermediate zeros as the steering speed increases. The electric power steering control device according to claim 1, wherein the zero point characteristic is changed.   The transfer function changing unit changes the low-frequency pole so that the pole frequency decreases with an increase in the steering speed, and a quadratic coefficient second-order coefficient in a second-order polynomial having the intermediate zero as a root. The electric power steering control device according to claim 1, wherein the intermediate zero point is changed by changing the ratio so as to increase the ratio to the coefficient.   The transfer function is configured such that the low-frequency pole and the two intermediate zeros can be changed based on a control constant, and the transfer function changing unit changes the low-frequency pole or the two two by changing the control constant. The electric power steering control device according to any one of claims 1 to 4, wherein a characteristic of the intermediate zero point is changed.   6. The electric motor according to claim 5, wherein the transfer function changing unit changes the control constant discontinuously and changes the state variable of the compensation operation unit discontinuously so that the current command does not change suddenly. Power steering control device.

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