ELECTRIC POWER STEERING APPARATUS

Abstract

An electric power steering apparatus that includes a function to perform a handle-returning control being consistent with an intention of a driver by using, for a calculation of a target steering angular velocity, only a steering input due to the intention of the driver which is included in a steering torque and the assist current or only vehicle motion characteristics based on the steering input, in the handle-returning control which performs a PID-control depending on a deviation between the target steering angular velocity and a steering angular velocity.

Claims

1-3. (canceled)

4. An electric power steering apparatus that calculates a current command value based on at least a steering torque, drives a motor based on said current command value and assist-controls a steering system by means of driving-control of said motor, comprising: a handle-returning control section which calculates a target steering angular velocity for a handle-returning by using said steering torque, said current command value, a vehicle speed and a steering angle, and calculates a handle-returning control current based on a deviation between a steering angular velocity and said target steering angular velocity; and interposes a filter, which attenuates frequency components which are equal to or higher than a steering input or which are equal to or higher than vehicle motion characteristics based on said steering input, in a calculation path of said target steering angular velocity, wherein said motor is driven by correcting said current command value by means of said handle-returning control current.

5. The electric power steering apparatus according to claim 4, wherein said handle-returning control section comprises: a target returning velocity calculating section to calculate a target returning velocity based on said steering angle and said vehicle speed; a steering torque gain section to calculate a steering torque gain based on said steering torque; a viscosity coefficient outputting section to calculate a viscosity coefficient of said steering system based on said vehicle speed; a vehicle speed gain section to calculate a vehicle speed gain based on said vehicle speed; a first steering system characteristic section to calculate a first target velocity value from an adding value of said steering torque and an assist torque which is a value being multiplied said current command value by a gain and said viscosity coefficient; said filter to filter-process said first target velocity value; a second steering system characteristic section to input a third target velocity value which is obtained by correcting said target returning velocity with a second target velocity value from said filter, and to calculate said target steering angular velocity from said viscosity coefficient and an inertia moment of said steering system; a handle-returning control gain calculating section to calculate a handle-returning control gain by multiplying a deviation between said target steering angular velocity and said steering angular velocity with said vehicle speed gain and said steering torque gain; and a handle-returning control current calculating section to calculate a handle-returning control current by performing at least one control calculation of a proportional control calculation, an integral control calculation and a differential control calculation to said handle-returning control gain, and output-limiting said calculated handle-returning control gain by means of said vehicle speed gain and said steering torque gain.

6. The electric power steering apparatus according to claim 4, wherein said filter is a low pass filter and a filter characteristic has an attenuation which is equal to or more than 3 dB at 10 Hz.

7. The electric power steering apparatus according to claim 5, wherein said filter is a low pass filter and a filter characteristic has an attenuation which is equal to or more than 3 dB at 10 Hz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the accompanying drawings:

[0018] FIG. 1 is a configuration diagram showing a general outline of an electric power steering apparatus;

[0019] FIG. 2 is a block diagram showing a configuration example of an electric power steering apparatus;

[0020] FIG. 3 is a block diagram showing a configuration example of the present invention;

[0021] FIG. 4 is a characteristic diagram showing an output gain example of a steering torque gain section;

[0022] FIG. 5A and FIG. 5B are characteristic diagrams showing output examples of a target returning velocity calculating section;

[0023] FIG. 6 is a characteristic diagram showing an output example of the vehicle speed gain section;

[0024] FIG. 7 is a characteristic diagram showing an output example of a viscosity coefficient outputting section;

[0025] FIG. 8 is a part of a flowchart showing an operation example of the present invention;

[0026] FIG. 9 is another part of a flowchart showing an operation example of the present invention; and

[0027] FIG. 10 is a gain diagram showing a characteristic example of a filter.

MODE FOR CARRYING OUT THE INVENTION

[0028] In an electric power steering apparatus, an operation is prevented by friction of reduction gears and a rack-and-pinion for transferring an assist torque, a handle does not return to a neutral position in spite of a running state to be returned to a straight running state and then it can be difficult for a vehicle to become the straight running state. In this connection, by correcting (compensating) a current command value by means of a handle-returning control current depending on the steering angle and the vehicle speed, it is possible to actively return the handle to the neutral position when the running state is returned to the straight running state.

[0029] The present invention defines a target returning velocity depending on a steering angle and a vehicle speed, adds a target velocity value calculated from a steering torque being applied to a column shaft and an assist torque (a current command value) to the target returning velocity, and calculates a target steering angular velocity by multiplying the added result with a transfer characteristic depending on a virtual steering system characteristic. The present invention further performs at least one control of a proportional control (P-control), an integral control (I-control) and a differential control (D-control) against a deviation between the target steering angular velocity and an actual steering angular velocity. By performing a feedback control with the target steering angular velocity that is calculated by correcting the target returning velocity by using the target velocity value calculated by dividing the steering torque and the assist torque (the current command value) with a viscosity coefficient, it is possible to realize a handle-returning control that a driver feels natural feeling even at a time of a steering intervention by the driver.

[0030] The present invention relates to the electric power steering apparatus (EPS) that calculates an assist torque (a steering shaft torque) by using the steering torque and the current command value (an assist current) and applies a handle-returning control current due to a deviation between the target steering angular velocity and the actual steering angular velocity. The present invention cuts noise components which the EPS has and a vibration due to a resonance, unnecessary road noise components, a torque variation which the driver does not intend, a vehicle variation and the like, and provides a smoother handle-returning performance by interposing a low pass filter (LPF) in a path which the target steering angular velocity is calculated from the assist torque (the steering shaft torque) so as to attenuate frequencies (10 [Hz]) which are equal to or higher than a steering input which the driver mainly intends, or which are equal to or higher than vehicle motion characteristics based on the steering input.

[0031] A simple virtual vehicle model in the present invention is a model that calculates the target steering angular velocity .sub.0 by applying a steering system transfer function depending on a virtual inertia moment J and a virtual viscosity coefficient C of the steering system to a sum of the target returning velocity t (t), which is calculated from the steering angle and the vehicle speed V, the steering torque Td and the assist torque Ta.

[0032] Since the virtual inertia moment J and the virtual viscosity coefficient C of the steering system can be set by using the virtual vehicle model, it is possible to determine vehicle characteristics, arbitrarily. Further, since the steering intervention by the driver, which is also taken into account for the assist torque Ta in the virtual vehicle model, is considered, the smooth handle-returning can be provided even in a state that the driver steers the handle.

[0033] Here, assuming that static friction, Coulomb friction and an elastic term are not existed in the steering system, an Equation of a force balance among a self-aligning torque SAT, the steering torque Td and the assist torque Ta is represented by a below Equation 1.

[00001]$\begin{array}{cc}\mathrm{SAT}+T_d+T_a=J.Math.\frac{.Math.d^2.Math.}{{\mathrm{dt}}^2}+C.Math..Math.\frac{d.Math..Math.}{\mathrm{dt}}& \left[\mathrm{Equation}.Math..Math.1\right]\end{array}$ [0034] Here, J is the inertia moment of the virtual steering system and C is the viscosity coefficient of the virtual steering system.

[0035] Since the actual steering angular velocity is a time differential of the steering angle , a following Equation 2 is satisfied.

=d/dt[Equation 2]

Then, the target steering angular velocity is set to .sub.0.

[00002]$\begin{array}{cc}\mathrm{SAT}+T_d+T_a=J.Math.\frac{.Math.d.Math..Math.{}_0}{\mathrm{dt}}+C.Math..Math.{}_0& \left[\mathrm{Equation}.Math..Math.3\right]\end{array}$

[0036] The above Equation 3 is satisfied, s is set to Laplace operator and a following Equation 4 is obtained. By rearranging the Equation 4, a following Equation 5 is obtained.

SAT+T.sub.d+T.sub.a=sJ.sub.0+C.sub.0[Equation 4]

SAT+T.sub.d+T.sub.a=(sJ+C).sub.0[Equation 5]

Then, the target steering angular velocity .sub.0 is represented by the above Equation 5.

[00003]$\begin{array}{cc}{}_0=\frac{\mathrm{SAT}+T_d+T_a}{\mathrm{sJ}+C}& \left[\mathrm{Equation}.Math..Math.6\right]\end{array}$

By rearranging the Equation 6, a following Equation 7 is obtained.

[00004]$\begin{array}{cc}{}_0=\frac{1}{\frac{J}{C}.Math.s+1}.Math.\left(\frac{\mathrm{SAT}}{C}+\frac{T_d+T_a}{C}\right)& \left[\mathrm{Equation}.Math..Math.7\right]\end{array}$

[0037] The target steering angular velocity .sub.0 is calculated by the above Equation 7. Here, SAT/C is the steering angular velocity generated by the self-aligning torque SAT, and it can be considered that SAT/C is set as returning steering angular velocity depending on the vehicle characteristics.

[00005]$\begin{array}{cc}\frac{1}{\frac{J}{C}.Math.s+1}& \left[\mathrm{Equation}.Math..Math.8\right]\end{array}$

An Equation 8 represents the transfer characteristic calculated from the virtual vehicle model.

[00006]$\begin{array}{cc}\frac{T_d+T_a}{C}& \left[\mathrm{Equation}.Math..Math.9\right]\end{array}$

[0038] An Equation 9 represents the steering angular velocity generated by the steering torque Td and the assist torque Ta.

[0039] Since the self-aligning torque SAT is generally determined based on the steering angle and the vehicle speed V, the returning steering angular velocity can be set depending on the vehicle speed V and the steering angle . Further, the steering torque Td can be detected by a torque sensor and the assist torque Ta can be calculated by considering a motor torque constant Kt from the current command value. In this connection, the steering angular velocity, which is generated due to the steering torque Td and the assist torque Ta, is calculated by dividing a sum of the steering torque Td and the assist torque Ta by the viscosity coefficient C of the virtual steering. By adding a term of SAT to a term of the steering angular velocity, the target steering angular velocity .sub.0 is obtained.

[0040] Although the steering torque Td and the assist torque Ta include variation components and so on due to the road surface disturbance, these are not derived from the intention of the driver. When these are reflected on the target steering angular velocity .sub.0, the behavior of the vehicle can be out of harmony with the intention of the driver and the driver may feel uncomfortable.

[0041] Accordingly, in the present invention, a filter (LPF), which attenuates frequency components which are higher than a steering input which the driver intends, or which are higher than vehicle motion characteristics (yaw, roll and the like) based on the steering input, is provided at a rear stage of the target velocity value .sub.1 calculated from the steering torque Td and the assist torque Ta, and thereby a stable control, a smooth returning and a steering feeling consistent with the intention of the driver are realized. Generally, since it is considered that a steering frequency of the driver and the vehicle motion by the steering of the driver are up to about 10 [Hz], the filter has an attenuation characteristic that reduces 3 [dB] or more from the gain 0 at 10 [Hz] as a filter characteristic.

[0042] Embodiments according to the present invention will be described with reference to the drawings.

[0043] FIG. 3 shows a configuration example of a handle-returning control section 100 according to the present invention. The steering torque Td is inputted into a steering torque gain section 110 that outputs a steering torque gain Th and an adding section 102, and the steering angle is inputted into a target returning velocity calculating section 120 that calculates the target returning velocity t. Further, the vehicle speed V is inputted into the target returning velocity calculating section 120, a vehicle speed gain section 130 that outputs a vehicle speed gain KP and a viscosity coefficient outputting section 133 that outputs the viscosity coefficient C. The actual steering angular velocity A is subtracting-inputted into a subtracting section 103. The current command value Iref is gain-Kt-multiplied at a gain section 111, and an output being a multiplied result is inputted into the adding section 102 as the assist torque Ta. Accordingly, an adding result of the adding section 102 is a sum of the steering torque Td and the assist torque Ta, and the sum value is inputted into a steering system characteristic section 150 that has a transfer function 1/C. The target velocity value .sub.1 from the steering system characteristic section 150 is inputted into the low pass filter (LPF) 151, and the frequencies (10 [Hz]) which are equal to or higher than the steering input of the driver, or which are equal to or higher than the vehicle motion characteristics based on the steering input are attenuated at the LPF 151. Then, the target velocity value .sub.2 whose frequencies are attenuated is inputted into an adding section 101.

[0044] The target returning velocity t that is calculated at the target returning velocity calculating section 120 based on the steering angle and the vehicle speed V, is inverted (t) a sign at an inverting section 121 and an inverted target returning velocity t is inputted into an adding section 101. A target velocity value .sub.3 that is an added result at the adding section 101 is inputted into a steering system characteristic section 160 that has a transfer function 1/(J/Cs+1). The viscosity coefficient C from the viscosity coefficient outputting section 133 is inputted into the steering system characteristic sections 150 and 160. The steering system characteristic section 160 determines the transfer function from the inertia moment J and the viscosity coefficient C in accordance with the above Equation 6, and the target steering angular velocity .sub.0 is calculated by multiplying the target steering angular velocity .sub.3 with the transfer function and is outputted from the steering system characteristic section 160. The target steering angular velocity .sub.0 is adding-inputted into the subtracting section 103. The steering angular velocity is subtracting-inputted into the subtracting section 103. The deviation SG1 between the target steering angular velocity .sub.0 and the steering angular velocity A is calculated at the subtracting section 103 and is inputted into a multiplying section 132.

[0045] Furthermore, the steering torque gain Th that is outputted from the steering torque gain section 110 is inputted into the multiplying section 132 and a limiter 142, and the vehicle speed gain KP from the vehicle speed gain section 130 is also inputted into the multiplying section 132 and the limiter 142.

[0046] A handle-returning control gain SG2 (a proportional control value) from the multiplying section 132, which is calculated by multiplying the deviation SG1 with the steering torque gain Th and the vehicle speed gain KP, is inputted into an adding section 104 and an integral control section comprising an integral section 140 for a characteristic improvement and an integral gain section 141, and is further inputted into the limiter 142 via the integral gain section 141. A signal SG4 whose output is limited depending on the steering torque gain Th and the vehicle speed gain KP at the limiter 142, is added to the handle-returning control gain SG2 at the adding section 104, and is outputted as the handle-returning control current HR. The integral at the integral section 140 compensates a low steering torque range that is easily influenced by the friction, and especially the integral is effective in the range that is largely affected by the friction in hands off state. The current command value Iref is added to the handle-returning control current HR at an adding section 105 and is corrected (compensated), and a corrected compensation current command value Irefn is inputted into a motor driving system.

[0047] A handle-returning control gain calculating section comprises the subtracting section 103 and the multiplying section 132, a steering system characteristic section comprises the viscosity coefficient outputting section 133, the steering system characteristic section 1 (150) and the steering system characteristic section 2 (160), and a handle-returning control current calculating section comprises the integral section 140, the integral gain section 141, the limiter 142 and the adding section 104.

[0048] The steering torque gain section 110 has a characteristic as shown in FIG. 4, outputs a constant value gain Th1 when the steering torque Td is from zero to T1, and has an output characteristic that the gain gradually decreases when the steering torque Td is larger than T1 and the gain is zero when the steering torque Td is equal to or larger than T2. In FIG. 4, although the steering torque gain Th linearly decreases, the gain may nonlinearly decrease.

[0049] Further, the target returning velocity calculating section, in which the vehicle speed V serves as a parameter, has an output characteristic that the target returning velocity t gradually increases when the steering angle is larger, as shown in FIG. 5A. As shown in FIG. 5B, the target returning velocity t varies with the output characteristic that the target returning velocity t does not gradually increase when the vehicle speed V is higher.

[0050] The vehicle speed gain section 130 has a characteristic as shown in FIG. 6 that the gain KP is a small constant gain KP1 when the vehicle speed V is from zero to at least V1, gradually increases when the vehicle speed V is equal to or higher than V1, and is a large constant gain KP2 when the vehicle speed V is equal to or higher than V2. However, the characteristic of the gain KP is not limited to such a characteristic.

[0051] Furthermore, the viscosity coefficient outputting section 133, in which the viscosity coefficient C depending on the vehicle speed V is changeable, has a characteristic as shown in FIG. 7 that the viscosity coefficient C is a small constant viscosity coefficient C1 when the vehicle speed V is from zero to at least V3, gradually increases when the vehicle speed V is equal to or higher than V3 and is equal to or slower than V4 (>V3), and is a large constant viscosity coefficient C2 when the vehicle speed V is equal to or higher than V4. However, the characteristic of the viscosity coefficient C is not also limited to such a characteristic.

[0052] In such a configuration, the operation example will be described with reference to flowcharts of FIG. 8 and FIG. 9.

[0053] At first, the steering torque Td, the current command value Iref, the vehicle speed V, the steering angle and the steering angular velocity , are inputted (read) (Step S1), and the steering torque gain section 110 outputs the steering torque gain Th (Step S2). The gain section 111 calculates the assist torque Ta by multiplying the current command value Iref with the gain Kt (Step S3) and the steering torque Td is added to the assist torque Ta at the adding section 102. The sum value is inputted into the steering system characteristic section 1 (150) (Step S4).

[0054] Further, the target returning velocity calculating section 120 calculates the target returning velocity t based on the inputted steering angle and vehicle speed V (Step S10), the inverting section 121 performs a sign inversion of the target returning velocity et (Step S11) and the inverted target returning velocity t is inputted into the adding section 101 and is added to the target velocity value .sub.2. The vehicle speed gain section 130 outputs the vehicle speed gain KP in accordance with the vehicle speed V (Step S12) and the viscosity coefficient outputting section 133 outputs the viscosity coefficient C in accordance with the vehicle speed V (Step S13). The viscosity coefficient C is inputted into the steering system characteristic section 150 (Step S14), the above sum value is multiplied with the transfer characteristic 1/C and the multiplied result is outputted as the target velocity value .sub.1 (Step S15). The target velocity value .sub.1 is inputted into the LPF 151 and is filter-processed (Step S16).

[0055] The target velocity value .sub.2 that is filter-processed at the LPF 151 is added to the target returning velocity t at the adding section 101 and the added target velocity value .sub.3 is inputted into the steering system characteristic section 160 (Step S30). The target steering angular velocity .sub.0 from the steering system characteristic section 160 is adding-inputted into the subtracting section 103, the actual steering angular velocity is subtracting-inputted into the subtracting section 103 and the deviation SG1 is calculated at the subtracting section 103 (Step S31). The deviation SG1 is inputted into the multiplying section 132 and is multiplied with the steering torque gain Th and the vehicle speed gain KP (Step S32). The handle-returning control gain SG2 is calculated by the above multiplication. The handle-returning control gain SG2 is integral-processed at the integral section 140 (Step S33) and further is proportional-processed at the integral gain section 141 (Step S34), and then a handle-returning control gain SG3 is outputted. The handle-returning control gain SG3 is inputted into the limiter 142 and is limiting-processed at the limiter 142 by using the steering torque gain Th and the vehicle speed gain KP (Step S35).

[0056] A handle-returning control gain SG4 that is limiting-processed at the limiter 142 is inputted into the adding section 104 and is added to the handle-returning control gain SG2 (Step S36), and then the handle-returning control current HR is outputted. The handle-returning control current HR is added to the current command value Iref at the adding section 105 and is corrected (Step S37), and then the compensation current command value Irefn is outputted (Step S38).

[0057] It is also possible to calculate the steering angular velocity by multiplying a motor angular velocity with a gear ratio, and the transfer characteristic of the virtual steering system model may be changeable depending on the vehicle speed, the steering angle, steering-forward, steering-backward and a steering holding state. Orders of data inputting, calculations and processes in FIG. 8 and FIG. 9 are appropriately changeable.

[0058] A frequency characteristic of the LPF 151 which is used in the present invention may has an attenuation of 3 [dB] or more at 10 [Hz], as shown in FIG. 10. The frequency characteristic of the LPF 151 is not limited to a characteristic A, and may be a characteristic B or C.

EXPLANATION OF REFERENCE NUMERALS

[0059] 1 handle [0060] 2 column shaft (steering shaft, handle shaft) [0061] 10 torque sensor [0062] 12 vehicle speed sensor [0063] 14 steering angle sensor [0064] 20 motor [0065] 30 control unit (ECU) [0066] 31 current command value calculating section [0067] 33 current limiting section [0068] 34 compensation signal generating section [0069] 35 PI-control section [0070] 36 PWM-control section [0071] 37 inverter [0072] 50 CAN [0073] 100 handle-returning control section [0074] 110 steering torque gain section [0075] 111 gain section [0076] 120 target returning velocity calculating section [0077] 121 inverting section [0078] 130 vehicle speed gain section [0079] 133 viscosity coefficient outputting section [0080] 140 integral section [0081] 141 integral gain section [0082] 142 limiter [0083] 150 steering system characteristic section (1/C) [0084] 151 low pass filter (LPF) [0085] 160 steering system characteristic section (1/(J/Cs+1))