Speed-following power-assisted control method and device for electric power steering system and medium

Abstract

The invention discloses a speed-dependent power-assisted control method and device of an electric power steering system and a medium. The method comprises the following steps: acquiring vehicle frequency sweep experiment data; processing the frequency sweep experiment data; frequency response characteristics from steering wheel torque to a motor torque instruction and from motor electromagnetic torque to the motor torque instruction are obtained; establishing a stability controller optimization model, and calculating stability controller parameters at zero vehicle speed; the damping ratio is adjusted, and stability controller parameters under other typical vehicle speeds are calculated; determining a speed-dependent assistance mapping relation; calculating an amplitude margin and a phase margin of the electric power steering system in a main vehicle speed range based on the speed-following power-assisted mapping relation and parameters of a stability controller; and when the amplitude margin and the phase margin meet the minimum threshold requirement, finishing controller design. According to the stability controller, the stability of the electric power steering system can be guaranteed, and meanwhile the expected steering hand feeling defined by the speed-following power-assisted mapping relation can be kept as much as possible.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power steering system control, and particularly to a speed-dependent assist control method, device, and medium for an electric power steering system. Background Art

[0002] An electric power steering (EPS) system is a steering system that provides auxiliary torque through an electric motor, and has advantages such as energy conservation, environmental protection, and easy layout. Against the backdrop of the rapid development of intelligent vehicle technology, the EPS system can effectively meet the requirements of advanced driver assistance systems (ADAS) and support the implementation of advanced functions such as automatic parking, lane keeping, and autonomous driving. Speed-dependent assist is one of the core functions of the EPS system, providing a light steering feel for the driver under low-speed conditions, while ensuring steering stability and reliability under high-speed conditions.

[0003] A high level of steering assist is the main cause of system instability, which may lead to steering wheel jitter, thereby affecting the steering feel and comfort. Moreover, when the system is extremely unstable, it may endanger steering safety. Therefore, designing a stability controller is the primary task in the development of the speed-dependent assist function of the EPS system.

[0004] In the "Control Method, Device, Equipment, and Storage Medium for an Electric Power Steering System" disclosed in Chinese Invention Patent Publication CN114194287A, this solution improves the amplitude margin and phase margin of the electric power steering system through a stability controller, enhancing the system's stability. However, this solution does not consider the impact of the phase lag and amplitude reduction introduced by the stability controller on the steering feel defined by the speed-dependent assist mapping relationship. This defect may cause the driver to significantly perceive the assist lag or insufficient assist phenomenon under low-frequency steering conditions, thereby affecting the driver's control experience. Summary of the Invention

[0005] In order to ensure the stability of the electric power steering system while reducing the impact of the stability controller on the steering feel defined by the speed-dependent assist mapping relationship and maintaining the desired steering feel, the present invention provides a speed-dependent assist control method, device, and medium for an electric power steering system.

[0006] To achieve the object of the present invention, a speed-dependent assist control method for an electric power steering system provided by the present invention includes the following steps:

[0007] (1) Obtain vehicle sweep test data;

[0008] (2) Preprocess the sweep test data to obtain the time-domain data of the steering wheel torque and the motor electromagnetic torque;

[0009] (3) Based on the time-domain data of the steering wheel torque and the motor electromagnetic torque, obtain the frequency response characteristics from the steering wheel torque to the motor torque command and from the motor electromagnetic torque to the motor torque command;

[0010] (4) Based on the frequency response characteristics of the electric power steering system from the steering wheel torque to the motor torque command, establish an optimization model for the stability controller and calculate the parameters of the stability controller at zero vehicle speed;

[0011] (5) Based on the parameters of the zero-speed stability controller, calculate the parameters of the stability controller at other typical vehicle speeds by adjusting the damping ratio;

[0012] (6) Determine the speed-dependent assist mapping relationship, which is used to describe the corresponding relationship between the vehicle speed, the steering wheel torque, and the motor assist torque;

[0013] (7) Based on the speed-dependent assist mapping relationship and the parameters of the stability controller, calculate the gain margin and phase margin of the system within the main vehicle speed range; when the gain margin and phase margin meet the minimum threshold requirements, complete the controller design; otherwise, return to step (4) and iterate cyclically until the gain margin and phase margin reach the minimum threshold requirements.

[0014] Further, in step (1), place the vehicle on a dry cement road surface or asphalt road surface, adjust the tire pressure to within the standard tire pressure range, and keep the steering wheel in a free state without torque input; excite the system by inputting sinusoidal signals with different frequencies through the assist motor, and measure the motor electromagnetic torque and the steering wheel torque as the system output variables.

[0015] Further, in step (1), in the frequency sweep experiment, the frequency of the motor torque command is 1, 2... 60 Hz or higher to obtain the system frequency response characteristics in a wider frequency band.

[0016] Further, in step (1), the motor electromagnetic torque is obtained by converting the measured value of the assist motor current; the steering wheel torque is measured by a torque sensor installed on the steering column, and the measured value should be close to but not exceed the measurement upper limit of the sensor to reduce high-frequency noise interference and improve the accuracy of the subsequent obtained system frequency response characteristics.

[0017] Further, in step (2), the preprocessing includes processing such as linear trend removal, zero drift correction, filtering, outlier rejection, and data dimensionality reduction.

[0018] Further, in step (2), the accuracy of the experimental data is consistent with the call period of the stability controller.

[0019] Further, in step (3), the frequency response characteristics of the electric power steering system are obtained by calculating the Fourier transform of the time-domain data.

[0020] Furthermore, in step (4), the stability controller adopted is a third-order lead-lag compensator, and its expression is:

[0021]

[0022] where p 1 、p 2 、p 3 are the first, second, and third-order poles respectively, ω n is the undamped natural frequency, and ζ is the damping ratio;

[0023] The open-loop transfer function expression of the electric power steering system is as follows:

[0024]

[0025] where T h is the steering wheel torque, T e * is the motor torque command, and K gain is the motor assistance gain;

[0026] To ensure the system stability, the designed stability controller needs to make the gain margin and phase margin of the open-loop transfer function of the electric power steering system greater than zero, and introduce a sufficiently deep notch near the system resonance frequency to suppress resonance; to ensure the steering feel, the stability controller should not introduce a large phase lag to avoid a sense of hysteresis in the steering assistance; nor should it introduce a large amplitude reduction in the low-frequency band to reduce the impact on the steering feel defined by the speed-dependent assistance mapping relationship; to determine the parameters of the stability controller that meet these constraint conditions, an optimization model based on the minimization of the objective function is established:

[0027]

[0028] where a i (i = 1, 2... 7) are the weight factors of each optimization objective, t j (j = 1, 2... 7) are the target values of each optimization objective, GM open is the amplitude margin of the system, PM open is the phase margin of the system, and f n is a specific frequency near the system resonance frequency.

[0029] The compensator parameters have different effects on the frequency response characteristics of the system; the first-order pole determines the overall response attenuation performance of the compensator and the introduced phase lag. When its value decreases, the response attenuation effect is enhanced, but the introduced phase lag also increases; the second-order pole and the third-order pole affect the phase lead performance of the compensator in the high-frequency band. As the values ​​of the second-order and third-order poles increase, the phase margin of the system is improved, but the suppression effect of high-frequency noise is weakened; the undamped oscillation frequency determines the notch position of the compensator, which is usually set near the system resonance frequency; the damping ratio affects the notch depth. The smaller the damping ratio, the greater the phase lag introduced in the low-frequency band. Its value needs to be determined in combination with the resonance peak of the system's frequency response characteristics from the steering wheel torque to the motor torque command; the upper and lower limits of the compensator parameters should be adjusted according to the frequency response characteristics of the system, and finally the stability controller parameters at zero vehicle speed are calculated through the optimization algorithm.

[0030] Furthermore, in step (5), the increment of the damping ratio is determined based on the resonance peak of the system's frequency response characteristic from the steering wheel torque to the motor torque command.

[0031] Furthermore, in step (7), the minimum thresholds of the system amplitude margin and phase margin are determined according to the system stability requirements, and the stability controller parameters at other vehicle speeds are calculated by linear interpolation.

[0032] The present invention also provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the aforementioned speed-dependent power steering control method for an electric power steering system when executing the computer program.

[0033] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the speed-dependent power-assistance control method of the electric power steering system is implemented.

[0034] Compared with the prior art, the present invention has at least the following beneficial effects:

[0035] 1) When obtaining the frequency response characteristics of the system from steering wheel torque to motor torque command, the accuracy of the data can be effectively verified through the frequency response characteristics of the motor electromagnetic torque to the motor torque command.

[0036] 2) The stability controller used can maintain the desired steering feel defined by the speed-dependent power assist mapping relationship as much as possible while ensuring system stability.

[0037] 3) The proposed speed-dependent assist control method is applicable to electric power steering systems with various mechanical structures, and can flexibly achieve different steering feel requirements by adjusting the stability controller optimization model, thereby improving the steering stability and comfort of the electric power steering system under various vehicle speed conditions. Brief Description of the Drawings

[0038] Figure 1 It is a flowchart of a speed-dependent assist control method for an electric power steering system provided by an embodiment of the present invention.

[0039] Figure 2 It is a schematic diagram of the frequency change mode of the motor torque command in the sweep frequency experiment in an embodiment of the present invention.

[0040] Figure 3(a) is a schematic diagram of the motor electromagnetic torque after processing the sweep frequency experiment data in an embodiment of the present invention.

[0041] Figure 3(b) is a schematic diagram of the steering wheel torque after processing the sweep frequency experiment data in an embodiment of the present invention.

[0042] Figure 4(a) is a schematic diagram of the amplitude-frequency characteristic of the system from the motor electromagnetic torque to the motor torque command in an embodiment of the present invention.

[0043] Figure 4(b) is a schematic diagram of the amplitude-frequency characteristic of the system from the steering wheel torque to the motor torque command in an embodiment of the present invention.

[0044] Figure 4(c) is a schematic diagram of the phase-frequency characteristic of the system from the motor electromagnetic torque to the motor torque command in an embodiment of the present invention.

[0045] Figure 4(d) is a schematic diagram of the phase-frequency characteristic of the system from the steering wheel torque to the motor torque command in an embodiment of the present invention.

[0046] Figure 5 It is a control block diagram of the speed-dependent assist control of the system in an embodiment of the present invention.

[0047] Figure 6(a) is a schematic diagram of the amplitude-frequency characteristic of the stability controller at zero vehicle speed and other typical vehicle speeds in an embodiment of the present invention.

[0048] Figure 6(b) is a schematic diagram of the phase-frequency characteristic of the stability controller at zero vehicle speed and other typical vehicle speeds in an embodiment of the present invention.

[0049] Figure 7 It is a schematic diagram of the speed-dependent assist mapping relationship determined in an embodiment of the present invention.

[0050] Figure 8(a) is a schematic diagram of the amplitude margin of the system at a vehicle speed of 0 - 200 km / h in an embodiment of the present invention.

[0051] Figure 8(b) is a schematic diagram of the phase margin of the system at a vehicle speed of 0 - 200 km / h in an embodiment of the present invention. Detailed implementation manners

[0052] To make the objectives, technical solutions and advantages of the present invention clearer and more definite, the following further describes the present invention in detail with reference to the accompanying drawings and by way of examples.

[0053] As Figure 1 shown, a speed-dependent assist control method for an electric power steering system provided by the present invention includes the following steps:

[0054] Step 1: Obtain vehicle swept-frequency experiment data.

[0055] In this step, the vehicle is placed on a dry cement road surface or asphalt road surface, the tire pressure is adjusted within the standard tire pressure range, and the steering wheel is kept in a free state without torque input; the electric power steering system is excited by inputting sinusoidal signals with different frequencies through the assist motor, and the motor electromagnetic torque and the steering wheel torque are measured as the output variables of the electric power steering system.

[0056] In the swept-frequency experiment, the frequency of the motor torque command is 1, 2... 60 Hz or higher to obtain the system frequency response characteristics in a wider frequency band. In some embodiments of the present invention, in the vehicle swept-frequency experiment of step 1, a motor torque command with an amplitude of 0.35 Nm is used, and the motor torque command is the expected output torque received by the assist motor controller; the swept-frequency range is from 1 Hz to 100 Hz, and its variation with time is as Figure 2 shown.

[0057] Among them, the motor electromagnetic torque is the torque actually output by the assist motor and can be obtained by converting the measured current of the assist motor; the steering wheel torque is measured by a torque sensor installed on the steering column, and the measured value is close to but does not exceed the measurement upper limit (10 Nm) of the torque sensor to reduce high-frequency noise interference and improve the calculation accuracy of the subsequent system frequency response characteristics.

[0058] Step 2: Preprocess the swept-frequency experiment data.

[0059] In some embodiments of the present invention, the preprocessing includes linear trend removal, zero-drift correction, filtering, outlier rejection, and data dimensionality reduction, etc.

[0060] The accuracy and stability of the swept-frequency experiment data in step 2 are consistent with the call period of the controller. In some embodiments of the present invention, the call period is set to 2 ms. In some embodiments of the present invention, as shown in FIGS. 3(a) and 3(b), the time-domain data of the steering wheel torque and the motor electromagnetic torque after processing the swept-frequency experiment data are shown for use in this step.

[0061] Step 3: Obtain the frequency response characteristics of the electric power steering system from the steering wheel torque to the motor torque command and from the motor electromagnetic torque to the motor torque command.

[0062] In some embodiments of the present invention, the frequency response characteristics of the electric power steering system from the steering wheel torque to the motor torque command and from the motor electromagnetic torque to the motor torque command are obtained by performing Fourier transform calculations on the time-domain data obtained in Step 2.

[0063] In industrial applications, the output characteristics of the assist motors used in most electric power steering systems are close to the frequency response characteristics of a first-order or second-order low-pass filter; therefore, the frequency response characteristics from the motor electromagnetic torque to the motor torque command obtained by Fourier transform calculations are close to the frequency response characteristics of a first-order or second-order low-pass filter, verifying the reliability of the experimental data.

[0064] In some embodiments of the present invention, Figures 4(a) to 4(d) The obtained system frequency response characteristics include amplitude-frequency characteristics and phase-frequency characteristics.

[0065] Step 4: Based on the frequency response characteristics of the electric power steering system from the steering wheel torque to the motor torque command, establish an optimization model for the stability controller and calculate the stability controller parameters at zero vehicle speed.

[0066] Among them, the stability controller adopted is a third-order lead-lag compensator, and its expression is:

[0067]

[0068] Among them, p 1 、p 2 、p 3 are the first, second, and third-order poles respectively, ω n is the undamped oscillation frequency, ζ is the damping ratio, H con is the transfer function of the stability controller, and s is the Laplace operator.

[0069] The control block diagram of the speed-sensitive assist control is as Figure 5As shown, based on the steering wheel torque measured by the torque sensor and the vehicle speed signal measured by the vehicle speed sensor, combined with the speed-dependent assist mapping relationship, the assist torque of the assist motor at the current moment (i.e., the motor assist gain) is determined; after the output torque of the assist motor is amplified by the worm and worm gear mechanism, a relatively high steering assist torque may cause instability in the electric power steering system, and further lead to steering wheel jitter and uncontrollable states; the stability controller can effectively improve the stability margin of the open-loop transfer function of the electric power steering system, thereby ensuring the stability of the system. Therefore, the motor assist gain needs to be adjusted by the stability controller to generate the final motor torque command. The assist motor outputs the motor electromagnetic torque according to this motor torque command to assist the driver in steering.

[0070] The expression of the open-loop transfer function of the electric power steering system is as follows:

[0071]

[0072] Among them, H open is the open-loop transfer function of the electric power steering system from the steering wheel torque to the motor assist gain; T h is the steering wheel torque, T e * is the motor torque command, K gain is the motor assist gain. In some embodiments of the present invention, the motor assist gain during calculation is set to 1.

[0073] To ensure the stability of the system, the stability controller needs to make the gain margin and phase margin of the open-loop transfer function of the electric power steering system greater than zero, and introduce a sufficiently deep notch near the resonance frequency of the system to suppress resonance; to ensure the steering feel, the stability controller should not introduce a large phase lag to avoid a sense of hysteresis in the steering assist; nor should it introduce a large amplitude reduction in the low-frequency band to reduce the impact on the steering feel defined by the speed-dependent assist mapping relationship; to determine the parameters of the stability controller that meet these constraint conditions, a stability controller optimization model based on minimizing the objective function is established:

[0074]

[0075] Among them, a i (i = 1, 2... 7) are the weight factors of each optimization objective, t j (j = 1, 2... 7) are the target values of each optimization objective, GM open is the amplitude margin of the system, PM open is the phase margin of the system, f n is a specific frequency near the resonance frequency of the system, H con (jw) is the frequency response function of the stability controller, j is the imaginary unit, and w is the angular frequency.

[0076] In some embodiments of the present invention, the weight factor a for each optimization objective is given i , the target value t for each optimization objective j is taken, and a specific frequency f near the system resonance frequency n is taken as 11 Hz, then the optimization model for the stability controller is:

[0077]

[0078] s.t. 55 < ω n < 75, 0.3 < ζ < 0.6, 20 < p 1 < 36

[0079] 1000 < p 2 < 1400, 1500 < p 3 < 1700

[0080] By solving the optimization model of the stability controller through an optimization algorithm, the parameters of the stability controller at zero vehicle speed are obtained. In some embodiments of the present invention, the optimization algorithm uses a particle swarm algorithm.

[0081] The compensator parameters have different effects on the frequency response characteristics of the electric power steering system; the first-order pole determines the overall response attenuation performance of the compensator and the phase lag introduced. When its value decreases, the response attenuation effect is enhanced, but the phase lag introduced also increases; the second-order pole and the third-order pole affect the phase lead performance of the compensator in the high-frequency band. As the values of the second-order and third-order poles increase, the phase margin of the system is improved, but the suppression effect on high-frequency noise is weakened; the undamped oscillation frequency determines the notch position of the compensator, which is usually set near the system resonance frequency; the damping ratio affects the notch depth. The smaller the damping ratio, the greater the phase lag introduced in the low-frequency band, and its value needs to be determined in combination with the resonance peak of the frequency response characteristics from the steering wheel torque to the motor torque command of the system; the upper and lower limits of the compensator parameters are adjusted according to the frequency response characteristics of the electric power steering system, and finally the parameters of the stability controller at zero vehicle speed are calculated through an optimization algorithm.

[0082] Step 5: Based on the parameters of the stability controller at zero vehicle speed, calculate the parameters of the stability controller at other typical vehicle speeds by adjusting the damping ratio.

[0083] In Step 5, the increment of the damping ratio is determined according to the resonance peak of the frequency response characteristics from the steering wheel torque to the motor torque command of the system. In some embodiments of the present invention, the parameters of the stability controller are shown in Table 1; the Bode diagrams of the stability controller at zero vehicle speed and other typical vehicle speeds are shown in FIGS. 6(a) and 6(b).

[0084] Table 1 Stability controller parameters at zero vehicle speed and other typical vehicle speeds

[0085]

[0086] Step 6: Determine the speed-dependent assist mapping relationship.

[0087] In Step 6, the speed-dependent assist mapping relationship is used to describe the corresponding relationship among vehicle speed, steering wheel torque, and motor assist torque. In some embodiments of the present invention, the speed-dependent assist mapping relationship is as Figure 7 shown.

[0088] Step 7: Based on the speed-dependent assist mapping relationship and the stability controller parameters, calculate the gain margin and phase margin of the electric power steering system within the main vehicle speed range; when the gain margin and phase margin meet the minimum threshold requirements, determine the stability controller parameters to complete the design of the stability controller; otherwise, return to Step 4 and iterate until the gain margin and phase margin reach the minimum threshold requirements.

[0089] In some embodiments of the present invention, the minimum thresholds for both the system gain margin and phase margin are set to 0, the upper limit of the calculated vehicle speed is set to 200 km / h, and the stability controller parameters at the remaining vehicle speeds are obtained by linear interpolation; Figures 8(a) and 8(b) show the determined system stability margins, with the color transitioning from red to blue, and the closer to blue, the greater the stability margin and the more stable the system; both the gain margin and phase margin being greater than the threshold of zero indicates that the stability controller obtained by the optimization algorithm can ensure the stability of the system; after discretizing the controller parameters in the continuous domain, they can be implemented in the vehicle ECU; thus, the design of the stability controller is completed.

[0090] In some embodiments of the present invention, a computer device is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the speed-dependent assist control method for an electric power steering system described in the foregoing embodiments.

[0091] In some embodiments of the present invention, a computer-readable storage medium is provided. The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, it implements the speed-dependent assist control method for an electric power steering system described in the foregoing embodiments.

[0092] The step numbers of the present invention are only set for ease of explanation and illustration, and no limitation is imposed on the order between steps. The execution order of each step can be adaptively adjusted according to the understanding of those skilled in the art.

[0093] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A speed-dependent power steering control method for an electric power steering system, characterized in that: The following steps are involved: (1) Obtain vehicle frequency sweep test data; (2) Preprocess the frequency sweep test data to obtain the time domain data of the steering wheel torque and the motor electromagnetic torque; (3) based on the time domain data of the steering wheel torque and the motor electromagnetic torque, the frequency response characteristics from the steering wheel torque to the motor torque command and from the motor electromagnetic torque to the motor torque command are obtained; (4) Based on the frequency response characteristics of the electric power steering system from steering wheel torque to motor torque command, a stability controller optimization model is established, and the stability controller parameters at zero vehicle speed are calculated; (5) Based on the stability controller parameters at zero vehicle speed, the stability controller parameters at other typical vehicle speeds are obtained by adjusting the damping ratio; (6) determining a speed-dependent power-assistance mapping relationship, wherein the speed-dependent power-assistance mapping relationship is used to describe a corresponding relationship between vehicle speed, steering wheel torque, and motor power-assistance torque; (7) Based on the speed-dependent power assist mapping relationship and the stability controller parameters, the amplitude margin and phase margin of the electric power steering system within the main vehicle speed range are determined; when the amplitude margin and phase margin meet the minimum threshold requirements, the stability controller design is completed; otherwise, return to step (4) and iterate until the amplitude margin and phase margin meet the minimum threshold requirements.

2. The speed-dependent power-assistance control method of an electric power steering system according to claim 1, characterized in that: In step (1), the method for obtaining vehicle frequency sweep test data is as follows: The vehicle is placed on a dry road, the tire pressure is adjusted to the standard tire pressure range, and the steering wheel is kept in a free state with no torque input; the electric power steering system is excited by inputting sinusoidal signals of different frequencies through the power-assisted motor, and the motor electromagnetic torque and steering wheel torque are measured as the output variables of the electric power steering system.

3. The speed-dependent power-assistance control method of an electric power steering system according to claim 1, characterized in that: In step (1), the electromagnetic torque of the motor is converted according to the measured value of the power-assist motor current; the steering wheel torque is measured by a torque sensor installed on the steering column.

4. The speed-dependent power-assistance control method of an electric power steering system according to claim 1, characterized in that: In step (3), the frequency response characteristics from steering wheel torque to motor torque command and from motor electromagnetic torque to motor torque command are obtained by performing Fourier transform on the time domain data of steering wheel torque and motor electromagnetic torque.

5. The speed-dependent power-assistance control method of an electric power steering system according to claim 1, characterized in that: In step (5), the increment of the damping ratio is determined based on the resonance peak value of the frequency response characteristic of the electric power steering system from the steering wheel torque to the motor torque command.

6. The speed-dependent power-assistance control method of an electric power steering system according to claim 1, characterized in that: The stability controller used is a third-order lead-lag compensator, and its expression is: Among them, p1, p2, and p3 are the first, second, and third order poles respectively, ω n is the undamped oscillation frequency, ζ is the damping ratio; The open-loop transfer function expression of the electric power steering system is as follows: Among them, T h is the steering wheel torque, T e * is the motor torque command, K gain Assist gain for the motor.

7. A speed-dependent power steering control method for an electric power steering system according to any one of claims 1 to 6, characterized in that: In step (4), the established stability controller optimization model is: In the formula, a i (i=1,2...7) is the weight factor of each optimization objective, t j (j=1,2...7) is the target value of each optimization target, GM open is the amplitude margin of the system, PM open is the phase margin of the system, f n is a specific frequency near the system resonance frequency, H con (jw) is the frequency response function of the stability controller, j is the imaginary unit, and w is the angular frequency.

8. The speed-dependent power-assistance control method of an electric power steering system according to claim 6, characterized in that: In step (7), the minimum thresholds of the amplitude margin and phase margin of the electric power steering system are determined according to the system stability requirements, and the stability controller parameters at other vehicle speeds are calculated by linear interpolation.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the speed-dependent power-assistance control method for an electric power steering system according to any one of claims 1 to 8 is implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the speed-dependent power-assistance control method of an electric power steering system according to any one of claims 1 to 8 is implemented.

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