System and method of controlling a steer-by-wire system having a road wheel reference angle generator

• US 6,757,601 B1
• Filed: 01/13/2003
• Issued: 06/29/2004
• Est. Priority Date: 01/13/2003
• Status: Expired due to Term

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Inventors: Yao, Yixin, Stout, Gregory J.
• Primary Examiner(s): Nguyen, Tan Q.
• Assistant Examiner(s): Tran, Dalena
• Application Number:
  US10/341,175
• Time in Patent Office: 533 Days
• Field of Search: 701/41, 701/42, 180/402, 180/446, 180/422, 180/443, 180/415, 180/421, 280/211, 280/5.51, 280/775, 280/779
• US Class Current: 701/41
• CPC Class Codes: B62D 6/002 computing target steering angles for front or rear wheels B62D7/159 takes precedence
• CPC Class Codes: B62D 6/008 Control of feed-back to the steering input member, e.g. simulating road feel in steer-by-wire applications

Abstract

The present invention involves a method of controlling a vehicle steer-by-wire system having two independent road wheels. In one embodiment, the method comprises providing a steering wheel control sub-system to generate road wheel reference angles and to produce steering feel for a vehicle driver. The method further includes providing a road wheel control sub-system with a servo control system structure including a multivariable road wheel controlled plant and a multivariable road wheel controller for controlling actual road wheel angles to track to road wheel reference angles. The method further includes generating left and right road wheel reference angles from the steering wheel angle with a road wheel reference angle generator using variable left and right steering ratios and calibration processes based on calibration variables, wherein the road wheel reference angle generator has first and second stages to produce left and right road wheel reference angle outputs. The present invention also involves a system for controlling a vehicle steer-by-wire system having a road wheel reference generator.

Claims

1. A method of controlling a vehicle steer-by-wire system having a road wheel reference angle generator, the method comprising:

providing a steering wheel control sub-system to generate road wheel reference angles and to produce steering feel for a vehicle driver;

providing a road wheel control sub-system including a multivariable road wheel controlled plant and a multivariable road wheel controller for controlling actual road wheel angles to track to road wheel reference angles;

modeling the nominal multivariable road wheel controlled plant as $\left[\frac{{\mathrm{\theta <br><br>}}_{r<br><br>\text{\hspace{1em}<br><br>}<br><br>l}}{{\mathrm{\theta <br><br>}}_{\mathrm{rr}}}\right]=\left[\frac{G_{\mathrm{ll}}<br><br>\left(s\right)}{G_{\mathrm{lr}}<br><br>\left(s\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{G_{\mathrm{rl}}<br><br>\left(s\right)}{G_{\mathrm{rr}}<br><br>\left(s\right)}\right]<br><br>\text{\hspace{1em}<br><br>}\left[\frac{u_l}{u_r}\right].$

wherein θ

_rl is indicative of the left road wheel angle, θ

_rr is indicative of the right road wheel angle, u_l is indicative of the left road wheel torque control variable, u_r is indicative of the right road wheel torque control variable, G_lr(s) is indicative of a first plant transfer function between θ

_rl and u_l, G_lr(s) is indicative of a second plant transfer function between θ

_rr and u_l, G_rl(s) is indicative of a third plant transfer function between θ

_rf and u_r and G_rr(s) is indicative of a fourth plant transfer function between θ

_rr and u_r;

implementing the multivariable road wheel controller as $\left[\frac{u_l}{u_r}\right]=\left[\frac{C_{\mathrm{ll}}<br><br>\left(s\right)}{C_{\mathrm{lr}}<br><br>\left(s\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{C_{\mathrm{rl}}<br><br>\left(s\right)}{C_{\mathrm{rr}}<br><br>\left(s\right)}\right]<br><br>\text{\hspace{1em}<br><br>}\left[\frac{e_l}{e_r}\right].$

wherein u_l is indicative of the left road wheel torque control variable, u_r is indicative of the right road wheel torque control variable, e_l is indicative of the left road wheel angle error, e_r is indicative of the right road wheel angle error, C_rr(s) is indicative of a first controller transfer function between u_l and e_l, C_lr(s) is indicative of a second controller transfer function between u_r and e_l, C_rf(s) is indicative of a third controller transfer function between u_l and e_rr, and C_rr(s) is indicative of a fourth controller transfer function between u_r and e_r;

applying road wheel angles and angular rates as feedback signals to construct a servo control system including the multivariable controlled plant and the multivariable road wheel controller to implement the left and right road wheel angles tracking for left and right road wheel reference angles;

generating left and right road wheel reference angles from the steering wheel angle with a road wheel reference angle generator using variable left and right steering ratios and calibration processes for the steering wheel angle, wherein the road wheel reference angle generator has one steering wheel angle input and left and right road wheel reference angle outputs, the generator having first and second stages to produce left and right road wheel reference angle outputs;

receiving left and right road wheel reference angles from the road wheel reference angle generator being based on the steering wheel angle command from the steering wheel control sub-system;

receiving the left and right road wheel angles from the multivariable road wheel controlled plant;

calculating the left and right road wheel angle errors based on the left and right road wheel angles and left and right road wheel reference angles;

generating the left and right road wheel torque control variables from the multivariable road wheel controller with servo control performances based on the left and right road wheel angle errors; and

applying torque control variables of the multivariable road wheel controller to the multivariable controlled plant to control tracking of the actual left and right wheel angles with the left and right road wheel reference angles, respectively.

2. The method of claim 1 wherein the road wheel control sub-system includes a servo control system structure having road wheel angular position feedback and road wheel angular rate feedback loops.

3. The method of claim 1 wherein calculating the left and right road wheel angle errors includes:

determining the left road wheel angle error between the left road wheel reference angle and the actual received left road wheel angle by using the left rod wheel angle negative feedback; and

determining the right road wheel angle error between the right road wheel reference angle and the actual received right road wheel angle by using the left road wheel angle negative feedback.

4. The method of claim 1 wherein generating the left and right road wheel torque control variables includes:

determining the left road wheel torque control variable based on the left road wheel angle error and the right road wheel angle error using the multivariable road wheel controller in the road wheel servo control system; and

determining the right road wheel torque control variable based on the right road wheel angle error and the left road wheel angle error using the multivariable road wheel controller in the road wheel servo control system.

5. The method of claim 1 further comprising:

applying actuation torque to the left road wheel based on the left road wheel torque control variable of the road wheel servo control system to independently control the actual left wheel angle and to track the left road wheel reference angle; and

applying actuation torque to the right road wheel based on the right road wheel torque control variable of the road wheel servo control system to independently control the actual right wheel angle and to track the right road wheel reference angle.

6. The method of claim 1 further comprising:

minimizing the left road wheel angle error between the actual left road wheel angle and left road wheel reference angle;

minimizing the right road wheel angle error between the actual right road wheel angle and right road wheel reference angle;

controlling actual left road wheel angle with quick time response for the left road wheel reference angle;

controlling actual right road wheel angle with quick time response for the right road wheel reference angle;

providing maximum servo stiffness to overcome the external disturbance torques in the left road wheal; and

providing maximum servo stiffness to overcome the external disturbance torques in the right road wheel.

7. The method of claim 6 wherein the external disturbance torques include disturbance torque from wind gust.

8. The method of claim 1 further comprising:

calculating left and right road wheel angular rates from the sensed left and right road wheel angles;

determining a left road wheel angular rats feedback control variable based on the left road wheel angular rate using a rate loop compensator;

determining a right road wheel angular rate feedback control variable based on the right road wheel angular rate using the rate loop compensator,

wherein the left road wheel angular rate feedback control variable is fed negatively to a left road wheel angular position loop for improving road wheel servo system damping performance and wherein the right road wheel angular rate feedback control variable is fed negatively to a right road wheel angular position loop for improving road wheel servo system damping performance.

9. The method of claim 1 wherein the multivariable road wheel controlled plant has coupled left and right road wheel angles with their input control variables.

10. The method of claim 9 wherein the left and right road wheel angles θ

_rl and θ

_rr, the left and right road wheel angles change if the left road wheel input control variable u_l changes and right road wheel input control variable u_r maintains un-change through the coupling, and the left and right road wheel angles change if the right road wheel input control variable u_r changes and left road wheel input control u_l maintains un-change through the coupling.

11. The method of claim 1 wherein a multivariable decoupling control is implemented in the steer-by-wire multivariable road wheel system to realize independent control of the left and right road wheel angles.

12. The method of claim 11 wherein calculating the left and right road wheel angle errors include:

receiving the left road wheel angle error based on the left road wheel reference angle from the road wheel angle generator and the actual left road wheel angle; and

receiving the right road wheel angle error based on the left road wheel reference angle from the road wheel angle generator and the actual left road wheal angle.

13. The method of claim 12 wherein generating the left arm right road wheel torque control variables includes:

determining a left road wheel torque control variable based on the left road wheel angle error and the right road wheel angle error; and

determining a right road wheel torque control variable based on the left road wheel angle error end the right road wheel angle error.

14. The method of claim 13 further comprising:

decoupling the left and right road wheel angles with the multivariable road wheel controller in the road wheel servo control system when the left and right road wheel torque control variables are generated;

wherein the multivariable road wheel controller controls the left road wheel angle based only on the left road wheal reference angle independent of the right road wheel reference angle and wherein the multivariable road wheel controller controls the right road wheel angle based only on the right road wheel reference angle independent of the left road wheel reference angle.

15. The method of claim 1 wherein a robust control is implemented in the multivariable road wheel controller to overcome affects of uncertainties and disturbances from the steer-by-wire system, vehicle, and external environment.

16. The method of claim 15 wherein the uncertainty of the road wheel controlled plant is given as G(s)=G₀(s)(I+Δ

G(s)) where G₀(s) is a nominal multivariable controlled plant model, Δ

G(s) represents a bound function of the uncertainty, and l is an identity matrix, and wherein the disturbance is represented as external disturbance torque to the left and right road wheels.

17. The method of claim 15 wherein the multivariable road wheel controller can be implemented using an H_∞

control being a robust system control design including road wheel system sensitivity transfer function matrices of S(s)=(I+G(s)K(s))<sup>−

1</sup> and road wheel system complementary sensitivity transfer function matrices of T(s)=G(s)K(s)(I+G(s)K(s))<sup>−

1</sup>=(I−

S(s)) where W₁(s) and W₂(s) are relative weighting functions.

18. The method of claim 17 wherein requirements of the control system performances and robust stability are specified by weighting functions W₁(s) for the road wheal system sensitivity function and W₂(s) for the road wheel system complementary sensitivity function.

19. The method of claim 1 wherein the multivariable road wheel controller is implemented with a gain scheduling property which the gain of controller changes automatically with vehicle variables, such as vehicle speed, to properly compensate a gain change of tie controlled road wheel plant to overcome non-linearity of road wheel system.

20. The method of claim 19 wherein the gain of the road wheel controlled plant changes substantially with vehicle dynamics, the road wheel controlled plant is non-linear, and the nominal road wheel controlled plant model is represented linearly in different operating points and a gain scheduling road wheel controller is designed based on each nominal linear controlled plant model.

21. The method of claim 1 wherein the first stage includes:

producing left and right road wheel angles from the steering wheel angle signal using left and right steering ratio gains in the reference command generator; and

implementing a variable steering ratio by using vehicle speed as a scheduling signal to change the steering ratio gains.

22. The method of claim 21 wherein the second stage including:

implementing a calibration for left and right road wheel angles according to vehicle geometry and steering performance requirements using vehicle variables in the reference command generator; and

obtaining the left and right road wheel reference angles in real time by using calibration variables including vehicle speed, yaw rate, and lateral acceleration.

23. A system for controlling a vehicle steer-by-wire system having a reference generator for generating road wheel reference angles from steering wheel angle and other vehicle variables, the system comprising:

a steering wheel control sub-system for generating road wheel reference angles based on a steering wheel angle command and for producing steering feel for a vehicle driver;

a road wheel control sub-system including a multivariable road wheel controlled plant and a multivariable road wheel controller for controlling actual road wheel angles to track to road wheel reference angles within a servo control system structure having road wheel angular position feedback and road wheel angular rate feedback loops; and

a road wheel reference angle generator cooperating within the road wheel control sub-system for generating left and right road wheel reference angles from the steering wheel angle using variable left and right steering ratios and calibration processes,

wherein the road wheel reference angle generator has one steering wheel angle input and left and right road wheel reference angle outputs, the generator having first and second stages to produce left and right road wheel reference angle outputs in real time by based on calibration variables,

wherein the multivariable road wheel controller receives left and right road wheel reference angles from the road wheel angle generator based on the steering wheel angle command from the steering wheel control sub-system and left and right road wheel angles from the multivariable road wheel controlled plant and calculates the left and right road wheel angle errors based on the left and right road wheel angles and left and right road wheel reference angles.

wherein the multivariable road wheel controller generates the left and right road wheel torque control variables from the multivariable road wheel controller with servo control performances based on the left and right road wheel angle errors, and

wherein the servo control system implements tracking of left and right road wheel angles with left and right road wheel reference angles and applies torque control variables of the multivariable road wheel controller to the multivariable controlled plant to control tracking of the actual left and right wheel angles with the left and right road wheel reference angles, respectively,

Wherein the nominal multivariable road wheel controlled plant is modeled as $\left[\frac{{\mathrm{\theta <br><br>}}_{r<br><br>\text{\hspace{1em}<br><br>}<br><br>l}}{{\mathrm{\theta <br><br>}}_{\mathrm{rr}}}\right]=\left[\frac{G_{\mathrm{ll}}<br><br>\left(s\right)}{G_{\mathrm{lr}}<br><br>\left(s\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{G_{\mathrm{rl}}<br><br>\left(s\right)}{G_{\mathrm{rr}}<br><br>\left(s\right)}\right]<br><br>\text{\hspace{1em}<br><br>}\left[\frac{u_l}{u_r}\right].$

wherein θ

_rl is indicative of the left road wheel angle, θ

_rr is indicative of the right road wheel angle, u_l is indicative of the left road wheel torque control variable, u_r is indicative of the right road wheel torque control variable, G_rr(s) is indicative of a first plant transfer function between θ

_rl and u_l, G_lr(s) is indicative of a second plant transfer function between θ

_rr and u_l, G_rr(s) is indicative of a third plant transfer function between θ

_rl and u_l, G_rr(s) is indicative of a fourth plant transfer function between θ

_rr and u_l; and

the multivariable road wheel controller is implemented as $\left[\frac{u_l}{u_r}\right]=\left[\frac{C_{\mathrm{ll}}<br><br>\left(s\right)}{C_{\mathrm{lr}}<br><br>\left(s\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{C_{\mathrm{rl}}<br><br>\left(s\right)}{C_{\mathrm{rr}}<br><br>\left(s\right)}\right]<br><br>\text{\hspace{1em}<br><br>}\left[\frac{e_l}{e_r}\right].$

wherein u_l is indicative of the left road wheel torque control variable, u_r is indicative of the right road wheel torque control variable, e_l is indicative of the left road wheel angle error, e_r is indicative of the right road wheel angle error, C_lr(s) is indicative of a first controller transfer function between u_l and e_l, C_lr(s) is indicative of a second controller transfer function between u_r and e_l, C_rr(s) is indicative of a third controller transfer function between u_l and e_r, and C_rr(s) is indicative of a fourth controller transfer function between u_r and e_r.

24. The system of claim 23 wherein, in the first stage for producing left and right road wheel reference angle outputs, the reference angle generator is designed to produce left and right road wheel angles from the steering wheel angle signal using left and right steering ratio gains in the reference command generator and implement a variable steering ratio by using vehicle speed as a scheduling signal to change the steering ratio gains.

25. The system of claim 24 wherein, in the second stage for producing left and right road wheel reference angle outputs, the reference angle generator is designed to implement a calibration for left and right road wheel angles according to vehicle geometry and steering performance requirements using vehicle variables in the reference command generator and obtain the left and right road wheel reference angles in real time by using calibration variables including vehicle speed, yaw rate, and lateral acceleration.