System and method of controlling a vehicle steer-by-wire system applying multivariable decoupling control

• US 6,691,009 B1
• Filed: 01/13/2003
• Issued: 02/10/2004
• Est. Priority Date: 01/13/2003
• Status: Active Grant

• 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:
  US10/341,176
• Time in Patent Office: 393 Days
• Field of Search: 701/41, 701/42, 180/402, 180/422, 180/443, 180/446, 180/415, 180/408, 180/400, 340/465, 340/576
• 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 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. Moreover, the method includes implementing the multivariable road wheel controller to control tracking of the actual left and right wheel angles with the left and right road wheel reference angles, respectively. The method further includes decoupling the left and right road wheel angles with the multivariable road wheel controller in the road wheel servo control system. The multivariable road wheel controller controls the left and road wheel angle based only on the left road wheel reference angle independent of the right road wheel reference angle. Also, 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.

Claims

1. A method of controlling a vehicle steer-by-wire system having two independent road wheels applying multivariable decoupling control, 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;

applying road wheel angles and angular rates as feedback signals to the road wheel control subsystem having a multivariable road wheel controller with the road wheel angle decoupling control to implement the left and right road wheel angles tracking for left and right road wheel reference angles respectively;

receiving left and right road wheel reference angles 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;

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

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

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

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

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; and

decoupling the left and right road wheel angles with the multivariable road wheel controller in the road wheel control system when the left and right road wheel torque control variables are determined by using a multivariable decoupling control which is implemented in the steer-by-wire multivariable road wheel system to realize independent control of the left and right road wheel angles,

wherein the multivariable road wheel controller controls the left road wheel angle based only on the left road wheel 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.

2. The method of claim 1 wherein the road wheel control sub-system including the multivariable road wheel controlled plant and the multivariable road wheel controller has a servo control system structure with road wheel angular position feedback and road wheel angular rate feedback loops.

3. The method of claim 2 further comprising:

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

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_ll(s) is indicative of a first plant transfer function between θ

_rl and u_l, G_rl(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

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

_rr and u_r; and

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

 

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 between the left road wheel reference angle θ

_refl and left road wheel angle θ

_rl, e_r is indicative of the right road wheel angle error between the right road wheel reference angle θ

_refr and right road wheel angle θ

_rr, C_ll(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_rl(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.

4. 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 road 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.

5. The method of claim 1 wherein determining 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.

6. 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.

7. 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 wheel; and

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

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

9. 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 rate 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.

10. The method of claim 3 wherein the multivariable road wheel controlled plant has coupled left and right road wheel angles with their input control variables.

11. The method of claim 10 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.

12. The method of claim 3 wherein the multivariable road wheel controller has decoupled left and right road wheel angles in the road wheel control system with road wheel angular position feedback and road wheel angular rate feedback loops.

13. The method of claim 12 wherein the left and right road wheel angles θ

_rl and θ

_rr, the left road wheel angle θ

_rl changes independently if the left road wheel reference angle θ

_refl changes and right road wheel reference angle θ

_refr maintains un-change through the decoupling control of multivariable road wheel controller, and the right road wheel angle θ

_rr changes independently if the right road wheel reference angle θ

_refr changes and left road wheel reference angle θ

_refl maintains unchanged through the decoupling control of the multivariable road wheel controller.

14. 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.

15. The method of claim 14 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.

16. The method of claim 14 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.

17. The method of claim 16 wherein requirements of the control system performances and robust stability are specified by weighting functions W₁(s) for the road wheel system sensitivity function and W₂(s) for the road wheel system complementary sensitivity function.

18. The method of claim 3 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 the controlled road wheel plant to overcome non-linearity of road wheel system.

19. The method of claim 18 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.

20. The method of claim 1 wherein a road wheel reference angle generator determines the left and right road wheel reference angles from the steering wheel angle using variable left and right steering ratio and calibration processes for l steering wheel angle.

21. The method of claim 20 wherein the road wheel reference angle generator has one steering wheel angle input and left and right road wheel reference angle outputs and has first and second stages to produce outputs of the left and right road wheel reference angles.

22. The method of claim 21 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.

23. The method of claim 22 wherein the second stage includes:

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.

24. A system for controlling a vehicle steer-by-wire system having two independent road wheels applying multivariable decoupling control, the system comprising:

a steering wheel control sub-system to apply a steering wheel angle command for generating road wheel reference angles 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,

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

 

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_ll(s) is indicative of a first plant transfer function between θ

_rl and u_l, G_rl(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

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

_rr and u_r;

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

 

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_ll(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_rl, C_rl(s) is indicative of a third controller transfer function between u_l and e_r,, and C_rrs) is indicative of a fourth controller transfer function between u_r and e_r, and

a servo control system with the multivariable road wheel controlled plant and the multivariable road wheel controller,

wherein the multivariable road wheel controller receives left and right road wheel reference angles being 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 determines 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 road wheel servo control system applies road wheel angles and angular rates as feedback signals for implementing 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 multivariable road wheel controller in the road wheel servo control system decouples the left and right road wheel angles when the left and right road wheel torque control variables are determined by using a multivariable decoupling control implemented in the steer-by-wire multivariable road wheel system to realize independent control of the left and right wheel angles, wherein the multivariable road wheel controller controls the left road wheel angle based only on the left road wheel reference angel 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.