US 20030055547 A1 Abstract A control system for a motor vehicle subsystem comprises a reference model and a feedforward controller. The reference model computes desired states of the subsystem. The feedforward controller computes a first control value based on input from the reference model, and computes a second control value based on yaw rate of the vehicle and a control variable for the subsystem.
Claims(20) 1. A control system for a motor vehicle subsystem, the control system comprising:
a reference model which computes desired states of the subsystem; a feedforward controller which computes a first control value based on input from the reference model, and computes a second control value based on yaw rate of the vehicle and a control variable for the subsystem; and means for affecting the subsystem based on the first and second control values. 2. The control system of 3. The control system of 4. The control system of 5. The control system of 6. The control system of 7. The control system of 8. The control system of 9. The control system of 10. The control system of 11. The control system of 12. The control system of 13. A method of controlling a motor vehicle subsystem, the method comprising:
computing desired states of the subsystem; computing a first control value based on the desired states of the subsystem; computing a second control value based on yaw rate of the vehicle and a control variable for the subsystem; and affecting the subsystem based on the first and second control values. 14. The method of 15. The method of 16. The method of 17. The method of 18. The method of 19. The method of 20. A method of controlling motor vehicle steering and braking subsystems, the method comprising:
computing desired states of the steering and braking subsystems; computing a first control value based on the desired state of the steering subsystem; computing a second control value based on the desired state of the braking subsystem; computing a third control value based on yaw rate of the vehicle and a control variable for the steering subsystem; computing a fourth control value based on yaw rate of the vehicle and a control variable for the braking subsystem; affecting the steering and braking subsystems based on the first, second, third, and fourth control values. Description [0001] This application is a continuation-in-part of Ser. No. 09/935,274, which was filed Aug. 22, 2001, the disclosure of which is hereby incorporated by reference. [0002] The present invention relates to control systems for motor vehicle subsystems, and more particularly to a system and method including a dynamic feedforward feature for integrated control of the motor vehicle steering and brakes. [0003] Unified or integrated chassis control systems have been proposed which control the brakes, steering, and suspension of a motor vehicle. The purpose of unified chassis control is to improve vehicle performance in all driving conditions by coordinating control of the chassis subsystems. Unified chassis control systems typically utilize a supervisory control concept that utilizes three fundamental blocks: a reference model, a state estimator, and a vehicle control. The vehicle control element normally incorporates a feedback control. This element computes control values by comparing actual states obtained from the state estimator with desired states from the reference model. [0004] It is well known that when brakes are applied during a steering maneuver, a yaw rate error is induced. It such circumstances, the conventional chassis control systems are relatively slow to compensate. [0005] The present invention is a system and method for controlling a motor vehicle subsystem. The control system comprises a reference model and a feedforward controller. The reference model computes desired states of the subsystem. The feedforward controller computes a first control value based on input from the reference model, and computes a second control value based on yaw rate of the vehicle and a control variable for the subsystem. [0006] Accordingly, it is an object of the present invention to provide a control system of the type described above which presents a standard methodology to integrate feedforward control into a unified chassis control supervisor that overcomes several known deficiencies. [0007] Another object of the present invention is to provide a control system of the type described above which improves control response. [0008] Still another object of the present invention is to provide a control system of the type described above which allows single-point tuning. [0009] Still another object of the present invention is to provide a control system of the type described above which incorporates dynamic feedforward logic. [0010] The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. [0011]FIG. 1 is a schematic view of a chassis control system according to the present invention for a motor vehicle; [0012]FIG. 2 is a block diagram of a dynamic feedforward control system using a first-order reference model; and [0013]FIG. 3 is a block diagram of a dynamic feedforward control system using a second-order reference model. [0014]FIG. 1 shows a control system [0015] The reference model [0016] The feedforward controller [0017] A dynamic feedforward control is represented by line [0018] [0019] where Φ and V [0020] The actual yaw rate Φ of the vehicle [0021] The variable P [0022] Assuming that the yaw rate is equal to the desired yaw rate Φ [0023] The transfer function to relate the yaw rate to the rear road wheel angle control, if present, is given by
[0024] If the desired yaw rate (or reference model) is represented as a first-order transfer function as
[0025] where the variables K Δδ [0026] Where
[0027] The dynamic feedforward control with a proportional term, a derivative term, and a diminishing integrator term of the steering wheel position is employed to achieve the desired dynamic feedforward control function. FIG. 2 shows a dynamic feedforward control method for a rear steer application assuming a first-order transfer function. The bicycle model parameters are first obtained at block [0028] If the reference model is modeled as a second-order transfer function as
[0029] then the dynamic feedforward term of the rear steer control becomes
[0030] and the rear steer control associated with the dynamic feedforward term is given by Δδ [0031] The parameters of the second-order transfer function are:
a _{5} =a _{11} a _{22} −a _{12} a _{21}
[0032] [0033] The variables ω [0034] The dynamic feedforward transfer function G [0035] The vehicle's natural frequency and damping ratio are preferably generally decreased as the vehicle speed is increased. An advantage of the second-order transfer function is that it allows a driver to choose a desired handling characteristic of a vehicle. FIG. 3 shows the dynamic feedforward control for the rear steer system assuming a second-order reference model. The dynamic feedforward control command can be represented as the summation of a proportional term, a differential term, and an integral term with a second-order transfer function as given above. The dynamic feedforward control gains can be represented as four separate table lookups. For example, when a vehicle handling characteristic with a desired natural frequency of 1.5 Hz and a desired damping ratio of 1.5 are specified, the control gains are represented as a function of vehicle speed and stored in the computer memory. The total rear steer control command is the summation of the static feedforward, the dynamic feedforward, and the feedback control. [0036] The transfer function to relate the yaw rate to the front active steering control is given by:
[0037] If the reference model is represented as a first-order transfer function, then the dynamic feedforward part of the front active steering control is given by:
[0038] The transfer function to relate the yaw rate to the active brake control is given by
[0039] If the reference model is represented as a first-order transfer function, then the dynamic feedforward part of the active brake control is given by:
[0040] The vehicle's transient handling performance, such as during step steer or slalom maneuvering, can be enhanced by choosing the natural frequency and damping ratio of the vehicle's reference model with a single point tuning approach. The dynamic feedforward control of the active steering and braking integration can be tuned to provide either a first-order or a second-order desired reference model behavior, which is a great benefit in systematically tuning the vehicle to a desired level of handling performance. The addition of dynamic feedforward control reduces the lag in vehicle yaw rate and lateral acceleration responses to steering inputs and enhances the vehicle's directional stability performance as compared to the static feedforward control alone. Furthermore, the feedforward control can provide many of the system dynamics benefits and tunability function if the closed-loop control system is disabled due to failure of, e.g., yaw rate or lateral acceleration sensors. [0041] While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein. Referenced by
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