Control system for steering a road vehicle having both front-wheel steering and rear wheel steering

A steering system for a road vehicle which vehicle has

• steerable front wheels disposed proximal a front axle;
• steerable rear wheels disposed behind the front wheels a distance l (in m), said distance being the wheel base;
• a steering wheel rotatable to a steering wheel angle .delta..sub.L (in rad);
• a mass m (in kg);
• a center of mass located a distance l.sub.v (in m) behind the front axle;
• and
• a rear cornering stiffness c.sub.h (in N/rad);
• said steering system comprising:
• a front actuator means for controlling a front wheel steering angle .delta..sub.v in rad;
• a rear actuator means for controlling a rear wheel steering angle .delta..sub.h in rad;
• a yaw sensor means for generating a yaw signal r proportional to a yawing angular rotation rate of the vehicle (in rad/s);
• a steering wheel angle sensor means for measuring the steering wheel angle .delta..sub.L (in rad);
• a speedometer sensor for measuring vehicle velocity v in m/s;
• and
• a processor means for accepting the r, .delta..sub.L, and v from said yaw sensor means, said angle sensor means and said speedometer and for controlling the front actuator and the rear actuator based thereon;
• the processor means incorporating
• a first calculation for calculating .delta..sub.v as a first function of r such that a front axle transverse acceleration a.sub.v (in m/s) is a function of .delta..sub.L only, the first calculation including compensation for the yaw signal r, and
• means for causing the front actuator to dispose the front wheels to .delta..sub.v, whereby steering is decoupled from yaw rate;
• a second calculation for calculating a command variable W.sub.h and calculating .delta..sub.h as a second function
• .delta..sub.h =(l/v-K.sub.D)(W.sub.h -r) (in rad)
• where K.sub.D is a parameter, and
• means for causing the rear actuator to dispose the rear wheels to .delta..sub.h ;
• the first calculation and the second calculation being decoupled;
• whereby yaw damping is independent of v and the yaw control does not affect the transfer function from the steering wheel to the front axle tracking.