acker98g - ADVANTAGES OF ACTIVE STEERING FOR VEHICLE...

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Unformatted text preview: ADVANTAGES OF ACTIVE STEERING FOR VEHICLE DYNAMICS CONTROL Professor J Ackermann, Dr T B¨unte, and D Odenthal, German Aerospace Center, D 99ME013 Keywords: active steering, robust control, yaw and roll disturbance atten- uation, rollover avoidance Abstract Yaw and roll dynamics of vehicles can be controlled efficiently by individual wheel braking or by active steering. Both approaches are compared on the basis of physical and application considerations. Two vehicle dynamics control concepts based on active steering are summarized. One of them focusses on the attenuation of yaw disturbances on the vehicle by robust unilateral decoupling of yaw and lateral mode. The other approach aims at rollover avoidance of road vehicles. There, in continuous operation, active steering improves the roll dynamics. In case of emergency an efficient strategy applies simultaneous steering and braking control. 1 Efficiency of braking and steering Driver assistance systems for vehicle dynamics primarily produce a compensating torque for yaw disturbances. Such control systems can react faster and more accurately than the driver, when an unexpected deviation from the desired yaw rate occurs. The deviation is taken between the desired yaw rate (generated by a prefilter from the steering wheel input and the velocity) and the actual yaw rate (measured by a rate sensor). Also critical rollover conditions may be measured and fed back into a driver assistance system. The actuation in such control systems is mostly by distribution of the brake force and in some cases also motor torque over the four wheels, ESP [1] is an example. The braking approach uses the existing ABS. Therefore only little additional hardware is required. The question now arises: Is it worthwile to consider active steering as an alternative or in combination with an active braking system? We will first discuss the potential of both systems for yaw disturbance attenuation in terms of physical limits. Three simplifying assumptions are made for this discussion: • The total force F max which is transmittable by the tires does not depend on the direction in which the force acts (Kamm’s circle). • The center of gravity (CG) is assumed to be midway between the front and rear axles of the vehicle. • The wheelbase ‘ is twice the trackwidth t . Fig. 1 compares the physical limits of braking and steering. Obviously, the limits for rear wheel contributions to the corrective torque are the same in both cases. The available torque from front wheel braking is F max · t/ 2 and from front wheel steering it is 2 F max · t . In other words: CG t F max CG F max F max ‘ = 2 · t Fig. 1: Torques by front wheel braking (left) and front wheel steering (right)....
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acker98g - ADVANTAGES OF ACTIVE STEERING FOR VEHICLE...

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