H∞-control of a rack-assisted electric power steering system

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H∞-control of a rack-assisted electric power steering system

Dannöhl, C. / Müller, S. / Ulbrich, H.

Today EPS is used more and more in passenger cars instead of hydraulic power steering. The steering assistance of the EPS has to be designed to satisfy the driver's requirements. Here, a controller that follows a reference steering torque is developed for a rack-assisted EPS. First, different models of the system with simplifying assumptions for the electrical subsystem are derived. Sometimes steering torque discontinuities, which are unwanted variations of the steering torque, are a problem. The root cause of this phenomenon is analysed using a detailed model, which contains a new model of the ball screw drive. For the examined steering gear, the nonlinear friction in the steering system is identified as the root cause of the discontinuities. In the next step, the requirements on the steering torque controller are specified by defining target values for characteristic control engineering parameters. An H∞-controller is developed and implemented as a steering torque controller. The controller's task is to follow an arbitrary reference steering torque quickly and accurately even if disturbances are present, for example, load forces from the road. Additionally, the controller shall compensate the friction to eliminate the effect of steering torque discontinuities. When designing the controller, frequency-dependent weighting functions are used. This makes it possible to establish different priorities of conflicting requirements in different frequency ranges. Other controllers (a PID controller, a cascade controller, and a linear quadratic compensator) are designed and implemented. Numeric simulations are performed and the different controllers are compared referring to the defined target values. The modified robust controller shows good behaviour in the numeric simulations, and it was also tested in a development vehicle with good results.