Vibration control based on dynamic compensation in an electric power steering system

The market for power assist steering in small cars is expanding exponentially [1]. Looking ahead to the next generation of small vehicles, having power assist steering with little to no fuel economy penalty will be necessary. In fuel efficient vehicles with tight package constraints mechanically driven hydraulic power assist steering will not meet all vehicle objectives. Therein lies the easily observable trend toward alternative power assist steering systems. By alternative systems we refer to systems that are driven in a way that reduces parasitic losses. Most common among the products rapidly coming to the market ale electrically powered hydraulic units and electrically powered systems that act directly on the steering column or on the steering gear [2]. Power assist steering systems must have sufficient bandwidth to respond seamlessly to the drivers fastest inputs while at the same time preserving the feel of the road [3] through the mechanical steering mechanism. Control system stability and system vibrations, of course, are always a concern. In the baseline hydraulic power assist system, hydraulic stability could often be achieved over a broad enough range by lowering assist gain or by increasing damping without seriously compromising the subjective feel of the system. Since greater opportunities for fuel economies are present in direct electrically actuated systems, the wholesale addition of damping is a less attractive solution. Further, since the electrically activated system can be developed and customized in software, the ability to provide operation over a wide range of vehicle dynamic strategies is desirable. The potential to reduce design time to production and prototype vehicle development expense are also driven toward reliable control design and simulation techniques that enable viable solutions the first time they are implemented in hardware. While the system synthesized in this paper is a straight-forward torque amplifier, the compensation necessary to assure control system stability, attenuate vibrations and preserve bandwidth can be a challenging control design problem. Although the system is inherently non-linear, it has been demonstrated that the linear approximation is sufficiently accurate to provide valid results over a useful range of operation. Validations have been performed on the bench and in vehicles to gain confidence in these techniques. The paper is organized as follows. In Section 2 we discuss the dynamic model of a power steering system. Dynamic compensator design and comparison analysis are given in Section 3. Earlier results obtained by authors on dynamic and stability analysis of a power steering system with an ideal actuator and development of quantitative measures of a steering system performance are presented in [3].