The complexity of the process of controlling the steering system assistance causes that the assistance units, designed as the control systems, change their characteristics depending on the frequency and amplitude of the torque applied to the steering wheel as well as on the frequency and amplitude of the force applied to the gear toothed bar. The investigation aiming to indicate this effect involved designing a special test stand enabling differentiation of the frequency and amplitude of the input functions applied to the steering wheel or to the gear-toothed bar. The results of the investigation are the characteristics diagrams and numerically determined friction force and compliance values. The test stand allows carrying out the research in two operation modes of the assistance device-normal mode and city mode as well as at simulated different driving speeds. The investigation permitted to identify the parameters of the model of a steering system with an electric assistance device. In the paper, the results of the investigation and their preliminary analysis are presented. The load input function from the side of the toothed bar showed a strong relationship between the frequency and the torque transmitted on the steering wheel. For example, high frequencies occurring during running fast through small irregularities of the road are strongly damped. The slow-variable runs are transmitted on the steering wheel as a feedback reaction.

The article presents a dynamic model of an electric device supporting the steering system of a passenger car; the model was verified through a series of bench tests. The research object was an integrated electric power steering system (EPS) mounted on a steering column and cooperating with a steering gear. The results of the theoretical analysis were compared with the results of tests performed on a specially built research bench fully reflecting the work of assistance in the car. A satisfactory level of agreement between the results of the model tests and the bench tests was obtained.

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