Research on Self-excited Vibration "Judder" generated in the Automotive Hydraulic Power Steering Systems

A hydraulic power steering system uses a hydro-mechanical bilateral servo mechanism to assist the driver in applying steering input commands to the vehicle. A violent self-excited vibration accompanied with sound, known as "judder", can occur during rapid steering maneuvers. It is explained explicitly using the present simulation model that the self-excited vibration is caused by the interaction of the hydraulic and mechanical (structural) dynamics of the vehicle. It is demonstrated that the main factors inducing the self-excited vibration are the response delay of wave propagation in the supply line, which can be represented decisively by the phase lag of frequency response characteristics of hydraulic impedance of the supply line at the servo valve end, as well as the sensitivity of the servo valve. Some specific analytical results that reveal the interaction of the hydraulic/mechanical dynamics are examined and compared with the experimental results performed on the bench test apparatus. Analytical results agree well with the test results. It is also explained that even if the system is dynamically unstable in substance the self-excited vibration can not occur owing to the coulomb friction acting on the rack bar, not when the vibration is excited over some threshold value. The proposed numerical simulation technique will be found to be very useful in searching the optimum parameters for steering design.