Performance Enhancement of a Friction Damper System Using Bond Graphs

This paper describes a study of the performance of a servo-driven dry-friction damper employed in a vehicle suspension application. The damper is force-controlled via an electrohydraulic pressure control system. Previous experimental work showed that the theoretically predicted system performance does not reflect actual system behaviour because of both the residual back-pressure which produces an undesired constant-amplitude damping force and reduced hydraulic system bandwidth due to presence of free air within the hydraulic fluid. Possible solutions are proposed and performance assessed against a simulated system with dynamic performance purposely worsened. The first issue can be addressed by spring-preloading the friction damper so as to compensate the constant force caused by the residual back-pressure. The second issue can be tackled by closing a local pressure control loop around the control valve. The friction damper is controlled by a variable structure scheme that performs spring force compensation tracking. Bond graphs are employed as a tool for modelling and designing the control system.