The full potential of active suspension has not yet been realized because of the limitations of available actuators. Current active suspension systems use hydraulic rams with the flow of hydraulic fluid to the ram controlled by a servo-valve to exert the desired force. The objective of this paper is to describe an alternate arrangement of hydraulic actuator which combines great compliance with short reaction times. The heart of the arrangement is a set of hydraulic rams which are connected between the body of the vehicle and one wheel assembly. Each of the rams is connected via a separate three-way, two-position valve to an accumulator, kept at a constant pressure, or a reservoir depending on the position of its valve. The force exerted by the actuator depends on the piston areas of those rams connected to the accumulator. The response time of the actuator depends only on the switching time of the solenoid valves, typically 25 milliseconds, independent of the compliance of the actuator. Three configurations of actuator are shown which incorporate a number of separate hydraulic cavities within a single unit. Not only is the compromise between isolation and response time eliminated, but the digital hydraulic actuator consumes significantly less power than a conventional hydraulic ram controlled by a servo-valve.
