Controllable active struts that would function as linear actuators with variable spring stiffness and damping have been proposed as components of advanced suspension systems of future wheeled ground vehicles. The contemplated advanced suspension systems would include computer-based control subsystems that would continually adjust the actuator responses to obtain optimal combinations of safety and comfort under operating conditions ranging from low speeds over smooth roads to high speeds over rough, unpaved ground. The proposed struts and suspension systems were originally intended for use in military vehicles, but there could also be a broad commercial market for them in trucks and sport utility vehicles.

A strut according to the proposal (see figure) would include an air spring in the form of a plunger sliding longitudinally in a pneumatic chamber. Compressed air would be supplied to the pneumatic chamber, from an external pump and accumulator, via a pneumatic hose and a high-speed valve. The chamber would be instrumented with (1) an electronic extensometer to monitor the axial displacement of the plunger and (2) an electronic air-pressure sensor. Notwithstanding the extensometer, the air spring would be used primarily to regulate the spring stiffness, rather than the length, of the strut. The diameter of the plunger would be small, so that only a small amount of compressed air would have to be pumped in or allowed to flow out to change the spring stiffness by a given amount. Because of the small amount of air needed and because the air spring would operate at moderate to high pressure, the required amount of air could be made to flow into or out of it rapidly and, hence, the spring stiffness could be changed rapidly on command. Image

The pneumatic chamber would also serve as a plunger that would slide longitudinally in a hydraulic chamber. Like the pneumatic chamber, the hydraulic chamber would be equipped with a pressure sensor, extensometer, and highspeed valve. However, instead of compressed air, hydraulic fluid would be supplied to this chamber from an external hydraulic pump, accumulator, and reservoir. The hydraulic chamber would be used primarily to adjust the length of the strut; secondarily, it could be used as a very stiff spring in the event of a malfunction of the air spring. The hydraulic fluid would be pumped in to extend the strut. Retraction would be effected by actuating the valve to allow the load on the strut to push hydraulic fluid back to the reservoir. Like the pneumatic chamber, the hydraulic chamber would have a small volume and would be operated at high pressure; hence, the length of the strut could be adjusted within a short response time.

A device denoted an actuator-restraining device (ARD) would provide controllable damping. The ARD would include (1) a set of plates, oriented in radial-axial planes, that would move with one end of the strut and (2) a set of pairs of plates, each pair parallel and close to one of the first-mentioned plates, that would move with the other end of the strut. The narrow spaces between the plates would be filled with a magnetorheological fluid, the effective viscosity of which would be controlled by the current in an electromagnet coil. In the absence of current, the plates would slide almost freely, so that any damping would be that attributable to friction damping in the air spring and the hydraulic actuator. In general, the amount of damping could be increased or decreased to almost any desired level by increasing or decreasing the current applied to the coil. By applying sufficient current, one could even obtain a damping or restraining force greater than the weight of the vehicle. The response time of the ARD would be an order of magnitude shorter than the response times of the pneumatic and hydraulic actuators.

This work was done by Gary L. Farley of the U. S. Army Research Laboratory for Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at under the Machinery/Automation category. LAR-16355-1