This paper presents a new type of hydraulic servo cylinder, which is charac
terized by its mechanical feedback structure. The feedback mechanism consis
ts of two poppet valves, a plate spring and a steel ball. The hydraulic pre
ssure in each cylinder chamber is controlled by a poppet valve, which is ac
tivated by a solenoid. The solenoid input current opens the valve, which re
sults in a pressure drop in the corresponding cylinder chamber, and initiat
es the piston motion. The piston is specially designed to generate a force
to close the valve in proportion to its distance from the cylinder center,
by means of a steel ball and a plate spring. Finally, the piston moves to i
ts new equilibrium position, controlled by the solenoid current. The charac
teristics of this hydraulic servo cylinder are studied by stability analysi
s, as well as the transient and steady-state responses to step inputs. Anal
ytical studies and computer simulations are executed to verify the effects
of the various design parameters on the performance. A prototype hydraulic
servo cylinder is developed and tested. The results confirm the successful
operation of the servo cylinder, and show consistency with the theoretical
studies. (C) 1999 Elsevier Science Ltd. All rights reserved.