Abstract: An improved motion simulator (10) includes a base (11), a capsule (14) having an at least partially-spherical smooth outer surface (15). The capsule is capable of roll, pitch and yaw angular motions about a pivot point (16) at the center of the spherical surface, and is also capable for upward and downward heave motion of the pivot point along a vertical axis (z--z) relative to the base. Actuators (33, 34, 35) are operatively arranged to selectively move the capsule relative to the base in any of four degrees of freedom. The capsule outer surface is configured such that no possible motion of the capsule in any of the permissible four degrees of freedom will cause any portion of the capsule to move outside of a motion envelope formed by an imaginary vertical cylinder (41 ) having a diameter (d) substantially equal to that of the spherical surface.

Abstract: The dynamic performance of hydraulic apparatus including a fluid-operated actuator controlled by a servo-controller, is improved by providing phase lead compensation to the controller in response to movement of the movable element of the actuator so that the low frequency response of the apparatus is raised.