Abstract: A machine for testing ball joints, elastomer bushings or other parts has a pair of mutually rotated gimbals attached to the outer shell or element for applying rotational displacements about two axes and a linear input attached to the inner element to apply a force along another axis. Each axis input is effected by a linear actuator and the gimbal inputs are specially designed to assure accurate independent control without parasitic motion between the inputs. Force and displacement transducers monitor each axis to provide feedback signals to control circuits. A heat chamber including the gimbal assembly has hot air applied from an external source through a hollow gimbal hub to hold the test specimen at a desired temperature.

Abstract: Structure for and method of compensating for a mechanical frequency response structure induced phase shift of an alternating electrical command signal in a closed loop control circuit so that magnitude errors independent of phase errors may be utilized in the control circuit. The structure includes a mechanical structure to be controlled having a mechanical frequency response to the cyclic command signal, an electronic frequency response circuit for producing a phase shift in the command signal substantially the same as that produced by the mechanical frequency response structure over the frequency range of the command signal, and structure for summing the error signal from the mechanical frequency response structure and from the electronic frequency response circuit to provide a magnitude error only signal, and means for providing an output signal when the selected error signal is outside of a permissible error signal range.