A rotary actuator for use in surgical instruments is provided, including a plurality of engine elements placed symmetrically around a rotation axis; a disk in contact with the engine elements, centered on the rotation axis, the engine elements providing a first rotation to the disk; an input shaft coupled to the disk to rotate with the first rotation; and a coupling mechanism to provide a second rotation to an output shaft from the first rotation of the input shaft. A surgical manipulator is also provided, including a base to provide stability and support; a first rotary actuator as above coupled to the base; and a first arm coupled to the rotary actuator.