An aircraft secondary control system comprises a rod arrangement of a driving actuator, configured for performing the motion. The rod arrangement is coupled to the aircraft secondary control member via a linkage arrangement. The driving actuator comprises a first engagement and disengagement device of a piston body and comprises a second engagement and disengagement device, each of which being configured for alternately and/or simultaneously engaging said rod arrangement. The piston body is arranged in a cylinder body and is configured to make a working stroke from a first position to a second position.

A pneumatic stepper motor includes a housing, said housing accommodating at least part of: a rack or geared axle comprising a plurality of gear elements; and two pistons, each comprising at least two teeth, said pistons being arranged to cooperate with said rack or geared axle. The racks may either be straight or curved. The pistons are preferably double-acting pistons. A device includes at least one, and preferably a plurality of, such pneumatic stepper motor(s). The device may in particular be an MRI-compatible robotic system, more in particular for example an MRI-guided breast biopsy device.