An MR-Safe Pneumatic Stepper Motor: Design, Control, and Characterization

Magnetic resonance imaging (MRI) can provide high contrast soft tissue visualization without ionizing radiation, which makes it an attractive imaging modality for interventional procedures. However, the strong magnetic and radio frequency (RF) fields impose significant challenges to the development of robotic systems within the magnetic resonance environment. Consequently, designing MRI-compatible actuators is crucial for advancing MRI-guided robotic systems. This paper reports the design, control, and characterization of a gear-based pneumatic stepper motor. The motor is designed with three actuating piston units and a geared rotor. The three actuating pistons are driven sequentially by compressed air to push the geared rotor and to generate bidirectional stepwise motion. Experiments were conducted to characterize the motor in terms of torque, speed, control, and MRI compatibility. The results demonstrate that the motor can deliver a maximum continuous torque of 1300 mNm at 80 pounds per square inch (PSI) (0.55 MPa) with 9 m air hoses. The closed-loop control evaluation demonstrates the steady-state error of position tracking was 0.81±0.52 deg. The MRI compatibility study indicated negligible image quality degradation. Therefore, the proposed pneumatic stepper motor can effectively serve as an actuator for MRI-guided robotic applications.