MR Safe Pneumatic Piston Step Motor (PneuStep-P)

Device compatibility with the magnetic resonance (MR) environment is highly demanding. In 2007, we reported an MR safe pneumatic step motor (PneuStep). A new 3-D printed version still using rolling diaphragms (PneuStep-D) is presented. We also report a novel motor with pistons (PneuStep-P) and a new MR safe encoder. 3-D printing enabled reducing the step size in both new motors to 2.5° (from 4°). Relative to one another, the piston motor has double the cylinder capacity of the diaphragm motor (0.90 versus 0.45 cm³) within the same overall size (70 mm outer diameter and 25 mm in length); maximum torque increased from 0.19 to 0.49 N·m, maximum speed from 60 to 108 steps/s, and max power from 0.15 to 0.5 W, respectively, all with 4.5-m long hoses and 0.48 MPa air pressure. Over a 78-min endurance test, PneuStep-P showed consistent cycle-to-cycle performance, no quantifiable wear, and position errors <1° under load. With short hoses, PneuStep-P generated up to 2.5 W. MR tests showed no SNR loss and less than 0.05% image change factors. The new piston model is simpler, more powerful, and relative to other MR safe motors, it offers a unique combination of high precision, power, and safety.