Magnetic Resonance Pneumatic Stepper Motor With Multiple Concentric Shafts Output

Abstract

Pneumatic stepper motors can be used to actuate MR safe robotics and other devices. Due to tight space requirements inside the MRI scanner, it is useful to combine several individual stepper motors into a single device with an efficient shared actuator mechanism, called the multiple concentric shaft pneumatic stepper motor. The demonstrated “multimotor” prototype has six geared shafts driven by four pneumatic cylinders and is controlled by five pneumatic lines. The dimensions are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\phi 58 \times 25}\rm \,{\text{mm}}$</tex-math></inline-formula> (excluding shafts) and the maximum torque at 1.5 bar system pressure is in the range 56 to 114 Nmm. The maximum unloaded stepping frequency is 50 Hz when using 0.3 m tubes and drops to 12 Hz when using 5.1m tubes and 15 N mm load on each shaft. The positioning accuracy is two steps and the most efficient path planning algorithm needs 1.7 times as many steps as six separate single-axis motors. An examplatory 6-DOF manipulator design concept driven by the proposed motor requires an order of magnitude fewer components than state-of-art robotic systems with comparable kinematics. While the multimotor concept offers significant advantages in motor dimensions and pneumatic tube count, the dynamic performance is fundamentally poor, accuracy is compromised and the coaxial output shafts require thoughtful kinematic design of robotic systems to make effective use of the motor.