Open Source MR-Safe Pneumatic Radial Inflow Motor and Encoder (PRIME): Design and Manufacturing Guidelines

Abstract

Actuators and encoders used in MR-guided robotic interventions are subject to strict requirements to ensure patient safety and MR imaging quality. In this paper, we present an open source computer aided design (CAD) of our MR-safe Pneumatic Radial Inflow Motor and Encoder (PRIME). PRIME is a parametrically designed motor that enables scalability based on torque and speed requirements for a wide range of MR-guided robotic procedures. The design consists of five primary modifiable parameters that define the entire motor geometry. All components of the motor are either 3D printed or available off-the-shelf. Quadrature encoding is achieved using a 3D printed housing and four fiber optic cables. Benchtop experiments were performed to validate the performance of the proposed design. To the best of our knowledge, this is the first open source MR-safe pneumatic motor and encoder in the field. We aim to share the design and manufacturing guidelines to lower the entry barriers for researchers interested in MR-guided robotics.

Fig. 1.

Three different scaled motor designs. (Left) Motor with 19 mm OD rotor (authors’ nominal preference). (Center) Motor with 44 mm OD rotor (motor size from [18]). (Right) Motor with 61 mm OD rotor.

Fig. 2.

(A) The full motor with air stream lines for inlet 1 (blue) and inlet 2 (red) can be seen. (B) The first set of jets that redirect the flow from inlet 1 can be seen in the sectioned motor. (C) The second set of jets that redirect the flow from inlet 2 can be seen in the sectioned motor. (D) The flow can be seen leaving the nozzles with an angle of attack of α.

Fig. 3.

The motor assembly in an expanded configuration with trace lines representing the assembly directions. Note that only the general housing shape of the planetary gearbox is modeled.

Fig. 4.

(A) Working principle of the quadrature encoder design. Note that receiver A and receiver B are on the same side to avoid cross-talk from the transmitters. (B) The oscilloscope screen capture depicts the electrical signal detected by the proposed encoder.

Fig. 5.

The printing setup in the PreForm software can be seen here. Note that this orientation is used to ensure the circular geometry of the rotor, stator, and cap are in the X-Y plane of the printer.

Fig. 6.

Schematic of the system setup for the motor inside the MRI scanner.

Fig. 7.

The torque (top) and power (bottom) curves of the three different motor designs from Fig. 1.

Fig. 8.

(Top) Tracking performance of the encoder (blue) to the commercial encoder (red). (Bottom) Error results of the MR-safe encoder to the off-the-shelf encoder. Note the mechanical backlash of the system was ±8°.

Fig. 9.

T₁-weight images of the phantom can be seen with no motor, with the motor in the scanner and off, and the motor in the scanner and running.