Development, control, and MRI-compatibility of the MR-SoftWrist

This paper presents the MR-SoftWrist: a parallel 3DOF MR-compatible wrist robot with compliant actuation. Through a design that aligns the wrist joint axes to the device DOFs and uses custom MR-compatible force-feedback actuation, the MR-SoftWrist can measure and support wrist movements during fMRI. The device has a circular workspace for wrist flexion/extension and radial/ulnar deviation with 18 deg radius, and is capable of generating 1.75 Nm joint torque. Control experiments validate the device´s workspace, along with position and zero torque control capabilities. In zero torque mode the maximum force felt by the user is 0.15 Nm, less than 10% of the device´s torque output. The device is shown to have no significant affect on imaging quality during fMRI.