  In this study, we introduce a novel MRI-compatible rotary series elastic
actuator module utilizing velocity-sourced ultrasonic motors for
force-controlled robots operating within MRI scanners. Unlike previous
MRI-compatible SEA designs, our module incorporates a transmission force
sensing series elastic actuator structure, with four off-the-shelf compression
springs strategically placed between the gearbox housing and the motor housing.
This design features a compact size, thus expanding possibilities for a wider
range of MRI robotic applications. To achieve precise torque control, we
develop a controller that incorporates a disturbance observer tailored for
velocity-sourced motors. This controller enhances the robustness of torque
control in our actuator module, even in the presence of varying external
impedance, thereby augmenting its suitability for MRI-guided medical
interventions. Experimental validation demonstrates the actuator's torque
control performance in both 3 Tesla MRI and non-MRI environments, achieving a
settling time of 0.1 seconds and a steady-state error within 2% of its maximum
output torque. Notably, our force controller exhibits consistent performance
across low and high external impedance scenarios, in contrast to conventional
controllers for velocity-sourced series elastic actuators, which struggle with
steady-state performance under low external impedance conditions.
