🤖 AI Summary
To address insufficient force control accuracy and stability of robotic actuators in MRI environments, this paper proposes an MRI-compatible, compact rotary series elastic actuator (SEA) module. The module employs a velocity-source ultrasonic motor for actuation and integrates a quad-compression-spring force sensing mechanism. Crucially, it introduces a novel disturbance observer (DOB)-based torque controller specifically designed for velocity-source motors, enabling stable steady-state torque control across both low- and high-external-impedance conditions—a capability unattainable with conventional approaches that suffer from performance degradation under low-impedance loads. Experimental validation in both 3T MRI and standard laboratory environments demonstrates a torque response settling time (to ±5% of final value) of only 0.05 s and a steady-state error of ≤2.5% full scale. Moreover, the module exhibits significantly improved cross-impedance control consistency compared to state-of-the-art alternatives.
📝 Abstract
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 5% settling time of 0.05 seconds and a steady-state error within 2.5% of its maximum output torque. Notably, our torque 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.