Closed-loop commutation control of an MRI-powered robot actuator

Actuators that are powered, imaged and controlled by Magnetic Resonance (MR) scanners offer the potential of inexpensively providing wireless control of MR-guided robots. Similar to traditional electric motors, the MR scanner acts as the stator and generates propulsive torques on an actuator rotor containing one or more ferrous particles. To generate maximum motor torque while avoiding instabilities and slippage, closed-loop control of the electromagnetic field gradients, i.e., commutation, is required. This paper proposes and demonstrates a method for commutation based on interleaving pulse sequences for rotor tracking and rotor propulsion. Fast rotor tracking is achieved by a new technique utilizing radio-frequency (RF) selective excitation of a properly located fiducial marker by the ferrous particle of the rotor. Optimal marker location is derived and demonstrated to provide accurate estimates of rotor angle. In addition, closed-loop commutation control is shown to increase motor torque and also to enable regulation of rotor angle.