Towards 5-DoF Control of an Untethered Magnetic Millirobot via MRI Gradient Coils

Abstract

Electromagnetic field gradients generated by magnetic resonance imaging (MRI) devices pave the way to power untethered magnetic robots remotely. This innovative use of MRI devices allows exerting magnetic pulling forces on untethered magnetic robots, which could be used for navigation, diagnosis, drug delivery and therapeutic procedures inside a human body. So far, MRI-powered untethered magnetic robots lack simultaneous position and orientation control inside three-dimensional (3D) fluids, and therefore, their control has been limited to 3-DoF position control. In this paper, we present a path-planning-based 5-DoF control algorithm to steer and control an MRI-powered untethered robot’s position and orientation simultaneously in 3D workspaces in fluids. Eventhough the simulation results show that the proposed optimal controller can successfully control the robot for 5-DoF, in the experiments, we observe a reduced 5-DoF controllability due to the robot manufacturing errors, which result in pitch angle to remain at around the neutral pitching angle at the steady state. The proposed controller was evaluated to track four different paths (linear, planar-horizontal, planar-vertical and 3D paths) generated by 3D Bezier curves. The worst-case path-tracking error was observed for 3D path-following experiments. For this case, the position-tracking error was 2.7±1.8 mm, and the orientation-tracking error was 13.5± 28.7 and 3.7± 10.2 degrees for yaw and pitch angles, respectively. The overall path is completed within 19.6 seconds with 23.6 mm overall displacement and 61.2 and 41.2 degrees of yaw and pitch angle rotation, respectively. Such robots can be used in future MRI-powered active imaging, laser surgery and biopsy robots inside a fluid-filled stomach type of organs.