A double-loop robust position and speed control of an electromagnetic actuator against nonlinear model uncertainties

Electromagnetic actuation (EMA) has been a reliable and powerful approach for the wireless manipulation of a small-sized robot in recent years. EMA with controllers for microrobot manipulation has been validated in various studies and demonstrated high precision in static environments. Nevertheless, from a control perspective, challenges remain in effectively compensating for lumped disturbances, which include nonlinear model uncertainties and unpredictable dynamics of target objects, thereby ensuring reliable performance in practical applications. In this article, we propose a double-loop robust controller to address these challenges. The proposed control architecture consists of an outer-loop sliding mode control (SMC) and an inner-loop disturbance observer (DOB). This controller functions as a force compensator, where the SMC provides primary feedback control for the motion of the magnetic object, and the DOB estimates and compensates for lumped disturbance forces. The controller's parameters and driving performance were comprehensively analyzed through dynamic simulations of the target object. The potential for practical applications was validated through experiments. The results demonstrated that position control accuracy reached 0.4 mm at a rated speed within 1 mm/s, showing a 71 % improvement compared to conventional control. Additionally, the comparative speed control performance could achieve a maximum speed of 11.4 mm/s resulting in relative effectiveness compared to other control approaches.