On Electromagnetic Variable Air-Gap Reluctance Actuator Motion System: Characterization, Modeling, and Feedforward Control

Reluctance actuators are being explored as a promising alternative to Lorentz actuators for driving advanced precision motion systems like lithography machine wafer scanners, primarily due to their capacity to generate higher magnetic force. However, the inherent force-to-current and force-to-gap nonlinearities pose significant challenges in designing their control systems. Also, negative stiffness in reluctance actuators can lead to instability and control complexities. Over the past decade, numerous studies have contributed to the modeling, design, and control of reluctance actuators. This article introduces designs for both C-core reluctance actuator (C-CRA) and E-core reluctance actuator (E-CRA), deriving analytical models for magnetic force and stiffness tailored for high-precision motion applications. Simulation and experimental findings are juxtaposed to elucidate and assess the proposed designs. The outcomes reveal that the C-CRA exhibits lower stiffness magnitudes at smaller air gap distances (below 0.5 mm) than the E-core variant. Furthermore, it demonstrates reduced force errors between measured and analytically modeled forces. Consequently, the C-CRA is recommended for short-stroke applications within high-precision motion systems. The study also investigates the force-to-gap relationship across different input currents. It presents frequency responses for systems with and without a feedforward controller, offering insights into the stiffness of the reluctance actuator.