Force-Balance Model and Stable Operating-Point Regulation of a Prestretched Repulsive Electromagnetic Elastomer Artificial Muscle.

Attractive electromagnetic actuators operated under open-loop control are susceptible to positive feedback instability because the attractive force increases as the separation decreases, making it difficult for the actuator to remain at an intermediate position. Here, we propose a prestretched repulsive electromagnetic-elastomer artificial muscle and establish a force-balance model based on the intersection between the electromagnetic repulsive-force curve and the elastomer restoring-force curve. In the repulsive configuration, the electromagnetic force decreases as the coil separation increases, whereas the elastic restoring force increases with tensile displacement; their intersection can therefore define a locally stable mechanical equilibrium position. Natural rubber was used for quantitative model construction and trend validation. Under a constant-voltage input of 36 V and an initial coil separation of 3 mm, the model predicted equilibrium displacements of 0.42 and 0.79 mm for the non-prestretched and 100% prestretched conditions, corresponding to theoretical actuation strains of 14.0% and 52.7%, respectively. Five repeated cycles using the same actuator yielded actual displacements of 0.285 ± 0.033 and 0.700 ± 0.033 mm, corresponding to actuation strains of 9.5 ± 1.1% and 46.7 ± 2.2%, respectively. Although the measured outputs were lower than the ideal model predictions, both the model and the experiments showed that prestretch shifted the working point toward a larger displacement and increased the actuation output. The force-balance framework further indicates that changing the electrical input can shift the electromagnetic-force curve and thereby theoretically regulate the intersection position. Because the repulsive force generated by the current coils was limited, a low-modulus two-part silicone rubber was additionally used to construct a macroscopic demonstration prototype. During a limited number of repeated on-off operations, a local elongation of 140-150% and shape recovery after de-energization were observed, demonstrating the feasibility of using the proposed configuration to drive a simplified movable structure. Coil heating was observed during repeated energization, suggesting that Joule heating may become an important engineering constraint on continuous operation and further increases in driving force; its quantitative effect remains to be investigated. These results provide an experimental basis and a theoretical framework for stable-equilibrium design and prestretch-based working-point regulation in repulsive electromagnetic-elastomer actuators.