Force sensing for compliant actuators using coil spring inductance

Coil springs are nowadays widely used in robotic applications, in particular in Series Elastic Actuators. The measurement of spring force, either via load cells or via position sensors, conventionally requires additional sensor hardware to be part of or parallel to the kinematic chain. In order to simplify measurement of spring deflection, we exploit the fact that helical spring are in fact solenoid coils, and as such exhibit inductance properties that change strongly with length. We investigate theoretical models for this effect, and we experimentally evaluate the accuracy of such models in predicting spring length from inductance, with and without additional calibrating measurements. Our preliminary results show that a sensing precision as low as 2% can be achieved, indicating that the principle could be suitable for force sensing of compliant actuators.