Sensing the motion of bellows through changes in mutual inductance

Bellows-like actuators are popular in soft robotic systems. Sensing the movement of these actuators with traditional sensors is challenging. This work proposes and tests a sensing system based on the changing mutual inductance of wire coils on bellows. A method for modeling the changes in mutual inductance between coils of tightly-packed wires is introduced. Changes in mutual inductance are measured through the self-inductance of a circuit made up of the coils in series. The self-inductance of the circuit measures the bellows bend-angle. The experiments show an approximately quadratic relationship between the bend angle and the measured inductance. From a bend angle of 121 ° to −67 ° the inductance of the circuit increases by 19 %. The bias-inducing effects of shear strain, torsional strain, non-uniform bending, and nearby metal are also explored.