Integrated capacitance sensing for miniature artificial muscle actuators

The use of McKibben pneumatic artificial muscles in surgical instruments is increasing due to their light weight, miniaturization potential, compliance and large actuation force. However, precise position and force control is still challenging due to the intrinsic muscle hysteresis. The present work proposes an easy-to-make integrated capacitance sensor for miniature artificial muscles by replacing two strands of the muscle braid with two conductive wires. By measuring the capacitance at high excitation frequency between both wires, the length of the muscle can be deduced. This new integrated sensor still allows the miniaturization of McKibben muscles. The miniature muscle with integrated capacitance sensing only loses 3% of its contraction length with regard to an original McKibben muscle. The sensor can predict the muscle length with a 0.31mm precision over 80% of the muscle total achievable contraction range in average.