Smart braid: Air muscles that measure force and displacement

In this paper, we introduce a novel method to measure the contraction length and force output of Pneumatic Artificial Muscles. Our key innovation is to make the braid of these fiber-reinforced actuators out of insulated wires that form a single electric circuit. Changes in the inductance and resistance of this circuit can be related to muscle contraction length and muscle force. We implemented the proposed approach in two McKibben muscles that were evaluated under a variety of air pressures (0-200 kPa) and loads (0-45 N). The experiments confirmed our theoretical prediction of a roughly linear relationship between the measured inductance and the contraction length, as well as a strong correlation between the measured resistance and the actuator force. We were able to determine force with a resolution of 5N and contraction length with a resolution of 0.5 mm. This technique can be used to create flexible, precise, and robust self-sensing actuators that benefit a multitude of robotic applications.