Design and Characterization of a Mckibben Pneumatic Muscle Prototype with an Embedded Capacitive Length Transducer

The McKibben muscle types are pneumatic actuators known to be intrinsically safe for their high power-to-weight ratio. For these reasons, they are suitable for robotic, biomechanical, and medical applications. In these application fields and, above all, in collaborative robotics, where safety must be ensured for human–robot interactions, the values of pressure, force, and length are necessary and must be continuously monitored and controlled. Force and pressure transducers are commercially available to be integrated into a McKibben muscle type. On the contrary, no commercial-length transducers can be adopted. This work presents a novel McKibben muscle prototype with an embedded capacitive-length transducer. The latter is a cylindrical capacitor made of a telescopic system composed of two tubes: one of its ends is connected to the muscle. A change in the length of the muscle causes a proportional change in the transducer capacitance. The paper reports in detail on the working principle of McKibben’s muscle, its fabrication, characterization, and validation of four prototype capacitive transducers. The results achieved from the experimental activities demonstrate that it is possible to control the variations of the muscle length relative to its elongation and compression for values less than 1 mm. This is the consequence of the ability to measure the transducer capacitance with a typical statistical relative indetermination better than 0.25%, which is a figure of merit for the reliability and mechanical and electrical stability of the proposed McKibben muscle prototype. Moreover, it has been demonstrated that the transducer capacitance as a function of the muscle length is linear, with maximum deviations from linearity equal to 2.44% and 5.22% during the muscle elongation and compression, respectively.