Pneumatic muscle actuator is a type of muscle-like actuator that mimics human skeletal muscle action through an airtight elastic tube. This unique type of actuator is able to generate a large output force with a very lightweight structure, and thus has been used in various robotic systems. In this paper, the authors further expand the performance and functionality of pneumatic actuator by introducing a unique modification to the actuator structure. Specifically, a telescoping insert is integrated to the center of the actuator. This insert serves dual purposes. First, by eliminating the central space in the pneumatic muscle, this structural change increases the force output and reduces the energy consumption of the actuator. Second, the insert incorporates an additional pneumatic chamber at the center of the actuator, which enables the actuator to generate an extension force and become a double-acting actuator. Comparative experimental results demonstrated the advantages of the new actuator over the traditional pneumatic muscle with respect to the actuation force over the entire range of motion. Furthermore, a design example, knee actuation mechanism in a powered leg prosthesis, is presented to illustrate the application of the new actuator. To provide the desired performance, a double-acting sleeve muscle drives the knee joint through an inverted crank-slider mechanism. A graphic comparison shows that the actuation system is able to provide sufficient torque to support a 75 kg user’s level walking and stair climbing.

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