Air muscle is essentially a robotic actuator which is replacing the conventional pneumatic cylinders at a rapid pace. Due to their low production costs and very high power to weight ratio, as high as 400:1, the preference for Air Muscles is increasing. Air Muscles find huge applications in biorobotics and development of fully functional prosthetic limbs, having superior controlling as well as functional capabilities compared with the current models.
The Air Muscle consists of an inner rubber tube, which is often made from pure rubber latex. It is surrounded by a braided mesh.
The header at each end of the muscle consists of an Aluminium ring, and a Delrin plastic bung, with a female thread. This thread can be used as a means of attachment, and to allow air into or out of the muscle. The muscle is supplied with two Delrin fittings also.
The inner rubber tube is inflated by entering air at a pressure, usually limited to 3.5 bar. The movement of this tube is constrained by the braid. When the tube gets inflated it experiences a longitudinal contraction. This would create a pull at both ends of the tube. Usually one end of the tube will be attached to somewhere so that force can be applied from one end. This pull when effectively utolised could provide the necessary motion. The working of the Air Muscle closely resembles that of the natural muscle and hence the name Muscle given to it along with Air. The figure below shows the physical appearance of the muscle at different stages of its working.
To measure the force-velocity properties of the McKibben actuator, a series of experiments were conducted with the axial-torsional Bionix (MTS Systems Corp., Minnesota, U.S.A.) tensile testing instrument. Actuators of three sizes were constructed and tested. Each experiment measured the force output at a constant pressure over the contraction range at various velocities. One end of the actuator was rigidly attached to the load cell while the other end was moved in response to the instrument’s digital controller. Step velocity profiles were applied such that one end of the actuator was rapidly accelerated and held to a constant velocity until the end of the actuator’s working length was reached. Input step velocity profiles tested included 1, 10, 25, 50, 100, 150, 200, 250, and 300 mm/s for concentric contractions and 1, 10, 25, 50, 100, and 150 mm/s for eccentric contractions. Up to 500 mm/s is possible; however, instantaneous fluctuations in velocity of 15 percent were measured during trails at 500 mm/s. The magnitude of these fluctuations decreased at lower velocities, and was less than 9 percent at 300 mm/s and 6 percent at 200 mm/sec. This anomaly is thought to arise from the hydraulic pump.