Sliding Mode Control Applied to a Novel Linear Axis Actuated by Pneumatic Muscles

Abstract

Pneumatic muscles are innovative tensile actuators consisting of a fiber-reinforced vulcanised rubber tubing with appropriate connectors at both ends. The working principle is based on a rhombical fibre structure that leads to a muscle contraction in longitudinal direction when the pneumatic muscle is filled with compressed air. This contraction can be used for actuation purposes. Pneumatic muscles are low cost actuators and offer several further advantages in comparison to classical pneumatic cylinders: significantly less weight, no stick-slip effects, insensitivity to dirty working environment, and a higher force-to-weight ratio. A major advantage of pneumatic drives as compared to electrical drives is their capability of providing largemaximum forces for a longer period of time. In this case electrical drives are in risk of overheating and may result in increasing errors due to thermal expansion. For these reasons, different researchers have investigated pneumatic muscles as actuators for several applications, e.g. a planar elbow manipulator in Lilly & Yang (2005), a 2-DOF serial manipulator in Van-Damme et al. (2007) or a parallel manipulator in Zhu et al. (2008). Pneumatic muscles are characterised by dominant nonlinearities, namely the force and volume characteristics. Hence, these nonlinearities have to be considered by suitable control approaches such as sliding mode control. In this contribution the sliding mode technique is applied to a novel linear drive actuated by four pneumatic muscles. This pneumatic linear drive allows for maximum velocities of approximately 1.3 m/s in a workspace of approximately 1 m. In Aschemann & Hofer (2004) and Aschemann et al. (2006) the authors presented the implementation of a trajectory control for a carriage with a pair of pneumatic muscles arranged at opposite sides of a carriage. Unfortunately, this direct actuation by pneumatic muscles suffers from two main drawbacks: On the one hand, the maximum velocity of the carriage is limited to approx. 0.3 m/s, on the other hand the workspace is constrained to the maximum contraction length of the pneumatic muscles, in the given case to approx. 0.25 m. To increase the available workspace as well as the maximum carriage velocity, a new test-rig has been built up. At this test-rig, a rocker transmits the drive force of the pneumatic muscles to the carriage, see Aschemann & Schindele (2008) or Schindele & Aschemann (2010). One disadvantage of this setup is the required height, necessary for the kinematics considered there. To reduce the overall size of the drive mechanism, now, the muscle force is transmitted to the carriage by a pulley tackle consisting of a wire rope and several deflection pulleys, see Fig. 1. The mentioned components are installed such that the required muscle force as well as the maximum workspace and velocity of the carriage are Sliding Mode Control Applied to a Novel Linear Axis Actuated by Pneumatic Muscles 19