Active Control of a Robotic Arm with Pneumatic Artificial Muscle Actuator

The paper is on the control of a robotic arm actuated by a pneumatic artificial muscle (PAM) system. A conventional proportional-integral-derivative (PID) controller with active force control (AFC) is employed to control the robot arm assigned to operate a prescribed task. Simulation results show the effectiveness of the proposed control strategy compared to the PID control only scheme. The results of the simulation study demonstrate the intense robustness and effectiveness of the proposed control scheme in countering the loading and operating conditions compared to the PID control scheme. INTRODUCTION Pneumatic muscle system was first developed by McKibben in the 1950's as soft actuators in artificial limbs and became commercially available in the 1980's (Caldwell et al., 1993). They have since been used as actuators in high-tech robotic applications, in physical therapy for functional healing and for strength escalation devices involving humans, where the devices have to be self-contained and carried for long distances. McKibben pneumatic artificial muscles (PAM) possess all the advantages of traditional pneumatic actuators (i.e, cheapness, quickness of response, high power/weight and power/volume ratios) without the main drawback (i.e. compliance or sponginess). For this reason, they are finding increased use in robotic systems. PAM technology is currently under study for use in exoskeleton suits to be WOl11 by humans for force and/or mobility assistance. A difficulty inherent in PAM technology for use in precision and/or force applications is the difficulty in controlling them precisely. This is because they are nonlinear and time varying (i.e., since they are made of flexible rubber or plastic, their characteristics vary with temperature and PAM temperature varies with use) (Lilly, 2003). Due to their high nonlinearities, advanced control strategies like adaptive control (Tanaka et al., 1999), adaptive control based on neural networks (Tao and Kokotovic, 1996), nonlinear pID control (Thanh and Ahn, 2006) have been investigated and explored. In this paper, AFC with PID scheme is proposed in order to improve the control performance of the PAM manipulator. The pID element provides the basic stable and reliable performance while the AFC strategy is used to suppress the disturbance so as to ensure a robust and effectiveness scheme is achieved to control the manipulator actuated by highly nonlinear actuators. The paper is structured as follows: First, the McKibben pneumatic artificial muscle (PAM) is briefly described in terms of its construction and operation. The next section is on the application of a feedback control method using AFC with PID to a single link mechanical arm. It is followed by modelling and simulation study. The results obtained from the study are discussed in the next section and finally a conclusion is derived with indication for future work and direction of the continuing research study. PNEUMATIC ARTIFICIAL MUSCLE The McKibben muscle was invented in the 1950s by physician Joseph L. McKibben with the initial aim of driving an arm orthosis for his poliomyelitic daughter. Even though, at this time, it has been marketed as an orthopedic device (Schulte, 1961). in the 1960s the invention was abandoned in favour of step motors. However, in the 1980s engineers of the Japanese tyre manufacturer Bridgestone proposed a redesigned and more powerful version of the McKibben muscle called the rubbertuator (Inoue, 1988), that was proposed to motorize both soft and powerful robot arms (Eskiizmirliler et al., 2002). Such an artificial muscle consisting of a braided sheath, according to a double-helix weaving, surrounding a rubber inner tube is mainly characterized by its initial length Ln, initial radius that is depicted in Fig. 1. ••:. '18 Fig.!. McKibben pneumatic actuators relaxed (top) and inflated (bottom)-Shadow Company (Aschenbeck et al., 2006)