Classical Sliding and Generalized Variable Structure Controls for a Manipulator Robot Arm with Pneumatic Artificial Muscles

Classical Sliding and Generalized Variable Structure Controls for a Manipulator Robot Arm with Pneumatic Artificial Muscles

Lamia Melkou (Automation and Robotics, Advanced Technologies and Development Center (CDTA), Algiers, Algeria) and Mustapha Hamerlain (Automation and Robotics, Advanced Technologies and Development Center (CDTA), Algiers, Algeria)
Copyright: © 2014 |Pages: 24
DOI: 10.4018/ijsda.2014010103
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Abstract

Service robotics is a domain in full effervescence because it allows a human being to interact directly with a robot while guaranteeing both safety and comfort to the human. The pneumatic artificial muscle (PAM), as an actuator, has become a solution increasingly adopted in the applications of service robotics because it provides a robot with joint compliance comparable to that of the human body. Although possessing known qualities, the PAM's nonlinearities make it one of the actuators the most difficult to model. This inconvenience limits the use of classical control as it can result in an unexpected or unwanted behavior of the system. It is thus advisable to opt for robust control algorithms to deal with these problems. Amongst robust controllers, the Classical Variable Structure (CVS) control generating a sliding mode is implemented. This control law is known by its robustness versus modeling errors, parametric uncertainties and external matched disturbances. However, the main disadvantage of this control is the appearance of high frequency oscillations once the sliding surface is reached. This phenomenon known as chattering can cause precision loss and premature actuators' wear. Results in both simulation and experiment show that these oscillations are due to the discontinuous component of the control. Numerous solutions exist for its attenuation. One is presented in this paper, the Generalized Variable Structure (GVS). The objective of this work is the synthesis and implementation of Classical and Generalized Variable Structure Control for a manipulator robot arm actuated by PAMs.
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Introduction

A number of applications in Robotics are mainly based on safer robots. A service robot is characterized by its ease of adaptation to its environment, its capacity to execute non-repetitive tasks and its ability to interact and cooperate with humans. Such a robot is actuated by what is called compliant actuators whether active or passive. The pneumatic artificial muscle is a passive compliance actuator. By analogy with the human muscle, robots using pneumatic artificial muscles (PAMs) were developed (Asami, 1994). Invented in the 1950s by physicist Joseph L. McKibben (Morales, 2008), the artificial muscle was intended for the motorization of orthotics human arm. Thereafter, it became increasingly adopted in applications of service robotics because it provided the robot with a natural compliance comparable to that of the human muscle. In this domain, our work focuses on a robot arm that retains the properties inherent to the human arm such as joint flexibility and natural capacity of contact. This robot arm is motorized by FESTO PAMs. A FESTO PAM is actuated by gas pressure and has linear movements. Despite their benefits, PAMs are complex dynamic systems which introduce strong nonlinearities inherent to the pneumatic systems, in particular hysteresis and dry friction. Because of this, their implementation in terms of automation is very delicate and they are extremely difficult to model. Modeling pneumatic artificial muscles has been the subject of several studies (Boitier, 1996; Swevers et al., 2000). However, any model obtained is only representative of the real system (Rezoug et al., 2011). The major inconvenience of the dynamic modeling is the calculation of the inertia matrix of the system. In order to calculate this matrix, it is necessary to know the geometrical shape, the dimensions (sizes), the mass and the volume density of each part constituting the robot and its center of gravity. Given that the flexible robot arms are made up of many different parts (geometry, size, mass,…), the dynamic model is relatively difficult to calculate. For that purpose, we shall by-pass this problem by identifying the parameters of the dynamic model. In this context, achieving a fairly accurate position control becomes complicated. Therefore, it is advisable to choose robust control algorithms to deal with these problems. Different approaches to control PAMs have been proposed for example: control by neural networks, adaptive control, the H1 control and variable structure control with sliding mode (Chettouh et al., 2008b; Hamerlain, 1995; Huang et al., 2013; Mefoued et al., 2011). Variable structure control by sliding mode (CVS) is one of the influential nonlinear controllers in certain and uncertain systems which are used to present a methodical solution for two main important controllers’ challenges, namely stability and robustness (Piltan et al., 2011a). The main characteristic of variable structure control with sliding mode is the commutation during synthesis on an a priori chosen surface, called sliding surface. The controlled system is then said to be in sliding mode and its dynamics becomes less sensitive to parameter variations, modeling errors and matched external disturbances (Braikia et al., 2010; Piltan et al., 2011b). Furthermore, it provides an independent behavior of linear or non-linear processes. All these features have encouraged us to implement the variable structure to control the robot arm actuators in our laboratory. However, the discontinuity of the sliding mode control generates oscillations which may excite the high frequency system dynamics. This phenomenon known as chattering is the major disadvantage of sliding mode control, because it can cause loss of precision and premature wear of the actuators (Utkin, 1993). As a cure for chattering, several solutions have been adopted. Some are second order sliding mode algorithms, proposed in (Emelyanov et al., 1993; Punta, 2005) for SISO nonlinear systems. Among them are the well-known twisting and super twisting algorithms (Braikia et al., 2011; Chettouh et al., 2008a; Rezoug et al., 2011), the one which is proposed in this paper is the control based on the GVS (Angulo, 2012; Defoort et al., 2012) detailed in (Fliess et al., 1990; Hamerlain et al., 2001). Our work is illustrated by practical experiments performed on a manipulator robot arm with two degrees of freedom actuated by PAMs. The obtained results show the performances of the control used. Robustness of this control versus external disturbances is proven by applying an external effort (push) on the arm.

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