Sliding Mode Control of Robot Manipulators via Intelligent Approaches

1.1 Robot manipulators Robot manipulators are well-known as nonlinear systems including strong coupling between their dynamics (Craig, 1996). These characteristics, in company with: 1) structured uncertainties caused by model imprecision of link parameters, payload variation, etc., and 2) unstructured uncertainties produced by un-modeled dynamics –such as nonlinear friction and external disturbances– make the motion control of rigid-link manipulators a complicated problem (Spong & Vidiasagar, 1989). Practice trajectory control is required in many of the sophisticated applications of manipulators (e.g. machining, welding, complex assembly). On the other hand, robot manipulators have to face various uncertainties in their dynamics and they are required to handle various tools and, hence, the dynamic parameters of the robots vary during operation. Thus, it is difficult to initiate an appropriate mathematical model for employing model-based control strategies. In general, the intelligent control approaches can attenuate the effects of structured parametric uncertainty and unstructured disturbance by using their powerful learning ability without a detailed knowledge of the controlled plant in the design processes. On the other hand, many intelligent control algorithms could have been found for the robot control system without including the actuator dynamics, while, actuator dynamics carry out a significant role in the complete robot dynamics and ignoring them may cause detrimental effects, especially in the case of high-velocity moment, highly varying loads, friction, and actuator saturations (Chang et al., 2008), (Chang & Yen, 2009). Since the electrical actuators are highly controllable in comparison with the other one, they are more convenient for driving manipulators. Also, in practical applications, the voltages or currents of the electrical actuators are accessible for applying control commands and consequently, torquebased control design confronts implementation problems when one intends to apply the torque control commands directly to actuators. Additionally, one constraint in the robot controller designs is saturation nonlinearity of actuators which is less considered in control design of robot manipulators.