| In nearly all machines with moving parts, problems such as non-negligible tracking errors, limit cycles and undesirable stick-slip motions are encountered which are not only inconvenient, but also unavoidable. Several strategies have been investigated in the literature to address this issue. We first present a historical review of the modeling and control strategies that have been developed for various frictional servo-systems.; We then propose our control scheme in a general and abstract form. The main component of this scheme is based on a dynamic output linear controller design and a LuGre friction observer dynamics, as the behavior of the LuGre model estimator is rich in terms of friction compensation and positioning-tracking, especially at zero crossings of the velocity. Our full-order dynamic output feedback controller is composed of two components: one that deals with the tracking problem and another that serves as a correction term in the observer for position and velocity errors. The analysis of the whole feedback dynamics is based on the following three features: interconnection, passivity and optimization. The subsequent linear state space controller matrices are found by using the Linear Matrix Inequality (LMI) approach. For purposes of simplification, we address the output feedback positioning-tracking of a simple mass moving with friction. Simulation results illustrate the effectiveness of the proposed compensator.; In an initial complex application, a design for a motion/force controller for a constrained servo-robot, based on a common modeling structure, is proposed. Since the contact between the end-effector and the environment is subject to friction, the control plant is based on the LuGre friction closed-loop observer. We therefore propose new nonlinear position and force input transforms, which are slightly different from classical computed torques, combined with a change of variable. The performances are validated experimentally on a 2R robot manipulator acting on a horizontal worktable with friction.; In a second application, we deal with the dynamic pneumatic actuators that are characterized by their complexity and their hard system control achievements. In the first step, we develop an accurate closed-loop force control technique for a pneumatic actuator, as this is an essential stage in the implementation of any positioning-control strategy. Since an analytical nonlinear structure, which is dependently affine on parameter uncertainties, generically characterizes pneumatic plants, a feedback linearization design is proposed to cancel out most of the resulting nonlinearities. We then propose a linear state feedback control and an additive nonlinear action to robustly bound the force-error dynamics. These devices are required to handle the further parametric uncertainties and exogenous unbounded disturbances that will arise on the deduced structure. The linear control gains are designed within robust closed-loop pole clustering using an LMI approach. In the second step, we deal with a high-friction pneumatic actuator positioning technique based on a LuGre friction closed-loop observer dynamics. The inner force control loop described above is regarded here as a complementary feature of the proposed technique. Our main goal is to establish the stability condition by using the passivity of interconnected linear and nonlinear sub-plants to deal with the varying and uncertain parameterization of friction modeling and exogenous bounded inputs resulting from the force-loop dynamics.; Various experimental results illustrate the validity of the components of our approach in different hard systems. |
