Model Based Compliance Shaping Control of Light-Weight Manipulator in Hard-Contact Industrial Applications

This Ph.D. Thesis investigates the interaction control problem for lightweight manipulators in industrial contexts, developing impedance control based algorithms to execute such interacting tasks, ensuring stability and improving performance of standard control schemes. Such Ph.D. Thesis has been carried out at the Institute of Industrial Technologies and Automation (ITIA) of the italian National Research Council (CNR), Intelligent Robot and Automation Systems group (IRAS). The control strategies design has been addressed developing dynamic models of the global plant based on the typical application scenario compliant robot base - controlled robot - compliant interacting environment. Each element of such interaction scheme has been studied in order to identify the complete model. Such interaction model has been used both for a theoretical analysis (i.e. stability proof and simulations) and for the implementation of the control scheme (i.e. observers definition). An in-depth study of the interaction problem for lightweight manipulator allowed the conceive of a novel family of impedance shaping controllers. Such controllers globally extend impedance variable controllers and overcome some limitations of such state-of-the-art interaction controllers. The defined controllers allow to track a target interaction force, both considering rigid and compliant robot bases scenarios, shaping the impedance of the global system, adapting the impedance control parameters while eventually compensating the dynamics of the compliant base. This is done tuning online both the position set-point and the stiffness and damping parameters based on both the force error and the estimate of the interacting environment (an Extended Kalman Filter is used to estimate the environment stiffness) and the robot base dynamics (a Kalman Filter is used to estimate the robot base position). The goals of such control strategies are (i) to avoid force orvershoots and instabilities (ii) while tracking a force reference and (iii) eventually compensating for the compliant robot base dynamics, (iv) maximizing dynamic performance during the free-space motion (i.e. the initial phase of the task, which leads to the first contact establishment). The novelty of the impedance shaping controllers family is inherent to the ability of the controllers to adapt whole the parameters while estimating the interacting environment and taking into account the robot base dynamics. To validate the proposed control schemes a real assembly task has been selected as application. The task has been performed without knowing the environment's geometrical and mechanical properties. Results show the effectiveness of the proposed control strategies, compared with control schemes in literature (in particular, a second order explicit force tracking controller based on the impedance control), that show force overshoots, instabilities and lower dynamic performance.