Nonlinear controllers for path following: application to grinding processes

The thesis project aims to develop and compare two control strategies, based on position control and force-position control, to execute a grinding process for a rubber gasket using the UR10 robot produced by Universal Robots Company. Due to the unavailability of an actual working cell in which the machining takes place, it is necessary to build a simulated environment that includes the robot and the gasket to test the control approaches. The presentation of the project its development can be divided into two phases: first, the construction of the simulated environment, and next the design and testing of the control strategies.\newline Due to the lack of data regarding the actual work cell, various parameters including its dimensions and structure are hypothesized based on the available data. Since grinding is a process that involves contact between two objects, in this specific case between the gasket and the grinding wheel, these must also be appropriately modelled and collocated in the simulated environment. The first part of the project provides details on how these three elements are modelled and how they behave in the simulation.\newline Once the simulated environment is completed, it is possible to prosecute the main task of the thesis: propose and compare two different control strategies. The first control strategy analyzes is the position control utilizing the well-known inverse dynamic control approach. The control is composed of an inner and an external loop. The inner loop acts as the feedback linearization of the system, which is the manipulator UR10, and the external one determines the final dynamics that we want to impose on it. Next, a new control strategy is proposed, which includes a force control loop in parallel to the previous position loop based on the inverse dynamic control. This strategy utilizes a force/torque sensor and compares the simulated gasket's force response with its expected values. Using this control strategy it is possible to achieve superior performances, especially in terms of precision achieved in the grinding process measured by the average error of the cutting depth. Following the evaluation of the force-position control results, the control strategy's robustness is tested, by perturbing some key the parameters in simulated environment. In particular, the gasket's force response is considered to be twice the expected one. The results achieved with this configuration confirm that the use of two parallel control loops ensures high robustness with respect to perturbation of the nominal conditions, a common occurrence in real applications.