|Seyed Ahmad Khalilpour|
Robust Control and Stability Evaluation of a Suspended Cable Robot considering the cable Dynamics
Known for their lower costs and numerous applications, cable robots are attractive research fields in the robotic community. However, considering the fact that they require an accurate installation procedure and calibration routine, they have not yet found their true place in real-world applications. This thesis aims to propose a new control strategy that requires no meticulous calibration and installation procedures and can handle the uncertainties induced as a result of that. It should be noted that cable-driven robots generally cover a large-scale workspace. As such, the cable dynamics can not be ignored and phenomena such as cable flexibility may cause undesirable vibration. Therefore, in large cable-driven robots, the challenge of kinematic uncertainty is not the only problem to be solved and in addition to it, the cable dynamics should also be considered during the design procedure of the controller. The problem becomes more complicated when we consider the fact that in a deployable robot, accurate and expensive measurement tools are not generally available. In the field of robotics, most of the common control systems are based on a cascade structure in which the inner controller is assumed to be a fast high-bandwidth controller, and the outer controller (a user-specified control loop) is responsible for the robot’s main objective: trajectory tracking. The effectiveness of such an approach depends on the assumption that the inner loop controller is fast enough, and thus its dynamics may be ignored. In practice, such an assumption generally does not hold. Therefore, In chapter 2 of this thesis, we analyze the whole structure of the cascade controller and investigate the effect of selected structures and coefficients for the inner loop controller. In addition to this, an inner loop controller is designed to mitigate the adverse effects of dynamic uncertainties (of the actuators and power transmission systems, i.e. pulleys) on force tracking. These effects are more notable when the cables are directly driven by the motors. Chapter 3 extends the study presented in the previous chapter by deriving a model for a spatial cable-driven robot with flexible cables and employing the singularity perturbation theory for analyzing it. In this chapter, assuming a flexible dynamic model for cables, we propose a robust cascade control method and investigate the effects of inner-loop force controllers on the robot’s performance. In chapter 4, the cable model is extended and the effects of both cable mass and flexibility are investigated simultaneously. In this chapter, we employ the passivity theorem to propose a new control law which exploits an information fusion method. Finally, in order to illustrate the performance of the proposed controller, we present the results of an experiment on a deployable suspended cable-driven robot, which shows the effectiveness of the proposed controller in the presence of the uncertainties.
|2019||Ph.D.||Parallel and Cable Robotics|