Abstract
In this paper, a method to control one degree of freedom lightweight flexible manipulators is investigated. These robots have a single low-frequency and high amplitude vibration mode. They hold actuators with high friction, and sensors which are often strain gauges with offset and high-frequency noise. These problems reduce the motion’s performance and the precision of the robot tip positioning. Moreover, since the carried payload changes in the different tasks, that vibration frequency also changes producing underdamped or even unstable time responses of the closed-loop control system. The actuator friction effect is removed by using a robust two degrees of freedom PID control system which feeds back the actuator position. This is called the inner loop. After, an outer loop is closed that removes the link vibrations and is designed based on the combination of the singular perturbation theory and the input-state linearization technique. A new controller is proposed for this outer loop that: (1) removes the strain gauge offset effects, (2) reduces the risk of saturating the actuator due to the high-frequency noise of strain gauges and (3) achieves high robustness to a change in the payload mass. This last feature prompted us to use a fractional-order PD controller. A procedure for tuning this controller is also proposed. Simulated and experimental results are presented that show that its performance overcomes those of PD controllers, which are the controllers usually employed in the input-state linearization of second-order systems.
Highlights
Robotic manipulators are used to assist in a wide range of tasks
The outer loop controller is designed using frequency techniques and, in particular, phase margin and gain crossover frequency specifications. We found that these two specifications have to be chosen carefully because they may lead to control systems with two problems: (1) closed-loop poles that are canceled by zeros, which implies that the robot is able to accurately track a trajectory without vibrating but, in turn, it is not able to damp the vibrations caused by external disturbances, even in the case that only small changes are produced in the robot state and (2) unfulfillment of the desired time specifications because direct correspondences between them and frequency specifications only exist in the case of low order simple systems
This work has developed a robust control system for a flexible link robots (FLR) with a single massless link that moves in the horizontal plane
Summary
Robotic manipulators are used to assist in a wide range of tasks. Most of them are designed in such a way that the vibration of the end-effector is minimized in order to achieve good position accuracy. The large thin links made of these materials that are used in FLR often have very small link-payload mass ratios and can be regarded as having a single vibration mode This greatly simplifies the dynamic models of these robots and facilitates the design of their controllers. This result was extended in [17] to a nonlinear robotic antenna with a single flexible link and two degrees of freedom (2 − DOF) by applying the input-state feedback linearization technique [18] In both papers, a control structure constituted by two nested loops was implemented: An inner loop was closed around the actuators which fed back the actuator position.
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