Experimental Study of Active Vibration Control of Planar 3-RRR Flexible Parallel Robots Mechanism

Hindawi Publishing Corporation Shock and Vibration Volume 2016, Article ID 4780181, 17 pages http://dx.doi.org/10.1155/2016/4780181 Research Article Experimental Study of Active Vibration Control of Planar 3-RRR Flexible Parallel Robots Mechanism Qinghua Zhang,1 Xianmin Zhang,2 and Junyang Wei2 1 Department of Mechatronics, Foshan University, Foshan, Guangdong 528000, China Key Laboratory of Precision Equipment and Manufacturing Technology of Guangdong Province, Wushan Road, Tianhe District, Guangzhou 510641, China 2 Correspondence should be addressed to Xianmin Zhang; Received 21 April 2016; Revised 15 July 2016; Accepted 20 July 2016 Academic Editor: Francesco Ripamonti Copyright © 2016 Qinghua Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. An active vibration control experiment of planar 3-RRR flexible parallel robots is implemented in this paper. Considering the direct and inverse piezoelectric effect of PZT material, a general motion equation is established. A strain rate feedback controller is designed based on the established general motion equation. Four control schemes are designed in this experiment: three passive flexible links are controlled at the same time, only passive flexible link 1 is controlled, only passive flexible link 2 is controlled, and only passive flexible link 3 is controlled. The experimental results show that only one flexible link controlled scheme suppresses elastic vibration and cannot suppress the elastic vibration of the other flexible links, whereas when three passive flexible links are controlled at the same time, they are able to effectively suppress the elastic vibration of all of the flexible links. In general, the experiment verifies that a strain rate feedback controller is able to effectively suppress the elastic vibration of the flexible links of plane 3-RRR flexible parallel robots. 1. Introduction Recently, flexible parallel robots have been intensively studied because of their excellent performance. However, the elastic vibration problem of such robots is serious due to their inertial and driving forces. As a result, suppressing the unwanted elastic vibration is currently a very significant and challenging problem. There are basically five design philosophies for suppressing unwanted elastic vibration that are classified into passive vibration control and active vibration control [1]. Passive vibration control means that unwanted vibration is suppressed by using advanced composite materials, optimizing the cross-sectional geometry of the elements, or using additional damping materials. Active vibration control means that unwanted vibration is suppressed by introducing a microprocessor-controlled actuator into the original mechanism or embedding some smart structures (actuators and sensors) into flexible elements. The surface-bonded lead zirconate titanate (PZT) sensor/actuator has been widely used for active vibration control of the flexible structure [2, 3]. PZT materials are able to dampen vibration and measure the vibration of distributed parameter systems because of their direct and inverse piezoelectric effect [4]. Photostrictive actuator as a new promising noncontact photoactuation technique was used for suppressing vibration of flexible structures [5]. During the past few decades, the question of active vibration control of flexible robots has been paid considerable attention, as indicated in survey papers [6–9]. Although there are many theoretical and experimental studies of active vibration control in space-based flexible structures and simple flexible beams [10–13], PZT materials are introduced into the vibration control of manipulators or mechanisms that only have a single link with a single actuator and sensor bonded [14, 15]. Meanwhile, compared with numerical simulation, the experimental study of the active vibration control of flexible manipulators is a more challenging work, especially for flexible parallel robots. Zhang et al. [16] have performed an experimental study of active vibration control of 3PRR flexible parallel robots, for which the elastic vibration of flexible links during motion is suppressed by an SRF 2 controller. The KED assumption is utilized. Zhang et al. [17] address the dynamic modeling and efficient modal control of a planar parallel manipulator (PPM) with three flexible linkages actuated by linear ultrasonic motors (LUSM). We performed studied dynamic modeling and dynamic analysis of planar 3-RRR flexible parallel robots [18–21]. The residual vibrations of the flexible links of planar 3RRR flexible parallel robots are studied in this paper. In fact, the amplitudes of the residual vibration are greater than the elastic vibration during the motion and will be verified in the experimental study of this paper, and these residual vibrations influence the repeat positioning accuracy of the system seriously. Thus, it is important to suppress the residual vibration of flexible links to improve the repeat positioning accuracy of the system. Essentially, the dynamic responses measured by PZT sensors include unmodeled or unknown dynamics. Therefore, the vibrations of the flexible elements are very complicated and contain many frequency components that are closely spaced. To measure the modal coordinates or velocities more precisely, it is desirable to use as many PZT sensors as possible. However, the number of PZT sensors is limited by the available space of the flexible links and the real-time computing power of the computer. A trade-off must be made between the real-time computing power of the computer hardware and the number of sensors [22]. To solve the above problems, an experimental study of the active vibration control of planar 3-RRR parallel robots with three flexible links, each of which bonds with two pairs of PZT actuators and one PZT sensor film, is implemented in this paper. The moving platform of the planar parallel robots moves along a given trajectory, control experiments are implemented in real-time, and the controller is activated when rigid motion stops. The active control system consists of an industrial computer, a dSPACE controller with many I/O ports, smart flexible links with PZT actuators and PZT sensors, and a PZT driven power and charge amplifier. A strain rate feedback control algorithm is adopted to suppress the elastic vibration of the flexible links of the system [20]. The remainder of the paper is organized as follows. The rigid-elastic coupling dynamic model is established in Section 2. According to the direct and inverse piezoelectric equation of the PZT material, a strain rate feedback control algorithm is given. Section 3 introduces an experimental system setup and describes the working principle of the controller system and each component. In Section 4, the results of an (...truncated)