University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2006 • 37-39
Prototype Micromanipulator of Space Robotics Applications
ABSTRACT—Piezoelectric actuators offer dramatic improvements in a variety of space-based robotics applications. Bending piezoelectric actuators offer a simple, lightweight, and reliable means for actuating end effectors. This research produced a single degree-of-freedom micromanipulator prototype which can be used to solve open issues in bending actuator control, especially in hysteresis modeling and control.
Piezoelectric actuators have considerable potential for space-based robots. This research produced a prototype single-degree-of-freedom (DOF) micromanipulator consisting of a piezoelectric bending actuator and a capacitive position measuring system. The actuator will facilitate research to solve open issues in piezoelectric actuator control, particularly in hysteresis modeling and control.
Premises of the Project
Piezoelectric actuation is ideal for space-based robotics applications requiring inherent lightweight features, simplicity, and immunity from magnetic fields. End effectors for miniature space-based robots must also be simple and lightweight and can also benefit from immunity from magnetic fields. Therefore, a piezoelectric actuator is an ideal candidate for an end effector.
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| Figure 1. Schematic of the Principle of Operation |
A piezoelectric bending actuator (Fig. 1) consists of two layers of piezoelectric material bonded together with opposite polarity in the form of a cantilever beam. The application of an electric field to the actuator causes one layer to extend slightly and the other layer to contract slightly.1 The differential length causes the beam to bend toward the contracting layer. By controlling the applied electric field precisely, it is possible to control the movement of the beam tip. The tip of this particular actuator has a range of motion of ±0.5 millimeters. Labview version 6.0 is used to generate voltages in the range +5V to -5V which are, in turn, supplied to a piezo-linear amplifier. The piezo-linear amplifier is used as a high voltage drive source for the piezoelectric actuating device. The capacitive sensor2 senses the motion and produces an analog voltage proportional to the distance between the capacitive probe and the piezoelectric actuator. Labview 6.0 is used to record the capacitive sensor output.
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| Figure 2. Block Diagram of the Experimental Setup |
Laboratory Apparatus
The UHCL micromanipulator system will facilitate research in dynamics and control of micromanipulators. The device uses a piezoelectric bending actuator that has a range of motion of approximately ±0.5mm. A block diagram of the apparatus is shown in Fig. 2.
The apparatus consists of the piezoelectric bending actuator mounted to a mechanical breadboard and driven by a piezo-linear amplifier (Model EPA 007). The EPA-007 is a very compact high voltage linear non-inverting amplifier, which is used as a high voltage drive source for the piezoelectric actuating device. The manipulator position is measured using a commercial high-resolution capacitive position sensor (Series 4000 Capacitec amplifier) mounted to the mechanical breadboard. Labview version 6.0 is used to generate drive voltages to the piezo driver and measure capacitive sensor output.
The experimental laboratory setup is shown in Fig. 3.
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| Figure 3. Micromanipulator Experimental Setup |
Research Plan
The main components of the prototype micromanipulator were available commercially. They were integrated mechanically, and a hardware-software interface was developed to permit experimentation. Three main research issues were addressed:
Results
The actuator, capacitive position sensor, actuator driver, and sensor amplifier were assembled as shown in Fig. 3. Labview 6.0 software was developed to control the actuator driver and capture the sensor amplifier output. Using Labview software, preliminary experiments were performed to verify the operation of the system throughout the actuator range of motion. For example, the approximate linear input-output relationship between the commanded position (actuator driver command voltage) and position (amplified capacitive sensor output voltage) was measured, as diagrammed in Fig. 4.
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| Figure 4. Response of Piezoactuator as a Function of Drive Voltage |
Future Work
An extensive literature is available on the control of piezoelectric micromanipulators. Among the control techniques studied are standard linear, non-model-based techniques such as proportional-integral-derivative control, linear control techniques including robust control, and adaptive control. The inherent precision feasible with piezoelectric micromanipulators is not attainable using these techniques because the piezoelectric actuators exhibit significant hysteresis. Preliminary results on dealing with the hysteresis have recently been reported3 but the techniques discussed in the literature deal only with repetitive motion. Therefore significant work remains in order to achieve the potential of these actuators for space-based robotics and other applications.
Piezoelectric bending actuators are also ideal for flapping-wing micro air vehicles. Among the issues to resolve are precise control of flapping amplitude and tailoring of flapping motion.
Acknowledgments
This study was supported by an ISSO mini-grant for the summer of 2006.
References
1M. Novotny and P. Ronkanen, "Piezoelectric Actuators," <http://www.ad.tut.fi/aci/courses/ACI-51106/pdf/Piezo/PiezoelectricActuators.pdf>
2M. A. Ayer, Operation/Maintenance Manual: Series 4000 Capacitec Amplifiers and Rack Accessories, Ayer, MA: Capacitec, Inc., 1998.
3J. Zhong and B. Yao, "Adaptive Robust Repetitive Control of Piezoelectric Actuators," 2005 International Mechanical Engineering Conference and Exposition, Orlando, FL, 2005.
4J. Yan, R. J. Wood, S. Avadhanula, M. Sitti, and R. S. Fearing, ‚"Towards Flapping Wing Control for a Micromechanical Flying Insect," 2001 Intl. Conf. on Robotics and Automation, Seoul, Korea, 2001.
Publications
Garud A., J. B. Dabney, and T. L. Harman, "Micromanipulator Modeling and Control: Initial Experiments," UHCL Systems Engineering Laboratory Report, 2007.
Funding
Dabney, J. B., ‚"Gulf Coast Consortium of Control and Dynamics Interactive Remote Laboratory," NSF Collaborative Proposal, $54,000 (Joint proposal with UH, Rice, UH-Downtown). (Not funded.)
Dabney, J. B., A. J. Meade (co-PI), "Micro Air Vehicle Modeling and Control," Air Force Office of Scientific Research, $81,694. (Pending.)
Institute for Space Systems Operations - Y2006 Annual Report
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