Copyright © IFAC Mechatronic Systems, Sydney, Australia, 2004 ELSEVIER IFAC PUBLICATIONS www.elsevier.comllocule/ifac MODELLING AND CONTROL OF A TlNI SHAPE MEMORY ALLOY (SMA) ACTUATOR Hugo Ramirez, Winncy Y. Du Departmenl of Mechanical & Aerospace Engineering San Jose State University San Jose, (',-1 95192-0087, USA Ramire=giles!ClJ, aol cam: ll'dll(memail. S;SIl. edll Abstract: TiNi SMA is a good candidate for nanoactuation due to its low operation voltage, large range of recoverable deformations, and high recovery forces. This paper addresses the modelling and control of a TiNi SMA wire in free-standing configuration. The dynamic behaviour of the wire during the cooling-heating phase under the normal operation condition (2- 3V) and the off- norlllal operation condition (1.2- 1.8V or 3.2- 3.4 V) are studied. A first-order model is employed to characterize the system. A hybrid input shaping, consisting of normal and off-normal operation voltage, is developed to achieve the ideal wire position control. A vision system measures the displacement of the wire, and then sends the position information to the controller to adjust the input to achieve the ideal wire position. Copyright © 200-llFAC Keywords TiNi Shape Memory Alloy (SMA), Nanoactuation, Input Shaping, Visual Feedback Control, Machine vision, System Modelling. I. INTRODUCTION Nanotechnology has received a great deal of attention due to its high potentials to bring more energy dense batteries. higher capacity disk drives. ultra small Sill art s ensors and arrays. etc. A mong the varied number of design solutions being otfered to address the current needs in nanotechnology , nano-actuators are considerably important. The existing materials and . properties, such as piezoceramics, piezoelectronics, magnetostrictives. thermal effect, and shape memory alloys (SMA), are being studied to investigate their potential appl ications as nanoactuators (Frantz, et al.. ::002; Gill, and 1\1 omoda, 200 I). Currently magnetostrictive actuators, ferromagnetic crystals that change their shape when subjected to a magnetic field. are in use. This actuation method offers large output forces and quick dynamic re s ponses at the cost of a small displacement. Similarly. piezoceramics, such as piezoelectric thin- tilms. in the form of sensors and actuators are well established. Their advantages over electrostatic, electromagnetic, and electrothermic actuators , are low energy loss, fast response, and less area consumption. On the other hand. thermal effect actuators such as nickel titanium thin film shape mcmOJY alloys. operate at TTL compatible voltages and produce more useful work per unit volume than all other actuating mechanisms . Furthermore, TiNi hils a large range of recoverable deformations, high rccovery forces. which are about 10 times greater than in piezoelectric materials, and high recoverable strains. These properties are desired for high precision M EMS manufacturing and assembling applications, Similarly, these properties make them 435 good candidates for nanoactuation (Grant and Hayward, 1997; Krulevitch, et al., 1996). Another important requirement for nanoactuators is to produce the actuation force as close as possible to the point of application when assembling nanomachines . In this arena, conventional actuators such as electromagnetic and piezoelectric, have not been found to scale well due to fabrication techniques such as photolithography and sputter deposition. However, the shape memory alloys do. In practice, it is important to note thilt the joule heating required to actuate miniature shape memory alloys is difficult to implement through electrical connections. Therefore. other means, such as energy beam actuation, needs to be exploited. An attempt at actuating this type of nanoactuator using an electron beam has already been successfully implemented (Johnson, 1990; Surbled, et af, ::00 I). Nevertheless , the development of a control strategy that would ultimately enable the use of a working device is still in the works. This paper aims at modelling the dynamic behaviour of a SMA wire under both normal and off-normal operation conditions and developing an effective control scheme to control the NiTinol SMA thin wire length (or the end position) using a vision feedback. The paper is organized as follows. Section 11 introduces the Shape Memory A Iloy actuators. Section III describes the experimental setup . Section IV focuses on the dynamic modelling of the SMAs. Section V devotes to the control strategies. Section VI discusses the machine vision feedback. Finally, Section VII gives the conclusions.