DESIGN, MODELING AND OPTIMIZATION OF PIEZOELECTRIC ACTUATORS

Open Access
Author:
Kommepalli, Hareesh Kumar Reddy
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 15, 2010
Committee Members:
  • Christopher Rahn, Dissertation Advisor
  • Christopher Rahn, Committee Chair
  • Srinivas A Tadigadapa, Committee Chair
  • Ashok D Belegundu, Committee Member
  • Md Amanul Haque, Committee Member
  • Christopher L Muhlstein, Committee Member
Keywords:
  • Tbeam
  • unimorph
  • piezoelectric actuators
  • uniflex
  • flextensional
  • MEMS
Abstract:
Microactuators provide controlled motion and force for applications ranging from RF switches to microfluidic valves. Large amplitude response in piezoelectric actuators requires amplification of the small strain, exhibited by the piezoelectric material, used in the construction of such actuators. This research studies a uniflex microactuator that combines the strain amplification mechanisms of a unimorph and flexural motion to produce large displacement and blocking force. The design and fabrication of the proposed uniflex microactuator are described in detail. An analytical model is developed with three connected beams and a reflective symmetric boundary condition that predicts actuator displacement and blocking force as a function of the applied voltage. The model shows that the uniflex design requires appropriate parameter ranges, especially the clearance between the unimorph and aluminum cap, to ensure that both the unimorph and flexural amplification effects are realized. With a weakened joint at the unimorph/cap interface, the model is found to predict the displacement and blocking force for the actuators fabricated in this work. This research also compares the performance of a uniflex actuator in terms of its displacement and blocking force with uniflex and flextensional actuators. Analytical models for displacement and blocking force for all the three actuators are used in optimization, to study their relative performance. The uniflex actuator outperforms both unimorph and flextensional actuators in displacement, but, the unimorph actuator generates more blocking force. The uniflex actuator can therefore be used in applications that demand higher displacement and lower blocking force compared to a unimorph actuator. This research introduces a novel T-beam actuator that can be fabricated by micromachining using a piezoelectric MEMS fabrication process or by dicing using a saw. With a T-shaped cross-section, and bottom and top flange and web electrodes, a cantilevered beam can bend in-plane and out-of-plane with unimorph actuation in both directions. Analytical models are developed to predict displacement, blocking force, and mechanical energy. Six prototypes of these T-beam actuators are fabricated by dicing and electrodes are deposited by photolithography and experimentally tested. The experimentally validated models are used to optimize the cross-section geometry for maximum displacement, blocking force, and mechanical energy. It is found that a cross section with ratio of web width b to total width s , b*, and flange thickness, t, to total height, h, t*, approaching zero produces maximum displacement. Also, the tip displacement is independent of bounding box of T-beam. The cross section with b*=t*=0.381 produces maximum blocking force, while, b*=0.25, t*=0.33 produces maximum mechanical energy. A properly designed T-beam has better free tip displacement per unit cross section area than a unimorph. Also, a flange actuated T-beam requires lower voltage than a unimorph to generate same electric field.