Micro-electro-mechanical system (MEMS) is a batch-fabricated integrated microscale system that may involve motional, electromagnetic, radiating energy and optical microdevices/ microstructures, driving/ sensing circuitry, controlling/ processing ICs, and requires multi-disciplinary process technology to create tiny integrated devices or structures that combine mechanical, electrical, and electronics components [1], [2]. MEMS devices are usually made using microfabrication techniques or using integrated circuit (IC) batch processing techniques on a single chip and can range in size from a few micrometres
to millimetres [3], [4]. Though MEMS have the ability to sense, control and actuate at the micro/nanoscale, they have far-reaching effects extending into large scale [2].
Based on applications, MEMS are divided into four functional elements, viz. microstructures, microsensors, microactuators, and integrated microelectronics as shown in Figure 1.1.
Microsensors and Microactuators are categorised as “transducers.” Generally, MEMS microsensors convert a mechanical signal to be measured into an electrical signal and are engineered to measure physical quantities such as temperature, pressure, radiation, magnetic field, and chemical detections [5].
(b) Micro Actuators
(c) Micro Structures
(d) Micro Electronics (a) Micro Sensors
MEMS
Figure 1.1: Micro-electro-mechanical system components.
Source: Adapted from (a) http://www.geekmomprojects.com/gyroscopes-and- accelerometers-on-a-chip/, (b) http://www.bacteria-world.com/high-force-mems-
actuator.htm, (c) http://www.eetimes.com/ document.asp?_mc= RSS_EET_EDT&
doc_id=1324827, (d) http://tzjwinfcha.pixnet.net/ blog/post/25040588.
An actuator is a type of motor for displacing or controlling a system or mechanism [6].
Generally, an actuator operates with an input typically in the form of an electric current or voltage, hydraulic pressure or pneumatic pressure, and converts the input energy into motion [7]. Microactuator is a mechanism in the microscopic domain that supplies and transmits energy for the operation of a secondary system or mechanism. The four main types of actuation options available in MEMS are electrostatic, thermal, magnetic, and piezoelectric [2]. The choice of actuation depends on the nature of the application, ease of integration with the fabrication process, the specifics of the system around it, and economic justification. The desired features for microactuators are large travel, high precision, fast switching, low power consumption, power-free force sustainability, micro-structurality, and integrability [3].
Popular microactuators developed include microresonators, micropumps to develop fluid pressure, microvalves for control of gas and liquid flows, optical switches and mirrors to
redirect light beams, interferometric modulation based displays, RF switches and others [5].
In 1948 Williams et al.[8] developed a piezoelectric motor consisted of four piezoelectric rectangle elements bonded to the four faces of a square bar as shown in Figure 1.2. The bending modes of the rectangular bar were excited by 90° out-of-phase signals resulting in a vibratory movement of a piezoelectric crystal element translated into rotary motion at one end of the bar with respect to the other end.
A piezoelectric motor was introduced by Lavrinenko in 1965 [9] as shown in Figure 1.3. The motor was made up of a piezoelectric plate pressed against a rotor and was unidirectional, i.e. only one mode on a vibrator is excited. One, or multiple, elastic vibration couplers oriented obliquely and attached either on a spinning element or a vibrating element transfer the vibration to the rotating element. In the previous structures, circular piezoelectric plates were used along with the rectangular piezoelectric plates, of which
Figure 1.2: Piezoelectric motor Source: Adapted from (a) Williams et al.[8]
longitudinal modes were excited, to convert radial vibratory motion into tangential motion via angularly-attached lamina [10] as shown in Figure 1.3 (b). The advance investigations of piezoelectric motors led to a recognition of fundamental design principles that following structures could be operated bi-directional [9].
Figure 1.3: Piezoelectric motor introduced by (a) V. Lavrinenko[9], (b) Wischnewskiy [11].
In 1988 Uchino et al.[12] invented a -shaped linear motor equipped with a multilayer piezoelectric actuator and fork-shaped metallic legs as shown in Figure 1.4 (a). A minor difference in the mechanical resonance frequency between the two legs, the phase difference between the bending vibrations of both legs can be controlled by changing the drive frequency. The walking slider moves in a way similar to a horse using its fore and hind legs when trotting.
In 2000 Koc et al. developed a piezoelectric micromotor known as windmill motor [13], in which uniformly electrode piezoelectric ring bonded to a metal ring is used as the stator as shown in Figure 1.4 (b). Four inward arms at the inner circumference of the metal ring transfer radial displacements into tangential displacements. The rotor ends in a truncated cone shape and touches the tips of the arms. A rotation takes place by exciting coupled modes of the stator element, such as a radial mode and a second bending mode of the arms.
The real potential of MEMS is in microelectronics where the micro– sensors, actuators, and structures can be integrated with circuits on a common silicon substrate. While the electronics are fabricated using integrated circuit (IC) process sequences, the
(a)
(b)
micromechanical components are fabricated using compatible "micromachining" processes that carefully etch away unwanted parts of the silicon wafer or adjoin new structural layers to form the mechanical and electromechanical devices [5].
This thesis mainly focuses on the research work carried out in the design, development, and analysis of the proposed dual friction-drive surface acoustic wave (SAW) motors, in which a slider is sandwiched between two piezoelectric stators producing surface waves that drive the slider with frictional force. The design, simulation, fabrication, and results of experiments on the linear and rotational types of proposed SAW motors with sliders with different shapes are presented in this thesis.