Microsensors and microactuators are considered to be the most crucial elements
of micro-electromechanical systems (MEMS) and devices. There has been growing interest
in the development of new microactuator technologies with an increasing requirement
for low cost microswitch arrays providing large air gap and large force at the
same time. In particular, large air gap/large force microactuators are essential for high
voltage switching in automobile electronics, test equipment switchboards and in network
remote reconfiguration. The necessity to reduce the size of actuators and at the
same time increase the force and the air gap has placed severe constraints on the suitability
of current microactuator technology for various applications. This has led to the
development of new actuator technologies based on novel materials or modifying existing
systems. As an effort in this direction, this thesis presents the details of the work on
the design, fabrication and testing of a new hybrid microactuator, combining electromagnetic
and piezoelectric actuation mechanisms.
The design and fabrication of electromagnetic actuators using planar coils and a
soft magnetic core has long been established. However, in many instances these designs
are constrained by difficulties in the fabrication of the multi layer planar coils, which is
tedious, often resulting in a low yield. Hence device performance is limited by the
maximum coil currents and thereby the maximum force able to be generated. In order to
overcome these problems, a hybrid actuator combining the electromagnetic system
along side of a piezoelectric actuation is proposed. This has been demonstrated to assist
in enhancing the total force and consequently achieving larger actuator displacements.
In this research a hybrid microactuator with a footprint of 10 mm2 was designed, fabricated
and tested. It can generate 330 쎠force and cover 100 쭠air gap as a microswitch.
Piezoelectric actuation has been used for many applications, due to its high precision
and speed. In these applications, piezo-ceramic materials, such as PZT and ZnO
were commonly used because they exhibit large piezoelectric coefficients. However,
there are also some difficulties associated with their use. Piezoelectric ceramic materials
are usually brittle, and have a relatively large Young?s modulus, thus limiting the
achievable strain. Furthermore, the deposition technologies required for preparing
thin/thick films of these ceramic materials need extensive optimization. Patterning these
films into required structures is also difficult. Hence, piezoelectric polymer polyvinylidene
fluoride (PVDF) is chosen in this work in spite of the fact that these materials
have relatively lower piezoelectric coefficients. However, the low numerical Young?s
modulus values of these polymers facilitates large strain in the piezoelectric actuators.
The hybrid microactuator designed in this work comprises a piezoelectric composite
polymer cantilever with a planar electromagnetic coil structure beneath. The
composite cantilever consists of polarized piezoelectric polymer PVDF with an electroplated
permalloy layer on one side. The device includes a permalloy core at the centre
of a copper micro coil with a permanent magnetic film attached on the other side of the
silicon wafer (substrate) and is aligned axially with the permalloy core. The cantilever is
suspended from an electroplated 150 mm high nickel post.
Initially the principle was tested using hand wound electromagnetic coils with
permalloy wire as the core. The performance of such a hybrid actuator was evaluated. In
the next stage, a microactuator was fabricated using completely planar micro technologies,
such as high aspect ratio SU-8 lithography, laser micromachining, microembossing,
as well as copper and permalloy electroplating.
This micro device was designed by modelling and finite element method simulation
using ANSYS 7.1 and CoventorWare electromagnetic and piezoelectric solvers respectively.
This helped in understanding the critical aspects of the design at the same
time leading to the determination of the optimum parameters for the cantilever, micro
coils and the core. An analytical model has also been developed to validate the numerical
results obtained from finite element analysis.
The devices were tested and the experimental data obtained were compared with
the simulation results obtained from both the finite element calculations and from the analytical model. Good agreement was found between the experimental results and the
simulation.
| Identifer | oai:union.ndltd.org:ADTP/216575 |
| Date | January 2005 |
| Creators | Fu, Yao, n/a |
| Publisher | Swinburne University of Technology. Industrial Research Institute Swinburne |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.swin.edu.au/), Copyright Yao Fu |
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