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Electrothermal microactuators operate under the principle of
resistive heating, meaning heat is generated in conducting
arms which in turn causes expansion and thus deflection from
the normal position. The devices normally use DC current
which when disconnected causes the arms to cool and return
to the normal position.
Polysilicon
was
the material
of choice
because it is suitable for surface micromachining techniques
and doping amounts can easily control its’ resistance.
Two designs were found to be the most successful; a hot-cold
arm actuator and an opposing hot arms design.
The best deflection was measured at 11.9µm for a hot-cold
arm design,
with
an input power of 39mW. The opposing beams design achieved a
deflection of 10.9µm
with
an input power of 303mW. The deflection characteristics of
both designs
were
in
good agreement
with theoretical model.
Hot-Cold Arm
(by
Gary O'Neill - MEng. Student 2003 and
Richard Parke - MEng. Student 2004)
The
actuator
design
is based on the bimorph effect, which relies on the
difference in thermal expansion of variable microstructure
sections. A narrow arm (‘hot’
arm)
is positioned parallel to a wider section (‘cold’ arm). The
electrical resistance of the ‘hot’ arm is higher than the
‘cold’ arm, so when electric current passes through the
actuator, the ‘hot’ arm is heated to a higher temperature.
The ‘hot’ arm expansion causes the tip of the device to
rotate about a flexure element.
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