Page 237 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim
P. 237
APPLICATIONS 217
Figure 7.59 Micrographs of SMA actuators made by MSL: (a) spherical elastic joint type placed
on experimental bench; (b) single-spring type; and (c) multiple overlapping spring type. From
Ballandras et al (1997)
a TiNi alloy, the phase-transformation temperatures are dependent primarily on the ratio
of Ti to Ni and are in the range of 10 to 100°C. The austenite phase has an associated
parent shape, so that when deformed martensite is heated, the material exerts a force to
re-attain the austenitic shape. This force is the basis of the actuation process. The amount
of shape recovery in unconstrained material depends on the level of strain in the deformed
martensite.
The assembly technique described in the preceding paragraph permits the fabrication
of various single module SMA actuators with dimensions of several millimeters. A device
with five modules is shown in Figure 7.59 (Ballandras et al. 1997) with a total length of
2 cm. By applying 2 V direct current (DC), a large displacement of 2 mm is obtained,
and this corresponds to 20 percent of the total length of the actuator. The maximum
current in the SMA wires was found to be 300 mA. The actuator dynamics appears to be
very stable with regard to the very large number of actuating cycles, and this means that
the MSL structures are stiff enough to ensure the return of the actuators to their initial
position. It should be pointed out that the fabricated actuators only weigh a few grams,
which is advantageous when using these actuators as manipulators in robots because less
energy is needed to move the actuators themselves.
A micropump has also been fabricated by MSL (Corrozza et al. 1995). The micropump
comprises a pump body and an actuator consists of a disk of piezoelectric ceramic disk
glued to a thin brass plate (Figure 7.60). The pump body is fabricated by MSL with a
complicated geometry to realise the optimal hydraulic performance and consists of two
ball valves, a pumping chamber, and inlet and outlet channels. Each ball valve has a
