Page 268 - Mechanics of Microelectromechanical Systems
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4. Microtransduction: actuation and sensing 255
multimorph. The SMA layer, being at low/martensitic temperature, can easily
be deformed, as driven by the PZT deformation.
If a temperature increase is subsequently applied so that the SMA’s
martensite phase transforms into austenite, the SMA layer will shrink,
becoming the actuator, and the multimorph will further bend. If no voltage is
supplied to the PZT during the SMA actuation, the PZT can act as a sensor
because the change in strain translates in a proportional voltage change. As a
consequence, the sandwich device can function as both actuator and sensor by
switching the functions of the PZT layer.
It is of interest to quantify the level of actuation, as well as the
relationship defining the voltage detection during the second phase, when
SMA is the actuator and the PZT layer is the sensor. By using the generic
multimorph model, it is possible to calculate the curvature radius R, as well as
the corresponding tip moment acting on the bender. At the same time, the
voltage generated through SMA-driven deformation into the PZT layer can be
evaluated by considering the strain on its lower fiber as being produced
through bending and axial deformation, namely:
where the axial force acting on the PZT layer is:
and the curvature radius corresponding to SMA-generated bending is
expressed as:
where:
and: