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Shape Memory Alloys In Micropositioning Applications, pp. 441-485 $100.00
Authors:  E. Asua, J. Feuchtwanger, V. Etxebarria and A. Garcia-Arribas
Shape memory alloys (SMA) are a special kind of smart materials whose dimensions
can be modified as the result of a temperature-dependent structural phase transition.
This property can be used to generate motion or force in electromechanical
devices and micromachines. The use of shape memory alloys in actuators offers the
opportunity to develop robust, simple and lightweight elements for application in a
multitude of different industries. Despite all these advantages, the accuracy of SMA
actuators is severely limited by their highly nonlinear stimulus-response characteristics.
Traditionally, they have been used as “on-off” electromechanical actuators due
to the non-linear and hysteretic nature of the martensite-austenite transformation. The
ideal solution to this problem is to model the hysteresis mathematically to compensate
for it in the control loop. However, useful models of phase transitions are difficult
to obtain. In this chapter, a Nickel-Titanium alloy (Nitinol) wire is considered as the
active element in micropositioning actuators. An electric current, applied through the
wire, heats it to induce the phase transition and the consequent contraction. The purpose
of this investigation is to finely control the wire contraction. The wire needs to
be able to contract and relax to an intermediate position within its range of movement
and the gradual contraction and relaxation needs to be controllable. An experimental
micropositioning device, based on a Ni-Ti wire, is controlled using several control
strategies. To account for the hysteresis effects, an inverse hysteresis model is obtained
using two different compensating methods. When used in a controller, the inverse
models nicely compensate the non-linear and hysteretic behavior of the wire, and
a proportional-integral with antiwindup control works adequately, obtaining position
accuracies around 3 micrometers. Once the real possibility of using these materials
as micrometric actuators is analyzed and tested, a SMA based actuator prototype grip
for a flexible robot is developed, so that it could be used in light applications. For this
purpose, the bulky LVDT (linear variable differential transformer) used in the previous
experiments to provide the position feedback to the controller must be replaced by
lighter alternatives. The first solution consists of deducing the strain of the SMA wire
from the readings of a set of strain gauges that are placed in the fingers of the grip.
Although the precision is limited by the strain gauge accuracy, position errors about
30 micrometers are achieved. The second one is the implementation of a sensorless
design, where the change in the resistivity of the SMA wire is used to determine the
strain of the active element. Position errors about 60 micrometers are achieved, with
the great advantage that the actuator is reduced to a single SMA element, specially
important if the goal is to reduce the overall size and weight of the actuator, like is the
trend in the miniaturization in mechatronics and robotics. The experiments presented
show that SMA wires as active elements are serious alternatives to be used as precision
actuators in micromachines. 

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Shape Memory Alloys In Micropositioning Applications, pp. 441-485