Shape Memory Alloys In Micropositioning Applications, pp. 441-485
Authors: E. Asua, J. Feuchtwanger, V. Etxebarria and A. Garcia-Arribas
Abstract: 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.