SMA Feed Axis With Predictive Control Strategies

In a current research project, the Chair of Production Systems is examining the use of multiple modular and standardized shape memory alloy (SMA) actuators in the feed axis of a small machine tool. Later on, these machines shall be used to manufacture small work pieces with high precision. Therefore, the actuators must be qualified to handle movements in the micrometer range. To achieve this goal, different measures to control the displacement are considered. To be applicable in a feed axis, the developed SMA actuator needs to be able to follow a given movement profile, as well as reach and hold every possible intermediate position during both activation and deactivation. Different control strategies are proposed to achieve this. For a simple control setup without feedback, an algorithm is devised which automatically generates a heating profile for a given displacement curve. This is done by iterating different heating currents in through a pulse duration modulation and analyzing the response of the actuator. The resulting actuator path lies within less than 40 µm of the given profile for an open-loop-control. This data can further be used as a basis for a closed-loop-control, in which the displacement of the actuator can either be measured by external sensors or by using the change in resistance in the SMAs during transformation.