INFLUENCE OF THE THERMAL-TREATMENT ON THE STABILITY OF PARTIALLY CONSTRAINED RECOVERY OF NITI ACTUATOR WIRE

NiTi shape memory wire may be used for actuation purposes in flexible robotic grippers, which have to be able to handle objects of different size, shape or weight. Therefore it is advantageous to develop an ele ctrically driven shape memory actuator, which may perform any combinat ion of shape change and exerted force within the following limiting bo undaries: free recovery: gripping of a very small and lightweight obje ct, constrained recovery: gripping of an object with maximum size and weight. Several NiTi actuator wires are fabricated and annealed betwee n 400 and 600 degrees C after cold working in the martensitic state. A fter prestraining each wire is embedded in a silicone matrix material. The polymer works as a bias spring and is able to store elastic defor mation energy. This paper investigates the influence of thermal treatm ent on the stability of the exerted force between the two boundaries o f completely free and constrained recovery, the ''partially constraine d recovery''. The stability of recovery strain and stress is measured in a test assembly, in which different modes of partially constrained recovery are simulated. The work is supplemented by dilatometric measu rements carried out with each actuator wire before and after the test procedure.