Electrical Shape Memory Actuators

2 Electrical Shape Memory Actuators Actuator Shape and Stroke 

The shape that the SM actuator recovers to when heated is imprinted into the alloy by an annealing process. For instance, to fabricate a coil spring a SM wire is wound around a mandrel and annealed for 1... 2 hours at 350. .. 500 °C. Annealing temperature and duration have a strong influence on the actuators properties, such as the trainable two-way effect, the effect stability, and the hysteresis behavior. 

The shape change between high-temperature and low-temperature shape defines the actuator stroke. Table 6.4 lists some commonly used actuator shapes and actuator strokes. 

The two-way effect will be stabihzed after 20... 100 thermal and mechanical cycles. Due to the ability of the martensite (low-temperature phase) to form a twinned crystalline structure, different areas of the actuator element may be strained in different ways extension, compression, or shear are deformations that will be reverted to by heating. This variety offers the interesting opportunity to adapt the actuators shape change to the special needs of the actuating task. By this means, transmission links or gears may be eliminated, which helps reduce the size and price of a system. 

The actuator stroke is limited only by the reversible strain that the martensitic structure can accommodate by de-twinning - otherwise irreversible strain will occur. The admissible strain is determined by the type of shape memory alloy as well as the desired number of activation cycles. If the effect is to be employed only once (for example, for tube connectors), NiTi-based alloys may be strained up to 8%. For actuator use with more than 100000 activations, only smaller strains are permitted, namely extensions adm 3%, shear 7adm 4%, and stresses up to Tadm 150N/mm or Tadm 100 N/mm. Table 6.5 gives an overview of the design data of the most commonly utilized SM actuator geometries. 

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The presence of oxygen-containing fimctional groups on the surface of the carbon nanofibers improved their dispersion in the TPU matrix. Electrical conductivity in TPU-CNF and TPU-CNFOX materials was a synergistic effect produced by the chaotic mixing and the morphology of these materials. Young s modulus at room temperature was controlled by PCL crystals, while at 60 °C it was eontrolled by the presence of carbon nanofibers. CNFOX composites provided better performance in thermally-induced shape memory, while CNF is reconunended for use with electrical shape memory actuation. 

The properties of electrically activated shape memory actuators described so far have indicated that these actuators are well-suited to drive mechanical mechanisms. The advantages and disadvantages of this kind of new actuator principle are smnmarized in Table 6.6. 

The analysis of the advantages and disadvantages reveals good feasibility and opportunities for electrically heated shape memory actuators, especially in two fields of application ... 

In this section some examples of precision engineering prototypes are presented that apply electrically heated shape memory actuators as driving elements. Further on flexure hinges of pseudo-elastic SM alloys will be presented. 

Hesselbach, J. Hornbogen, E. Mertmann, M. Pittschellis, R. Stork, H. Optimization and Control of Electrically Heated Shape Memory Actuators. Proc. 4th Int. Conf. on New Actuators, June 15-17, Bremen, Germany (1994), pp. 337-340... 

Tactile display of spatial patterns on the skin uses three main types of transducers [Kaczmarek et al, 1991 Kaczmarek and Bach-y-Rita, 1995]. Static tactile displays use solenoids, shape-memory alloy actuators, and scanned air or water jets to indent the skin. Vibrotactile displays encode stimulation intensity as the amplitude of a vibrating skin displacement (10-500 Hz) both solenoids and piezoelectric transducers have been used. Electrotactile stimulation uses 1-100 mm -area surface electrodes and careful waveform control to electrically stimulate the afferent nerves responsible for touch, producing a vibrating or tingHng sensation. 

Besides the intrinsic conductive polymers, some deformable polymers, such as shape-memory polymers, are usually activated by heating. After incorporating with conductive fillers, such as carbon nanomaterials, they can be simulated by the electricity through Joule heating (Liu et al., 2009 Hu and Chen, 2010 Koerner et al., 2004). This kind of electro thermally active polymer composites can produce expansion/contraction and bending behaviors upon with the electricity. Moreover, these actuators can work durably... 

Figure 2.3 Shape-memory properties of MWNT-TPU composites, (a) Stretched (800%) 1 wt% MWNT-TPU composite ribbon, tied into a loose knot and heated at 55 °C. The knot closes on strain recovery, (b) Strain recovery and curling of the 1 wt% MWNT-TPU composite ribbon upon IR irradiation within 5 s. (c) Comparison of the stress recovery before (left) and after (right) remote actuation by IR irradiation. Neat TPU (M) bends and does not recover. In contrast, the 1 wt% MWNT-TPU composite (PCN) contracts on exposure to IR irradiation (arrow indicates moving direction), (d) Electrically stimulated stress recovery of a 16.7 wt% MWNT-TPU composite. Reprinted by permission from Macmillan Publishers Ltd. ...

Ikuta, K., M. Tsukamoto, and S. Hirose, Shape Memory Alloy Servo Actuator System with Electric Resistance Feedback and Application for Active Endoscope, in Computer-Integrated Surgery, R. H. Taylor et al. (eds.), 1996, MIT Press, Cambridge, Mass, pp. 277-282. 

By far the most common actuator for electrically powered prostheses is the permanent magnet dc electric motor with some form of transmission (Fig. 32.10). While there is much research into other electrically powered actuator technologies, such as shape memory alloys and electroactive polymers, none is to the point where it can compete against the dc electric motor. A review of the available and developing actuator technologies with their associated advantages and disadvantages as well as their power and force densities can be found in Hannaford and Winters (1990) and Hollerbach et al. 

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