Development of shape-memory alloy actuators. Performance assessment and introduction of a new composing approach

A servo actuator using shape-memory alloy (SMA actuator), which performs special functions not found in conventional actuators, is expected to be a new driving source for future robots. This paper evaluates its performance and discusses the fundamental issues in designing such actuators. Being driven by the thermal martensitic transformation of the SMA itself, an SMA actuator is a driving system which can in principle, perform without any frictional parts, can be designed at a high output/weight ratio especially when a light-weight actuator is to be constructed But has low energy efficiency due to its mechanical characteristics of operating at a small temperature difference, and has a relatively low response. Our study shows some ways of overcoming these disadvantages by a special design approach. The method is called ξarray and is characterized by SMA wires being connected in parallel structurally and in series electrically. This approach enables the response to be enhanced due to the increased surface area. The diameter of the power-supplying lead can be reduced due to the increased electrical resistance of the SMA. The power source and power control system can be simplified, and the SMA sensor function can be used positively. The above observations are reviewed by the experimental production and operation of an antagonistic SMA servo actuator system using control based on the ξ-array. Consequently, the resistance-feedback is able to produce a servo less susceptible to variations in ambient temperature.