Force Control Method of Soft Actuator Using Multiple Super Elastic Alloy Wires

Super elastic alloy (SEA) has good flexibility with moderate rigidity and then is widely used in eyeglasses, bras and so on. SEA’s flexibility is based on the phase transformation between austenite and martensite structures of material metallurgy, and it is known that the transformation can be controlled by heating and cooling the material. The active deformation of SEA is already reported as controllable by the heating and cooling under pre-stressed condition, but it is known that the deformation is on / off control because the material has very strong nonlinearity in their stress-strain relationship. Then the development of smooth control of SEA’s active deformation is studied to realize soft actuator in this report. The smoothness of the control is realized by the multiplying the wires of SEA in actuating unit of 1DOF actuator. The theory of SEA controller is formulated by using the constitutive equation of the metallo-thermo-mechanics which represents the kinetics of phase transformation between austenite and martensite structures. The developed controller with the formulation is applied to actuate 1DOF actuator with multiple wires of SEA for the verification of the realization. The qualitatively match between theory and experimental results is observed here but quantitatively mismatch has occurred in this report. In the discussion of this paper, it is shown that the mismatch is occurred because the formulation is based on the ideal condition without the deformation of actuator unit. Then this report shows that it is important to design the soft actuator not only their actuating element but also the other elements which affect the deformation behavior of the actuator as a total unit.