Research on the mechanism of variable stiffness of the twisted and coiled polymer actuator during saturated contraction

Twisted and coiled polymer actuator (TCPA), manufactured from low-cost material such as fishing line, has been widely applied and playing a significant role in soft robotic. Therefore, it is not surprising that it has been attracting much attention of researchers. The main consideration for driving TCPA lies in the heating process. However, the heating process causes stiffness degradation and precisely leads its structural to be unstable. Accordingly, it is proposed through this paper, a method of adjusting the pre-stretching to reach the saturated contraction. The pre-stretch is adjusted to bring the TCPA to saturated contraction within the given heat. Due to the contact of adjacent coils of TCPA, there is not enough space to contract, thus TCPA is compressed into the hard structure by increasing the heat. Initially, the stiffness will be decreased and then be increased during the entire heating process. So the flexibility of the motion state and the stability working state can be achieved. Besides that, the proposed method can also improve the rigor of TCPA by more than two times during saturated contraction. It has been shown by some relevant results of a custom platform experiment.