Control of twisted-coiled actuators via multi-DOF PID

Abstract

In the last decade, twisted coiled actuators (TCAs) made from nylon have drawn the attention of the community as an attractive electroactive mechanism mimicking the performance of human muscles, with notably favourable output power densities. The feasibility of the TCA, in terms of performance, size, safety, and scalability has been evaluated in previous studies, but they exhibit substantially non-linear behaviour, thereby requiring a sophisticated control system. Furthermore, it is desired that these actuators are able to rapidly actuate to match biological muscle levels of capability. To this end, the efficacy of a linear physics-based model of TCAs was tested to elucidate the relationship between temperature, force, and displacement in these actuators. The accuracy of this modelling approach is discussed in the context of position control theory using a two-degree- of-freedom proportional-integral-derivative (PID) controller to switch between Joule heating and active cooling with a continuously controlled fan to enhance actuator response time. This model was subsequently employed to simulate the TCA response to heating and cooling in the MATLAB-Simulink® environment while referencing the experimental results reported by Takagi et al. as a benchmark. Results from this investigation indicate a 5% offset error, which is attributed to the non-linear nature of the TCA. Finally, results confirm that TCAs augmented with active cooling exhibit significantly improved cycle times relative to conventionally heat-controlled TCAs.