Rotational Angle Control of a Twisted Polymeric Fiber Actuator by an Estimated Temperature Feedback

A twisted polymeric fiber (TPF) actuator often referred to as a fishing line/sewing thread artificial muscle, is one of the soft actuators which is made by twisting and heating a nylon fishing line. There are mainly two types of the TPF actuator, one is to make a contraction motion, which is sometimes called a twisted and coiled polymeric fiber actuator, and the other is to make a rotational motion, called simply a TPF actuator. This letter focuses on the latter one and proposes an estimated temperature feedback control method to regulate a torsional angle of the TPF actuator. The TPF actuator is very lightweight and low cost, but the advantage would be lost if some external sensors, such as a thermal sensor or encoder is used. In order to control the torsional angle without the use of the external sensors, a temperature of the actuator is estimated by measuring the change in the Ohm resistance of a heater wrapping around the actuator. By feedbacking the estimated temperature, the torsional angle can be regulated indirectly. First, the two types of models are proposed. One is to derive a desired temperature of the actuator from the desired angle. The other is to estimate the actuator's temperature from a change of the resistance of the actuator's heater. Next, the temperature feedback control law is composed using these two models. Finally, experiments of the torsional angle regulation are conducted using a prototype of the actuation module, which consists of antagonistically embedded two TPF actuators to demonstrate the usefulness of the proposed controller.