Design of an Active Cooling System for Thermally Activated Soft Actuators

A new type of actuator made from twisting a silver-plated nylon thread presents new possibilities for the way wearable mechatronic rehabilitation devices are designed. The twisted coiled actuator (TCA) has been previously shown to provide a power density up to 100 times that of biological muscles, while also encompassing biomimetic characteristics. However, since TCAs require heat to contract, the main drawbacks preventing this type of actuator are its inherent low efficiency and slow reaction times. To combat both of these issues, a simple tube enclosure was designed to provide active cooling using forced air. The two main parameters affecting the efficiency and bandwidth are the cooling air pressure and tube diameter. This study presents a two-way repeated measures test to compare these parameters on the cooling and heating rates of the TCA system, as well as the thermal capacitance with three pressure levels (10, 15, and 20 psi) and three tube diameters (4, 4.5, and 5 mm). The results show that an increase in pressure significantly improves the rate of cooling, while a decrease in tube diameter has negative effects on the efficiency and cooling rate of the system. The mean values of the cooling time $(\tau_{\text {cool}})$ were 2.972, 2.210, and 2.682 seconds for 4, 4.5, and 5 mm diameters, respectively. These results indicate that a decrease in diameter improves the cooling rate up to the point at which the walls of the tube become so close to the TCA strand, that they prevent rapid heat transfer while cooling.