(J-376) Polycaprolactone-Based Thermoresponsive Variable Impedance Actuators: Prioritizing Active Heating/Cooling for Effective Offline Impedance Control

Introduction:: Variable Impedance Actuators (VIAs) are a new generation of robotic drive trains that are suitable for assistive technology in physical human-robot interactions. These actuators can adjust their level of impedance to safely and efficiently interact with unknown environments. Typically, this change in impedance requires multiple actuators to control the two degrees of freedom of (i) position, and (ii) stiffness (impedance). This paper proposes a one-of-a-kind thermal-based variable impedance actuator that eliminates the second actuator and instead regulates its impedance through controlling the temperature of a thermoplastic polymer, Polycaprolactone.

Materials and Methods:: The actuator was constructed using Polycaprolactone (PCL) (melting point = 60degC, glass transition = -60degC), a thermoplastic polymer with temperature-dependent viscoelastic properties. Its temperature was regulated using flexible, embedded Peltier elements. These elements create a heat flow when an electrical current passes through, producing a hot and cold side that can be reversed by switching the current's direction, enabling active cooling. The actuator's design was simplified, consisting of 3D-printed PETG components, a fixed-stiffness compression spring between the actuator and the output link, and a PCL sponge for variable stiffness. The PCL is strategically placed next to each Peltier unit to facilitate temperature alterations without overheating other parts. The PCL's viscoelastic properties were tested under various temperatures (30-60degC) in a climate chamber to determine stiffness and damping values, aiding in the system identification of our mechanical model, which was confirmed with a simple step response.

Results, Conclusions, and Discussions:: The actuator prototype demonstrated the capability to regulate both elasticity and viscosity in an off-line fashion. The simplicity and lightness of the design open up new possibilities for developing the next generation of variable impedance actuators for physical human-robot interaction applications. The design of this actuator is also highly scalable, making it a suitable candidate for applications where the actuator’s size, weight, and compactness are the main concerns.

The proposed thermal-based variable impedance actuator presents a unique solution to the limitations of current actuation technologies. By leveraging the temperature-dependent properties of polycaprolactone and the capabilities of Peltier elements, the actuator can effectively regulate its impedance in an off-line manner. This innovative approach opens up new possibilities for the development of more efficient and adaptable robotic systems.

Future work will focus on evaluating the sinusoidal response of the VIA to understand the dynamics of the impedance regulation. Additionally, the effects of scaling down the size of the actuator on the rate of impedance adjustment will be analyzed. The speed of the heat transfer through the PCL sponge is the main concern moving forward, as the dynamics of the Peltier are dependent on external variables such as ambient temperature. The potential of this technology in various applications, particularly in physical human-robot interaction applications, will also be explored.

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