Highly Dynamic Shape Memory Alloy Actuator for Fast Moving Soft Robots (original) (raw)

2019, Advanced Materials Technologies

However, soft robot locomotion tends to be relatively slow and typically relies on external hardware for power and control. This is largely due to current limitations with the "artificial muscle" actuators that are used to place the battery-powered electrical motors (e.g., DC motors, servos) that have been traditionally used in robotics. For example, untethered soft robots that use fluidic or dielectric elastomer actuators require bulky on-board hardware for power and control that result in a high payload and slow locomotion speed. [3,8,9] While ionic polymer-metal composites require low voltage and can be controlled using miniaturized electronics, they have not been shown to generate the forces required for a cm-scale robot to walk in dry conditions. [10,11] By contrast, soft robot actuators composed of elastomers embedded with wires or springs of thermally activated smart materials such as shape memory alloy (SMA) can generate large forces in an adequately short time interval and be directly powered and controlled with portable, lightweight electronics. [12] Moreover, they can reversibly transition from being mechanically compliant in their natural (unactuated) state to being stiff and load-bearing when actuated. [12] Although promising, SMAs have only been used as actuators for untethered soft robot in a limited number of cases. [13,14] A key challenge has been the limited frequency with which SMA-based actuators can be activated. This is due to the long duration of time required for the alloy to cool down and return to its natural shape and compliance following electrical activation. As a result, soft robots powered with SMA actuators either have sudden but infrequent bursts of motion, [14] slow steady-state locomotion gait cycles with long recovery times, [15] tethered hardware [15] or take advantage of marine environments for active cooling. [13,16] In this work, we address this challenge by examining how materials selection, actuator design, and operating conditions can be used to improve the frequency bandwidth of an SMA actuator. In particular, we find that encasing SMA wire in thermally conductive rubber can allow for more rapid heat transfer and enable soft robots to exhibit robust dynamical motion over extended operating times (Figure 1a). The SMA actuator is capable of a rapid transition