RESUMO
We introduce a soft robot actuator composed of a pre-stressed elastomer film embedded with shape memory alloy (SMA) and a liquid metal (LM) curvature sensor. SMA-based actuators are commonly used as electrically-powered limbs to enable walking, crawling, and swimming of soft robots. However, they are susceptible to overheating and long-term degradation if they are electrically stimulated before they have time to mechanically recover from their previous activation cycle. Here, we address this by embedding the soft actuator with a capacitive LM sensor capable of measuring bending curvature. The soft sensor is thin and elastic and can track curvature changes without significantly altering the natural mechanical properties of the soft actuator. We show that the sensor can be incorporated into a closed-loop "bang-bang" controller to ensure that the actuator fully relaxes to its natural curvature before the next activation cycle. In this way, the activation frequency of the actuator can be dynamically adapted for continuous, cyclic actuation. Moreover, in the special case of slower, low power actuation, we can use the embedded curvature sensor as feedback for achieving partial actuation and limiting the amount of curvature change.
RESUMO
Soft robots represent an emerging class of biologically-inspired machines that are primarily composed of elastomers, fluids, and other forms of soft matter. Current examples include crawling and swimming robots that exhibit the mobility, mechanical compliance, and deformability of various classes of soft biological organisms, ranging from cephalopods and larvae to marine fish and reptiles. Rather than using electrical motors, soft robots are powered with "artificial muscle" actuators that change shape and stiffness in response to controlled stimulation. In recent years, conductive shape memory materials have become especially popular for soft robot actuation due to the ability to stimulate these materials with on-board microelectronics and miniature batteries. Here, we review recent progress in the development of artificial muscle using shape memory materials that can be stimulated through electrical activation. This includes the use of shape memory alloy (SMA) to create fully untethered soft robots capable of biologically-relevant locomotion speeds as well as recent progress in engineering liquid crystal elastomer (LCE) composites that are capable of robust electrically-powered actuation.
