In-Situ Sensing and Dynamics Predictions for Electrothermally-Actuated Soft Robot Limbs

Abstract

Untethered soft robots that locomote using electrothermally-responsive materials like shape memory alloy (SMA) face challenging design constraints for sensing actuator states. At the same time, modeling of actuator behaviors faces steep challenges, even with available sensor data, due to complex electrical-thermal-mechanical interactions and hysteresis. This article proposes a framework for in-situ sensing and dynamics modeling of actuator states, particularly temperature of SMA wires, which is used to predict robot motions. A planar soft limb is developed, actuated by a pair of SMA coils, that includes compact and robust sensors for temperature and angular deflection. Data from these sensors are used to train a neural network-based on the long short-term memory (LSTM) architecture to model both unidirectional (single SMA) and bidirectional (both SMAs) motion. Predictions from the model demonstrate that data from the temperature sensor, combined with control inputs, allow for dynamics predictions over extraordinarily long open-loop timescales (10 min) with little drift. Prediction errors are on the order of the soft deflection sensor’s accuracy. This architecture allows for compact designs of electrothermally-actuated soft robots that include sensing sufficient for motion predictions, helping to bring these robots into practical application.

Safe Supervisory Control of Soft Robot Actuators

“…have recently been deployed by the authors for both open-loop [29,54] and closed-loop [55] control as a free-standing manipulator. For eventual application in locomotion, this article uses the proposed supervisory controller to maintain…”

“…Three sensors are located on the limb: one for the body's pose, and one each for the temperatures of the wires. Temperature is sensed by thermocouples (Omega Engineering, type K, 30 AWG) affixed to the SMA coils at the rear of the limb using thermally conductive epoxy (MG 8329TCF) via the fabrication procedure described in our prior work [54], see Fig.…”

“…We calibrate eqn. ( 27) for each SMA from data collected in hardware, using the same procedure as in our prior work on this hardware platform [29,54].…”

Multi-strand Fibers with Hierarchical Helical Structures Driven by Water or Moisture for Soft Actuators

“…In organisms, a muscle that consists of a bundle of muscle fibers exhibits large load capacity, energy storage, and rapid response to mobility by simultaneous contraction and stretching of multiple muscle fibers. , Such a muscle has become a research hotspot for use in imitation and transcendence through flexible actuating materials. − Such materials, which can perform reversible motions such as bending, rotation, contraction, and twisting spontaneously in response to external stimuli, , such as light, − humidity, pH, − magnetism, − and chemicals, have attracted increasing attention for the wide applications of artificial muscles − and biomimetic robots. , Many materials have been investigated for actuators with high performance, including shape memory and conducting polymers, which are made in different forms, such as films, , fibers, and hydrogels . However, although, like living organisms, film and hydrogel actuators can spontaneously respond to external stimuli, their low stability, long response time, poor strength, and toughness limit their application as strength-requiring actuators. , On the contrary, fiber-based actuators that are closest to biological muscles have been of interest to researchers as these actuators leverage strong responsiveness, lightweight, great mechanical properties, and superior flexibility, which enables them to be twisted, knotted, and weaved without any structural failure …”

“… 1 , 2 Such a muscle has become a research hotspot for use in imitation and transcendence through flexible actuating materials. 3 − 5 Such materials, which can perform reversible motions such as bending, rotation, contraction, and twisting spontaneously in response to external stimuli, 6 , 7 such as light, 8 − 10 humidity, 11 pH, 12 − 14 magnetism, 15 − 17 and chemicals, 18 have attracted increasing attention for the wide applications of artificial muscles 19 − 21 and biomimetic robots. 22 , 23 Many materials have been investigated for actuators with high performance, including shape memory 24 and conducting polymers, which are made in different forms, such as films, 25 , 26 fibers, 27 and hydrogels.…”