Caterpillar-inspired soft crawling robot with distributed programmable thermal actuation

Abstract

Many inspirations for soft robotics are from the natural world, such as octopuses, snakes, and caterpillars. Here, we report a caterpillar-inspired, energy-efficient crawling robot with multiple crawling modes, enabled by joule heating of a patterned soft heater consisting of silver nanowire networks in a liquid crystal elastomer (LCE)-based thermal bimorph actuator. With patterned and distributed heaters and programmable heating, different temperature and hence curvature distribution along the body of the robot are achieved, enabling bidirectional locomotion as a result of the friction competition between the front and rear end with the ground. The thermal bimorph behavior is studied to predict and optimize the local curvature of the robot under thermal stimuli. The bidirectional actuation modes with the crawling speeds are investigated. The capability of passing through obstacles with limited spacing are demonstrated. The strategy of distributed and programmable heating and actuation with thermal responsive materials offers unprecedented capabilities for smart and multifunctional soft robots.

Fig. 1.. Bioinspired crawling motions.

(A) Schematics of the forward locomotion of a caterpillar. (B) Schematics of the reverse locomotion of a caterpillar. (C) Snapshots of the crawling robot in one cycle of actuation for reverse locomotion. (D) Snapshots of the crawling robot in one cycle of actuation for forward locomotion. (E) infrared image of the crawling robot with 0.05-A current injected in channel 1 and the tilted view of the crawling robot. (F) Infrared image of the crawling robot with 30-mA current injected in channel 2 and the corresponding tilted view of the crawling robot.

Fig. 2.. Design and fabrication of the caterpillar inspired crawling robot.

(A) Fabrication steps of the caterpillar inspired crawling robot. PI, polyimide. (B) Cross-section view of the fabricated sample. The AgNWs are half-embedded below the top surface of PDMS/CB composite. The mesogens in the LCE ribbon are aligned through tensile stretching. (C) Top view of the crawling robot with symmetric actuator A and actuator B. Each actuator contains two conductive channels (1 and 2).

Fig. 3.. Heating performance of the soft crawling robot.

(A) Photograph of the heating pattern with two sections on each actuator. (B) Diagram of the two-channel electrical circuit corresponding to the heating pattern in (A). (C) Photograph of the bimorph before (left image) and after (right image) the heater is turned on. (D) Curvature of the bimorph cantilever with respect to time with different current applied. (E) Curvature of the bimorph cantilever with respect to time with different thickness ratio between the two layers of the bimorph. (F) Theoretical prediction of maximum curvature compared with the experimental results. (G) Temperature of the bimorph cantilever with respect to time with different current applied. (H) Temperature of the bimorph cantilever with respect to time with different thickness ratio. (I) Relationship between the curvature and temperature of the bimorph cantilever.

Fig. 4.. Two crawling modes of the caterpillar robot.

(A) Comparison between the images and simulation results (color bar representing the normalized out-of-plane deformation) of the robot in forward mode. (B) Comparison between the images and simulation results of the robot in reverse mode. (C and D) Friction force on two ends of the crawling robot in forward mode and reverse mode normalized by the self-weight of the robot.

Fig. 5.. Locomotion speed of the crawling robot.

(A) Forward mode and (B) reverse mode with different current and actuation frequency. (C) Locomotion speed of forward and reverse mode with different current at a constant actuation frequency of 0.2 Hz. (D) Locomotion speed of the forward and reverse mode with different frequency at a constant current of 30 mA. (E and F) Locomotion displacement (at 30 mA and 0.2 Hz) of the crawling robot in forward and reverse mode, respectively.

Fig. 6.. Demonstration of the crawling robot passing through a shallow deep gap.

(A) Side view of the crawling robot during the transition from actuator A to actuator B. (B) Overlapped photographs showing the history of motion in (A) and corresponding schematic showing the obstacles that the crawling can pass through. (C) Comparison between the crawling robot and the obstacle, which forms a confined tunnel with the floor. (D) Snapshots of the robot passing through this confined tunnel and reversely passing again to return to the initial location.