Confined space searches such as pipeline inspections are widely demanded in various scenarios, where lightweight soft robots with inherent compliance to adapt to unstructured environments exhibit good potential. We proposed a tubular soft robot with a simple structure of a spring-rolled dielectric elastomer (SRDE) and compliant passive bristles. Due to the compliance of the bristles, the proposed robots can work in pipelines with inner diameters both larger and smaller than the one of the bristles. Firstly, the nonlinear dynamic behaviors of the SRDE were investigated experimentally. Then, we fabricated the proposed robot with a bristle diameter of 19 mm and then studied its performance in pipelines on the ground with inner diameters of 18 mm and 20 mm. When the pipeline’s inner diameter was less than the outer diameter of the bristles, the bristles remained in the state of bending and the robot locomotion is mainly due to anisotropic friction (1.88 and 0.88 body lengths per second horizontally and vertically, respectively, in inner diameter of 18 mm and 0.06 body length per second in that of 16 mm). In the case of the pipeline with the larger inner diameter, the bristles were not fully constrained, and a small bending moment applied on the lower bristle legs contributed to the robot’s locomotion, leading to a high velocity (2.78 body lengths per second in 20 mm diameter acrylic pipe). In addition, the robot can work in varying geometries, such as curving pipes (curve radius ranges from 0.11 m to 0.31 m) at around two body lengths per second horizontally and on the ground at 3.52 body lengths per second, showing promise for pipeline or narrow space inspections.

Inspired by the fact that flying insects improve their power conversion efficiency through resonance, many soft robots driven by dielectric elastomer actuators (DEAs) have achieved optimal performance via first-order modal resonance. Besides first-order resonance, DEAs contribute to multiple innovative functions such as pumps that can make sounds when using multimodal resonances. This study presents the multimodal resonance of a rectangular planar DEA (RPDEA) with a central mass bias. Using a combination of experiments and finite element modeling (FEM), it was discerned that under a prestretch of 1.0 × 1.1, the first-, second-, and third-order resonances corresponded to vertical vibration, rotation along the long axis, and rotation along the short axis, respectively. In first-order resonance, superharmonic, harmonic, and subharmonic responses were activated, while only harmonic and subharmonic responses were observed in the second- and third-order resonances. Further investigations revealed that prestretching tended to inhibit third-order resonance but could elevate the resonance frequencies of the first and second orders. Conveniently, both the experimental and FEM results showed that the frequencies and amplitudes of the multimodal resonances could be tuned by adjusting the amplitudes of the excitation signals, referring to the direct current (DC) amplitude and alternating current (AC) amplitude, respectively. Moreover, instead of linear vibration, we found another novel approach that used rotation vibration to drive a robot with soft bristles via hopping locomotion, showcasing a higher speed compared to the first-order resonance in our robot.

Adopting smart materials and structures for locomotion could minimize the body size of pipeline robots, thereby enhancing their coverage, especially in long-distance and small-diameter regions. Pneumatic pipeline robots and electric active ones show good potential, still they suffer challenges in long-distance travel due to increased system damping, and adaptation to a large diameter ratio due to insufficient active deformation, respectively. In this article, we developed a single chamber minibot that integrates a dielectric elastomer as the vibration source and pneumatic hinges for the elastic support of bristles. The active pressurized chamber serves as: an antagonism in the axial direction to provide actuation stability for DE resonance, and elastic hinges in the radial direction to support and regulate the bristles. A contact force analysis indicates that the elastic hinges play a significant role in adapting to various diameter pipelines by reducing the normal contact force compared to the traditional bending of flexible bristles. A finite element model is developed to optimize the structural design and analyze the mechanisms of contour variation by regulating the initial angle of bristles and chamber stiffness. The primary prototype demonstrates rapid locomotion, achieving more than 0.4 body length per second in vertical pipelines with inner diameters of 12–16 mm (diameter ratio of 1.33), satisfying the critical value of 1.26 in a typical parent-twin pipeline as predicted by Murray’s Law. The compact structure offers advancements for locomotion in long and small pipelines with various diameters, presenting new opportunities for the miniature design of pipeline robots.