A Torsion-Bending Antagonistic Bistable Actuator Enables Untethered Crawling and Swimming of Miniature Robots

Abstract

Miniature robots show great potential in exploring narrow and confined spaces to perform various tasks, but many applications are limited by the dependence of these robots on electrical or pneumatic tethers to power supplies outboard. Developing an onboard actuator that is small in size and powerful enough to carry all the components onboard is a major challenge to eliminate the need for a tether. Bistability can trigger a dramatic energy release during switching between the 2 stable states, thus providing a promising way to overcome the intrinsic limitation of insufficient power of small actuators. In this work, the antagonistic action between torsional deflection and bending deflection in a lamina emergent torsional joint is utilized to achieve bistability, yielding a buckling-free bistable design. The unique configuration of this bistable design enables integrating of a single bending electroactive artificial muscle in the structure to form a compact, self-switching bistable actuator. A low-voltage ionic polymer-metal composites artificial muscle is employed, yielding a bistable actuator capable of generating an instantaneous angular velocity exceeding 300 °/s by a 3.75-V voltage. Two untethered robotic demonstrations using the bistable actuator are presented, including a crawling robot (gross weight of 2.7 g, including actuator, battery, and on-board circuit) that can generate a maximum instantaneous velocity of 40 mm/s and a swimming robot equipped with a pair of origami-inspired paddles that swims breaststroke. The low-voltage bistable actuator shows potential for achieving autonomous motion of various fully untethered miniature robots.

Fig. 1.

Torsion-bending antagonistic bistable design. (A) Schematic of torsion-bending antagonistic bistable design. It is built by attaching 2 living hinges in between the frames of a prestretched LET joint. The prestretch yields bending deflection in the 4 LET beams, while the rotation of the LET joint produces torsional deflection. (B) A strain energy barrier. The maximum position (I) is produced through prestretch, and the strain energy drops when the joint rotates in either direction, generating 2 local minimum positions (II and III). The 2 local minimum positions on the total strain energy curve correspond to 2 stable states while the maximum position to unstable equilibrium state of the design. (C) Strain energy curves of the LET beams due to bending and torsion.

Fig. 2.

Design for self-switching. (A) Principle of self-switching. The living hinges are replaced by 2 low-voltage electroactive IPMC strips to provide bending actuation. (B) Prototype of the bistable actuator. IPMCs are attached to the frames by clamping pads. The switching between the 2 stable states is achieved by alternating the voltage applied on IPMCs. (C) Parameter determination of the bistable LET design. Left: The switching moment and stable angle are plotted with respect to the aspect ratio of beam cross-section and the normalized prestretch deflection, in which the zone surrounded by the dashed lines represents the feasible region (T_max ≤ 0.26 mN·m). The experimental results are represented by small cubes, and the optimal design is marked as a red dot. Right: The moment curve of the finalized bistable design.

Fig. 3.

Characterization of self-switching. (A) Switching of bistable actuator between the stable states and the unstable equilibrium state. The bottom end of bistable actuator is fixed and the deflection of the free end is recorded. (B) Rotation angle and angular velocity of bistable actuator under an alternative voltage. The sign of the applied voltage is distinguished by the background color. (C) Rotation angle and angular velocity of bistable actuator with voltages of different amplitude (3.5, 3.75, and 4 V). (D) Total strain energy curve of the bistable actuator with stop blocks. Stable 2' (the rotation angle is 2°) is the new stable state between the unstable equilibrium stable and stable 2. (E) Rotation angle and angular velocity of the bistable actuator with stop blocks.

Fig. 4.

Bistable crawling robot. (A) Prototype of the bistable crawling robot. An untethered 3-leg robot employing the bistable actuator, with a carry-on lithium battery and a carry-on control circuit is fabricated. (B) Directional locomotion of robot: from stable 1 to stable 2, the bistable actuator switches downward (3.75 V), during which the rear feet are pinned to ground due to the sharp corner while the front foot moves forward; from stable 2 to stable 1, the bistable actuator switches upward (−3.75 V), during which the front foot is pinned to ground while the rear feet move forward. (C) Measured displacement and velocity of the center point of the robot. The sign of the applied voltage is distinguished by the background color. (D) Crawling on sandpaper with different grit numbers (240, 320, and 400).

Fig. 5.

Bistable swimming robot. (A) Origami-inspired paddles. The Miura-ori pattern is selected to amplify the rotation angle of the bistable actuator and the mountain crease (red solid line) is actuated by the bistable actuator. (B) Measured paddle angles φ₁ and φ₂. The stop blocks are employed to confine a workspace as the bistable actuator switching between stable 1 and stable 2' (the nearly flat state). The sign of the applied voltage is distinguished by the background color. (C) Left: Propelling of the robot by the origami-inspired paddles. The robot can swim the breaststroke in a straight line under an alternative voltage. Right: Recorded propelling displacement and velocity.