Bridging Hard and Soft: Mechanical Metamaterials Enable Rigid Torque Transmission in Soft Robots

Abstract

Torque and continuous rotation are fundamental methods of actuation and manipulation in rigid robots. Soft robot arms use soft materials and structures to mimic the passive compliance of biological arms that bend and extend. This use of compliance prevents soft arms from continuously transmitting and exerting torques to interact with their environment. Here, we show how relying on patterning structures instead of inherent material properties allows soft robotic arms to remain compliant while continuously transmitting torque to their environment. We demonstrate a soft robotic arm made from a pair of mechanical metamaterials that act as compliant constant-velocity joints. The joints are up to 52 times stiffer in torsion than bending and can bend up to 45°. This robot arm can continuously transmit torque while deforming in all other directions. The arm's mechanical design achieves high motion repeatability (0.4 mm and 0.1°) when tracking trajectories. We then trained a neural network to learn the inverse kinematics, enabling us to program the arm to complete tasks that are challenging for existing soft robots such as installing light bulbs, fastening bolts, and turning valves. The arm's passive compliance makes it safe around humans and provides a source of mechanical intelligence, enabling it to adapt to misalignment when manipulating objects. This work will bridge the gap between hard and soft robotics with applications in human assistance, warehouse automation, and extreme environments.

Figures (14)

• Figure 1: Fig. 1. TRUNCs conceptual overview. We present a joint that is torsionally rigid yet compliant in bending and extension. The joint can be directly connected to a motor to create soft torque actuators. Multiple joints can then be composed to create soft arms with continuous torque actuation.
• Figure 1: Fig. S1. TRUNCs are based on tilings of the double arrowhead.(A) The double arrowhead can be made as a linkage where links have a width $w$ and thickness $t$. (B) The equatorial cell is based on a tiling where $N=4$ and $M=2$ while the truss cell ($N=4$ and $M=3$) has an additional row of arrow heads.
• Figure 2: Fig. 2. The two TRUNC variants and mechanical properties. (A) The equatorial TRUNC has a single band of joints along its equator (B) and the truss TRUNC has two bands of joints on either side of the equator. This doubled structure allows for shear internal to the cell, which is disallowed in the equatorial structure. (C) Moment-rotation in twist and bending modes and force-displacement in extension for the equatorial cell (D) and truss cell demonstrates soft bending and extension compared to twist. (E) A truss TRUNC performing a screwing operation at various bend angles, demonstrating torque transmission while bending. An internal conical spring provides restoring force.
• Figure 2: Fig. S2. TRUNCs are constant-velocity joints. TRUNCs maintain a constant angular velocity between input and output shafts when (A) bending and (B) extending. Input and output signals are plotted in radians (modulo 2$\pi$).
• Figure 3: Fig. 3. Composition of TRUNCs.(A) Joints can be chained in series to create flex shafts that transmit torque while bending and extending. (B) TRUNC flex shafts remain torsional rigid under extension. The shaded region represents the min and max bounds for $N=5$ trials. (C) Joints can also be concentrically nested. Nested joints are coupled in bending and extension but (D) rotate independently. (E) Nested joints can be chained to create flex shafts that transmit multiple torques. (F) Each column of the flex shaft rotates separately, allowing multiple independent rotational degrees of freedom.