Soft and lightweight fabric enables powerful and high-range pneumatic actuation

Abstract

Soft structures and actuation allow robots, conventionally consisting of rigid components, to perform more compliant, adaptive interactions similar to living creatures. Although numerous functions of these types of actuators have been demonstrated in the literature, their hyperelastic designs generally suffer from limited workspaces and load-carrying capabilities primarily due to their structural stretchability factor. Here, we describe a series of pneumatic actuators based on soft but less stretchable fabric that can simultaneously perform tunable workspace and bear a high payload. The motion mode of the actuator is programmable, combinable, and predictable and is informed by rapid response to low input pressure. A robotic gripper using three fabric actuators is also presented. The gripper demonstrates a grasping force of over 150 N and a grasping range from 70 to 350 millimeters. The design concept and comprehensive guidelines presented would provide design and analysis foundations for applying less stretchable yet soft materials in soft robots to further enhance their practicality.

Fig. 1.. Construction of the soft pneumatic actuators using soft, lightweight fabrics.

(A) Comparison of the proposed fabric actuator-based gripper’s grasping range and force with various previously developed soft grippers, including the ones with jamming parts (–58), ones reinforced by fiber (–63), ones integrated with phase-changing materials (36, 38, 64), ones with soft-rigid hybrid structures (, , –68), and ones consisting of multiple pouches (47, 69, 70). (B) Responses of different soft pieces to the same tensile load. The passive stretching of the fabric piece is negligible compared with those of silicone rubber pieces. (C) Expanded height versus input pressure of a silicone-based airbag and a fabric-based one. As the pressure increases, the fabric-based airbag rapidly reaches its saturation point without future obvious height increase, while the silicone-based one continues to expand until the explosion. (D) Bending comparison between a fabric pneumatic cylinder and a steel tube with similar mass and identical size. The steel tube is readily crushed by a 3-kg weight, while the fabric chamber keeps stable upon the same payload under 45-kPa pressure. (E) Fabrication process of the fabric actuator. Step 1: Two pieces of thermoplastic polyurethane (TPU)–coated fabric are tightly attached to the 3D printed polyvinyl alcohol (PVA) mold. Step 2: Weld the overlapped fabric edges with an electric soldering iron. Step 3: Dissolve the internal PVA mold in 60°C water. Step 4: Air dry and seal the chamber together with a pneumatic pipe. The internal structure is shown in the bottom right corner. (F) Simulated deformation statuses of different actuators with various body lengths and rotation angles. These deformations can be predicted through both the finite element analysis (FEA) and a theoretical model based on robot kinematics. (G) Actual deformation statuses of three actuators with geometric parameters same as the simulated ones.

Fig. 2.. Mechanical properties of the fabric actuator.

(A) Response of a 250-mm-long actuator upon inflation. (B) Required times for reaching deformation limits. Five actuators with body lengths between 100 and 300 mm were tested. (C) Tip blocked force of three actuators in different body lengths at different pressure values. (D) Tip blocked force of three actuators with different rotation angles at different pressure values. (E) Comparison of an unloaded bending actuator and a 1-kg loaded actuator with input pressure of 10 and 50 kPa. (F to H) Stiffness curves of actuators in different body lengths at different pressures. Actuator stiffness showed positive correlation with pressure and negative correlation with body length. (I) Simple, quick repair process of a leaked/damaged actuator. The patched actuator can almost restore to its original performance.

Fig. 3.. Programmable actuation modes and their combinations in a single actuator.

(A) Three basic actuation modes of the actuator: straightening, bending, and twisting. (B) Fabrication method and outcomes of serial combination. Segmented, complex motions can be achieved on a single slender actuator. (C) Fabrication method and outcomes of parallel combination. The top chamber is fabricated after the mold inside the bottom chamber is dissolved. A single prototype can then exhibit various motion modes and freely switch among these modes according to different scenarios.

Fig. 4.. Windable structure results in tunable actuator length.

(A) Schematics of the fabric actuator with tunable length. A winding mechanism is designed to control the effective length of the actuator. (B) Actuation process of a fabric chamber with variable effective lengths. A 600-mm actuator can be fully wound inside a small base box and inflated at any length. (C) Tuning the effective length to better conform to different target objects. The selected objects have diameters from 60 to 220 mm.

Fig. 5.. Powerful soft robotic gripper with variable grasping range.

(A) Schematics of the gripper. (B to D) Pull-off force of the gripper at different finger lengths. (E) Grasping and lifting a stool with a big teddy bear. (F) Tested objects. The diameters of the objects are from 70 to 350 mm, and their weights are from 50 to 7050 g.