Soft Semi-active Back Support Device with Adaptive Force Profiles using Variable-elastic Actuation and Weight Feedback

Abstract

Portable active back support devices (BSDs) offer tunable assistance but are often bulky and heavy, limiting their usability. In contrast, passive BSDs are lightweight and compact but lack the ability to adapt their assistance to different back movements. We present a soft, lightweight, and compact BSD that combines a variable-stiffness passive element and an active element (an artificial muscle) in parallel. The device provides tunable assistance through discrete changes in stiffness values and active force levels. We validate the device's tuning capabilities through bench testing and on-body characterization. Further, we use the device's tuning capabilities to provide weight-adaptive object lifting and lowering assistance. We detect the weight handled by the user based on forearm force myography and upper-back inertial measurement unit data. Furthermore, electromyography analyses in five participants performing symmetric object lifting and lowering tasks showed reductions in back extensor activity. Preliminary results in one participant also indicated reduced muscle activity during asymmetric lifting.

Figures (18)

• Figure 1: Overview. (a) Our proposed back support device combines a variable-stiffness passive element and an active element (a pneumatic artificial muscle) in parallel. (b) The device implements weight-adaptive assistance using discrete passive and active force profiles. (c) The lightweight and compact soft structure enables assistance during lifting and lowering, while allowing unrestricted mobility during activities such as sitting and walking.
• Figure 2: (a) Device assembly with the VS resistance band (passive), IPAMs (active), and origami muscle (slack tuning). (b) Device and portable pneumatic/electronic units worn by a user.
• Figure 3: (a) Design of the VS resistance band with an integrated electroadhesive (EA) clutch. (b) Principle of operation of a single interface of the EA clutch: When 300 V is applied between the positive and negative electrodes, the electro-adhesion at the interface prevents sliding. This reduces the active length of the resistance band and increases stiffness. The EA clutch includes eight such interfaces.
• Figure 4: (a) IPAM design: It consists of a silicone tube helically wound with fiber reinforcement and sealed at both ends. Our device uses two such IPAMs in parallel. (b) Working principle of the IPAM: When pressurized, it extends axially; when deflated, it contracts, thereby generating an active force.
• Figure 5: (a) Origami muscle assembly for slack tuning. (b) Working principle of the origami muscle: On vacuuming, a zigzag skeleton housed in a sealed fabric enclosure contracts, shortening the device. (c) Working principle of the EA muscle brakes: When 300 V is applied between the positive and negative electrodes, the electro-adhesion at the interface prevents sliding, locking the origami muscle length after slack tuning. This enables compact actuation without constant power draw. The EA muscle brakes include five EA interfaces on either side of the origami muscle.