Soft Pneumatic Exoskeleton

Che-Wei Wang

The Soft Pneumatic Exoskeleton is a soft and lightweight wearable pneumatic muscle suit for the lower extremities. Pneumatic muscles are worn around the leg to assist the user in lifting loads, muscle reinforcement and walking. Unlike other exoskeletons, this application is untethered and constructed primarily of soft materials, making the device lightweight, portable, and comfortable. The system is built to sustain an idle-power state and is activated as muscle assistance is needed. Primary concerns are weight, comfort, and flexibility.


Classes Wearables Studio

The Pneumatic Soft Exoskeleton is worn by strapping components of of the system on parts of the leg to align the assistive muscle to major muscle groups in the leg. By attaching a few pneumatic muscles to assist gross movement of the lower extremities, properly timed actuation of the assisting muscles add to the user's own movements provide support and power. Each assistive muscle coincides with existing muscle groups and transfer power to tension lines that wrap around the leg in such a way that the forces transfer in a similar fashion to the muscle-tendon-bone hierarchy. The system is powered by a 'pony' size scuba tank and is triggered by the user's motions through flex and force sensors worn on the body. An electronic circuit, powered by a battery senses and triggers movements in the worn artificial muscles.

Pneumatic systems are clean, safe, lightweight, and reliable. In a pneumatic electronic hybrid, electric components simply control the flow of air pressure, removing the burden of weight and kinetic actuation from electric to pneumatic power. The result is a lightweight low idle-power system with high-power kinetic impact.

Powered exoskeletons, currently developed within research groups around the world, are focused on assisting human locomotion through a wearable machines.1 Actuated parts of the machine coincide with the body and gross muscle groups to help lift heavy loads. A suit for the upper extremities has been created by Hiroshi Kobayashi, a roboticist from the Science University of Tokyo.2 Dr. Daniel Ferris and Dr. Riann Palmieri-Smith lead a group of researchers at the University of Michigan in creating pneumatically powered exoskeletons for the lower limbs.3

Pneumatic muscles work by inflating a silicon tube within a plastic braided sleeve. The inflation of the tube shortens the overall length of the assembly as the braided sleeve increases radially.4 Pneumatic muscles are a relatively recent development in air powered actuation, lead by the Shadow Robot Company, and FESTO Corporation. They were originally commercialized by The Bridgestone Rubber Company in the 1980’s.5 Pneumatic muscles use simple materials that have a low cost to manufacture and are extremely lightweight. A fully assembled muscle can potentially have a 1:400 weight to strength ratio (compared to the 1:16 ratio of pneumatic cylinders and DC motors).6 The assembly is also flexible, cushioned, and operates smoothly, making it an ideal candidate as an artificial muscle for a wearable application.

The Soft Pneumatic Exoskeleton does not use off-the-shelf air muscles, since it requires custom lengths. The result is a more affordable air muscle that can be tailored to specific lengths and strengths. Pneumatic muscles are strapped to the calf and quad muscles on each leg with a nylon reinforced leather holster. Air flow of each pneumatic muscle is controlled by a single tube from a 3-way solenoid valve which controls the air flow in and out of the pneumatic muscle from a portable air reservoir. Each solenoid is controlled by outputs from a battery powered Arduino board. Switches from the user’s inputs are fed to the Arduino board to control the actuation of the artificial muscles.

User Scenario
Potential uses for the Soft Pneumatic Exoskeleton follow much of the current applications for powered exoskeletons. These wearable machines can assist lifting and locomotion. The added benefit of the Pneumatic Soft Exoskeleton is its weight and flexibility. By making the system lightweight and soft, its appearance is less obtrusive and less of a burden to fit to the body. Components of the system are readily available, relatively affordable and highly customizable. The complete system weights approximately 6.5 pounds (3kg). If constant muscle assistance isn’t needed, a 12-25 gram CO2 cartridge can replace the 5 lb scuba tank, dropping the entire system’s weight to less than 2 lbs.