Suunto compass re attached to a custom leather strap.

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.

Soft Pneumatic Exoskeleton: Prototype 03 from che-wei wang on Vimeo.
The Soft Pneumatic Exoskeleton (developed in the Wearables Studio at ITP, NYU) 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.

The Pneumatic Soft Exoskeleton is worn by strapping components of of the system on parts of the leg to align the pneumatic muscle to major muscle groups in the leg. Synchronized actuation of the pneumatic muscles add to the user’s own movements providing support and power. 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. A force sensor under the foot activates the air muscle around the calf and a flex sensor behind the knee activates the air muscle around the quads.

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.¹ 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.² 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).³ The assembly is also flexible, cushioned, and operates smoothly, making it an ideal candidate as an artificial muscle for a wearable application.

Powered exoskeletons, currently developed within research groups around the world, are focused on assisting human locomotion through a wearable machines.⁴ 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.⁵ 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.⁶

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.

Uses
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.

Technical concerns
Since the system relies on an air reservoir, it is likely that a user may find the system insufficient in its capacity to perform continuously. This concern can be address with a larger reservoir, but that would add more undesirable weight and volume.

There may be a potentially harmful side-effect to the body due to repeated unfamiliar stress on bones and muscles. The softness of the system is intended to dampen any impact that may be harmful, but repeated stress points due to the power of the assistive muscle or the location and transfer of forces to the limbs may be damaging.

Precedents
Here’s a nice intro to the subject from engineeringtv.com
Human Neuromechanics Laboratory at The University of Michigan
HAL at the University of Tsukuba
Muscle Suits at Koba Lab
Wearable Power Assist Suit at Kanagawa Institute of Technology, Robotics and Mechatronics

1. Lightner, Stan, et al. The International Journal of Modern Engineering. Volume 2, Number 2, Spring 2002, Jan.28 2008 <http://www.ijme.us/issues/spring%202002/articles/fluid%20muscle.dco.htm>
2. Lightner, Stan, et al. The International Journal of Modern Engineering. Volume 2, Number 2, Spring 2002, Jan.28 2008 <http://www.ijme.us/issues/spring%202002/articles/fluid%20muscle.dco.htm>
   Air Muscle videos: http://www.youtube.com/watch?v=w77YDDTXfRc&NR=1, http://www.imagesco.com/articles/airmuscle/AirMuscleDescription03.html
3. Shadow Robot Company: Air Muscles overview. Shadow Robot Company. Jan. 28 2008. <http://www.shadowrobot.com/airmuscles/overview.shtml>
4. exoskeletons: http://bleex.me.berkeley.edu/index.htm, http://www.newscientist.com/article.ns?id=dn1072, http://spectrum.ieee.org/print/1974, http://www.youtube.com/watch?v=0hkCcoenLW4
5. BBC. BBC News: Health, Jan. 28 2008 <http://news.bbc.co.uk/1/hi/health/2002225.stm>
6. Human Neuromechanics Laboratory, Dr. Daniel Ferris and Dr. Riann Palmieri-Smith. Jan. 28,2008.

The ankle support is a low profile sleeve around the foot and a strap at the heel to connect to the air muscle. A shorter 12″ muscle connects the ankle support to the calf. The calf attachment is now constructed out of a single piece of leather with nylon reinforcement at the top to prevent buckling and stretching when the air muscle is actuated. It’s ideal to have the solenoid valve as close to the air muscle as possible, so I’m going to have to make room for that somewhere on the calf.

The most common point of failure in all the test have been the connection at the collar around each end of the air muscles. The inflation of the muscle along with the tension force rip through the threads around the collar, so I’ve reinforced those points with rivets with some room to expand.

This is what happens when you pump compressed air directly into an air muscle without a pressure regulator. Good thing I wasn’t wearing it when it exploded.

After the first round of testing, I realized assisting jumping with pneumatics is nearly impossible. The size of the tubes and tank necessary to get a high enough cfm to actuate the air muscles at the speed of jumping seems too large for a lightweight wearable application. So, I’m going to concentrate on assisting walking. Maybe speed walking.

Soft Exoskeleton V01 Test from che-wei wang on Vimeo.

I finally have a full assembly of all the major components sewn with thick canvas and leather. The small scuba tank provides the air pressure, controlled by an Arduino and solenoid valves. The basic operation of pneumatic muscle works. Air pressure at about 100psi inflates the muscle, creating a contracting motion of about 2 inches, enough to make my leg kick out (although the speed of inflation needs to be faster).

The harness for the scuba tank and a guide for the pneumatic muscle are still issues to be dealt with. The strap for the thigh needs to extend further down to help guide the placement of the pneumatic muscle and the scuba tank needs to be attached in a more comfortable way.

I finally got the solenoid valve hooked up to an air compressor and and arduino board. Air pressure is regulated at 100 psi. It looks like I’m going to need bigger pipes to get more cfm for faster muscle actuation. The shorter muscle has an over sized braided sleeve, which I thought might help the actuation distance, but it seems like it just takes more air to fill up and doesn’t make a noticeable difference.

Pneumatic Muscle: Solenoid Valve Test 02 from che-wei wang on Vimeo.

The first set of home brew pneumatic muscles actually work. There’s no leakage and it looks like it holds well over 60 psi. The tiny air reservoir barely holds enough air for 2 actuations, so it looks likes a small scuba tank or a tiny air compressor is going to be needed. Weight tests coming soon.

Pneumatic Muscle : Pressure Test from che-wei wang on Vimeo.

Here’s my first set homebrew pneumatic muscles. I’m not sure how strong these are going to be, but it looks promising. The compression fittings aren’t being used the way they were meant to be used, so it’s likely that connection will be the first point of failure when it goes under stress.

As soon as the assembly is tested, I’ll be posting instructions on how to make your own.