Pneumatic Soft Exoskeleton

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

My initial encounter with pneumatic systems like inflatable shelters and floatation devices exhibited an enormous potential for creating lightweight systems that can be reconfigurable and transportable. Pneumatic systems are used in various critical applications that require immediate response, reliability and flexibility. Air bag systems, inflatable life vests, and self-inflating rafts are of particular interest to me.

In terms of pneumatic systems as a wearable technology, a few possibilities come to mind (nomadic shelters, impact protection, an expressive suit, and a powered exoskeleton, just to name a few). Each have potential uses that interest me and many have been explored by others. Motoair sells pneumatically actuated vests and jackets for motorcyclists and other high impact sports to cushion accidents. ((Motoair, MOTOAIR, Jan 28.2008, <http://www.motoair.com/>)) Powered exoskeletons, currently developed within research groups around the world, are focused on assisting human locomotion through a wearable machine. ((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)) 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. ((BBC. BBC News: Health, Jan. 28 2008 <http://news.bbc.co.uk/1/hi/health/2002225.stm>)) 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. ((Human Neuromechanics Laboratory, Dr. Daniel Ferris and
Dr. Riann Palmieri-Smith. Jan. 28,2008. ))

The goal of the Pneumatic Soft Exoskeleton project is to create a set of soft and lightweight wearable pneumatic muscles for the lower extremities. A series of pneumatic muscles are worn around the legs to assist the user in lifting loads, walking, running, and perhaps jumping. Unlike other exoskeletons, this application would be untethered and constructed of primarily soft fabric, making the device more lightweight and flexible. The system will be built to remain primarily in an idle state to be activated only as needed. Primary concerns are weight, comfort, and flexibitily. As a benchmark, if I can wear the device and beat the world record for any track event, I would consider the device a success.

Features of the wearable system
The system will be powered by a pneumatic reservoir and triggered by the user’s motions or an external switch to activate the system only at desired moments. An electronic circuit, powered by a battery will sense and trigger movement through a series of artificial muscles. 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. ((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>))

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. ((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)) 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). ((Shadow Robot Company: Air Muscles overview. Shadow Robot Company. Jan. 28 2008. <http://www.shadowrobot.com/airmuscles/overview.shtml>)) The assembly is also flexible, cushioned, and operates smoothly, making it an ideal candidate as an artificial muscle for a wearable application.

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 can add to the user’s own movements to achieve greater results in speed and power. Each assistive muscle would coincide 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 quadriceps femoris muscle group will be the primary group of focus. As the system is perfected, other muscle groups will be identified and addressed.

Components of the system
The system consists of pneumatic muscles of varying sizes which attach to the main wearable frame. The wearable frame is a large fabric that is wrapped tightly around the thigh and crus. The frame consists of straps that are sewn in a pattern to distribute forces from muscle-to-tendon junctures to the rest of the frame. The straps are collected at several junctures to accept a tension connection from a pneumatic muscle.

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 worn at the hip of the user. 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 Pneumatic Soft 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. The system is potentially more affordable and highly customizable because of the simplicity of the components.

Technical concerns
The Pneumatic Soft Exoskeleton must be lightweight and perform reliably to assist locomotion. 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