Air muscles (also known as a McKibben artificial muscle or braided pneumatic actuators) were originally developed by J.L. McKibben in the 1950's as an orthotic appliance for polio patients.
Here's how they work:
The muscle consists of a rubber tube (bladder or core) that is surrounded by a tubular braided fiber mesh sleeve. When the bladder is inflated the mesh expands radially and contracts axially (since the mesh fibers are inextensible), shortening the overall length of the muscle and subsequently producing a pulling force.
Air muscles have performance characteristics very similar to human muscles- the force exerted decreases as the muscle contracts. This is due to the change in the interweave angle of the braided mesh as the muscle contracts- as the mesh expands radially in a scissors like motion it exerts less force due to the weave angle becoming increasingly shallow as the muscle contracts (see the diagram below- figure A shows that the muscle will contract to a greater degree than figure C given an equal increase in bladder pressure).The videos show this effect as well. Air muscles can contract up to 40% of their length, depending on the method and materials of their construction.
Gas law states that if you increase pressure you also increase the volume of an expandable cylinder (provided temperature is constant.) The expanding volume of the bladder is ultimately constrained by the physical properties of the braided mesh sleeve so in order to create a greater pulling force you need to be able to increase the effective volume of the bladder- the pulling force of the muscle is a function of the length and diameter of the muscle as well as its ability to contract due to the properties of the mesh sleeve (construction material, number of fibers, interweave angle) and bladder material.
I constructed two different sized muscles using similar materials to demonstrate this principle- they both were operated at the same air pressure (60psi) but had different diameters and lengths. The small muscle really starts to struggle when some weight is put on it while the larger muscle has no problems at all.
Here are a couple of videos showing both of the constructed air muscles in action.
Now let's go make some muscles!
Step 1: Materials
You'll need an air source:
I used a small air tank with a pressure regulator but you can also use a bicycle air pump (you will have to make an adapter to make it work with the 1/4" poly hose.
Air tank- Amazon
Pressure regulator (will require a 1/8" NPT female to 1/4" NPT male adapter)- Amazon
1/4" high pressure poly tubing- Amazon
multitool (screwdriver, scissors, pliers, wire cutters)- Amazon
for the small muscle:
1/4" silicone or latex tubing- Amazon
3/8" braided nylon mesh sleeve (see above)
1/8" small hose barb (brass or nylon)- Amazon
small bolt (10-24 thread by 3/8 in length works well)- Amazon
steel safety wire- Amazon
for the large muscle:
3/8" silicone or latex tubing- Amazon
1/2" braided nylon mesh sleeve- Amazon
1/8" or similar sized drill bit- Amazon
21/64" drill bit- Amazon
1/8" x 27 NPT tap- Amazon
1/8" hose barb x 1/8" pipe thread adapter- Amazon
small hose clamps- Amazon
3/4" aluminum or plastic rod to construct the muscle ends- Amazon
Safety note- make sure you wear safety glasses when testing your air muscles! A high pressure hose that pops off a loose fitting could cause a serious injury!
Step 2: Making the Small Muscle
Now cut the 3/8" braided sleeve about two inches longer than the silicone tube and use a lighter to melt the ends of the braided sleeve so it doesn't fray apart.
Slide the braided sleeve over the silicone tubing and wrap each end of the tube with the safety wire and tighten it.
Now make some wire loops and wrap them around each end of the braided sleeve. As an alternative to using wire loops on the ends of the muscle, you can make the sleeve longer and then fold it back over the end of the muscle, forming a loop ( you have to push the air fitting through)- then tighten the wire around it.
Now connect your 1/4" high pressure tubing and pump a little air into the muscle to make sure it inflates without leaking.
To test the air muscle you have to stretch it to its full length by putting a load on it- this will allow it maximum contraction when it's pressurized. Start adding air (up to about 60psi) and watch the muscle contract!
Step 3: Making the Large Air Muscle
Now cut a 8" length of 3/8" rubber tubing for the air bladder and slide one end over one of the machined fittings. Then cut some 1/2" braided sleeve 10" long (remember to melt the ends with a lighter) and slide it over the rubber tube. Then slide the opposite end of the rubber tube over the remaining machined air fitting. Now securely clamp each end of the tubing using hose clamps.
The larger muscle works just like smaller version- just add air and watch it contract. Once you put it under load you'll immediately realize this larger muscle is much stronger!
Step 4: Testing and Additional Info
Stretch out the muscles so they reach their maximum extension by adding weight. A good test rig would be to use a hanging scale- unfortunately I didn't have access to one so I had to use some weights. Now slowly start adding air in increments of 20psi until you reach 60psi.
The first thing you notice is that the muscle contracts a progessively smaller amount with each incremental increase in air pressure until it fully contracts. Next you'll find that as the load is increased the ability of the muscle to contract decreases at an increasing rate until it can no longer lift the increased load. This is very similar to how a human muscle performs.
It is immediately noticeable that a change in the size of the muscle has a huge effect on the performance of the muscle. At 22lbs. @60psi, the smaller muscle can still lift, but it is nowhere near obtaining full contraction while the larger muscle can very easily obtain full contraction.
The dynamics of air muscles are fairly difficult to mathematically model, especially given the number of variables in their construction. For further reading I recommend having a look here:
Several applications of air muscles include robotics (especially biorobotics), animatronics, orthotics/rehabilitation and prosthetics. They can be controlled by microcontrollers or switches using three way solenoid air valves or by radio control using valves operated by servos. A three way valve works by first filling the bladder, holding the air pressure in the bladder and then venting the bladder to deflate it.
The thing to remember is that air muscles must be under tension to work properly. As an example two muscles are often used in conjunction to balance each other to move a robotic arm. One muscle would act as the bicep and the other as the tricep muscle.
Overall, air muscles can be constructed in all sorts of lengths and diameters to suit a wide variety of applications where high strength and light weight are critical. Their performance and longevity varies according to several parameters regarding their construction:
1) Length of muscle
2) Diameter of muscle
3) Type of tubing used for bladder- testing I've read states that latex bladders tend to have a longer service life than silicone bladders, however some silicones have greater expansion rates (up to 1000%) and can hold higher pressures than latex (much of this will depend on the exact tubing specification.)
4) Type of braided mesh used- some braided meshes are less abrasive than others, improving bladder life span. Some companies have used a spandex sleeve between the bladder and mesh to reduce abrasion. A tighter woven mesh allows for more even pressure distribution on the bladder, reducing stress on the bladder.
5) Pre stressing of the bladder (the bladder is shorter than the braided mesh)- this causes a reduction of contact area (and hence abrasion) between the bladder and braided mesh sleeve when the muscle is at rest and allows the braided mesh to fully reform between contraction cycles, improving its fatigue life. Pre stressing the bladder also improves the initial contraction of the muscle due to initial lower bladder volume.
6) Construction of muscle end housings- radiused edges reduce stress concentrations on the bladder.
All in all, given their power to weight ratio, ease/low cost of construction and ability to mimic the dynamics of human muscles, air muscles offer an attractive alternative to traditional means of motion for mechanical devices.
Have fun building them! :D