How to make air muscles!

I needed to create some actuators for an animatronics project I'm working on. Air muscles are very powerful actuators that work very similar to a human muscle and have a phenomenal strength to weight ratio- they can exert a pulling force up to 400 times their own weight. They will work when twisted or bent and can work under water. They're also easy and cheap to make!

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

All of the materials are readily available on Amazon.com, with the exception of the 3/8" braided nylon mesh- it is available from electronics suppliers. Amazon does sell a braided sleeving kit with several sizes of braided mesh but the exact material is not stated-
Amazon

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
lighter

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!

Comments

[Ghunter10]

i remember seeing this instructable when it was first shown in the email send out as a featured ible and i favorited it, i have now come back and am designing a "gauntlet" that someone puts their arm into and the muscle will hopefully add to the strength of their motion, thank you for the inspiration.

[Honus]

That sounds like a really cool project- make sure to post pics or video when it's done!

[Ghunter10]

here is what i have so far for a design for a bicep, what do you think?

[pharseid]

I think it needs a rotational joint at the elbow. And the cables should be attached to a disk or something to translate the contraction of the muscle into torque.

If this works out, do you plan to extend the exoskeleton to more body parts? I just read Wearable Robots: Biomechatronic Exoskeletons and I suppose I'm interested in working on the same sort of thing. Also, I just found this website, are other people here working on exoskeletons?

[Honus]

That looks pretty interesting- how exactly will it work?

[Ghunter10]

i intend to have the system set up where a button operated valve will be on the ring in the bottom left of the picture set up where it faces downward towards the arm inside. when the arm flexes the valve will open and provide air for the air muscles (depicted as black cylinders) which would contract, as they contract cables attached to the muscles and the ring with the valves would hopefully pull the arm up into a flexed position, i then need to design another system on this that would involve more muscles that will create the opposite motion to bring the arm and frame back down.

[strongbino]

In order to decrease the weight of the system, can the air compressor only be used in the initial filling of a muscle and then removed and then have the air flow into the adjacent muscle or another container and then the adjacent muscle?

[pharseid]

I would think that would complicate the valve system, one set of valves to prefill the muscle and the other set to take it to full pressure. Plus there's no guarantee that a muscle will need to deflate at the time another muscle needs to inflate. But if the muscles all deflated by releasing into a common line, you could put a compressor on that line which exhausts into the system's holding tank. The compressor would have to do less work when the air at the inlet was at higher than atmospheric pressure.

[Honus]

I don't know about that- I guess you would just have to try it and see.

[strongbino]

I just feel like its a waste to expel the air from the muscle into the atmosphere. It would be more efficient to have it circulate through the system to another muscle so that the pressure remains constant.

[jibblesmgee]

I want to scale this up a little. I had the idea of using sections of bicycle innertube, and finding a substitute for the nylon mesh (or just some heavy duty wide diameter nylon mesh) and seeing what I could do with it. Has anybody tried this? I'll let everyone know how it goes when I get around to it.

[Graceria]

I'm working on a set of animatronic angel wings and I need it to be entirely free of any showing tubes or wires and I need to be able to move freely without being attched to anything. These air muscles look like they could be the solution to a problem I've been having with the wings not looking natural enough. Could these be made using a small air canister with a limited supply of air and is there a way I could set it up so that the air would return to the canister when the muscles relax? I'm not quite sure how I could get the air moving the way I need it to.

[Honus]

Sorry for the late reply- been super busy! I think they would work really well for your application. You could make a small air tank from a plastic soda bottle (I think they're good to at least 60psi.) I'm not sure about the air return as I haven't yet done anything like that. The smaller muscles don't consume that much air so it might not be that big a problem unless you need it to go for several hours or over say fifty cycles. A lot of it is going to depend on your design.

[Tolderian]

Only a few years behind the times but... check out EARS... stands for (E)something (A)ir (R)eturn (S)ystem. Not sure how to incorporate it... but, that's why I'm telling you. Surely you, or someone on these sortsa pages, can come up with something for the common man. w00t!

[Graceria]

Thanks! I'll check that out!

[Honus]

Exhausted Air Return System is a really great idea- I'll have to do some research on that to see if it would work.

[Tolderian]

Well if you figure out a way to Macgyver it, that'd be pretty sweet.

I made a couple of the muscles that chewee threw up here that worked well but it just seems like an awful lot of wasted air that could be recycled and make any reserve tank last longer.

Just throwing that out there.

I come back along from year to year to see if anyone else has pooped another gem of amazing. First time I've ever posted. Good luck!

[kikiclint]

 as long as you have some kind of quiet pump, capable of the necessary psi, you could just have it continually fill the bladder, and just let a solenoid valve release the air.  Either that, or you will have to pump the air back from the muscle.  It wont be that important to recycle the air, unless you are in space. 

[nidobrito]

Hello i have a question.
How exactly the pressure regulator works with the air muscle?
I presume that when i turn the regulator for one side, the muscle contracts, when i turn to the oderside, releases the air inside it and relaxes.
It's correct?

[Honus]

That is correct.

[nidobrito]

So, like this image i posted, is the linkage correct? and if is correct, i just add a modified servo, so when turn servo the muscle contracts/relaxes.

[Honus]

Technically that is correct but I think you would have a difficult time making that work. Your best bet is to use a solenoid valve-
http://www.mcmaster.com/#air-solenoid-control-valves/=dpt784

[nidobrito]

Yeah i tried with the solenoid valves but "Mother of God!", even in ebay they are very expensive, do you know another solution for it, that can be electronically activated?

[Honus]

Have you tried Clippard? They have a valve for under $25-
http://www.clippard.com/store/byo_electronic/byo_mouse_valves.asp

There's tons of companies out there but you have to do a Google search and do some serious digging to get all the info/pricing.

[nidobrito]

Still a little expensive, what most impress-me is the fact of the inexistence of any solenoid valve tutorial on instructables...we have almost everything on here, eletromagnetic rifles, cookies, robots...but not valves...

If someone achive sucess in building a cheap and simple substitute to the solenois valves in this kind of project i hope that he posts the tutorial here...

=D

[Honus]

The only problem with making your own electronic valves is that they would probably cost more than the commercial ones! I've seen a three way aquarium valve operated by a servo but the problem is they can't hold high pressure- and it still cost nearly $15 to make.

[NightHawkInLight]

Very cool. This could allow for very quiet operation, especially if a lightweight liquid was used rather than gas.

[tilmen]

cool actuator for a robot

[maxxion]

Okay, I'm trying to make a "puppet" of sorts (Idea already start failing from here, BUT wait it gets worse). Its design has four legs, however Im having massive trouble trying to make it: Walk, keep balance, and turn. Someone suggest I use these, and I can potentially see how to walk and turn it......Maybe. The big problem is, controlling all four legs in sequence so it actually moves forward...and making it balanced so it doesnt fall over when it tries to move. Any ideas?

[maxxion]

Something I forgot to add.....Im trying to make it able to support me on top of it. Would possibly some sort of hydraulic system work better?

[Honus]

Boy I don't know. You would have to use a microcontroller to send signals to control the valves at just the right time and then have an accelerometer/gyro IMU to try and stabilize it. I don't know of anyone that's ever built anything like that before. The closest thing are I've seen are some of the systems that Festo builds.

[Rasna2011]

how exactly the air releases from the air muscle?Please answer me soon .i need this for my project............

[Honus]

Air is released when you open the air valve from your air source. Both two way and three way air valves allow you to vent the air. If you have any other questions just let me know!

[Rasna2011]

sir we have the three way air valve but that is of ''T'' shape where the air is entering through the single way and bifurcating to the both silicon tubes.after entering the air the silicon tube will inflate.Then how we will able to release the air through the valves?

[Honus]

Which valve are you using? Can you post a link to it?

[carlo.rousseeuw]

Goodday,

I've built de muscle, but if i put pressure on unloaded muscle, it does not contract alot at 30psi. Instead of other eamples where you say 60psi is allready enough to get the maximum displacement.

There is air flow, although i think not enough pressure. Or the tube is to strong ?
It is a tube of 2mm wall thickness, in stead of your 1/16 inch or 1,5mm.

I'm asking cuz i took a scare when the previous air muscle (without sleave) bulged out the end of the muscle untill it exploded there. And the muscle did not blow up homogenously.
Any thoughts ?

[Honus]

It sounds like you're using the wrong type of tubing for the bladder- what type is it?

[carlo.rousseeuw]

i'm sorry for my usage of the metric system:
All tubes are made of sillicone tubing and i've got 3 sizes:
8mm ID 12mm OD
10mm ID 14mm OD
6mm ID 10mm OD
(every wall is thus about 2mm thick)

IIf I may ask, about what maximum pressure have you put on the muscles ?
My muscle is about 30cm long made with the 8x12mm for the moment.

I don't have a weight on the muscle, so maybe that's the problem I't doesn't contract that much ( as you stated in your instructable)
thanks

[Honus]

The maximum pressure I've used is 60psi. The muscle really needs to be under tension in order for it to work properly so try using a weight or attach a strong spring to one end of the muscle.

[techno guy]

Where can I get some braided nylon mesh? Is there a store that I can buy this at? Are you sure that I cant buy it at amazon?