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Picture of How to make air muscles!
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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!
 
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Does anyone have any ideas/pointers on how the system could be made quieter?

Honus (author)  KirklandYuknis12 days ago
You could put something like a small filter or a baffle over the air exhaust port to quiet it.

Maybe even a closed-loop system of some sort?

p_scheidegger3 months ago
Do you have any info on how to control multiple air muscles via a micro controller, such as an Arduino?
Honus (author)  p_scheidegger3 months ago
The easiest method would probably be to connect multiple solenoid air valves to the Arduino output pins with a transistor to turn each valve on and off.
Do you know of anyone that sells VERY SMALL solenoid valves for use in this way? I looked in my Jameco Electronics catalog, and they only have ONE solenoid valve for sale, and it's like 1.2". Looking for something much smaller that also uses less power.

ALSO, do you think "air muscles" could be used with water inside rather than air? My intention is to make an extremely smooth operating cluster of "muscles", that mimic biological organisms accurately.

Thanks sooo much for your help!!!

i see you are trying to build a muscular system for an robot of some sort that moves and function like the real thing. EXCELLENT! do you have any interests what so ever in exo-suits?

Honus (author)  p_scheidegger3 months ago

I would just do a search for micro or subminiature solenoid valves. Most really small valves can't handle high pressure though.

You might be able to use water inside but I really don't see any advantage as you're adding a fair bit of weight and you would need a pump. If you want to go that route it would probably just be better to use traditional hydraulic cylinders.

Starsword72 months ago

Wow, amazing Instructable, wonder why they were never used in robotics.

Honus (author)  Starsword72 months ago

Several companies have use them (Festo immediately comes to mind) but they are nowhere near as common as traditional electric motors or even hydraulics. You have to have an application that specifically requires their particular properties.

Because they are very hard to control.

WarrenP22 months ago

Expanding on something mentioned here, would it be possible to run these using water? What sort of pump would you need? My thoughts are that a small pump would be more portable than an air compressor and a tank.

if accurate-yes

Honus (author)  jmacalupu herrera1 year ago

I don not know where to buy materials in Peru. Maybe try a hardware store or a place that sells aquarium supplies?

as it would be for you to bring it to the silicone tube peru because I find

Honus (author)  jmacalupu herrera1 year ago

Sorry I don't understand. Are you looking for silicone tubing in Peru?

Honus thank you tell me where each material conseguistes especially the silicone tube and not the meeting and thanks in advance

could give me the exact link of the materials or detallarmelos nesecito thanks in advance what to draft an exoskeleton

Honus (author)  jmacalupu herrera1 year ago
All of the materials are listed in Step 1.

me podrian dar exactamente el link de los materiales o detallarmelos gracias de antemano lo nesecito para un proyecto de un exoesqueleto

temmerson1 year ago
what does the strength of the muscle depend on?
Honus (author)  temmerson1 year ago
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.
temmerson1 year ago
Hi i was just wondering how you managed to get the aluminium at the ends of the muscle into the shape they are, did you buy them like that or what?

Thanks in advance
Honus (author)  temmerson1 year ago
I turned them from Aluminum rod using a lathe.
Ghunter103 years ago
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 (author)  Ghunter103 years ago
That sounds like a really cool project- make sure to post pics or video when it's done!
Ghunter10 Honus3 years ago
here is what i have so far for a design for a bicep, what do you think?
bicep of suit.gif
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?
I would love to turn this into an exoskeleton (and hope to), the problem I' ve had is cost so far, so a decent amount of time is taken saving up for the different parts.
You should enter the Jack Daniel's contest. $25,000 would buy a lot of parts.
You might think so.
Hey just spotted this and thought it would be really cool if you could do this, but if you use this for an exoskeleton then note that the air muscles can fail and cause a lot of pain, trust me if done something like this and couldnt bear the pain it gave me, when it collapsed it had caused a metal 'pinch' and fractured my arm. Just watch out for that but good luck with the attempts. :)
Honus (author)  Ghunter103 years ago
That looks pretty interesting- how exactly will it work?
Ghunter10 Honus3 years ago
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.
ExoFOX2 years ago
im building the iron man suit obvsly minus anything power by the fictional arc reactor , any tips or comment are welcome , just looking to talk and make sum friends in the process of this exp,......thank you all
Honus (author)  ExoFOX2 years ago
I'm currently working on a full animatronic Iron Man suit for a friend so I'll have a complete instructable up when it's finished. Everything is powered by servos though- no air muscles.
ExoFOX Honus2 years ago
thats whats up i would like to see the process of your exp' i myself am using linear force and gravity to make mines work sum hydrolics or shock obsobvers but not batteries,
ExoFOX2 years ago
im using old technology like the pharros back in egypt days meaning i wount be using battery i am using motors but not run on batteries or liquids just motion and pure physics
strongbino3 years ago
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?
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.
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