COTS Air Muscles and How Not to Do Internal Position Feedback

Intro: COTS Air Muscles and How Not to Do Internal Position Feedback

Air muscles are great because they are inherently shielded from dirt and moisture. However, they are normally controlled with exposed position sensors (potentiometers). You also can't use a potentiometer if you have a compliant joint. My goal was to create an inexpensive, position feedback system that would be just as waterproof as the muscle itself and was compatible with compliant joints.

The air muscles I built here a sort of a hybrid of cheewee2000's project and Honus'.

They were part of an investigation to see if I could get position feedback from an internal LED and photo-transistor. I learned that is not a very reliable way to get position feedback. At least not in my setup. My purpose in publishing this Instructable is so that you can be more wise than I have been.

Maybe this can be a crowd-sourced problem.

STEP 1: Concept of LED Diffusion

The idea was to create end pieces that held either a red LED or a photo-transistor. I poked holes in a rigid tube and attached that to the inside diameter of a a barbed fitting I got of McmasterCarr. The holes let the air flow into the muscle even with the end of the tube topped of with the diode. Really you just need holes on the side that has the air supply.

I connected the photo-transistor to a Wheatstone bridge to measure the the variation as the muscle contracted. Despite the silicone tubing (but probably because of my translucent fitting) I had lots of interference from ambient light. The readings were all over the place. I suspect that this was due to misalignment of the LED and photo-transistor.

In the future, I want to try position feedback with internal capacitance instead.

What do you think? leave a comment below

STEP 2: Building the Air Muscles

The parts you'll want (to just make a regular ol', blind-to-position air muscle) are listed below:

What cheewee2000 told me to get:

High-Temperature Silicone Rubber Tubing Soft, 3/8" ID, 1/2" OD, 1/16" Wall
mcmaster.com # 5236K15 | $1.31 per foot

Heavy Duty Polyester Expandable Mesh Sleeving 1-1/2" ID, 3/4" to 2" Bundle Dia, 3'L, Black
mcmaster.com # 9142K34 | $5.83 each

Though I think the mesh is too big, one of my muscles popped after about 50 contractions (see picture below). I also wasn't using a pressure regulator.

Other fittings you might find helpful: (also from mcmaster carr)

44705K36 Low-Pressure Aluminum Threaded Pipe Fitting, 1/8 Pipe Size, Coupling  2.13

2 2808K25  White Polyethylene Single-Barb Tube Fitting, Adapter for 3/8" Tube ID X 1/8" NPT Male Pipe, Packs of 10,  5.53 Per Pack

5116K114 FDA White Nylon Single-Barbed Tube Fitting, Plug for 3/8" Tube ID, Packs of 10, 2.55 Per Pack

And some hose clamps.

I used dental floss for tendons. It's super strong and it makes your air muscles smell minty fresh.

STEP 3: Valves and Air

I used solenoid valves available at BYU's EAAL lab.

Because they required AC, I chose to trigger them with a solid state relay.

Thanks to pdf below I was able to figure out how to use the valves.

I used two valves one to open pressure and one to open exhaust.;

Comments

[sparten11]

I have an idea that might help. instead of going from mounting point to mounting point devise a system that will measure the OD of the expanding bladder. if you know your diam you know your length. by testing different diameters you get a basic feel for the amount of movement for any given amount of pressure. all you need is a benchmark and to keep an eye on calibration. this could work very easily. You need to get y our variables and create a spreadsheet to run the numbers. let me know i can help ya. Also if you pick up a load with the muscle all you need to do is again measure the added pressure and you know how much loss or deviation from norm you have and can add more pressure to compensate and maintain position. We as humans use our eyes more than anything. yet another option would be to use the newest gigapixle camera's to create a unit to measure with.

just some thoughts.

thanks
Warren

[rickharris]

This type of sensing is quite possible to do although I have only done it with opaque rubber tubing and measured the light loss when the tube was stretched and so narrowed.

The results were quite repeatable.

[W_world]

That's good to hear. I'll have to try it again with more opaque tubing and ends. Do you suppose I should use a diffuse lens on my LED?

[rickharris]

I didn't just a simple red LED and a sensor at the other end we used an LDR.