Low Cost String-pot Displacement Transducer

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Introduction: Low Cost String-pot Displacement Transducer

Prototyping on a student budget is always challenging, especially when it comes to prototyping pneumatic robotics and wanting to implement linear motion feedback. To overcome the high costs of industrial linear displacement transducers I decided to build them myself and use them in a working principle prototype. The linear displacement transducers as shown in this instructable are low cost, flexible and versatile however they do not function according any standards.

Step 1: Materials

1) A potentiometer of your choosing

2) A badge reel of your choosing

3) A “servo mounting thingy”

4) A piece of aluminum square tubing of your choosing

5) A small screw

Additional stuff: Glue, Drill, Drilling machine, metal saw, (the nut comes with the potentiometer)

Step 2: Construction

1) Drill the appropriate holes in the aluminum tubing.

2) Take apart the badge reel and keep the reel and the spring.

3) Glue the “servo mounting thingy” to the reel.

4) Place the reel in the aluminum tubing and attach the small screw to the spring.

5) Put the potentiometer to the hole in the aluminum tubing and screw on the nut loosely.

6) Press the head of the potentiometer in the “servo mounting thingy” (it should make a tight fitting such that the potentiometer turns when you pull the string on the reel)

Lastly) Fasten the nut on the potentiometer and voila: A low cost string-pot displacement transducer.

Step 3: Voila

I used the above mentioned string-pot displacement transducer as position feedback in a servo pneumatic robotics working principle prototype and they did the job perfectly. In my next instructable I will show you how to build your own low cost servo valves (up to 8 bar) for servo pneumatic prototyping purposes.

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14 Comments

is that a servo horn?

if it's then where does that fit in.

I'm interesting in making a steering angle sensor for my 1992 Porsche 968 track car, and this might be a good approach, in that I could attach it to the shaft on the steering rack that's attached to the tie rod. Just one question - could you please describe the "servo mount thingy" you used in a little more detail? Thanks.

I need something like this to be able to measure the velocity of the wire. 2 types of questions:

1. Where did you get that badge reel? It looks sturdy How long is it? I can't find any, the only thing I can think to do is to get a retractable dog leash and take it apart. I did it once but it broke :(

2. Would a rotary encoder work as well?

you can buy a badge reel at most office supply (web)stores. Depending on your application a retractable dog leash might be perfect. In my case a dog leash would have been too big.
If you only need velocity, an encoder is perfect. If you never worked with encoders, this will require some time getting used to. There should be able to find some madlab or Arduino tutorials for this.

I have this on hand
http://www.amazon.com/gp/product/B00KHTLY7M?psc=1&redirect=true&ref_=oh_aui_detailpage_o03_s00

However its a cheap one I intended to use just to practice and generate some code to read the signals until I buy a more robust rotary encoder


This is the one I probably will buy, could you give me a suggestion for this one, or similar?

http://www.amazon.com/iiMash-Incremental-Encoder-DC5-24V-Voltage/dp/B00IJ788AA/ref=pd_sim_sbs_421_2?ie=UTF8&dpID=41cFrN8r5RL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=135Q5ND45GHRVHK5Z1BN

The link to the dog leash is missing. The iiMash encoder has
a rod that could support axial forces and possibly shear forces, I have doubts
about the cheaper one. Since the forces applied by the reel are quite
significant I definitely recommend the iiMash.

Firstly, drill a hole in the housing of the dog leash to connect
the housing of the encoder. In case of the iiMash encoder, four holes (one for
the rod and three for the connecting screws). Next, it might be possible to
drill a press fit in the reel to connect the rod to the reel. In any other case
you need to search for an appropriate coupling.

I hope this helps

I am comfortable reading electrical output from the rotary encoder, I'm just at a loss at how I would attach the rotary encoder to the dog leash so that it spins as it retracts..

Well, that really depends on the encoder you use. Probably you
need an axle/rod to connect the encoder to the reel of the dog leash. If you
send me a link of the encoder, I can think of specific suggestion to solve the
issue.

Have a look here at a picture of the dog leash I have

How did you affix the spring?