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electrical 360 potentiometer

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Allen

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I need a potentiometer with 360 electrical degree and a infinate mechanical turn. What would also be good was if it had a very low physical resistance. Can anyone offer any suggestions of a good one and where I can get one in the UK.
 

mechie

New Member
Potentiometer alternatives ?

Does this need to be a carbon(or plastic etc. etc) track to give ohms out or could you use a shaft encoder from a mouse and pulse-count on a microcontroller?
Or a Grey (or Radix) code wheel (8 bits will resolve to 1.5 degrees)?

The only pots I've seen that allow 360 rotation tend to either give a 'blind spot' or ramp up then down again, maybe 200deg up and 160deg down as the shaft is rotated. They are as common as hens teeth.
 

Allen

New Member
I did eventually find a pot that did the job at RSS (but they're £10).

And I have also now been looking into using a pulse counter to do the job, so with regard to that is there anyone that knows of any good tutorials about making such things, I need it to get the direction movement as well.

For this any info on where to get the bits in the UK would be greatly apprechiated as well.
 

mechie

New Member
360 degree encoder

A computer mouse contains two rotary encoders for X and Y axes, each gives two pulse trains in quadrature. This allows the computer to resolve direction by seeing which pulse leads the other.
The problem I see with using one of these is that some form of mechanical drive is required to gear the input shaft rotation up for improved resolution and a second detector would be required to identify a 'start position' of maybe 0 degrees. This isn't a problem for a mouse but to get absolute position it will be required.

A Grey code wheel system is fairly easy to do electronically but making the wheel will take time. The wheel needs marking out with something resembling a binary pattern (twisted to avoid two outputs changing at the same time). Easy for four or five bits but fiddly for eight bits!
More inf at http://www.nist.gov/dads/HTML/graycode.html
Picture of a code wheel (5 bit) http://www.ida.net/users/tetonsl/railroad/graycode.htm
Code wheel (8 bit) http://www.peg.si/electro/angle.html
Grey-to-binary converter http://www.ulib.org/webRoot/Books/Numerical_Recipes/bookcpdf/c20-2.pdf

So what resolution do you need ?
Do you require an absolute value from power-up ?
How compact must it be ?
... Do I see a ham mast rotator ??
 

Allen

New Member
I'm in the process of developing a number of art projects that have rotation effect what happens on a number of video screens. On is a pendulum with a monitor as the weight, the idea is that the image on the screen will remain horizontal to the floor while it swings. This one I think Ihave sorted out now since it's quite easy to do with a pot. The other more elaboate piece involves a number of screens stacked up on top of each other but revolving indently which is where I need to head towards the 360 issue.
The gray code looks interesting. Originally I had been pulling old mice apart and playing around with limited success. But I have a question with regard to this. The bit I have uncovered about quadrature involve two IR LED and receivers for distance and direction and yet the mice only seemed to have one, is it the case that the LED in the mouse actually emits from two points?
Also is it worth me continuing with the mouse dissectio do you think. Or to put it another way what IR would enable me to developthis myself considering I only really need one axis.
Does any of that make sense?
 

mechie

New Member
Quadrature !

Kill a mouse!
Each encoder wheel assembly has a single plastic 'blob', this contains the source and both recievers. They are built this way to ease the manufacturer's assembly lines, the two outputs are bound to be about right as far as phase is concerned.

I reckon the full-rotation effect of multiple screens could be accomplished with one of these sensors and a master 'reset' sensor to ensure any counting errors are corrected once per revolution, although I would suspect a six-bit Gray code wheel (about 5 degree resolution) could be used. You pays yer Monet, you takes yer choice!.

I'd love to see the finished display!
 

Allen

New Member
Great thanks for your help.
I will keep you informed of my progress and send you an invitie to my grand opening in the Tate (which I'm sure I'll be having one day (ha, ha)).
 

Allen

New Member
I said I would keep you informed as things developed and so I am, by asking another related question.
Update: Most of my time lately has been given over to my job (dam it) and trying to get some funding for the project, which is going OK.
I have also developed conceptual aspects of the project and I now have a more specific idea of what I need it to do, technically I believe it actually makes it simpler.
I intend to have a manually operated crank, this would be ratchet so one way only. The individual would turn the crank which would stimulate the image change.
What I believe would be the best way to do this would be using the IR pulse counter, much like a mouse. Each change of state (or maybe each return to state) in the IR switch would send the pulse via serial for a pc to receive.
So much like my normal questions are their any basic tutorials that demonstrate this activity, what IC would be best for carrying this out?

Thanks for any help, advice.
 

mechie

New Member
Serial interface ??

This sounds like your biggest problem will be software ... :?

A bog standard old serial mouse should supply you with the encoder wheel, interface electronics, and possibly even a software driver :!: (if you need it).

If you remove the guts, throw away the ball and drive one (either) wheel directly, a shaft extension or a rubber-tyred wheel giving a friction drive to the mouse wheel axle ...

All your custom software needs to do then is look for a mouse movement as if it were still a normal mouse :shock:

I'm off to dissect one now ...

ps. Nice to hear you are still persuing the idea - i've bought my Tate entrance ticket ...
 
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