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Pot based position system

Posted by Anonymous User 
Anonymous User
Pot based position system
October 04, 2008 04:01AM

I have been posting about a pot based positioning system over in [forums.reprap.org] and since then found the time to build a new frame and design for my testing. Essentially what i have is a piece of mdf sliding with guilds and a powered chain driving it to create a moving platform. On the right side i have linear 100k pot connected to a swing arm which is used to feedback the pot value depending on how far it swings up or down. I then read this value and get the y distance value using the following formula.

float potRatio = 0.245;
float yArmLength = 196.5;
float pi = 3.141593;

float convertUnitsToDistance(float units)
float deg = potRatio * units;
float rad = pi * deg / 180;
float distance = sin(rad) * yArmLength;
return distance;

This is a video of it working in test mode [picasaweb.google.com.au] (Click the image on the left to get it working if it does not start automatically) It moves back and forth to exactly the same position along the Y axis. I have coded it to work similar to an elevator so that the closer to each end it gets the slower it goes, and thus allowing it to travel faster when moving larger distances. I am playing around with optimum settings for speed ect and would like to think that with an extruder head that can vary its output accordingly you should be able to extrude fast when appropriate and slower for more accuracy when desired.

There is no end points, or need to move the platform to a predefined starting point. A lot of feedback i have read is in relation to time take to print parts, i hope that with some work and feedback i can get this to move in an accurate and fast manner.

Re: Pot based position system
October 04, 2008 11:27PM
I like it. My big hold up on printing speed isn't the extruder as I thought it would be but the cart-bot. If you can get it to traverse at a fair clip you have gone half way to solving a big problem that I, at least, have been having. Also, since it appears you are using a simple brushed DC motor and gear train, you cut out some fairly expensive components.

I'm very interested in seeing the x-axis stacked on this and the two working together. Are you gonna have it accept G-Code input so that you can use the standard software chain Zach has been working on putting together? Keep us posted on your work.

as I read this...
what do you think of making our own potentiometers?
A very long (travel of the bot per axis ~30cm) linear would be nice.

We have that ni.chrome wire
So we "just" need one meter of a nylon threaded rod and six Nuts (nylon)
cut the rod to the desired length, wrap the wire around and fix it with the nuts,
should be precise enough for our needs (depending on pitch, wich is .8 for a m5 thread p.e. // smaller pitch available)
oh yes, a small pickup is needed too winking smiley

Anonymous User
Re: Pot based position system
October 05, 2008 08:11AM

Sid, that
well you'll find some nylon threaded rods in most hobby or hardware stores.
nichr wire is just the same that is used for the extruder;
proxxon is selling 30m of .2mm nichr at most hardwarestores.

let's guess the length of the wire...
r=2.2mm (core of a m5 thread should be around 4.4mm
so it's about 1.4cm/rev we need about 30cm to be covered,
assuming 0.8cm pitch that'd be about 38 revolutions that's ~54cm of wire
(that's not too much should be around 22 Ohms)

connect on end of the wire and a small piece of copper to a multimeter, then move the copper along the thread that's pretty much it.
we'll just have to find a way to mount that to the bot, but I don't think that's a problem at all, is it winking smiley

you can connect that to your electronics just like you connected the trimpot,
just find the max and min value of resistance by reading them with the multimeter,
you'll find the difference of resistance to be around 0.56 per 0.8cm of movement.
unfortunately you may find, that sometimes the copper is not connected to the nichrome at all, to prevent that, you can add another piece of copper that catches the other side of the thread.
for error-correction you can also connect both ends of the wire, just take care that you have two different ports for measurment winking smiley


PS I'm sorry, i cannot be more precise at the moment, I'm not at home and I don't have any computer nearby to quickly draw an image or something.
Re: Pot based position system
October 05, 2008 11:57AM
That is a nice idea, Sid.

Re: Pot based position system
October 05, 2008 12:06PM
So, back home again

farnell #7067513 (m5 nylon studding 1 meter)
farnell #7016967 (m5 nylon nuts)
and the wire:

Hope that helps a bit


PS 1 meter just to have enough length to cut it in three pieces that are all long enough to fit our needs of 30cm per axis winking smiley

Edited 1 time(s). Last edit at 10/05/2008 12:41PM by sid.
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