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linear magnetic encoder

Posted by travel 
linear magnetic encoder
June 19, 2008 09:11PM
we need a encoder to know the real position (xyz) of the extruder and make a optical encoder is hard.

if unroll a magnetic tape from a compact cassette recorded whit a tone 11900hz we had 2 micrometers peak to peak,

plus stereo channel for index maybe even digital modulation on this channel with FSK (frequency shift keying) or PSK (phase shift keying) .

3 or 5 sensors(heads) and the electronic for set up the signal for the adc is cheap.
magnetic tape maybe must be reel in sometimes like did the printers to avoid damage. the mechanism for do this can be borrow from a old radio cassette.

another option is keep the tape running and compare the frequency and the index from a static head only 60 minutes for reprap and you have to rewind the tape smiling smiley

from wikipedia [en.wikipedia.org]
The tape is 3.81 mm (0.150 in) wide, with each stereo track 0.6 mm wide and an unrecorded guard band between each track. The tape moves at 4.76 cm/s (1 7/8 in/s) from left to right.

a few screws and one spring to catch the tracks and can "hear" the movement

can this work?

(sorry for my english)
Re: linear magnetic encoder
June 19, 2008 11:35PM
travel Wrote:

> can this work?
> (sorry for my english)

Sounds interesting! Try it and see! smileys with beer
Re: linear magnetic encoder
June 20, 2008 03:10PM
That sounds great!
(no pun intended)
Re: linear magnetic encoder
June 25, 2008 06:29AM
Well, the interesting point is that signal strength of the head depends on speed of movement over the tape. This is probably not very constant.


Airspace V - international hangar flying!
[www.airspace-v.com] for tools and toys
Re: linear magnetic encoder
June 26, 2008 01:55PM
Great idea, why not take one step further?

"glue" the tape on the axis and mount the head on the sled.
Now you're close to what other linear encoders work like...

should be possible

Re: linear magnetic encoder
June 29, 2008 07:34AM
I would be a little worried about wear on the tape, not only the usual physical wear and tear, but also I believe the process of reading a tape actually causes the tape to lose signal gain each time. As I recall the tape works by aligning lots of microscopic magnetic particles to create magnetic fields and the read head converts some of the magnetic energy into electrical energy, but the energy has to come from somewhere (or conversely the entropy has to go somewhere) and so the magnetic field becomes a tiny bit weaker. This is why over time magnetic tape recordings lose their crispness and volume. How soon this would effect your device I don't know, and since tape is cheap it might not matter much, just replace your position tapes every 12000 fabrications as it were...
Re: linear magnetic encoder
July 06, 2008 06:59AM
Nice thinking! I wonder what would be easier, optical or this magnetic way.

Now only to find tape cassette... haven't seen those anymore in ages smiling smiley


Erik de Bruijn
[Ultimaker.com] - [blog.erikdebruijn.nl]
Re: linear magnetic encoder
July 07, 2008 01:00PM
I really like the idea!

Somewhere in the dark ages, before cellphones became a commodity, I relied on an answering machine. I remember that thing had an endless tape with a total time of a few minutes. One problem could be to know where you where, in case you should miss a period or two on the 11900 signal. You could modulate a data stream on the tape (also used in the dark ages to store data at about 1500 bps on a casette player) keeping track off the position at large.

Wear and tear would be a problem, but since the same head is used for playback and recording one could simply re-record the data stream when it becomes weak.

Main problem though being that magetic media is a definte end-of-life product.

Hmm maybe chemical film could be exposed with a laser or similar to give the tight resolution, another end-of-life product :-(
Re: linear magnetic encoder
September 14, 2008 01:39AM
What you may want to do is us a new, cheap digital solution - sensors. These things are tiny, cheap, and can detect minute variations in the structure of a surface.



With one of these along some guideline material strips on the x and y axes, they could do on a minute digital level what the human eye does with the ruler guidelines along the sides of the glass in photocopiers. And because it detects minute variations, it will help when RepRap gets into really fine, delicate things like microcircuitry, microwiring, and other detail work.

I can't help thinking of how they used to calibrate joysticks' X and Y axes when computers were just coming out. You'd be told to move all the way to the top and press the joystick button, all the way to the bottom and press the button, all the way left and press the button, and all the way to the right and press the button. The computer would then know what joystick information corresponded to what position in its range, exactly and precisely. The materials the sensors used for guidestrips could simply be a very long strip with a diagonal edge along it - so it would look like a really stretched triangle. The RepRap would calibrate itself by running along the strips and collecting data from the sensor about the width of the strips at given points along the X and Y axes. Then it would just collect information from the sensors at any point on the work surface, and be able to know exactly where it was at any time, extremely precisely.

Re: linear magnetic encoder
September 14, 2008 05:35AM

With one of these along some guideline material strips

Making a robust, precise guideline is in itself a difficult precision operation winking smiley Making something that can reliably and accurately to sub-millimetre precision with a high degree of repeatability isn't simple. The magnetic strip approach is nice, because it doesn't require fancy manufacturing capabilities or super high precision machining to work.

The usual commercial solution seems to be to use steppers or servos and a drive train that offers a high degree of repeatability. This is the sort of thing that I'd lean towards... adding additional expenses and complexities to the reprap should be avoided where possible!
Re: linear magnetic encoder
September 16, 2008 06:33PM
No chance. The magnetic tape only works while moving. Magnetism is only picked up by the coil (reader head) if the polarisation *changes*. So below a certain speed, your signal will be lost entirely in the elektronic noise.

Get an old inkjet printer instead and remove the "gray" looking optical band that is tied along the printhead together with the optical sensor. This is used to trigger the nozzles on the printhead while the head moves across and has half as many lines as your printer's horizontal resolution.
Re: linear magnetic encoder
September 16, 2008 07:51PM
I remember doing an electronics project a while ago that could work for this. It was based on resistance - using a pencil, we made a thick graphite line on a piece of paper. One probe wire was touched to one end of the line, and as the second probe was slid along it the resistance would change - and the circuit would emit a tone that would slide up or down based upon the resistance.

I have to wonder if that kind of variable resistance could be picked up along a piece of thick wire, or perhaps copper pipe, and used as the basis for a cheap, variable-resistance-based gauge for the X and Y axes. To increase the sensitivity, you'd just use a more sensitive circuit and possibly a slightly better conductor.

Maybe a good use for that magnetic tape?

You could also rig a simple circuit to measure the rotations of a disc, and have the RepRap's belts movement turn the disc. Basically, what you'd have is a very simple rheostat. A dimmer switch, with the resistance level at any given time telling you where you were along the X and Y axes. This would get around the problem of a tapehead only giving information as it was rubbed along a strip of cassette tape. Rheostats are simple and cheap, and making one spin from the movement of the belts would be easy - connect it directly to whatever turns the belts.
Re: linear magnetic encoder
September 18, 2008 07:48AM

the basis for a cheap, variable-resistance-based gauge

The difficulty is making it *precise*. As it stands, you have to make a strip, and then you need to calibrate it carefully using and external measuring system which you need to be able to do to sub-millimetre accuracy.

You then need to recalibrate at regular intervals in the future. The advantage with magnetic and optical (and possibly capacitative, though I'm not so certain about the way they work in this case) position encoders is that they do not require physical contact, and so don't get worn down.


You could also rig a simple circuit to measure the rotations of a disc, and have the RepRap's belts movement turn the disc

At which point your positioning system governs the accuracy of your encoder, rather than the other way around. This is what is done with servo-driven ballscrews, for example... it is assumed that backlash or slop in the system is negligible, and so a simple revolution count can be used to accurately calculate position.

Attaching a super-high quality servo drive and rotary encoder to a crappy positioning system just leaves you with an empty wallet and a crappy positioning system.

And eventually we come back to the servo vs stepper discussion, and counting steps that the stepper driver takes is a perfectly good guage of location in a low-force, low-speed device like the reprap.
Re: linear magnetic encoder
October 30, 2008 04:08PM
Hi there!

The magnetic tape could be great. But not with a cassette recorder head. Instead a Hall sensor (or MR, but those are used to have a bias magnet which would demagnetise the tape) should be used. They are relatively cheap, easy to get. Even from samples (Allegro).

Re: linear magnetic encoder
November 01, 2008 04:43PM
how about just attaching a high qualtity laser mouse so it reads its movement for positional correction. not realy sure right now how to implement this with just the arduino setup but just a thought.
Re: linear magnetic encoder
January 28, 2013 10:04PM
By far this encoder tapewith optical sensor found in the printers sounds to be the best idea by far, that why its already used in printers IMO.

Why not buy it from a manufacture?r, they are for sure produced at consumer scale an therefore they sh bould be cheap enough.

Precission is built on precision, so after some reprap generations it will be possible to print out our own stripe smiling smiley


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