Idea - DIY Linear Magnetic Encoder

[axis_control]

Hi,

Can the old-timer type magnetic cassette and head be used to make a DIY linear encoder?
Since commercial linear encoders work on the principle of alternating north and south poles on the strip and a head that reads the pitch between two magnetic poles on the strip. This pitch represents a certain linear distance and is calibrated into the head electronics to give an output signal equal to the linear distance (mostly digital signal)

Look at the magnetic cassette, head and the commercial Magnetic strip type linear scale. Don`t you see a possibility ?

[axis_control]

helpful link to understand the linear encoder principle of working.


Electromagnetic Linear Encoders to the Rescue | Control Engineering

[axis_control]

This is an Hall effect sensor manufactured by Honneywell

[eSilviu]

Originally Posted by axis_control Hi, Can the old-timer type magnetic cassette and head be used to make a DIY linear encoder? Since commercial linear encoders work on the principle of alternating north and south poles on the strip and a head that reads the pitch between two magnetic poles on the strip. This pitch represents a certain linear distance and is calibrated into the head electronics to give an output signal equal to the linear distance (mostly digital signal) Look at the magnetic cassette, head and the commercial Magnetic strip type linear scale. Don`t you see a possibility ?

I like your idea, It should work if the magnetic head/sensor can work at very low speed.

[RomanLini]

I think you will have 3 problems;

1. resolution. The tape head is designed to have metal poles so it can pick up smaller sized magnetic details (ie high audio frequencies). But the Hall sensor does not have that ability and it's silicon sensor is a couple mm deep inside the body so the smallest magnetic detail on the tape it can pickup is going tobe large (maybe 2mm?).

2. Quadrature. To work properly as an encoder you need quadrature output which means 2 sensors mounted 90' out of phase in comparison to the magnetic detail.

3. Signal strength. The magnetic field on the tape is VERY small! Tape heads use very sensitive pickup coils and massive amounts of audio AC gain. The hall sensors are meant to pickup a much larger but constant (DC) magnetic fields. It's going to be difficult to get it reliable.

There are also some issues of the tape being very delicate and the tape field degrading over time.

A better option might be to use a fine metal "rack" with teeth and 2 hall proximity sensors that can detect the rack teeth. Like the tape you may still be limited to about 1mm or 2mm resolution but it would be a much better system.

Considering you can buy cheap DROs for $30 or so and just hack the signal from one that is still the best option as you can get high resolution.

[axis_control]

Originally Posted by RomanLini I think you will have 3 problems; 1. resolution. The tape head is designed to have metal poles so it can pick up smaller sized magnetic details (ie high audio frequencies). But the Hall sensor does not have that ability and it's silicon sensor is a couple mm deep inside the body so the smallest magnetic detail on the tape it can pickup is going tobe large (maybe 2mm?). 2. Quadrature. To work properly as an encoder you need quadrature output which means 2 sensors mounted 90' out of phase in comparison to the magnetic detail. 3. Signal strength. The magnetic field on the tape is VERY small! Tape heads use very sensitive pickup coils and massive amounts of audio AC gain. The hall sensors are meant to pickup a much larger but constant (DC) magnetic fields. It's going to be difficult to get it reliable. There are also some issues of the tape being very delicate and the tape field degrading over time. A better option might be to use a fine metal "rack" with teeth and 2 hall proximity sensors that can detect the rack teeth. Like the tape you may still be limited to about 1mm or 2mm resolution but it would be a much better system. Considering you can buy cheap DROs for $30 or so and just hack the signal from one that is still the best option as you can get high resolution.

First of all, I am Thankful to you for pointing me to the right direction.
Reading your reply, I have concluded that the idea of using the cassette tape and head is useless and there is nothing to do with it .

If we are looking for 1 micron precession, then I guess a mechanical rack teeth design won`t be able to give that unless we had nano tech to fabricate the profile.

I checked on ebay for DRO and mostly the cheap ones are second hand. If I can get a new one for $30 then I would go banana`s. I am sure you are referring to a second hand one. I got a quote from a local manufacturer (he says so) for a 300 mm travel Glass scale linear encoder for $ 305 (USD). The accuracy specified was 5 microns.

Apart from the above I checked out some Chinese products on alibaba and some other business to business websites. The thing that scares me most about buying these products is the reliability and the other is getting a different product when you have ordered one. I have seen some people being cheated for good.
heres one I was looking at
Buy Linear scale, linear glass scale, linear encoder scale, FREE SHIPPING Linear scale DC10 301-500mm at Aliexpress.com

Most companies buy components from large manufacturers with resources and assemble it at their site and put a brand name of their own and sell it as their product. Using the same concept I think it is possible to assemble a magnetic or glass linear scale system.

[axis_control]

I am searching on the internet for suppliers of
1] Encoder strip.
2] Encoder strip sensor.

I am getting good results (at least for now and the accuracy is unknown)

[axis_control]

Do printers have magnetic linear scale inside them?
I have been getting quite a lot of sites pointing at printer components when I am looking for linear magnetic scales.

[Al_The_Man]

If Mitotoyo is available in your part of the world, then check on the measuring scales linear tape with quadrature output.
Particularly the ST24/44 and ST36.
http://www.mitutoyo.ca/files/2009%20...0section_0.pdf
Al.

[axis_control]

Originally Posted by Al_The_Man If Mitotoyo is available in your part of the world, then check on the measuring scales linear tape with quadrature output. Particularly the ST24/44 and ST36. http://www.mitutoyo.ca/files/2009%20...0section_0.pdf Al.

Al Thanks for the info.
By the way I loved your Avatar before this one, the baby with a cigarette
It was outstanding and one could remember you with that Avatar.
The new avatar is good too and projects one of your work side that is watching out. I know you do much more than watching out and that is helping so many like me on this forum.

[RomanLini]

Originally Posted by axis_control ... I checked on ebay for DRO and mostly the cheap ones are second hand. If I can get a new one for $30 then I would go banana`s. ...

No, that's new. I received a junk mail a while back that had Chinese 500mm DRO for about $30 (if I remember right).

I expect it would be as accurate as any of those Chinese verniers with digital read outs, the electronics bit looked like the same type of thing but with a longer "stick".

[TarHeelTom]

Originally Posted by axis_control Do printers have magnetic linear scale inside them? I have been getting quite a lot of sites pointing at printer components when I am looking for linear magnetic scales.

Not an expert, but many printers seem to have an optical linear scale inside.

Tom