DRO Clean - 1 Tiny Chip Wrecked a Part

[MrCrankyface]

It's not often you read about problems like these. I had it happen ~1 year ago.
I'm running a CNC-controller alongside with my DRO and couldn't understand why the numbers had stopped matching.
Had me really confused troubleshooting all kinds of things for at least an hour.
The problem? Just like for you, a couple of chips on the magnetic strip..

 

[BladesIIB]

It's not often you read about problems like these. I had it happen ~1 year ago.
I'm running a CNC-controller alongside with my DRO and couldn't understand why the numbers had stopped matching.
Had me really confused troubleshooting all kinds of things for at least an hour.
The problem? Just like for you, a couple of chips on the magnetic strip..

Thanks for sharing. Took me a while to trouble shoot as well, since it seems it never happens. Also so easy to solve, hopefully this will help someone else trouble shoot faster.

 

[ycroosh]

I'm a bit late to this party, but hopefully this will help y'all.
The thing about magnetic scales being low maintenance is a 100% not true. The have many benefits, compared to glass scales. Namely, they are less sensitive to shock and vibration (as long as the distance and angle between the reading head and the strip stays the same), they are easy to cut to size, and they are almost immune to coolants, lubricants and general dirt. Low maintenance is not one of them.
Magnetic scales are VERY sensitive to ferrous chips anywhere between the sensor and the magnetic strip. These scales read analog magnetic field with 90 degree offset (basically sine/cosine signal) and then use sophisticated interpolation to convert sine waves with period of 5mm to 5um quadrature reading. Any deviation in the magnetic field will throw them off. High-end stuff (and I mean "really-really expensive") uses very sophisticated circuits and algorithms with a dedicated error correction track that can work around some minor glitches. The biggest problem is that when chips get under the sensor array (which is very small, compared to the reading head), they introduce local error that is hard to notice (it's not an abrupt position jump or skip usually).

Magnescale (the people who invented these scales) describe them as: "A position detection system that is resistant to adverse environments due to its magnetic detection method. They are not affected by condensation or oil, which are common to machine tools, and they continue to have high vibration resistance and shock resistance characteristics."

One more (unrelated) thing to keep in mind is that for comparable quality and accuracy, magnetic scales are (and should be) 3-5 times more expensive. The low end stuff is so cheap because manufacturers cut corners, particularly in the electronics. "Affordable" interpolation modules cost $35-$50 just for the sensor chip (without any support electronics), compared to about $10 for a full "carriage" assembly for glass scales.

I'm not saying that magnetic scales are bad, but they are not magical. They have their place, and in some applications they are the only option, but there is also a lot of marketing BS around them.

Regards
Yuriy

P.S. I'm the schmuck who developed TouchDRO, and I've heard a lot of stories along the lines of "some cast iron dust got in and my scale was reading all over the place".

 

[BladesIIB]

I'm a bit late to this party, but hopefully this will help y'all.
The thing about magnetic scales being low maintenance is a 100% not true. The have many benefits, compared to glass scales. Namely, they are less sensitive to shock and vibration (as long as the distance and angle between the reading head and the strip stays the same), they are easy to cut to size, and they are almost immune to coolants, lubricants and general dirt. Low maintenance is not one of them.
Magnetic scales are VERY sensitive to ferrous chips anywhere between the sensor and the magnetic strip. These scales read analog magnetic field with 90 degree offset (basically sine/cosine signal) and then use sophisticated interpolation to convert sine waves with period of 5mm to 5um quadrature reading. Any deviation in the magnetic field will throw them off. High-end stuff (and I mean "really-really expensive") uses very sophisticated circuits and algorithms with a dedicated error correction track that can work around some minor glitches. The biggest problem is that when chips get under the sensor array (which is very small, compared to the reading head), they introduce local error that is hard to notice (it's not an abrupt position jump or skip usually).

Magnescale (the people who invented these scales) describe them as: "A position detection system that is resistant to adverse environments due to its magnetic detection method. They are not affected by condensation or oil, which are common to machine tools, and they continue to have high vibration resistance and shock resistance characteristics."

One more (unrelated) thing to keep in mind is that for comparable quality and accuracy, magnetic scales are (and should be) 3-5 times more expensive. The low end stuff is so cheap because manufacturers cut corners, particularly in the electronics. "Affordable" interpolation modules cost $35-$50 just for the sensor chip (without any support electronics), compared to about $10 for a full "carriage" assembly for glass scales.

I'm not saying that magnetic scales are bad, but they are not magical. They have their place, and in some applications they are the only option, but there is also a lot of marketing BS around them.

Regards
Yuriy

P.S. I'm the schmuck who developed TouchDRO, and I've heard a lot of stories along the lines of "some cast iron dust got in and my scale was reading all over the place".

Great insight, thanks for sharing. I will be a little quicker to sort it out next time for sure. I agree to many benefits over glass but nothing is perfect I guess.

 

[JimDawson]

I have found mag scales to be pretty bullet proof. I don't cover mine, I just leave them exposed to everything. Maintenance is a quick wipe with a finger.

 

[BladesIIB]

I have found mag scales to be pretty bullet proof. I don't cover mine, I just leave them exposed to everything. Maintenance is a quick wipe with a finger.

View attachment 425467

That is a great pic of what they can handle right there Jim! Thanks.

 

[ycroosh]

I have found mag scales to be pretty bullet proof. I don't cover mine, I just leave them exposed to everything. Maintenance is a quick wipe with a finger.

View attachment 425467

Well, my uncle believes that doing oil changes in his truck is a marketing scam, so he just tops it off as needed. Never had a problem, apparently

Now, as far as your scale is concerned, you can cover it with iron dust and it will still read something. That "something" might even be close to the actual measurement. This particular scale (and many others in this form factor) use interpolation method called "Vector Tracking Conversion". It can be made fairly cheaply and precisely. The main drawback of Vector Tracking Conversion is lack of signal filtering, so any deviation from pure sine/cosine input will result in conversion errors. In mots cases you won't notice (or care) this error, since it will be very localized, unless the chip gets stuck to the head and moves with it.
The alternative for linear scales would be Continuous Sampling A/D conversion, which has much better signal filtering and can to a degree compensate for small local glitches. It is much more expensive, especially if the output has to be close to real-time, so you usually find it in $$$$ scales.

If you read the docs for a decent magnetic scale (for example Electronica/EMS) link, you will find things like:

"Avoid using the magnetic stand or any permanent magnet close to the magnetic scale any time during its use or during maintenance. This may damage the magnetic scale permanently."
and
"Ensure the supplied cover is properly mounted to protect the scale and reader head for the entire length to ensure the reliability and performance of the system."

I tend to think that a DRO is a precision measurement instrument, and some basic care and maintenance won't hurt anything. I have magnetic scales on two lathes (on the cross slide), and on the milling machine on the quill. If kept clean, they work very well. Keeping them clean is not that much work...

Regards
Yuriy