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Diference between DIY MSP430 LaunchPad and TouchDRO Quadrature board?
#13
(09-21-2016, 08:18 AM)Retromachinist Wrote: Hi Yuriy,


Quote:I personally would go for a transformer power supply. 90%+ of all problems I've seen with noise come from ground loops and a transformer would isolate if from the mains ground (eliminating the biggest round loop).

Ok, this is valuable Info. I was planning on using a transformer type Power Supply,
but this really makes it a one way decision.




Quote:If you go for a switching power supply and put it in the case, my biggest worry would be the wires that go to the D-Sub connectors. At the very minimum twist A/A',B/B', and Vcc/Ground wires together so if they pick up any noise it will cancel itself out.

Another very valuable piece of Information!!! As you may already have understood, i'm not the electronics
guy, so i did not think about twisting the pairs inside the Enclosure. But now i will definitely do this!

Quote: The board itself should be fine. I tested it with HAM radio transmitter antenna wrapped around it and the board didn't care much.

This sounds pretty bullet-proof! Very reassuring! Thanks!

I made some assumptions:
Would it be preferable that nothing connected to the board - ground, touches any grounding of the
mill? As to prevent any noise from the mill/AC-Network entering the ground of the Board?
I hope the scales have no physical connection between the the Enclosure and the ground
lead that goes to the Sub-D connector. I'll check this.


And hey, thanks a lot for the dimensions!!! I'll feed them directly in to the CAD! Big Grin

Cheers, Arne.

Arne,
The board needs to be grounded, ideally to the same ground the machine is at. This way there will not be any voltage differences between the two grounds. Basically you want to avoid the big ground loop through the mains neutral line. Machine motor can draw enough current to create voltage difference between the two legs of the ground that is big enough to reverse scale polarity momentarily. With iGaging scales that is very visible as a position jump. With glass scales it will cause skipped pulses and errors of up to 4x the resolution (20 microns for 5 micron scales), etc.

If you got "Chinese" glass scales (Ditron, Sino or alike), metal cable shrowd should be connected to the ground only on one end, either to the machine or to the board's ground so it provides EMF sheilding without a ground loop. The reader head's 0V is by default isolated from the scales ground, so you should be fine there. If that is the case you will be fine.
On mine I have everything mounted and grounded to the mills column (I have square column Sieg SX4) and isolated from the mains.

I don't mean to scale you. In practice you might never see those problems even with ground loops and unshielded cables, but it's s bummer when you're working on a critical dimension and loose 0.02mm without knowing it over and over again. For this reason I err on the side of caution and try to eliminate possible failure points.
I don't have enough data to tell you how likely you are to have problems. Anecdotally, i would say roughly 10% of the emails I get related to the DIY builds are about noise issues, and majority of them are using calipers or other capacitive scales. I know of three cases when a person was loosing counts on a 1 micron scale due to excessive noise in the line. All three of them were using resistor dividers for level shifting and had bad grounding schemes. I recon there are at least a few thousand people using quadrature scales with TouchDRO, so it's not a bad ratio. I haven't heard of any issues with the pre-built boards, so it could be a moot point, but I've sold relatively few quadrature boards so far, so I don't have a good data set yet.

Regards
Yuriy
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RE: Diference between DIY MSP430 LaunchPad and TouchDRO Quadrature board? - by Yuriy - 09-21-2016, 07:32 PM

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