Emr/emi
kelvin james
Posts: 531
Anyone had any concerns with this when designing a product? I am concerned with the amount of EMR/EMI that a motor (stepper or otherwise) gives off, when in close proximity to another sensitive electronic device. I was wondering if this is something i should be testing, or maybe i'm just paranoid from reading too much about potential problems. I found this meter at jameco, a lot cheaper than a fluke unit, and thought it might be better to put out some coin for this to be on the safe side. Any knowledge/opinion would be appreciated.
www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=127407
Post Edited (kelvin james) : 2/11/2005 7:04:52 AM GMT
www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=127407
Post Edited (kelvin james) : 2/11/2005 7:04:52 AM GMT
Comments
Based on what you've told us, it is hard to help you make a decision about your need for this meter.
I guess the first question I would ask is, when you know how much EMI the motor is putting out by using this expensive meter, what will you do with the information? Do you have a standard by which to judge if it is too much for your application? Maybe you should let the results of your project first dictate whether you have a problem.· -Just a thought.
now I'll go back in my box...[noparse]:)[/noparse]
Bruce
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Name: Bruce Clemens
Work:· Clemensb@otc.edu
Good Stuff on my Bolg: http://theDeadBug.journalspace.com
bugg
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Really weird kid
We once had a wind system that had some hydro lines nearby.· We were picking up 60Hz hum which was being sampled at a rate that we'd suddenly have a 40knot·gust (guessing at the number) for one sample period and then was down to 1 or 2 knots (normal days wind).· We'd sometimes encounter a steady 12 knots of speed but our direction was moving all around (which is an indicator that there really isn't enough wind to keep the direction stable).
Anyways, we were running underground/water cables so couldn't replace them with newer shielded cables but the easy fix was to use a differential measurement on both wires rather than a single wire and ground.· The noise, in differential mode, would get measured/canceled and you were left with the regular sensor voltage.· luckily our logger had this capability or more work would have to go in to it.
We ran in to trouble at another site with connecting the shield on a long run cable.
If you connect one end, the shield is basically connected like an antenna....so would bring noise in to our systems ground.
If we connected both ends, we ended up with a ground loop that did no better.
I can't quite recall what our exact solution was....it was either an R/C filter on our data lines with the shield floating or we used an isolation transformer somewhere....wish I remembered!
As frustrating as noise issues are....it's a great feeling when you solve them.
Certainly when designing anything with oscilators/crystals you want to build some isolation in to them.· Whether these are coupling capactiors or faraday cages (look in an old cableTV converter) it's always about experimenting for the fix from the hobby end.· Unless you've been doing it for years.
A nice thing to get in to is HAM stuff.· There are some really nice books published by some Amatuer radio guys.· ARRL Antenna book is a big one and you can find old ones with good information at HAmfests for cheap!· Hints & Kinks is another one too.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·
Steve
http://members.rogers.com/steve.brady
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
Quick definition of terms:
EMF: Electromotive Force
EMR: Electromagnetic Radiation
EMI: Electromagnetic Interference
Distance from motor to sensor and its signal lines·is always the best option. Like Steve said, using differential lines will help.
The theory of differential lines is if you have two lines that travel the same path, any interference picked up by one will be picked up by the other, assuming the distance to the radiator is·much larger·than·the distance·between the lines, the EMI picked up by both lines will be equal (this is just·an RF·radiation source assumption, the energy of RF decreases at a rate proportional to·1/R^2. If d is the distance between the two wires and if R >> d·(much greater than), the energy on the two lines is equal). If you then subtract one signal line from the other, the EMI which appears as noise in the signal·goes away. Usually differential lines will operate at +V,-V·with the two voltages changing polarities to indicate the two logic states for digital communication (this is what ECL logic does and can operate in the GHz) or V2 = -1*(V1) in analog systems. You could just leave the second line undriven and would act as an antenna trying to pick up the same energy as the driven line, though it will also pickup any quick signal transitions of your signal line as well, which may produce undesired effects (it think it will act as a low pass filter reducing your effective switching rate on your signal line, but I'm not certain on this point).
Now none of this may be nessessary, if your sensitive sensor's output changes slowly over time wrt the frequency of the EMI you can simply put a low pass filter on the end of the line before measuring the voltage.
Paul
Thanks for the info guys and good to see you can figure out what i was getting at. Sometimes i have trouble putting thoughts into words and will have to work on that. I think i understand the concept of the wiring, will have to let it sink in a bit more and the box/ shielding was something that i had considered, but did not know if it was necessary. Probably is best to do it anyway to be on the safe side. Wouldn't the use of a meter like this be useful to detect possible excessive noise before it turns out to be a issue after the fact? I am kind of surprised no one uses something like this when doing a prototype on a circuit/ device. Like how else would i know the noise was there, unless i had a means to measure it. Maybe i'm worrying over nothing, but i think i will purchase one anyway, just to satisfy my own curiousity. I will even be able to see how radiation i am absorbing sitting in front of this pc monitor, too much of the time. The trick will be is how to apply that info to the real world, maybe just taking random samples of various electronic appliances, to see what an acceptable amount is? Hopefully it comes with some good documentation. Anyway, i will report back on it's usefulness in the near future. Thanks again.
kelvin
I know I've seen an "RF sniffer" somewhere.· It's not a sold product, it's a DIY thing.
Something along the lines of a handmade coil of wire and a coat hanger with an old analog voltmeter to catch the peaks (Digital meters average out the pulses).
I've got a depot of old circuits on my hard drive and I've attached something that MAY work.
Actually, an AM radio would be pretty good at picking up noise (in a limited band...but noise nonetheless).
I'd certainly experiment with lesser cost items rather than drop ~$200.· Because $200US is like $500Cdn!!· Our sled-dogs would go hungry if we spent that!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·
Steve
http://members.rogers.com/steve.brady
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."