Load cell noise from motor

[bmcbride]

I have two load cells setup to measure different voltages on my system. As far as I know, everything is grounded correctly with the myDAQ, the wires are twisted pair and shielded, which is also grounded.

There’s a variable speed motor about 20ft away that introduces a lot of noise (+/- 1mv on my 0-20mV system) into the system whenever the motor is in use. What would cause this with this specific motor? There is a massive, constant speed 15 Hp motor 2 feet away from the load cells that causes no issue at all.

[walker.p]

bmcbride,

I can't speak to what exactly is causing the noise, but here are a few NI documents that talk about different noise considerations:

1. http://www.ni.com/product-documentation/3344/en/
2. https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P9K1SAK&l=en-US
3. http://www.ni.com/newsletter/50262/en/

Also, if you've got a brushed DC motor they are "notorious for throwing out EMI", as per this link: https://electronics.stackexchange.com/questions/232567/interference-from-motors-on-pwm-signal

They suggest using a good bypass capacitor right across the power pins of the motor. May or may not be applicable, but worth including as a reference.

[bmcbride]

walker.p, 

I tried this without my success. I re-read through the different documents provided with myDAQ and found they specify signal wires should be 12" or less when connected to the 20-pin board on the myDAQ. 

The wires I have are 24 feet long, and I suspect they are picking up a lot of interference from the nearby motor. 

If there any sort of hardware filtering that can be put in place? Both load cells have a shield, which is grounded.  

[walker.p]

1 mV of noise is (generally) pretty good - obviously that changes based on your application/system setup though. What is it about your application that requires significantly less than 5% SNR? You could amplify the signal that you're outputting to reduce that ratio further, but it won't affect your absolute noise obviously. 

24 feet is a lot, especially considering the required specifications of the device. Outside of using the recommended/required 12" wires, there's not much more to be done that I can think of.