Ground loop + BNC 2120 behaving like antenna(pr​oblem)

Question

Hi,
I am using DAQ 6024E, and using two sensors and one error message "data is overwritten before it could be read
by the system try divide the signal before measurement" --it took me whole day as to what data is going inside that my computer with 900Mhz processor is unable to handle. I started tearing my hardware step by step. I looked at my range sensor which has one LED which blinks as it throws waves out - but the moment my motors gains speed all of a sudden my labview generates error that "data overwritten before it could be read by the system", now at the same time I looked at my range sensor and instead of blinking slowly like persec it was blinking continuously without break so I took the wire out which was going into the BNC 2120 and the sensor becomes stable again.
I thought that may be the wire is picking up RF signal from motors so I changed all the wires with shielded insulated wire but again as I switched my motors on labview generates the same error. If motors stays off there is no error. I felt my BNC is acting like antenna and is catching motor RF signals. I took motor out of the circuit and supplied separate voltage from another power supplier and it confirmed me that moment my motor speeds up the range sensor speed of blinking increases but If I take out the signal wire going into BNC 2120 then there is no effect on range sensor and it works fine steady. moment I connect BNC with sensor and motors ON then sensor behave erratically with led blinking extremely rapidly.
So it is confirmed that nothing is wrong with hard ware of computer but some strange phenomenon.
Now next I powered my controller with a battery and motors with another power supply to break the loop but problem not solved, I think there is some ground loop thing going on and that is making BNC 2120 as antenna for RF signal created by motor.
That's why initially at less voltage/speed VI works fine but as motors speed up VI stops working and generating the error " data is overwritten before it could be read
by the system try divide the signal before measurement"
I hope now you are in better situation to understand what is happening and how can I stop BNC wire to act like this, remember I am using good insulated shielded wire.
Thanks

Answer 1

I've got fast fingers, I play lead guitar in a rock n roll band (nah, just wish I did, but I do play lead guitar - just a bit rusty at it right now). I've spent the last 20+ years being a test engineer at various places. I learned from some of the best at AT&T and Bell Labs (when they were a big dog in the 80's). Before that I was a mobile radio repair technician for 8 years. Learned RF and electronic repair on the job. Troubleshooting background has helped me tremendously in the test engineering world. Also spent three years doing SCADA engineering so I am familiar with PLCs (Motorola, Allen Bradley, and GE Fanuc) and HMI software (Wonderware and GE Cimplicity). SCADA was fun, but I prefer the RF world.
Getting back to the problem, you give good advice about grounding only one end of the shielded cables. Grounding both ends does cause problems with non RF signals. A scope will go a long way to determine where the problem is coming from. But since he mentioned ultra sonic sensors, I am wondering if the problem is actual sonic noise from the motor. I would not be able to hear it because I lost all hearing above 12KHz from all that loud guitar playing when I WAS in a rock n roll band.
BTW, your name reminds me of the Rush song, Analog Kid, and also of the progression of analog synthesizers to digital ones, and now there is a movement back to the analog dinosaurs again.
- tbob
Inventor of the WORM Global