motor acting like an antenna

[polak1]

Hello,

Here is the set-up I use: I have a X-ray lead cabinet (to protect the outside from the X-ray beam) where sits a X-ray source (225 kV). A mechanical shutter is used to cut the X-rays and so the motor associated to this shutter (AC servo motor MAC800)is placed near the X-ray source, as well as a cooling radiator, at the top of the cabinet. The detector I use is a photodiode. (normally used to measure the light coming from a scintillator crystal)
Here is the problem: if I measure the voltage coming from the photodiode with a scope and switch on the shutter motor (X-ray are off and no crystals sits on the photodiode), I get spikes at a frequency~ 0.7 Hz. If I look carefully at these spikes they are made of ~40 kHz ripples. This arises each time I switch on the motor. These waves radiate all over the room like an antenna.
I tried to make a Faraday cage with mesh steel around the motor, that I grounded, but it didn't work.
If someone has a clue what is happening there, please let me know.

Thanks,

 

[dbaird]

Your motor manufacturer advertises "EMC safe. Switching noise remains within motor." I would contact them with your problem to get their take/fix.

http://www.jvl.dk/default.asp?Action=Details&Item=703#MAC400

David Baird
mrbaird@hotmail.com
Sr Controls Engineer
EET degree.
Journeyman Electrician.

 

[Higgler]

Switching power supplies often use 10 khz to 200 khz for regulating their power supply voltages (maybe the 225kv source power), hence my first thought is that you are using a power supplies in your setup?
If you locate your source of the 40 khz, that'll help alot.
In many low frequency rf problems due to ground cable radiations. you need battery sources to eliminate the problem.
There's an outside chance that you will need to change your motor from AC plugged into the wall to (battery plus AC inverter).
Just alot of guess work.
Let me know if you measure the 40 khz on some power supply.
kchiggins

 

[analogkid2digitalman]

I have had some experiences with photodiode and noise pickup.

I was using a Si photodiode and a transimpedance amplifier.
Noise was abundant on my output signal. I ended up building an enclosure around the photodiode use copper clad PCB material and soldering it all together to make a box. Made sure photodiode case was connected to shield and grounded too. This greatly decreased the amount of noise.

So, you may want to try shielding the photodiode instead of the motor, just a suggestion.

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