Sensing Corona discharge.

BigAl

Lifetime Supporting Member
Join Date
Nov 2002
Location
Syracuse
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I am hopeful that some one may be able to help with this application. I have an automated dial table which uses 2 Corona generators to treat injection molded parts before they are printed. The corona is created in by shorting a high voltage electrical charge about 2 inches away from the face of the part. Compressed air is used to dissapate the corona onto the part. The parts are then printed, and the corona allows the ink to ahere to the part.
Last week I was notified that some of the parts were not being treated with the corona. Some parts were randomly tested and it was found that the ink would rub off. I noticed that there were no alarm outputs or other I/O on the generator that could be interfaced with the PLC. I contacted the manufacturer of the units, and he suggested installing a current sensing relay on the 120VAC feed to each of these units, and monitoring this with the PLC. This seemed like a good solution, so I went ahead and wired in two current sensing relays, and interfaced them with the PLC. In the event the Corona generator was triggered, but I did not see the output from the current sensing relay, I take the machine out of Auto, and sound an alarm. The operators have been instructed to remove all product from the table, and reject it before proceeding.
I tested this logic several times, and everything worked great. On our next trial we experienced one alarm, and the operators followed the procedure to reject the parts. However we still had parts that did not get the Corona treatment. Again I contacted the manufacturer, and he explained that it could be possible, that even though the current relay was activated, corona was not generated at the tip of the machine. Keep in mind that the Corona generators mounted on this are very old.
We are now trying to find some method to detect Corona directly at the tip. I have searched the web for some kind of sensor, but have not been able to find something.

Any ideas would be greatly appreciated.

BigAl
 
Do you have an idea of what the wavelength of light is from the corona?

Select a sensor for that wavelength from International Light. Use output of instrument as a "sensor" to determine
1. Did corona generator work?
2. How much "corona"
3. Is it enough to do the job?

Wont be cheap about 5K or so, but that is the cost of doing business.
Dan Bentler
 
BigAl,

Another thought:

Can the corona light up a flouresent light without disturbing the process? If it can perhaps an a_m_b_i_e_n_t light detector that is sheilded from all other light. A Banner SBAR1 or SBAR1GH would be one type of reciever that might work on the glowing floresent tube.
 
Thomas.


That is a good question. To be honest I do not have an extensive background in the printing end of things.The person who originally took on this project has retired. This is the only machine in our facility which has any type of Corona generator mounted on it. I was involved with mounting the power relays and integrating them into the logic. When this did not fix the problem, it was assumed that there was some type of malfunction with the corona generators themselves.

Would I be correct in the assumption that the air pressure is pretty critical?
 
If I had the fredom to design, I would install a circuit that detects voltage drop. The high voltage will go high when powered, then drop during the discharge. This should last a few milliseconds or so.
Current also goes up during discharge. The sensor you have now should have worked reliably if sensitivity is set corretly.
Try some experiments with a 30MM capacitive proximity sensor. The spark should cause quite an upset in the capacitive field.
 

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