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Corona Monitor

Wily was just about to have some lunch when his phone rang.  It was from a local medical equipment company.   They made kidney dialysis machines.  They also made lots of the non-reusable blood filters, which were used on their machines.  The filters were rather complex canisters of plastic sheets, all glued together.  To enhance the glue adhesion, the company ran the plastic sheets through a special machine, which bombarded the material with a high voltage 40KV 20KHz discharge.  This “corona discharge” broke up the plastic bonds on the surface, allowing the glue to stick.
The company had been using their corona machine for several years without too many problems.  However, on occasion, the machine might hiccup for a few seconds.  The glitch lasted long enough to allow sections of the filter plastic sheets to pass through the system without being properly treated.  One or more blood filter assemblies would then be completed and only later rejected due to leaks in the filter, caused by the glitch. The company wanted a more robust system, which would monitor the corona and stop the take-up rollers for the plastic sheets, whenever a glitch occurred.  The operator could then correct the problem, pull the plastic sheets back a few feet, and start up again. Could Wily come up with something?
  Dialysis Machine   Dialysis Filter
   
Corona Treatment System Corona Light
   
The company had been using their corona machine for several years without too many problems.  However, on occasion, the machine might hiccup for a few seconds.  The glitch lasted long enough to allow sections of the filter plastic sheets to pass through the system without being properly treated.  One or more blood filter assemblies would then be completed and only later rejected due to leaks in the filter, caused by the glitch. The company wanted a more robust system, which would monitor the corona and stop the take-up rollers for the plastic sheets, whenever a glitch occurred.  The operator could then correct the problem, pull the plastic sheets back a few feet, and start up again. Could Wily come up with something? 

The medical company explained that they wanted Wily to design for them a corona detection system, which would tie into their computer. Their computer controlled the whole corona treatment process.  They already had an audible alarm generator and a big emergency stop button.  That button also acted as a rest button, when pulled.  All they needed from Wily was a logic signal that would swing low when no corona was detected.

Wily scheduled a visit to the company’s production line the next day.  He took with him some light measurement equipment and a camera, to capture some images of the corona machine in operation.  When Wily was shown the system in operation he immediately noticed that the corona discharge produced a very bright violet light. The light was a mixture of visible blue and invisible ultraviolet light.  He took some quick light output measurements with the corona on and off, using a bare silicon photodiode, connected to a multimeter.  Based on those quick tests, Wily decided that detecting the visible and the invisible ultraviolet light produced by the corona would be the way to go. Wily collected some physical data from the corona treatment machine and then went back to his lab and started the designing process.
In addition to light, Wily was told that the corona discharge produced a large amount of broadband radio frequency noise. This electrical noise could interfere with any sensitive electronic circuit placed nearby.  In fact, to prevent RF interference problems, the medical company had placed all the control corona system electronics inside large vertical metal racks, which were positioned on either side of the corona treatment station. The steel racks shielded the electronics inside from the electrical noise. Wily decided that the light sensor and associated electronics should also be housed inside one of those equipment racks, which was about 6 feet away from the corona.  He saw a good spot for his corona monitoring box, just above one of the system’s DC power supplies, on an empty rack shelf. 
19 Inch Equipment Racks

To collect the light from the corona, Wily decided to use an optical fiber bundle about 10 feet long and about 0.40 inches in diameter. The 0.2 inch bundle inside was made of many tiny strands of optical fibers. The bundle assembly was held inside a plastic jacket.  Wily carefully removed the jacket from a section of the bundle.  He then slid it into the center barrel of a coax type connector and glued the bundle in place with an optical grade epoxy.  That connector would later be used to shine the collected light onto a photodiode, to measure the light intensity.  The connector would make it easier to service the system, if needed.  On the light collection end, he removed a small section of the plastic jacket and covered the bundle with more optical grade epoxy.  Once the epoxy cured, using various sandpaper grits, he then polished both ends of the bundle, to a smooth finish.  This treatment improved the light collection and coupling. 

The medical company fabricated a custom plastic holder for the fiber bundle and installed it onto part of the corona bar.  The bundle was positioned so it pointed down toward the corona to within a half inch of the discharge. The fiber bundle would then collect the light emitted by the discharge and send it to the other end of the bundle. 

Wily carefully positioned the photodiode inside the corona monitor’s metal box so when the fiber bundle connector was installed onto the box’s matching connector receptacle the fiber bundle end lined up with the center of a bare silicon PIN photodiode.  Wily chose a photodiode about 0.5cm x 0.5cm in size.  As shown below, the photodiode had a response curve, which included ultraviolet light.

Corona Light  Optical Fiber Bundle
Silicon PIN Photodiode 
 
Photodiode Response Curve
 

Wily’s light corona monitoring and alarm circuit was pretty simple.  The circuit was powered by +5v and had an on-board voltage regulator as a reference voltage.  The current from a reversed biased photodiode was converted into a voltage using an op Amp circuit.  That output voltage was fed to a voltage comparator circuit.  Wily included an adjustment in the current to voltage converter, so he could calibrate the circuit, once it was installed on the corona treatment machine.  He would set the gain of the circuit so it would produce a one volt output signal for a typical corona discharge.  The comparator’s threshold voltage could also be adjusted, so the device would change state when the corona intensity dropped by about 20%.  The alarm output of the monitoring circuit was connected to the company’s computer through a three pin connector.  That alarm signal controlled the treatment process.  When the computer sensed the corona alarm signal, it would shut down the system.  A human operator would then back up the plastic film a few feet and rethread it into the take-up rollers.  Finally, he would press the reset button, which told the computer to start up the film treating process again. 

 

April 2010     Issue 8

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Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products Rants &
Raves
What the World
needs Now
Wily Widget


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