The Adventure of the Arcless Painting

While investigating how to avoid blowing up paint rooms, an engineer finds an easy solution by studying a plot of results taped to his wall

By Radcliffe Cutshaw, Contributing Writer
In the early eighties, I was working for a consulting firm doing RF design when I got an assignment to see how a non-arcing electrostatic spray painting system could be built. The contract was with a company that manufactured electrostatic spray painting systems. They had tried and failed to come up with a system. But they had been offered a company-wide sale to a Japanese car manufacturer if they could guarantee that no arcs would occur if a work piece came near a spray head.

I was told that the car company had blown up a “oh, just a few” painting rooms in the past.

An electrostatic spray painting system has a spray head that puts an electric charge on the paint, which is attracted to the work piece, even on the side away from the paint head. If the work piece comes too near the spray head, an arc can result, causing a fire or even an explosion due to the paint fumes present in the reoom.

When the power supply arrived, it was about the size of a refrigerator only much heavier. It could produce over 300 kV at 100 ma. After several impressive arcs and my noting that the current rose as the work piece approached the paint head, I tried various ways of measuring the electric fields around the paint head, which produced more arcs and blew electronics. After learning how to protect the electronics from the arcs, I still could not measure the changes in the paint head’s electric field.

I paid a visit to the customer’s headquarters, met the chief engineer, and found that I had duplicated the work that they had done and reached the same result–nothing. Measuring the electric fields around the spray head only distorted the paint spray patterns and provided no useful data. Other than learning how to insure the survival of sensing electronics under extreme conditions, which was not the objective of the project, no progress had been made.

Also, I found that the power supply that I had been equipped with was very small compared to other products that the company made. I left the headquarters with a file of their work. Over the next week, I went over the company’s material and found the same graphs of current vs. distance that I had generated.

But something continued to nag at me in the back of my mind.

For about two weeks, I worked on other projects with the current vs. distance graphs taped in front of me on the cubical walls. Then I realized what was nagging at me: All you had to do was sense the current already built into the power supplies and shut down the power supply at a given current level. The current would rise when spraying, but normal levels of current could be calibrated to the spray level.

After calling the customer’s chief engineer, I explained my idea and he agreed to try it out in the lab where they tested such things. It worked. Other models of the painting power supplies, including the ones that the company wanted to sell to the Japanese car company, had a microprocessor control built into it that sensed, among other things, the current and could shut down the power supply.

From this, I learned a few lessons: Never overlook the obvious; consider all alternatives, even if they seem stupid. Try the simple things first. Learn as much as you can about the problem. Don’t jump to conclusions. And distract yourself by working on something unrelated so your thinking does not get in the way of finding the solution.

Mr. Cutshaw, a serial entrepreneur, is currently a private consultant specializing in RF and analog design and development. He has been involved in many areas of engineering throughout his career.