The Case of the Jumping Gyro

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July 14, 2011

In 1979, I was designing lighting control and navigation systems for remote-controlled underwater vehicles (RCVs). I watched with great interest the development of other parts of the system. The RCV had a heading servo referenced to a gas-rate gyro, which had a trimmable dc offset. Every once in a while, the heading would take a jump of 10 to 15 degrees. The crusty old engineer in the lab was assigned to track it down.

He connected the gyro output to one channel of a multi-channel analog strip chart recorder, a second channel on the reference trimmer output, and the third channel to the local power supply. He put the gyro in a plastic bag in a bath of ice water, which warmed up very slowly. He sat there with his stack of magazines, a thermos of coffee, and pack of cigarettes, while he watched the strip chart recorder all morning. Sure enough, every once in a while, the output jumped. When the trimmer output jumped, the gyro output jumped with it. The power supply was stable.

The fault was a wire-wound potentiometer. As temperature changed, the pot mechanically skipped from one wire to another, causing an offset. He replaced the pot with a resistive divider. The gyro stabilized, and the problem was solved.

Fast forward to 1991 at a customer's lab. We saw the same jumping calibration problem with a blood pressure meter (the type with an arm cuff). It was being used in a dialysis machine. I knew what the problem was, but I was wrong. It turns out the voltmeter being used had a fault where on very slowly moving voltages it would count 149, 149, 151, 150, 152...

I called a co-worker who had designed the software in that specific voltmeter years earlier. He confirmed the software fault in the voltmeter (it was not a noise issue). The customer replaced his voltmeter, the blood pressure meters passed, and all was well.

This entry was submitted by Dennis Seguine and edited by Rob Spiegel.

Dennis Seguine designed underwater systems, machinery monitors, and medical instruments for several decades. He is now an applications engineer with Cypress Semiconductor, trying to fit every possible analog and digital application into a PSoC micro controller.

Tell us your experience in solving a knotty engineering problem. Send examples to Rob Spiegel for Sherlock Ohms.

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