I am tasked with supporting a capacitor voltage withstand test system built by an outside contractor. The system measures the voltage across 6 capacitors under test, these are very large value capacitors, say up to 10mF and up to 2KV. The system uses a PXI chassis, 4071 DMM, with a 2527multiplexer. The voltage across the capacitors is sensed via LEM sensors and transformed to a maximum output current ~50mA through precision 100ohm resistors. These voltages are fed into the multiplexer. The system has been on line and operational since 2010. We have seen in the past months a gradual deterioration in voltage accuracy and repeatability. Discussions with LEM educated us to the fact that LEM had problems with the original design voltage sensors, new ones were available with those issues "designed out". Replacing the sensors, and installing linear power supplies provided much improved uniformity of measurements with less random variation in measured voltage. When the test system is turned on, it works as designed. As production lots of 6 capacitors are tested, the measured voltage falls equally on all 6 parts. The programmable Lambda supplies are sourcing the exact voltages requested. It seems that this is a DMM problem. Before I submit a purchase request for a ~$3500 4071 module, I'm open to any other possible ideas to resolve this issue. I am completely unfamiliar with how to program NI systems. There are no error messages displayed on the tester GUI, the tester is controlled by a generic desktop PC. The software runs without problems. Any ideas? I sure thank any folks for thinking about this issue. terryatsbe
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