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Tester predicts when encoders will fail

01 March, 2001

Tester predicts when encoders will fail

An American company has developed an instrument that checks the operation of quadrature encoders and can predict when they are likely to fail. Lynn Electronics` Quadrater can detect problems such as scratched or dirty scales, and timing errors between pulses, which can result in production line faults if they are not spotted in time.

After encoders have been in use for a while - typically for one to five years - their performance deteriorates as a result either of damage to, or dirt on, the glass scale, or because of changes in the output and sensitivity of the LEDs and sensors. The traditional way to check encoders has been to use an oscilloscope but this is not always done because it requires a skilled operator and can disrupt production.

Lynn, which also makes CNC controls, originally developed the Quadrater for in-house use because it found that its equipment was often being blamed for faults when the problem lay with the encoders. The company subsequently patented the technology and is now selling the testers.

An ideal quadrature encoder should provide square-wave signals that are 90 degrees out of phase with each other. However, as the encoder ages, the phase shift decreases, and the machine has to slow down to ensure proper counting. At zero phase shift, the counter does not know which way the machine is going and will miscount.

"If you start missing counts, you start getting bad parts," says John Lynn, the company`s president and chief executive.

The Quadrater - essentially a phase measuring device - senses the shifts in phase angle and displays these as a value from 0 to 9 on a LED counter. "When you start seeing fives, you need to get something done," says Lynn.

"It will tell you if you`re missing a pulse - something that an oscilloscope can never do," he adds.

The Quadrater is designed to work with almost any linear glass scale or rotary quadrature encoder. "It won`t check resolvers, but checks the circuitry that makes square waves out of resolvers," Lynn explains.

Lynn argues that on some high-value production lines, the $600 cost of the tester can be recouped simply by preventing one bad component being produced. "If it saves one part in five years, it has paid for itself."

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