Hi:

For a couple of months now I'm experimenting some problems with the signal of an encoder. It seems like some times the encoder increment the count number a little bit, it can be douring movement (the counts grow more than it should) or when the encoder is static. I checked the Cable routing, even change it, the encoder and the HSC card, and it didn't work.

Anyone habe been experimenting problems like this or hyave any idea of test that I could make to fix this.

Tanks.

Are you using a quadrature encoder (two input channels A and B)?

Yes I'm using one.

If your encoder count register in the PLC is increasing with the machine stopped, then the high-speed counter card is counting pulses from somewhere. Most likely electrical noise from improper grounding. Inspect the encoder wiring into the counter card and make sure the ground lead is floating at the encoder and landed at the counter card (Float In Field). A lot of electricians want to land every wire in a cable, but that pesky ground loop will cause you lots of havoc (like higher count rates and counting while machine stopped).

There are many ways false counting can arise.

With the encoder still, noise can arise from:
1. Wiring ground loops (as kaiser_will above correctly pointed),
2. Common mode noise entering through the encoder power supply,
3. Faulty encoder output electronics,
4. Absent/wrong termination at the input card (with differential signalling), and
5. Faulty/latched-up input card.

With the encoder moving, in addition to the above the most common cause is so-called dithering or a worn/damaged encoder optical disc.

You may find our paper (pdf, 238K) of interest as it discusses these problems.