Encoder with a heart beat?

[Philadelphia Frank]

We have a new upgrade going on. We've replaced our existing resolvers with KUBLER encoders connected to CompactLogix 1769 HSC cards. We are not getting consistent movement results.

Something that I noticed is the HSC input LEDs are blinking slowly even though the encoders should be still. When I say blinking, I mean a pulse of 0.5s on 0.5s off. Those values change somewhat, 0.3 on, longer off.

We have about a dozen encoders. Some never blink, the others blink consistantly for many minutes, maybe hours. They are not blinking in unison. Most of the time, it is only one channel. At least once though, both channels were blinking at the exact same time. The pulses were not out of phase. I use a scopemeter to view the pulses.

Like most problem situations, I'm probably leaving important information out. There is too much going on to cover every possible problem. I guess I'm hoping sopmebody will recognize an encoder "heartbeat" and have an answer.

Some more clues:

We are only using the A and B channels and need 4 wires. The pinout should be: brown= +24VDC , white=common, green=A channel, and grey=B. Unfortunately are using the existing resolver wiring and may have applied 24 VDC to the A-NOT channel. We found a few encoders not working and warm to the touch. We think the 24 VDC to that A-NOT channel might have been the cause. We fixed that problem and have not burned up any more encoders.

I tested the remaining encoders with a 9 volt battery and a scopemeter. I applied the 9V to the power leads and monitored the output channels A and B looking for a pulse stream. The voltage for the actual job is 24 volts DC however.

The system is working somewhat. The axis that are not reliable are still moving and in the proper direction.

I know there are several possible reasons for the movement problems. I'm just stumped by the heartbeat pulse coming from the encoders.

Has anyone seen a situation like I've tried to describe, a steady 1/2 second pulse when the encoder is not moving?

Thank you for your time and help;

Frank

 

[BryanG]

Have often seen one channel output pulsing on encoders. The encoder can get to the point where it is just on a change from 0 to 1 or 1 to 0 and the slightest movement/vibration in the shaft can cause it to flash. It depends on the pulses/rev and the slack in the shaft/drive. Using quadrature input means that the single channel flashes are ignored and any double channel flashes should be taken care of by the up/down counting. If you are getting two channels flashing synchronously then I would think that you are looking at pickup on the cable run, make sure signal and power cables are run separately and cross at right angles. You can sometimes get around pickup by adding extra load onto the system using resistors just make sure you don't overload the encoder output, not the best method but it can get you out of the sh*t.

Bryan