Encoder Counter Module

ELarsen

Member
Join Date
Dec 2002
Location
Gardena, CA
Posts
35
Greetings to all,
I am curious about this module which does something odd from time to time. It doesnt affect the machine operation. It is an AB 1771-IJ which is in a PLC5 rack and sends info to a Quick Panel. The module is reading the shot stroke on a Die Cast Machine. The labeling which corresponds to the lights is as follows from top to bottom on the module: CH A, CH B, MARKER, LIMIT SWITCH, OUTPUT 1, OUTPUT 2, FAULT.
I have noticed that CH A light will stay on and sometimes it will turn off. Like I said the HMI still receives the proper signals and shows the encoder is reading properly, i.e. zero to 5.5 inches, and it always resets to zero. Now for my question. Why does this happen?
Thanks in advance.

Ed
 
An encoder provides pulses to the counter module. A 'pulse' consists of an 'ON' section and an 'OFF' section. These should be equal in distance/rotation (depending on how the encoder is coupled to the item being measured). For accuracy there will be a great many of these pulses during the extension of the stroke. (During the extend the CH A light will appear continous because the frequency of the pulses is to fast to register to the eye.) The more pulses there are, the more precise the reading. The end of a stroke may end up in the 'ON' section or the 'OFF' section of a cycle. If there were very few cycles then the chances would be that the stroke would always end up in the 'ON' or 'OFF' section each time. But when there are a great many pulses, the natural variance of the process will cause the end of the stroke to possibly be in one or the other. The counter will still count them with the variance of the partial cycle figured in.

It may be possible that the CH B light turns on also which provides a mode called 'quadrature' which is used in processes which need accurate measurements during both extend and retract secctions.
 
Thanks Bernie, that is exactly what happens as far a CH A and CH B. A is blinking very fast and B does come on as well. I appreciate your response.

ED
 

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