basic count

I need some help with a basic count problem. I am tracking the daily infeed number on a production line. The photoeye counts jars that are spaced about 0.5 to 0.7 seconds apart. Sometimes the count is too high and others it is too low. What could be causing this?

I am researching specialized sensors for accurate counting, but could there be a programming issue as well? Would program scan time affect the counting accuracy?(...even though the jars are spaced out fairly well?)

Help is appreciated.
rgeorge

rgeorge,

It could be any number of things. Please try to narrow down the possibilities by providing as much information as you can about the system (i.e what kind of sensor currently in use, what kind of PLC, the ladder logic if available, etc.). This will make the problem easier to tackle.

Jeff.

0.5 seconds spacing should not cause scan-time problem. However, you may want to check how long (on-time) the piece stays in front of the sensor? If it is too short i.e >20ms, you may have a scan-issue.

In your program, are you using a one-shot. If not, that would explain why you count is too high. One object gets counted multiple times if the input stays high for more than one scan time.

Jar-Jars?

You mention jars. If you are using a photo-eye sensor to look at glass, and the sensitivity of the sesor is "edgy", you might be picking up multiple triggers on a single jar. If clear, you might be looking through it.

What about an overhead mounted sensor looking down?

TM

You said "the photoeye counts jars". Does this mean that the photoeye has a built-in counter or that the photoeye is wired to a PLC input and you're using the PLC's 'Up Counter' function? How do you verify that you're getting overcounts and undercounts? What model PLC are you using?

Overcounts are generally caused by multiple signals from the sensor. If it were a mechanical switch, you'd call it 'contact bounce'. It's possible that the signal from the photocell is varying enough to be interpreted by the PLC as multiple OFF-ON-OFF transitions.

Undercounts are typically caused by the signal not being True long enough to be sensed by the PLC.

Usually a single application like this consistently counts over or under, but doesn't count over on some days and under on others. Please provide as much information as possible so we can help you figure things out.

I would suggest putting a 0.25 to 0.35 second timer in to hold the high signal in and put a one shot in line with this. This will make sure to eliminate any "bounce"(not really bounce in this application but it could be inconsistency in the jars causing the photoeye contact to "bounce") in the contact and make sure that your counter only sees one pulse/jar!