Not my usual kit, I do Siemens panels mostly, but this one landed on me. Bagging line, MicroLogix 1400 with RSLogix 500, photoeye at the discharge into I:0/3, CTU C5:0 counting bags for the shift report.
The count runs 15 to 20 percent high against the pallet tally. A lot of bags count as 2 and some as 3, and online the counter sometimes jumps by 2 for a single bag.
I swapped the eye for a new one of the same type and got exactly the same, and on the bench the input LED goes on once and off once when I wave a bag past it, which looks clean enough to me.
Is the CTU doing something odd on this old firmware, or should I be counting another way? Would a CTU on a MicroLogix maybe pick up something on that input the LED isn't showing me?
Where's the eye looking. Square across the bag, or down at it? And is it seeing the heat seal?
Could the scan be too fast and the PLC counting the same bag on two scans? We had something like that on a line at college, or I think we did.
No. A CTU counts the rising edge of its rung, one per false to true, however many scans the rung then stays true. So the eye's giving you more than one edge per bag and the counter is doing its job.
You haven't proved the signal yet. The bench test with your hand is slow. On the line the edge is a few ms and you'll never see that on the input LED. Do this: add a second counter on the same input with a TON in front, 30ms preset, the DN bit into the counter. Run both for a pallet and compare against the hand tally. If the timed one matches, the eye's giving double edges.
Ran both for 300 bags. Raw counter 351, the one behind the 30ms timer 302, hand tally 300. So the timed one is near enough right and the raw one's way over.
So the eye's bouncing? its a new eye.
Not bouncing, seeing two things. The heat seal on those woven bags is glossy. Eye sees bag, then the seal reflects and it drops out for a few ms, then bag again. Two edges. Had it on a feed mill line with the same bags. Tilt the eye 10 degrees off square and it stops catching the seal.
Both, then. Tilt the eye as carl says and keep the timer, because any eye will give you a short edge sooner or later.
Final rung: I:0/3 into a TON, T4:5, time base 0.01, preset 2, then T4:5/DN as the only contact in front of C5:0. 20ms is enough for a seal edge and short enough that a bag at your speed still counts. If the bags ever come faster than about one per 50ms you'll need to look at the falling edge instead with a minimum on-time check, but you're nowhere near that.