Intermittent Counter Problem

teppe · Mar 14, 2008

I have two counters in my program to count the number of parts entering the system and the number of parts exiting.

99.9% of the time, the counters work fine, but occasionally a part enters or exits without incrementing the count. I have confirmed that it's a problem with the controller since I have a digital counter installed in parallel with the input that activates the counter pulse. The digital counter is working fine. The Micrologix controller sometimes skips a count.

Any ideas?

I am using RSLogix 500 with Micrologix 1200 PLC.

Thanks,
Tom

Ron Beaufort · Mar 14, 2008

Greetings Tom ...

welcome to the forum ...

any chance that you can post your .RSS file? ... we'd be glad to take a look ... maybe we can spot something minor that you're overlooking ...

teppe · Mar 14, 2008

code

PLC code attached.

The PP_Pusher (I:0/8) is a photo-eye that senses the part is present, which then actuates the cylinder (O:0/8).

Thanks,
Tom

kamenges · Mar 14, 2008

Just so that we're all clear, you aren't counting off the input, you are counting off the output. Are you sure the external counter isn't getting too many counts? You have filtering on the input prior to the pusher action that you (presumably) don't have on the external counter.

Keith

teppe · Mar 14, 2008

That's correct. I'm counting off the output, and yes, the external counter doesn't have a delay. I see your point, but the external counter is wired to the output O:0/8, the same as the cylinder is. If the problem were due to the lack of a delay, wouldn't the external counter be more likely than the Micrologix controller to skip a count, rather than the other way around?

-Tom

kamenges · Mar 14, 2008

Originally posted by teppe:

I see your point, but the external counter is wired to the output O:0/8, the same as the cylinder is.

That's different. In your initial post you stated the external counter was wired to the INPUT that activates the counter pulse. I made a bad assumption based on that.

So how are you determining that the plc really is incorrect? You could still be getting false counts off the external counter based on noise from the valve when it disengages. The logic you posted seemed pretty solid to me. Do you have an end of travel prox on the pusher that you could hook the counter to? I'm kind of confused by this too.

Keith

rlmts · Mar 14, 2008

Hi Tom,
Which Micrologix 1200?
Is output O:0/8 a relay output or FET?

Can you actually physically count the parts by hand to see which counter is in error, or is it too intermittent to track?

Try slugging the counter (24vdc?) with say.. a 22ohm R in series with it and say.. something like a 4.7uF Cap across the counter input.

(Guessing values.. experiment with it)

Rich

teppe · Mar 14, 2008

kamenges said:
So how are you determining that the plc really is incorrect? You could still be getting false counts off the external counter based on noise from the valve when it disengages. The logic you posted seemed pretty solid to me. Do you have an end of travel prox on the pusher that you could hook the counter to? I'm kind of confused by this too.

Keith

I verified that the external counter was correct by physically counting the number of parts that were pushed, which agreed with the external counter, but not the PLC counter.

There is not a prox sensor at the end of the cylinder. It's pretty basic.

-Tom

teppe · Mar 14, 2008

rlmts said:
Hi Tom,
Which Micrologix 1200?
Is output O:0/8 a relay output or FET?

Can you actually physically count the parts by hand to see which counter is in error, or is it too intermittent to track?

Try slugging the counter (24vdc?) with say.. a 22ohm R in series with it and say.. something like a 4.7uF Cap across the counter input.

(Guessing values.. experiment with it)

Rich

I'll try that Monday.

Thanks for the suggestion,
Tom

rlmts · Mar 14, 2008

Hi Tom,

Well, to be honest you have already removed the need to expriment with slugging the external counter (I thought perhaps the ext counter could have been receiving relay bounce inputs from the MLgx relay, but you are saying that you have physically checked the parts, and the ext counter is definitely accurate? Is this correct Tom? Have another think and confirm that you are sure.
(If you are sure we need to start thinking 'sideways' here and revisit your PLC hardware and code, because at this time that code looks fine?)

Rich

swimmy67 · Mar 14, 2008

Hello Tom,
If i would have to guess, i would say that when you are resetting the counter it does not reset to zero. I would put a ONS instruction on rung 0121. This way the count is only active for one program scan. Otherwise the counter could count to one if the counter is reset and the pusher output is energised.

See if this helps.

Intermittent Counter Problem

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