keeping track of two timings

I have a Micro830, but I imagine my problem is just locical and probably applicable to most any PLC

I have a conveyor with a photocell at one end and a reject blowoff at the other.

the problem is that the next object arrives at the photocell before the last object is at the blowoff. the timing is always the same but I need to know when to start it and how to keep track of it so that I can reject the object if necessary.

I have been pulling my hair out over this... It seems that it should be fairly easy, but I have not found it to be so.

the objects arrive at random times up to 120 per minute... every thing I try seems to have some sort of flaw

would some one help please.

thank you

realolman,

if there is a space between the boxes (i would say at least .05 sec because i'm not used to high speed), then i would consider using 2 bit shift commands. #1 is for the box and #2 is to accept or reject it.

if there is no space ans no way to put a space between the boxes, then i would use an encoder.

you could use a timer option, but i think that's to messy.

regards,
james

thanks for replying... This is an application that already exists. the PLC went bad and the company that made the equipment is not available.

So somebody already did it... And I gotta work with what is there... among other things, a photocell at the beginning of the conveyor and a blowoff at the other

The distance between the photocell and the air blast blowoff is always the same, but I need to keep track of at least two of the objects seperately. If the objects reached the blowoff before the next one hit the photocell it would be much easier, but the next object hits the photocell before the previous one hits the blowoff.

What I thought I could do was have two timers enabled alternately with every other blockage of the photocell, but I have not been able to transfer that thought to ladder logic. I was hoping to get some help with that.

If the time it takes the object to get from the photoeye to the reject will ALWAYS be the same, then using timers may be enough. I have done this on a slow moving machine, and it worked fine. Attached is a clip of the ladder I used. This one can handle up to 4 objects between the photocell and reject.

For your application, my 'Reject Part at Discharge One-Shot' will be the one-shot from your photocell.

Basically, it's just 4 timers in sort of a FIFO arrangement. When timer 1 is 'in use' by the first object, timer 2 gets allocated to object 2, and so on.

The 'Reject Air Start Delay' timers determine the time between the photocell and reject. All get set to the same preset. The 'Reject Air Blast Duration' is how long the reject blowoff actuates.



-Eric

Nice Eric. Do you mind if I borrow it for my 'just in case it's needed bag of tricks' file?

Please do... I'm sure I've borrowed a few of your ideas over the years, Bernie...



-Eric

In another similar thread I had posted a program that may work in your case as well.

Here is the Link: http://www.plctalk.net/qanda/showpost.php?p=541427&postcount=14
Read from "Notes on the program"

realolman said:

I have a Micro830, but I imagine my problem is just locical and probably applicable to most any PLC

I have a conveyor with a photocell at one end and a reject blowoff at the other.

the problem is that the next object arrives at the photocell before the last object is at the blowoff. the timing is always the same but I need to know when to start it and how to keep track of it so that I can reject the object if necessary.

I have been pulling my hair out over this... It seems that it should be fairly easy, but I have not found it to be so.

the objects arrive at random times up to 120 per minute... every thing I try seems to have some sort of flaw

would some one help please.

thank you

Click to expand...

The photocell detects the presence of an object. Is the object determined good/bad at this point? How many objects between detection and reject? You may need to track every object or time-slot if the timing is "always the same".

The object is not good or bad at that point... but I want to know where it is on the belt so I can reject it if necessary. The photocell itself will qualify it as to length, and there are two more sensors after that

Eric Nelson, Anirban Hazra ... thank you...

Eric I havn't looked at your attachment yet, but if Bernie Carlton likes it, it's gotta be good.

From your explanation it sounds like exactly what I need. I think I only need to track two items, but four would be almost twice as good. ;-)

thanks to everyone... keep your computers on... I imagine I'll probably be back

Eric, I looked at your logic.... and I'll have to look at it some more

Anirban Hazra... I just looked at your link... thank you I understand your logic perfectly and I think it is just what I need... thank you for posting it.

Eric Nelson thank you for posting yours... I appreciate it very much, and as I said before, if Bernie likes it , it must be good... but it is unfamiliar to me... It's no disgrace to be dumb, but it is awfully unhandy.

I think I'll just make myself a tagline that says: thanks to everyone, you are the best

I am hesitant in being referred to as some kind of great authority on good code just by looking at it. You can't imagine the number of times some code looks good to me at my desk but totally goes south when loaded into a machine. (Though they are getting fewer as I tend to remember my mistakes and I'm slowly using up the more common methods of messing up. But I'm assured that the actual ways of making mistakes are infinite compared to the ways of getting it right.) But Eric's code did look simple and effective.

@Bernie about the bag of tricks file, now that's something I want to see haha!!

the actual ways of making mistakes are infinite compared to the ways of getting it right

Click to expand...

Amen to that.

Being forgetful as well, I am even able to repeat many of them, making it (like the tv commercial with the little kids ) infinity plus