Newbie question

Hi Guys just looking for some direction. I am doing a conveyor control project. The lay out 3 conveyors infeed, scale and outfeed. Problem is correct pacing of the product as it enters the scale to prevent 2 cases on the scale at once. My original idea was use a counter and 2 photo eyes an infeed eye and an outfeed eye. My though was the infeed eye would trigger a counter the outfeed would clear the counter. If the counter reads 2 it stops the infeed conveyor until it clears the scale (trigggering the outfeed eye) once outfeed eye is triggered the infeed starts up again. Worked great with 2 boxes. Once I introduced 3 boxes on top of each other the third box doesnt stop like it should. I already figured out that the counter is cleared and counts the third box as one. Sorry Im doing my best to explain this I hope it makes sense. Anyways my question is am I barking up the wrong tree with the counter ( I think I am now) and any direction you can give me on where I might start to lean would be appreciated. Thank You

Yes it sounds like your barking up the wrong tree. As your test shows the counter is owrking exactly like you programed it to do. What you need to figure out now is how to seperate your boxes. I do not have enough details to give you a very specific answer so hopefully something vauge will help. Instead of looking at the entrance of the scale why not look across the scale itself. In other words can you look across the scale section and detect more than one box?

Thanks Clay. I can always vary the speeds of the conveyors to help assure spacing but as the boxes are filled and put on by people the spacing can vary alot. I was hoping to use this as a safety net so to speak. I was thinking of a timer but Im gonna have to kick this around in my head some more.

It may be easier to put spacers on the conveyor to separate the boxes. Another thought if you can have the conveyor do indexing in increments of less than half a box. The indexes could be triggered by your off load eye. If all else fails use a vision system.

If you move the P/E from the Outfeed conveyor to the Charge end of the scale bed you might be able to get a timer to work. If you have back to back cartons you might need to install a metering or gaping belt conveyor. That requires too much work though, check out the attatchment and see if that gives you any ideas.

You have a 3-position conveyor. I have done a few of these. You need three photoeyes. I don't know how you'd make it bulletproof without an eye for each position. It might not apply in your situation but this also takes care of problem of operator uses forktruck to add or remove boxes.

Here are some ideas and suggestions. As peoplehouse said you need a gapping belt conveyor on the infeed to insure that a gap is there always. This is a mechanical fix where the infeed conveyor is driven by a single motor and uses two sections, one for entrance and one for exit, coupled together with a chain or belt between the two sections. They should about a 2:1 ratio on the pulleys or sprockets. The exit section being the faster of the two. The 1st P.E. would locate at the end of the exit section of the infeed unit. The scale conveyor will require enough P.E.'s to see the product as long as it is on the scale. The outfeed conveyor will require 1 P.E. to determine if the outfeed belt is clear so that the scale can discharge the product.
No counters or timers would normally be required in the logic, you would sense presence or absence of the product in the various locations to determine when to run the different conveyors. Of course, the scale will also have to have an output to indicate that it is finished weighing and an input to know a new product is ready to be weighed.
I know this is nowhere near complete but maybe it will give you some ideas and direction.

Matrix,

This project has the possibility of never performing well due to poor mechanical design.

Depending on throughput and how fast the scale is. You may need several conveyors inline feeding into the scale. First you need to assure that there are gaps between the packages (jrwb4gbm suggested this). Then you may need some "non contact accumulating conveyor" sections.
http://www.tkf.com/accumulaton-conveyor/
http://www.automation.com/content/f...&pagePath=00000000,00000307,00000316,00001418

If you are not correct mechanically there is no control solution.

Edit: I not saying your three conveyor system will not work. But it has the possibility of never working well. You already indicated personnel problems. If they are like the personnel I work with it will be a uphill battle forever. At about the time you get an employee trained to do the proper thing. They are are promoted to their next level of incompetence at some other work station.

I agree with mildrone. If you don't nail the mechanicals, this system is going to be nothing but trouble.

A common approach would be to feed the scale with a split ratio gapping conveyor.

The conveyor has one motor and drives two sections that are mechanically geared to run at different speeds. The induct section is used to meter incoming product, and the discharge section runs faster, creating a predictable gap between each product. For example a 1:2 ratio between the induct and discharge will give you a gap equivalent to the length of your product.

Then as an additional layer of control you could outfit the conveyor with a brake/clutch and a photo-eye at the discharge measuring the gap. If the gap is insufficient, engage the brake long enough to create the necessary gap.

Update

First I want to say thanks to everybody who responded you insights where invaluable. I did get the problem resolved by speeding up the scale and out feed conveyor and slowing down the infeed. I did work out logic using a timer that I think would work. I plan on still testing maybe end of the week depending on how busy things are. I'll update on that when Im done testing. Probably wont use it here but hey you never know it may come in handy some day right? Thats always my justification when I cant let something go... Thanks Guys