Thick customers. :)

Just done a straightforward conveyor system for a bedding company.

Nothing difficult (I have done a lot of conveyor systems.)

There are 20, 4M X 2M conveyors in a straight line from the factory to the warehouse.
The operators make mattresses either side of the conveyors and throw them on the conveyor when they have finished one.
Each conveyor will take 2 mattresses....... and this is where the problem started..... each conveyor will take 2 mattresses.

Each conveyor has 2 photo cells - one at the end and one in the middle
If an operator puts a mattress in the middle, it will take it to the end of that conveyor leaving a gap for a second one to be put on. They will then travel on their way to the warehouse.

I get a call that it is not working right and is leaving gaps and he reckons he is 200 mattresses down a per day due to this?

I am now dealing with an irate customer who thinks he is not getting what he paid for.

We are talking - shouting at me.

He shows me the long line of conveyors packed with mattresses all queuing beautifully but with the odd conveyor with only 1 mattress on (The gap he is so angry about)
I tell him that an operator did not put a second mat' on that conveyor. It must have moved on before he had finished.
'you said there would be no gaps'
no, i said you can fit 2 mat's on one conveyor and the system will know where they are and queue them.

He takes me to a gap - this conveyor has one mat' on it. The one in front has 2 and the one behind has 2
'why can't you make it close that gap'?
'because the mat'is at the front and 2 conveyors have to run to transfer from one to the other. It would try to push the front mat' into the ones before it and eventually they would fall on the floor at the end.

'I am 200 mattresses a day down. This system is rubbish.
I say 'Take a look at the unloading end - they cant cope with the amount of mattresses coming down stream - the system is always backing up.
You need another man down there to unload.

By the way - he has taken on a lot more mattress makers but no extra unloading staff. He cannot get his head around the half empty conveyors because someone didn't put one on there.
The mattress are at the unloading end in more volume than they can handle but I am the reason he is 200 mattresses a day down.

Thick!

l would make a programming change so that each conveyor does not unload until a third mattress is ready to go on it, thus having 2 mattresses on each.

There would be no gaps and the problem would be unloading. Maybe that would get through the thick headed customer.

OP: Does this mean the mattress makers stand waiting for a gap to throw their mattress on to the conveyor?

Your customer is right, you shouldn't move on a half full conveyor, unless the unloading station has no mattresses.
The only reason you wouldn't want this is if it takes a long time to travel the 80m. If so, have a fixed number of mattresses after which the loading station waits on a second mattress. The optimal number can be calculated if you know the maximum unloading rate and the travel time.

200 mattresses is probably a bit rich though. And unless 'finding the absolute optimization for the mechanical system presented' is in your contract, offer to implement his improvement at a discount rate. Ofcourse, only apply the discount after you have added your rude-customer surcharge. You don't yell at control systems engineers.

Don't know if I understand the whole process, what tells the converyer to run etc. But to close the gap couldn't you move the conveyor with the gap and all after. Leaving you with a conveyor that is empty, and then move all conveyors before to fill the empty one?

So you will needa queing system to get all matresses in a row. It can be done with a few sensors, so the belt will stop until the belt before is loaded with matress, That way the cue will built up, A better way is to make a stock system so they can load and unload independent.

Write the code to close the gap after the last loading point.
You will need a gap at the loading area so you can place a matresses on the conveyor without stacking them .
When your customer sees the conveyor is full then he will understand he need more help at the unload point.
Maybe you could sell him on the idea of an accumulator to handle the overflow.

run it as an indexing system where the downstream, conveyor only advances as the upstream is sending a unit. if there is no unit, the downstream doesnt advance. this will never leave a gap at the discharge end, and it will open gaps so operators at the loading end never have to wait, even if their gap had already pulled forward past their loading station.

I don't think he is looking for a solution to the queuing problem. Closing gaps when you have two items on a single conveyor that are not evenly spaced is a nightmare.

Customers don't understand how queuing works without accumulation. They think there is some magical thing that allows the incoming rate to be different to the outgoing just by changing speeds.

That's right geoff White.
It is impossible to close a 1 mattress gap on a conveyor that takes 2 - if the operators do not put 2 on it.
It really doesnt matter anyway as they accumulate at the end of the conveyors.
When the gap comes - the next mat' is quickly sent to the end before they have properly removed the single one.
I suspect his 200 mat's down statement is to do with how many each operator can make in a shift. (as I said - he has employed a lot of new men)

There are none left on the conveyors at the end of the shift.
No mattress maker is ever stood waiting for a gap as they are all taken to the end leaving the working area clear.

The owner has to blame somebody for his lower than expected output - who better to blame than the system designer.

So did this evil thought go through your head: Just program the conveyor to never move unless there are two mattresses on it? At least the root cause of the gaps would be obvious.