I am required to program the system in the picture, but need a little help with maintaining spacing using the conveyor belts. How should I go about programming the conveyors to prevent any clog ups or problems? I have run a bunch of scenarios through my head, but cant seem to figure out something that would properly maintain spacing. I am not trying to ask for anyone to do it for me I am just trying to seek guidance, as I haven't encountered a problem like this yet. Any help is greatly appreciated.
Conveyor Belt
- Thread starter Hodak04
- Start date
mendonsy
Member
OK, for starters has the carton spacing already been established at the labeler or do you have to create the spacing as the cartons move between conveyors??
I would never "assume" anything, if in any doubts, go back to the designer of the machine or exercise and ask...
Here's a thought for you to chew on... why are there two conveyors ?
What would happen if they ran at different speeds, specifically what would happen if the conveyor that feeds the palletiser robot ran faster than the labeller conveyor ?
JRoss
Lifetime Supporting Member
The palletizing systems I've seen are usually pretty robust in terms of spacing. You just need to make sure there is enough room for the robots end effector to get in and pick up the case. So most likely, all you will be doing is stopping/starting the conveyors based on the two photo eyes so that the case don't slide into each other.
However, it's never safe to assume, so definitely confirm. At a minimum you will need to find out fast the cases are coming (case per minute) and the fastest cycle time of the robot. You'll need that information so that you don't risk presenting cases faster than the robot can handle.
However, it's never safe to assume, so definitely confirm. At a minimum you will need to find out fast the cases are coming (case per minute) and the fastest cycle time of the robot. You'll need that information so that you don't risk presenting cases faster than the robot can handle.
Unfortunately, all I have is the picture and information given. Also, I am taking this class through a military distance learning program and do not have a teacher I can consult.
I have thought about the different speeds and that is part of what is tripping me up I think. I keep thinking of different ways the conveyor could get backed up. I also can't decide if it would be smarter for both belts to run when the machine is taken to run or have the second belt be triggered by the photoeye. I am leaning towards the photoeye triggering the second belt, but I am struggling in figuring out how to keep it running and when to stop it. My worry is another box will just trigger the photoeye and negate a stop programmed in. Please understand that this class is an introduction and I havent really covered anything very advanced, so if I am stupidly missing something please point it out.
I have thought about the different speeds and that is part of what is tripping me up I think. I keep thinking of different ways the conveyor could get backed up. I also can't decide if it would be smarter for both belts to run when the machine is taken to run or have the second belt be triggered by the photoeye. I am leaning towards the photoeye triggering the second belt, but I am struggling in figuring out how to keep it running and when to stop it. My worry is another box will just trigger the photoeye and negate a stop programmed in. Please understand that this class is an introduction and I havent really covered anything very advanced, so if I am stupidly missing something please point it out.
Ah, the fact that it's a theoretical school project makes a difference 
As far as keeping the spacing intact, I'd probably look at something along the following lines:
- The second conveyor (the one closest to the packing robot) stops when a carton is at the end of it. Once the robot removes the carton, the conveyor will restart. This bit seems straightforward
- The first conveyor (furthest from the robot) stops if a carton appears at the end of it, and the downstream conveyor is stopped OR the previous carton has not progressed far enough along the second conveyor to maintain required gap. This one is a little more complicated. The "stop if a carton gets to the end and the second conveyor is stopped" bit is easy, but the "maintain the required gap" bit is not so much. First, you'll have to take a stab at a "time gap" - for example, let's say that based on the conveyor speeds, the cartons need to be a minimum 2 seconds apart to maintain the required physical gap. So you need to know that since the previous carton left the first conveyor, the downstream conveyor has had more than two seconds to take it away before your first conveyor can discharge the next carton. If a second carton appears within that two second window, your first conveyor has to stop until that two seconds has elapsed.
Remember, that two seconds has to be two seconds with the downstream conveyor running. So if it's been two seconds since it left, but the downstream conveyor was stopped for one of those seconds, you still have one second to go once the downstream conveyor restarts, before your first conveyor can discharge a carton.
Hopefully that will get you moving
As far as keeping the spacing intact, I'd probably look at something along the following lines:
- The second conveyor (the one closest to the packing robot) stops when a carton is at the end of it. Once the robot removes the carton, the conveyor will restart. This bit seems straightforward
- The first conveyor (furthest from the robot) stops if a carton appears at the end of it, and the downstream conveyor is stopped OR the previous carton has not progressed far enough along the second conveyor to maintain required gap. This one is a little more complicated. The "stop if a carton gets to the end and the second conveyor is stopped" bit is easy, but the "maintain the required gap" bit is not so much. First, you'll have to take a stab at a "time gap" - for example, let's say that based on the conveyor speeds, the cartons need to be a minimum 2 seconds apart to maintain the required physical gap. So you need to know that since the previous carton left the first conveyor, the downstream conveyor has had more than two seconds to take it away before your first conveyor can discharge the next carton. If a second carton appears within that two second window, your first conveyor has to stop until that two seconds has elapsed.
Remember, that two seconds has to be two seconds with the downstream conveyor running. So if it's been two seconds since it left, but the downstream conveyor was stopped for one of those seconds, you still have one second to go once the downstream conveyor restarts, before your first conveyor can discharge a carton.
Hopefully that will get you moving
Okay I think I see with the first two examples you explained. I believe I should be able to program those somewhat easily. Keep the belt running with a XIO instruction and have it linked the the stopper photoeye. As far as creating a "time gap", could a timer be used for that? Like have a timer start counting as soon as the box leaves the belt #2 photoeye? But if i did that and the boxes on belt #1 were to close would it negate that? I dont know any other functions besides a timer right now that I could use for a time gap.
Also, I would like to use a counter for the boxes that would stop the system after a certain amount required (we'll say 20 boxes) have reached the stopper photoeye. That way when the pallet is full, the pallet has to be replaced and system reset before it starts again. To do that I could place the counter done bit on the first rung to make the rung false and open a deenergize an internal relay right?
Also, I would like to use a counter for the boxes that would stop the system after a certain amount required (we'll say 20 boxes) have reached the stopper photoeye. That way when the pallet is full, the pallet has to be replaced and system reset before it starts again. To do that I could place the counter done bit on the first rung to make the rung false and open a deenergize an internal relay right?
You're on the right track. Look up retentive timers vs non-retentive timers for a hint on how to deal with the downstream conveyor stopping halfway through your timing period.
The only way this would not work is if the boxes on the first conveyor were so close together that the PE could not see a gap between them. But if that's the case, the problem is occurring before your first conveyor, and is therefore outside the scope of this problem. As long as the photoeye can see a gap, you just reset your timer as soon as you see the PE cell go clear. If the PE cell is blocked again before your two-second window has elapsed, you need to stop your conveyor and wait for that two second gap to complete.
Yes, you could do that with a counter - another approach (rather than forcing it to be reset) could be to just watch for the pallet to be taken away, and then replaced again. Then automatically restart. Even if you want to force a manual restart, it would be good practice to watch for the pallet to be taken away and replaced again before allowing the restart, to prevent the system starting again with the same pallet in place
The only way this would not work is if the boxes on the first conveyor were so close together that the PE could not see a gap between them. But if that's the case, the problem is occurring before your first conveyor, and is therefore outside the scope of this problem. As long as the photoeye can see a gap, you just reset your timer as soon as you see the PE cell go clear. If the PE cell is blocked again before your two-second window has elapsed, you need to stop your conveyor and wait for that two second gap to complete.
Yes, you could do that with a counter - another approach (rather than forcing it to be reset) could be to just watch for the pallet to be taken away, and then replaced again. Then automatically restart. Even if you want to force a manual restart, it would be good practice to watch for the pallet to be taken away and replaced again before allowing the restart, to prevent the system starting again with the same pallet in place
One important point is that the bottom-line restriction on the conveyors is the average speed of the robot cycle, or how long the robot needs to (1) pick up package, (2) rotate and move, (3) stack package.
Once you determine the robot cycle time* (see below), then you know that there is no need to space the packages on either conveyor any closer (because the robot would not be able to keep up and a backlog would develop on Conveyor #1). So use the robot cycle time to determine the time between packages. Each time the robot picks up a package, start a timer that is set for the average robot cycle time. Then wait for that time before starting Conveyor #2 to release the next package onto Conveyor #1. This will automatically maintain a space on Conveyor #1. Conveyor #1 should start and run until a package is seen at the Case Present Photoeye at the Stopper.
*Don't know the robot cycle time? Never fear, help is here! Each time the robot picks up a case (Case Present Photoeye goes from ON-to-OFF, and if there is a case waiting at the Conveyor #2 Photoeye, then start and run Conveyor #2 until the next case triggers Photoeye #2. This will maintain the correct spacing on Conveyor #1.
You could measure the robot cycle time by starting a retentive-type timer when the Case Present Photoeye goes ON-to-OFF (case is removed by robot), and run it until the next case is removed (measuring one round trip of the robot). This is the time (or space) that you can allow between cases and not slow up production. Any closer, and the cases will pile up on Conveyor #1 until they cause a jam on Conveyor #2, or some fall onto the floor. Any farther apart, and the robot will be waiting. Time wasted is money not earned, even for a robot. You can use this time to set the spacing on the next package. That way, the average spacing will be continuously adjusted to allow for any small speed increases or decreases of the robot due to different times in stacking on the pallet.
Once you determine the robot cycle time* (see below), then you know that there is no need to space the packages on either conveyor any closer (because the robot would not be able to keep up and a backlog would develop on Conveyor #1). So use the robot cycle time to determine the time between packages. Each time the robot picks up a package, start a timer that is set for the average robot cycle time. Then wait for that time before starting Conveyor #2 to release the next package onto Conveyor #1. This will automatically maintain a space on Conveyor #1. Conveyor #1 should start and run until a package is seen at the Case Present Photoeye at the Stopper.
*Don't know the robot cycle time? Never fear, help is here! Each time the robot picks up a case (Case Present Photoeye goes from ON-to-OFF, and if there is a case waiting at the Conveyor #2 Photoeye, then start and run Conveyor #2 until the next case triggers Photoeye #2. This will maintain the correct spacing on Conveyor #1.
You could measure the robot cycle time by starting a retentive-type timer when the Case Present Photoeye goes ON-to-OFF (case is removed by robot), and run it until the next case is removed (measuring one round trip of the robot). This is the time (or space) that you can allow between cases and not slow up production. Any closer, and the cases will pile up on Conveyor #1 until they cause a jam on Conveyor #2, or some fall onto the floor. Any farther apart, and the robot will be waiting. Time wasted is money not earned, even for a robot. You can use this time to set the spacing on the next package. That way, the average spacing will be continuously adjusted to allow for any small speed increases or decreases of the robot due to different times in stacking on the pallet.
Last edited:
RET
Member
pallent not present at photoeye -> disable system
case present at labeler, or conveyor #2 photoeye case present-> stop conveyor
case present at labeler and conveyor #2 stopped -> apply label
label applied-> enable conveyor #2 start
conveyor #2 enabled and case not present at #2 photoeye -> start conveyor
case present at robot -> stop conveyor #1, start pacakging robot
case not present at robot -> start conveyor #1
conveyor #1 running and package present at conveyor #2 -> start conveyor #2
or something like that.
case present at labeler, or conveyor #2 photoeye case present-> stop conveyor
case present at labeler and conveyor #2 stopped -> apply label
label applied-> enable conveyor #2 start
conveyor #2 enabled and case not present at #2 photoeye -> start conveyor
case present at robot -> stop conveyor #1, start pacakging robot
case not present at robot -> start conveyor #1
conveyor #1 running and package present at conveyor #2 -> start conveyor #2
or something like that.
GlenGineer
Member
I haven't finished reading all the replies to your question yet but the problem reminds me of one that a fluid dynamics professor gave me. She set up the problem but only gave us half the information. What she wanted was for us to make some logical assumptions and based on those come up with an answer. The problem was fire sprinklers we were given all sorts of info but not initial pressure or pipe diameters. So I used the google machine to find reasonable numbers and then I corrected when I found out my pipe size assumptions were too small.
So if I were you I would spec out the equipment. For instance what type and speed capability is the robot. Also stacking accuracy is a concern the robot might not be capable of accurately stacking the cases at top speed. (also top speed is hard on devices) Hope this helps.
So if I were you I would spec out the equipment. For instance what type and speed capability is the robot. Also stacking accuracy is a concern the robot might not be capable of accurately stacking the cases at top speed. (also top speed is hard on devices) Hope this helps.
GlenGineer
Member
Also remember that if the robot has on board vision the conveyor does not need to stop....oh and if you make some assumptions be sure to list them.
Since it is a learning exercise, let me add another 2c...
It is generally "bad" practice to cycle conveyor motors start/stop continuously... Motors and contactors would need to be uprated for the duty-cycle.
From the limited information given in the picture you posted, I'm guessing the exercise is about spacing the boxes for the robot, hence the 2 conveyors instead of one. Any other scenario could be solved with just one conveyor.
It is always preferable, and more energy efficient, to keep the components of a system running at constant speeds whenever possible. The more you can achieve that, the more reliable the machine or process will be.
Since you've been given no data on the robot performance, I would assume that it can handle what is fed to it. On that assumption, I see no reason to be stopping conveyors at all, so I stand by what I said earlier, the robot conveyor runs faster to create "spacing"....
It is generally "bad" practice to cycle conveyor motors start/stop continuously... Motors and contactors would need to be uprated for the duty-cycle.
From the limited information given in the picture you posted, I'm guessing the exercise is about spacing the boxes for the robot, hence the 2 conveyors instead of one. Any other scenario could be solved with just one conveyor.
It is always preferable, and more energy efficient, to keep the components of a system running at constant speeds whenever possible. The more you can achieve that, the more reliable the machine or process will be.
Since you've been given no data on the robot performance, I would assume that it can handle what is fed to it. On that assumption, I see no reason to be stopping conveyors at all, so I stand by what I said earlier, the robot conveyor runs faster to create "spacing"....
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