Starting & Stopping Conveyors in Sequence

Hey guys and gals, I have a question about programming 3 conveyors in sequence each starting seconds apart and when the stop button is hit stopping in sequence.

Could someone give me some guidance. I have a start, stop and each conveyor starting correctly however how do I get the to stop in sequence? Mind you conveyor 3 starts first and therefore stops last.

Any suggestions will be appreciated, and please feel free to assume I know the basics. (this is a training scenario by the way)

Im using Logixpro...

Thank you for your time, lets keep adapting my friends!

Paul Godines

It's normal to have the conveyors interlocked so that if one stops it stops all the ones that preceed it, this usually happens immediately otherwise the material piles up at the transfer point.

I'm thinking mining here, the industry I'm most familiar with.
It gets a bit trickier when you have re-circulating loads like in a screen house or crushing plant.

I'm in the machine rebuilding industry for recycling companies myself. I'm thinking that conveyor 3 starts then 2 then 1, now my prob is that when I hit the stop button, I cannot figure out how to keep power going until the conveyors stop in reverse sequence (may not make sense but I hope this helps).

Thanks for your time.

Should I use jmp controls? And if so do I use a Selector switch?

iadapt said:

I'm in the machine rebuilding industry for recycling companies myself. I'm thinking that conveyor 3 starts then 2 then 1, now my prob is that when I hit the stop button, I cannot figure out how to keep power going until the conveyors stop in reverse sequence (may not make sense but I hope this helps).

Thanks for your time.

Click to expand...

You probably should use 2 stop buttons, 1 for sequenced stop and the other for quick or crash type stops. I don't believe you need JMPs anywhere. I will try to write a sample program and post it a little later.

I would use an up down counter to turn the conveyors on/off in sequence.

Up count to turn on
Down count to turn off
and use an equal to compare the counter .ACC to a value to turn on and off the motors.

Alan

iadapt said:

Should I use jmp controls? And if so do I use a Selector switch?

Click to expand...

No, you should use timers. A start button to the "Start Conv. Sequence" timer, seal it in (branch around a momentary start input) with the "timer not done" instruction, an break the main branch with the "Stop Conv. Sequence" timer done bit.

For the "Stop Conv. Sequence Timer" rung, use the stop button and branch around it with the "timer timing" instruction(s). If you are transporting discrete objects, you may want to ensure the downstream most conveyor starts first and stops last.

The logic driving the outputs to the motor starters would have a comparision instruction for the timer accumulator in series, so the downstream most belt would be running as long as the stop timer is not done. The next one upstream would be running as long as the start timer accumulator was greater than its designated delay, and as long as the stop timer accumulator was less than its designated time.

The above is only good if you have no feedback. In the real world, you want to monitor a fault contact (spare starter pole, VFD "running" contact, disconnect contacts, all of the above), and ensure that before each motor starts, the downstream motor "proof" contact dwell time is done, and before it stops, the upstream conveyor must be stopped for another timer done. Two timers per belt in each train, but very easy to predict, control, and debug.

Here is a sample program using timers. There are probably more effective ways to write this. I am just trying to show timer sequencing. As OkiePC said, you really need proof inputs to verify each motor is running. I did not include that.

You guys are awesome, the solutions have been great, thank you. This was my first time posting here, I believe. However I am impressed! Thank you for your precious time, I am grateful! By the way no VFD's.

Do you have any product position feedback on the conveyors? Proximity Switches, Photoeyes, etc.

I did some modification to programs to "automagically" turn on/off conveyors based on traffic control downstream/upstream of each conveyor after a delay with no movement.

No reason to keep running motors and gearboxes if the operators weren't pulling parts.

Yes, a timer measured to the length of the conveyor, use an RTO (retentive timer) enabled by motor running proof, conveyor empty, or go way out there in abstract space and map the contents and associated measurements of each discrete item with pointers to data maps of each belt contents. Once identified, tracking forward stop, reverse can be done, missing packets detected by photocell. This is really sweet with encoder feedback, but pseudo (measured accel times and decel times of hte objects) code can suffice if the spacing to object ratio is within the resolution of your maps. I have bullet proof package tracking that works with motor starters and one sloppy scalar (V/Hz) VFD doing the spacing control, it is literally tracked in educationally estimated (hand tach calibrated) inches from the "birth eye" a polarized retro sensor before the scale. Once born a new box entry is popped into existence and it position updated in a timed interrupt routine for accuracy. You can even go so far as to display them animated on the HMI and let the operators remove them manually from the system without disturbing the under-achieving BSL or FIFO style of product tracking. Since I have three short belts it made more sense to keep track of up to fifteen packages, but with a long continuout belt, I just drop numbers in a data table at an offset from the pointer which is tied to calculated distance as a real, and then carefully rounded to a float for its index position in the rotating file. You have to have a cleanout mechanism to zero out the end of the belt and move the contents of the files (weight, barcode, whatever else) to the next map input offset. Your units must be optimize-able to fit within the constraints of the file size in a SLC/Micro/PLC-5 to avoid crossin file boundaries, but often, I have found that 256in, meters, feet or some other division, like half-foot resolution.

As long as this map has resolution finer than the size of the packages and gaps it will work. It's much more accurate and efficient than FIFO logic, but hard to explain and follow without a good understanding of how it works.

Conveyors are often underestimated! Also bear in mind when passing a product from conveyor A to Conveyor B you must know that there is space on conveyor B! It sounds like you will rely on timers but you could make it more intelligent with sensors; this might enable you to stack products on conveyor and move them as a batch too.

Just a thought,

Nick

Any suggestions will be appreciated, and please feel free to assume I know the basics. (this is a training scenario by the way)

Im using Logixpro...

Click to expand...

Paul, Did you get your LogixPro conveyor program working? You may post it here if desired. ZIP (compress) it to save as a LogixPro *.RSL file, or print it out to a PDF or JPG file. LogixPro has an option to "File, Save Rungs as Picture" which saves a JPG file that can be uploaded to PLCtalk.net.

Here is the method that uses only two timers, a Start-Up Timer and a Shut-Down Timer, and motor interlocks to prevent running out-of-sequence. Using 2 timers, this method can be used for any number of conveyors or other machinery that needs to be started and stopped in a certain sequence. This LogixPro program can be run using the LogixPro Silo Simulator, or with the I/O Simulator.