PLC ladder Logic help

Thread Starter

aakanni

Joined Apr 19, 2008
10
I am fairly new to PLC programming. I have a fairly complex assignment I am trying to write ladder logic for and need some assistance. Any help would be greatly appreciated. The task is as follows:

1. A pallet will be loaded on position 1.
2. The Operator will press the start button.
3. Conveyor 1 will move the pallet until it breaks the position 1 light sensor.
4. If position 2 is free, the pallet has to move until it breaks the position 2 sensor.
5. Once a pallet is on position 2, the centering bars have to close and remain closed for a predetermined time (3 seconds) before opening again.
6. Once the load has been centered, and if the sensor on the machine is free, the pallet will move under the machine. Once the pallet is under the machine we have to send a signal to start the machine
7. The pallet will stay there until the machine sends a signal that it has finished hooding the pallet.
8. If conveyor 4 is free the pallet will move until it breaks the position 4 light sensor and then the pallet will stay there.

Inputs and Outputs - all inputs are normally open
I1: PB Start I7: Pos 2 Sensor O1: FWD Pos1
I2: PB Stop I8: Centering Closed O2: FWD Pos2
I3: E-Stop I9: Centering Opened O4: FWD Pos4
I4: SafetyDoor1 I10: Pos 3 Sensor O3: FWD Pos3
I5: SafetyDoor2 I11: Machine Done O5: Centering Close
I6: Pos 1 Sensor I12: Pos 4 Sensor O6: Centering Open
O7: Run Machine

Consider the following in your logic:
The system has two safety doors and an e-stop; when any of those are activated all movement must stop. When the safety doors and e-stop are cleared the operator must press start again to continue line automation.
If no data type was specified for some information then you can choose the data type yourself.
You may add buttons/inputs/outputs if you need to.

I have attached an image for more clarity. I have also attached a file containing the ladder logic I have come up with so far. Can a more experienced programmer look it over and let me know if my program will work? Any tips or suggestions? Please any help is appreciated.

Thank you.
 

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MaxHeadRoom

Joined Jul 18, 2013
28,506
A couple of things jump out, your first four rungs all do the same thing, they are all going to latch in with the start PB. IOW they can be represented by one rung internal register bit.
May not apply in this case but where you use multiple rungs with the E.Stop PB, a MCR can be used.
Can you identify the sensors with the input/output No.'s Such as Sensor B = 1:xx etc.
Max.
 

Thread Starter

aakanni

Joined Apr 19, 2008
10
I couldn't get all the commands into a single rung without getting an error. I'm not sure what I am doing wrong. Also, what us a MCR?
Pos1 sensor = I:0/6
Pos2 sensor = I:0/7
Pos3 sensor = I:0/10
Pos4 sensor = I:0/12
Conveyor 1 motor=O:0/1
Conveyor 2 motor=O:0/2
Conveyor 3 motor=O:0/3
Conveyor 4 motor=O:0/4
I have not yet added the E-stop and safety doors.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,506
You don't get all the commands on a single rung, you use the start stop p.b's and latch them in with a internal register or BIT file coil.
You can then use that single BIT coil for all the other output.
A MCR or Master Control Relay is a coil set on by the system start/ready and is set to off with the E-stop and/or stop PB, all output between the initial MCR coil and a end of zone unconditional MCR rung turns off all non-retentive outputs within that MCR zone.
Max.
 

Thread Starter

aakanni

Joined Apr 19, 2008
10
If I have the start and stop PBs, the E-stop PB and the safety doors all on one rung controlling a single bit coil and that coil controlling the outputs, wouldn't I still need 4 different rungs for each conveyor start relay?
 

MaxHeadRoom

Joined Jul 18, 2013
28,506
One way you would then use the (e.g.) B3:3.0 internal storage bit to set on all 4 outputs in parallel .
You save a few rungs and extra instructions.
Max.
 

Thread Starter

aakanni

Joined Apr 19, 2008
10
I made some adjustments to the program. I put all PBs and the safety doors on one rung controlling an internal bit coil B3:1/0. I then use that coil to turn on the starter relays for all the motors. Is this what you meant?
 

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MaxHeadRoom

Joined Jul 18, 2013
28,506
Can you mark up the dwg showing the actual position of the sensors?
The clamping device shows 1 either side, I assume to confirm it has clamped or unclamped?
The first four B3 relays are redundant as the logic is identical to all three so you only need one, as they all will be latched at PB start.
Is there a hooding pallet sensor?
What is the reason for the position relay outputs to register the sensor inputs? You could just use the input itself on the rung, without making an output e.g. B3:01
It so far.
Max.
 

Thread Starter

aakanni

Joined Apr 19, 2008
10
The position sensors for each conveyor are the red lines in the diagram. The centering device has two inputs, closed and open. The diagram confuses things a bit but it is clearer in the instructions. I removed the four B3 relays in the follow up program I sent after your advice. I'm not sure what you mean by a hooding pallet sensor. I see what you mean about the position relays. I'll make some more changes.
 

Thread Starter

aakanni

Joined Apr 19, 2008
10
Yes. I see that those rungs were redundant. I made some more changes to the program and was able to get it down to 11 rungs. I know it works but with no way to test it I don't know how well it works. Unfortunately my deadline is up and I have to submit it. I really appreciate your help and input. Thanks a lot!
 
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