Help with a Simple PLC ladder diagram.

Thread Starter

Linium

Joined May 28, 2017
4
Hi everyone, I've been tasked with writing a simple PLC program to control 3 motors which turn on and off in sequence.
Here is the brief:
The problem is a conveyor belt system that comprises of 3 conveyor belts, each driven by a separate motor : M1, M2, M3 and must meet the following specifications
Starting:
∑ One pushbutton must start the motors in the sequence M3, M2, M1. This ensures that material from the previous belt is carried off before more is added
∑ M3 must start immediately when the start button is pushed
∑ After a 20 second delay M2 must start
∑ After another 20 second delay M1 must start

Stopping:
∑ One push button must stop all motors in reverse order : M3, M2, M1 to allow material to be cleared from the belts
∑ M1 must stop immediately when the stop button is pressed
∑ After a 20 second delay, M2 must stop
∑ After another 20 second delay M3 must stop

Start button must be N.O
Stop/Reverse Button must be N.C
Emergency Button must be N.Clogix.png
I've gotten this far but i cannot get the stop button to turn off the M1 and have it stay off in order to set up my timers to turn off the remaining 2. Any help at all would be welcomed!
Thanks
 

MaxHeadRoom

Joined Jul 18, 2013
22,831
For a start I would use the normal start rung you have but with a internal bit register in place of the M3.
From the program description I assume there is no E-Stop, just a sequential stop?
Max.
 

Thread Starter

Linium

Joined May 28, 2017
4
No restriction on timers. Yes, there has to be an E-Stop.

Start button must be N.O
Stop/Reverse Button must be N.C
Emergency Button must be N.C

Its set out this way because once if got it simulated i have to write in an program for a Mitsubishi plc and i have just those 3 switches available.
 

MaxHeadRoom

Joined Jul 18, 2013
22,831
Then the stop button you have in the first rung will start the M3 but it will also stop it first instead of allowing it to stop last, this is the reason for the normal stop/start rung by internal relay. (post #3)
Isn't that program you show for a AB RSlogix ?
Max.
 

Thread Starter

Linium

Joined May 28, 2017
4
Thanks, the program is Logixpro which is an AB sim i think. I was instructed to use this program, I am very very new to PLC's as you can tell. On the first rung is the E-stop which i think works well, i don't have stop/reverse button in there at all yet. That is where i am stuck.

Thanks again.
 

panic mode

Joined Oct 10, 2011
1,881
There are problems with this...

First of all definition is incomplete (stop sequence description is contradictory, it mentiones both M1,M2,M3 and M3,M2,M1) and very unusual (and unsafe) for a real world application - You would NOT want same button to work as both STOP and REVERSE. Better idea is to have three buttons FORWARD-STOP-REVERSE

I know that i would not want controls like this, because it does not offer modes of operation (well, i may be spoiled). I would definitely also want additional controls to be able move each belt independently (manual control or override).

ESTOP need to be done on hard wiring but can be also used in logic to make it easier to troubleshoot.

your timers are wrong, you are using 0.1sec base (just like default in Mitsubishi) but presets are wrong. 20*0.1 sec = 2 sec. you said you need to get 20 and 40 sec.

This is what (part of this) could look like in Mitsubishi:


first rung is simple start/stop
next two rungs generate needed events (T1, T2 are start delays, T3, T4 are stop delays)
finally we get three rungs that control outputs,
since your description was unclear, two alternatives are shown

EDIT ----

S#%^T, did not realize this was a homework section, here is slightly blurred version
 

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Thread Starter

Linium

Joined May 28, 2017
4
There are problems with this...

First of all definition is incomplete (stop sequence description is contradictory, it mentiones both M1,M2,M3 and M3,M2,M1) and very unusual (and unsafe) for a real world application - You would NOT want same button to work as both STOP and REVERSE. Better idea is to have three buttons FORWARD-STOP-REVERSE

I know that i would not want controls like this, because it does not offer modes of operation (well, i may be spoiled). I would definitely also want additional controls to be able move each belt independently (manual control or override).

ESTOP need to be done on hard wiring but can be also used in logic to make it easier to troubleshoot.

your timers are wrong, you are using 0.1sec base (just like default in Mitsubishi) but presets are wrong. 20*0.1 sec = 2 sec. you said you need to get 20 and 40 sec.

This is what (part of this) could look like in Mitsubishi:


first rung is simple start/stop
next two rungs generate needed events (T1, T2 are start delays, T3, T4 are stop delays)
finally we get three rungs that control outputs,
since your description was unclear, two alternatives are shown

EDIT ----

S#%^T, did not realize this was a homework section, here is slightly blurred version
Hi there, thanks for the reply. Yes, the stop button should just be a reverse. I am limited to using just 3 buttons, forward, backwards and e-stop. I understand my 2 timers only run for 2 secs, thats ok it was just for quickness in testing. I will have a go with the info you provided.

Thanks.
 
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