control garage door - simple example

[Pandiani]

Hi guys,
after some time (I was busy passing exams) I'm back to PLC programming.
I'm trying to write a simple ladder program to control a garage door.
There are two limit switches Limit_Up and Limit_Down to signal door's final
positions, two buttons: Up and Down and two outputs: Moving_Up and Moving_Down.
I manage to do something (in pdf document) but I have problem. I want following behaviour:
when door is moving up and user press button down door will stop and when he (she) press
button down again then the door will start moving down. As you can see in my example if
door is moving up and down is pressed door immediately start moving down which is not
what I want.
Can you please help me by giving advice how to find solution. I was thinking to introduce
another state - stopped but not sure if that's way is done in real world.

Please help
Thanks in advance

 

[Ken Moore]

Try adding this as network 3:

 Moving_Up      Moving_dn     stopped

--] / [-----------] / [-------(   )-

Then in Network 1 add a NO stopped in the logic after the one shot, but before the branch:

 

    UP       M3.0       Stopped

--]   [------(P)---------]   [---|--

                                 |

 

 

Do the same for Network 2.

So...if the door is moving up, and the down button is pressed, the first scan will drop out the up latch, the down network will be false, since the stopped bit is still false, on the next scan the stopped bit will be true, but since the button is still depressed the one shot will be false, so the nework is false.
Once the button is released, and then depressed again, the network will become true, and the door will start to go down.


Hope this helps, I'm better at doing than explaining.

 

[Pandiani]

It works, thanks...
It is better for me to see ladder because English is not my native and therefore it's easier for me to understand when see ladder rungs than to read explanation which is btw, excellent.
Thanks again