watch dog (output check)

hi ,


i am new to plc and its applications
i want to know wat is meant by watch dog ( output check), because
in our purchase order its been mentioned that plc should have watch dog ( output check), and wat r its applications


regards

jose

Go to the bottom of the forum topics page and type "watch dog" in the "search this forum" text box. There was a good explanation of it just a few days ago.
PC

The idea is to set alarm if PLC is trying to move or turn on something and no action is detected in specified amount of time.
For example you have clamp that is operated by pneumatic cylinder
(or whatever). Then you would have outputs to open and close it.
You would also have sensors to tell you if the opening or closing is complete.
So if you say "close" and it takes 3 seconds to do it, you can setup WDT for say 6-10 second to set alarm and let you know
if operation times out or doesn't complete as expected
(like when it's jammed or stuck)
There are slight variations on how to do it (latch or seal etc.), here is simple one:

        Y       X              T 

-------| |-----|\|-----------[TIM]--

 



        T                      B

-------| |------------+-------( )--

                      | 

                      |

                      |

        B        R    | 

-------| |------|\|---+



Y= output (such as clamp close)

X= related input (such as clamp closed)

B= Alarm bit

R= Reset bit (Alarm Reset push button)

 

We've only used 'watchdog timers' to output an alarm, should communications between a HMI computer & PLC/network. Should the PLC fail to communicate with the HMI, The HMI would output an alarm. Should the HMI fail to communicate with the PLC, the PLC would output an alarm.

In the clamp example above, we do use 'timers' to set alarm outputs shoould it take longer then normal to complete an open/close command. But we don't consider these as watchdogs.

We use physical watchdog timers to monitor a PLC, a PLC digital output is connected to a "pulse input - off delay" timer and the PLC output is pulsed to keep the timer energised. If the PLC stops pulsing (either on or off) then the timer times out and either sets an alarm or trips the e-stop etc. The timer we use is a Crouzet 88865185 function N.