School Project Watchdog Timers

[lanif79]

I am working on a school project, I have most of my program done but i need to add another operation. the teacher called it a watchdog timer but was explaining it on an individual basis. the instruction is "Operation stops upon detection of lost puck"
we are using trainers that sort pucks, what i got out of it is

flow chart:
if after a certain time a sensor is not tripped
move to specified state and alert operator

how do i turn this into code?

thanks for any help

 

[gclshortt]

A watchdog timer will keep resetting as long as the conditions are met. If the timer expires, then stop the process and turn on an alarm.

Regards,
Garry
http://www.accautomation.ca

 

[Lancie1]

How do i turn this into code?

The details to do that will depend on which brand and model of PLC that you are using for your school project.

 

[shooter]

when detecting a puck at the start, start a timer, if timer is elpased before the puck is seen on the other end, stop the belt and report a lost puck.

 

[boneless]

The same thing in other words. You start an action. You expect this action to be done within x seconds and detect your puck at the next station to start your next action.

If you have started the first action and have not detected the puck at the next station in x + y seconds you will have an alarm.

y should be less than the time between two separate pucks.

Another application is in communication. One value should be updated regularly to detect a drop in communication.

 

[lanif79]

The rest

Thank you for your replies
I am using a mitsubishi PLC
with gx developer fx

so would it look something like

stl 120
set m1
ld m1
set t1 k100
ld x1
rst t1
ld t1
ani x1
set s130

 

[boneless]

lesson 1 in PLC's, add comments

 

[lanif79]

stl s120 ; enter state 120
set m1 ; set memory 1
ld m1 ; load memory 1
set t1 k100 ; start timer 1 10 sec
ld x1 ; load input 1
rst t1 ; reset timer
set s10
ld t1 ; load timer
ani x1 ; and inverse input 1
set s130 ; go to state 130

 

[lanif79]

sorry sent that before i could finish

s10 = restart process
s130 = inform operator of error

 

[boneless]

I'm not familiar with GX, but wouldn't RST force a 0 and SET force a 1, no matter what you are comparing to prior?

I might be very wrong..

 

[lanif79]

no the way i was taught to look at it was

ld = if
set = latching
rst = reset

so if input 1 is high
then reset timer 1
and go to specified state 130

if timer high and input 1 not high
then go to specified state 10

 

[boneless]

got ya. then I guess it looks good? can you get some hands on time?