lack of pulsing input to enable output

[controlphreek]

I have device which pulses a NO contact when it is in alarm (roughly 2 second pulse every 15 seconds). I need to move a 3-way valve when when my device is in alarm status and return the valve to "normal" position when my device is not in alarm status. I intend to use the KM Easy (ladder logic)...Any suggestions

 

[Eric Nelson]

That's a pretty lousy alarm indicator...

Think about how you can tell when the alarm is on and off. When the alarm pulse turns ON, you have an alarm, but when it turns OFF, the alarm isn't necessarily gone. You have to wait 15 seconds to make sure it doesn't pulse again...

Here's one (of many) ways to do it:

|  ALARM         +---------+

|---]/[----------| TIM 1   |

|                | 15 SEC. |

|                +---------+

|

|  ALARM     TIMER

|---] [---+---]/[----(LATCH)

|         |

|  LATCH  |

|---] [---+

|        

|  LATCH

|---] [--------------(VALVE)

Attempt at an explanation:

First rung:
When the alarm is OFF, TIM 1 starts timing. When it finishes, the TIMER contact on the second rung will open. When the alarm is ON, TIM 1 will not run, and therefore the TIMER contact on the second rung will remain closed.

Second rung:
When the ALARM input is ON, the TIMER contact will be closed, causing the LATCH coil to turn ON. The LATCH contact will keep the LATCH coil energized (ON) when the ALARM input turns OFF, because the TIMER contact is closed. Once TIM 1 finishes, the LATCH coil will turn OFF because the TIMER contact opens.

Third rung:
The VALVE output simply follows the state of the LATCH.

Get it?...

beerchug

-Eric

P.S. You can probably shorten the TIMER preset by however long the ALARM input is ON. The timer begins timing when the ALARM input turns OFF, therefore the ALARM input should turn ON again in roughly 13 seconds (based on your "roughly 2 second pulse" statement). This will just let you know the alarm is OFF, as quickly as possible.

 

[controlphreek]

Thanks Eric. Yes, you're right...it's not a very impressive alarm. But, then it's only suppose to be a visual and audible annunciation that the swimming pool chemicals are out of whack. There is an LED display of the exact reading on the device. Thanks again.