how to retain values of bits when PLC lose the power then power up again

[MaGoOoDy]

hi,

i have a lift station project which consists of 2 pumps working in Lead/lag every 7 days or duty switch cycle when one of the pumps have 200 working Hours more than the other one.

i have almost completed the project when i realized that how could i rely on my timers and counters when the PLC lost the power , how could be sure that the values/Accumulators retain their values when the PLC power up again, exactly i am using retentive timers, counters and some Math functions.

the second question, for calculating the 7 days duty cycle, i am just putting a retentive timer directly on the rung so if the PLC powered up it will keep timing regardless the mode of the HOA switch, is this a proper way to implement this 7 days duty cycle?
and i am wondering how others program the 7 days duty cycle ?



i am using AB 1100 PLC .

 

[Helliana]

Did you purchase the Realtime Clock module for your Micrologix1100? If you have the module, you can use the realtime clock to set your cycle instead of depending on timers. That way you can make the change happen at 8:00 AM every Monday morning instead of every 604800 seconds.

 

[daba]

hi,

i have a lift station project which consists of 2 pumps working in Lead/lag every 7 days or duty switch cycle when one of the pumps have 200 working Hours more than the other one.

i have almost completed the project when i realized that how could i rely on my timers and counters when the PLC lost the power , how could be sure that the values/Accumulators retain their values when the PLC power up again, exactly i am using retentive timers, counters and some Math functions.

the second question, for calculating the 7 days duty cycle, i am just putting a retentive timer directly on the rung so if the PLC powered up it will keep timing regardless the mode of the HOA switch, is this a proper way to implement this 7 days duty cycle?
and i am wondering how others program the 7 days duty cycle ?



i am using AB 1100 PLC .

I am thinking that you don't need to account for periods of time that the processor is off, since neither pump will be running anyway. On that basis, you probably want to changeover your pumps after 7 days of cumulative run time, not at a specific time each week. I'm also thinking you don't really need this to be that accurate, a few seconds, or even minutes, either way isn't going to make much difference to the pumps !!

Without the real-time clock module, you can generate a pulse every 168 hours (7 days), using the attached logic.

The timer will never time-out, so the internal storage of elapsed time will take care of any scan issues.

Another thing to consider when "balancing" the run-times of pumps is that you increase the chance of them requiring some maintenance, servicing, or repair at the same time. Whenever I've done this sort of thing, I have introduced an offset in the run-hours, by running one pump continually for xxxx hours, then kick-in the "balancing" software. In a perfect world, let's say the bearings on the pumps fail at 10,000 hours, you don't want both pumps failing at the same time.