Logix 500 Totalizer?

I have seen a few postings here and all give me pretty good ideas.. I can usually hammer these little problems out fairly quick but this time i am in pretty deep..... A project that was SUPPOSED to be due on Thanksgiving is now due LABOR DAY WEEKEND..... I am reading in a 4-20 signal 0-200gpm from a flow meter... I need to totalize this in my program so i can fire Chemical pumps at a given total flow... What would be the best way I can do this? Any examples would be appreciated.. Thanks

Think about it a little bit first... How does a totalizer work? Can you draw it up in a flow chart or write an explanation of it???

Now, plc's really don't totalize an analog signal very well. It's an extremely difficult application to do accurately. Basically, what you want to do is to periodically take a look at the flow rate, calculate the volume over the time period, then add it to a totalizer. See any problem with that??? (What happens if the flow rate changes during the period...??) So, you take more readings more often. How accurate do you need to be? How fast can you tolerate to make the calculations?

Obviously, we don't know what flow meter you're using, but there's an easier way -- depending on the meter. Alot of flow meters can also be configured to give a pulse/volume. This makes it very easy -- you just count pulses. I.e. -- you need 1500 gallons in your batch. The flow meter is configured to give 1 pulse / 5 gallons. You simply count 500 pulses. This tends to be more accurate that having the PLC totalize.

If you are not able to get the flow meter to pulse per unit as OZEE sugested then the next is to use his other idea and integrate. To save you some headaches look at using an STI (look in online help) to place the totalizer logic (remember keep the STI code SIMPLE).

Darren

See example below. The ladder in the example below is controlled by an STI (selectable timed interrupt) at one second intervals.

Note the rate in the example is a daily rate hence the divide by 86400, yours is minutes so divide by 60. The example also has a running hourly total and a running daily total. They are buffered hourly and daily to provide a previous hour and previous day total, and then reset.

You can then use compare type instructions to operate your pump.