Ladder logic flow integration

[Steve Bailey]

There have been many threads started about integrating the signal from an analog flow sensor to get a total volume along with comments (some by me) that it's not the most accurate or reliable method. And if the flow meter has an option to output a pulse at some increment, to use that signal as the more reliable.
So, I set out to see how well I could integrate a signal in ladder logic. I used a 25 year-old GE Fanuc 90-30 CPU351. I didn't bother with an analog signal. Instead I started with constant flow rate of 100 (let's say GPM, but it could represent anything that varies with time) and imposed a +/- 10 percent sinusoidal variation on that rate so that it varied between 90 and 110 at some frequency.
I've implemented three methods; integration over a time period with samples taken at a constant interval, integration over a number of samples taken at a constant interval, and integration over a fixed time period with samples taken every scan.
I've included some results along with the ladder logic in the attachment. If anyone cares to look it over, I've got a pretty thick hide, so comment away. If you've got any suggestions for other approaches or other tests you'd like to see, I'm open to suggestions.

 

[Peter Nachtwey]

I like the simulations and testing different methods. I would simply count/accumulate pulses over time if counts are provided. I would use the real time clock for measuring the time period. The only assumption is that the flow sensors can respond accurately to higher frequency changes inflow. Another issue is often do you need the total volume?

The analog only problem requires short intervals between the samples. Otherwise, there could be a short burst of flow that could be missed between samples. I would use a 10 ms timed interrupt.

Something I would look at is the bandwidth of the analog flow sensor and the analog input card. I would be tempted to put a scope in the analog output of the flow sensor to see how much noise there is or how the data is filtered.

 

[drbitboy]

So, I set out to see how well I could integrate a signal in ladder logic. ...

Nice work!

 

[IO_Rack]

Very nice work Mr. Bailey! ...and noted!

As you mentioned, the best accuracy will come from a pulse output. I've did a job in the fuel industry where they used a highly sophisticated mass flow meter that compensated for temperature and pressure. (Just to mention other factors)

As for the analog, I share Peter's concerns about noise. I've found the more accurate you measure, the more you factor in, including....

Wouldn't the scan-based sampling be much faster than the analog conversion time?

There are a lot of great points to be noted from your experiments. Thanks for sharing!

 

[Steve Bailey]

Wouldn't the scan-based sampling be much faster than the analog conversion time?

That will depend on the analog module and the scan time. In the 90-30 the spec for an analog module was 2.5 mS conversion for all channels. My application was running longer scan times, so no problem. With a newer, faster model PLC it could have been an issue.