Totalizer program without pulse from totalizer

[zxced]

Hi,

So we have a flowmeter installed but doesnt have a feature to send a pulse to plc for it to compute the total volume. I want to somehow compute the total flow and will try an internal pulse, since there's no signal/pulse from the totalizer. Is there a way on how to do this? I know that it may not accurate and stable but I want to show them that it is possible but it will have a problem with the accuracy and stability.

Thanks

 

[Steve Bailey]

Do you get an analog signal (0 - 10 volts or 4 - 20 mA) from the flow meter? If so, what are the flow rates at minimum and maximum signal levels? You can capture the flow rate at some point in time, multiply it by the elapsed time since the last capture, and apply appropriate scale factors to calculate a volume. Add each incremental volume to a running total. The method implies that the flow rate has been constant over the time interval between captures. The more the flow varies, the less accurate the calculation.

 

[drbitboy]

If the PLC has the flow rate as an input, some PLCs have totalizer instructions, or you can construct your own using the general approach mentioned by @Steve Bailey.

The first part is minimizing roundoff errors. One strategy is to totalize in integers, using at least one 32-bit integer tag, or perhaps two, or a double-precision floating-point value. Single-precision 32-bit REALs are rarely good enough, but may work if the total is reset frequently.

The second part is choosing the sample period, but that may feed back into the roundoff errors.

But in the end it comes down to managing time.

I suspect if you searched for the keyword "totalize" on this forum you would find some ideas. I don't know what brand of PLC is in play here, or the character of the flowmeter values, or the character of the totalized values i.e. how often they will be reset, so recommending a detailed approach at this point would be premature.

 

[cardosocea]

I know that it may not accurate and stable but I want to show them that it is possible but it will have a problem with the accuracy and stability.

In my experience this is actually quite accurate and a good way to go about it without buying additional hardware or worrying that you're not sampling your input channel quick enough.



Which PLC are you using?