Programming problem for indication of alarm

TheoPap

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Join Date
Jun 2016
Location
Thessaloniki
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Hi guys, I am new with PLC programming so I want your lights.. We suppose to install a flowmeter( with analog current output 4-20mA), after an on/off solenoid valve. Connecting the flowmeter to AI module I will get indication in SCADA for the flow. My question is that I want a Set Point for the flow, below which I will get 1) an indication of alarm in SCADA , 2) an output signal for the alarm to activate a signaling device and 3) 1 and 2 only when the valve is open.We use Siemens PLC. Any relevant information or basic steps for how to do the programming part will be helpful. Thank you in advance
 
In this situation, I would make the alarm setpoints tags that can be edited in the SCADA and put the alarm logic in the PLC.

I normally have a time delay before triggering the alarm bit, and then seal in the alarm to hold it true once the time delay has been satisfied and release the seal in branch when the alarm acknowledge button on the SCADA has been pressed. The timer prevents nuisance alarms in case the analog signal bounces a little past the setpoint momentarily, but if you really want to trap and alarm that situation you can set these debounce timer presets to zero.

There would be two alarm bits that the SCADA monitors, one for high flow and one for low flow.

I don't know Siemens PLCs but I suspect you will need to be more specific about the exact hardware and software you are using for both PLC and SCADA to get someone to post some good examples.
 
Last edited:
The Siemens PLCs I use (300 and 400 series) the analog inputs and outputs are scaled from 0 to 27648 bits and are stored as an integer value. We convert from a word to a real with the logic in the attached image. The type of number you are using matters in the Siemens world when doing compare logic. You must compare two real values, you cannot compare a real to a integer. Most likely the signal coming from the SCADA will be real unless you make it an integer for some reason.

My recommendation would be to convert the incoming 0-27648 to a real value and then scale that number to engineer units that make sense for the flow application. Use this scaled real number in the comparison block to whatever alarm limit you want.

A note about the Step-7 software when you're adding logic to a function call. If you highlight something and press the F1 key it will give you a good help file for whatever you have highlighted.

Siemens Word to Real.JPG
 

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