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Asked:18 Oct 2025

LOGO! - Software: Pulse counter Movement Computation

Hi, I want to measure the flow from a meter with a pulsed output, I saw the solutions on the forum, but whenever the time base is fixed, it does not will not be possible to calculate the flow as a function of the time interval between two pulses? using a PLC OBA6? or 7?
It is unfortunate that the program does not contain a tachometer function, the problem would have been solved.

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Siemens Test Added an Answer on18 Oct 2025

Flow Measurement with Pulsed Output & Siemens PLCs (OBA6/7)

You are correct to identify the challenge with fixed time bases when calculating flow from pulse outputs. Directly relating pulse frequency to flow rate requires accounting for varying time intervals between pulses, which a fixed time base struggles to handle accurately.

While the OBA6/7 PLCs lack a dedicated "tachometer" function, you can achieve the desired flow calculation using these methods:

  • Pulse Counting & Time Stamping: Capture each pulse and record its timestamp. Calculate the interval between pulses. Flow is proportional to the inverse of this interval.
  • Interrupt Routines: Utilize PLC interrupt routines triggered by each pulse for precise time stamping.
  • Scaling & Linearization: Determine the pulses per unit volume from your meter's specifications. Combine this with the pulse interval data to calculate flow.

See this Siemens Forum discussion for related insights. Also, refer to your PLC's programming manual (available via SIEPortal) for interrupt handling and timer functionalities.

Consider using a fast timer with high resolution to accurately measure the pulse intervals.

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