I am new to Programmable Logic Controllers (PLCs), so please excuse any errors in my terminology. Currently, we are facing an issue at my workplace involving a flow meter calibrated for 0 to 60 million gallons per day (MGD). To check if the signal is reaching our Supervisory Control and Data Acquisition (SCADA) system, we generated a 20 milliamp signal. However, upon applying the signal, the flow reading displayed 47.184 MGD instead of the expected 60 MGD. The analog value showing as 32,000 when the 20 mA was applied indicates a potential issue with how the information is being stored in the register. Can someone offer insight into what might be causing this discrepancy and how it can be resolved for accurate readings?
Are you utilizing a 4-20 mA signal or a 0-20 mA signal? What is your method for determining the MGD value? Are you performing calculations using integer arithmetic or floating point operations?
What is the range of raw values for your analog input, starting from 6400 and going up to 32000? Can you explain the logic behind how you scale these values?
Ken Moore inquired about the raw counts being 32000 and the range of the input raw value for the analog input, which may be between 6400-32000. He also questioned the scaling logic. It seems like the issue could be related to an old TI 505 PLC not applying the 20% offset for a 4-20mA signal. Regardless, a value of 32000 indicates full scale and would result in a reading of 60. It would be beneficial if the original poster could specify the PLC and analog module being used.
As a seasoned veteran in the field of industrial automation, I couldn't help but reminisce about the good old days when the numbers "32000" were mentioned. TI 505 PLCs are equipped with convenient built-in scaling capabilities, making it puzzling as to what might be causing any issues. While I personally do not advocate for this approach, I have observed some individuals opting to handle all scaling tasks within an HMI or SCADA system.
The TI scale function comes with a 20% offset and Bipolar selection options, allowing users to accurately determine if an input signal is within range. CTI acquired the rights to the TI architecture, which is still being utilized by customers today. Many users are gradually transitioning to Logix for enhanced performance and functionality.
It sounds like you're on the right track by checking the 20 mA signal, but there could be several factors at play here. One possibility is that the scaling in your SCADA system might not be set up correctly. If the system is interpreting the 32,000 count incorrectly due to a mismatch in scaling, it could lead to the unexpected flow reading. Make sure that the conversion from the analog input (20 mA) to the flow rate is properly configured in your SCADA settings—check the scaling factor and ensure it's aligned with the flow meter’s calibration. Another potential issue could be a problem with the wiring or the input card itself, so double-check those connections as well. It might also be helpful to consult the flow meter’s manual for specific calibration details or to test the meter independently to rule out any hardware faults.
It sounds like you're dealing with a classic scaling issue! The discrepancy between your expected flow rate and the actual reading could stem from how the flow meter's output is scaled in your PLC or SCADA system. Since the flow meter outputs a 4-20 mA signal, you'll want to ensure that the scaling in the PLC is set correctly to interpret that signal for the full range of 0 to 60 MGD. It might be worth checking the configuration settings in the PLC programming to make sure the input channel is properly calibrated to match the flow meter's specifications. Additionally, confirm that there are no other factors at play, such as signal noise or issues with the feedback loop. Debugging those settings could certainly help bring your readings into alignment!
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Answer: The discrepancy could be caused by issues in how the analog signal is being stored in the register or potential calibration errors in the flow meter.
Answer: The discrepancy in the displayed flow rate could be due to inaccuracies in signal processing, calibration settings, or errors in how the information is being transmitted to the SCADA system.
Answer: Resolving the issue may involve recalibrating the flow meter, adjusting the settings in the PLC to handle higher flow rates accurately, or investigating potential communication errors between the flow meter and the SCADA system.
Answer: Troubleshooting steps may include verifying calibration settings, checking for signal processing errors, inspecting the communication protocols between the flow meter and the SCADA system, and potentially adjusting the scaling of analog signals to match the expected flow rates accurately
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