There can be a few reasons for variance between the output of a load cell and the input reading on a PLC. Here's a breakdown of potential causes and solutions:

1. Signal Scaling Mismatch:

• Issue: The load cell's output voltage (often in millivolts) might not be directly compatible with the PLC's analog input range (e.g., 0-10V).
• Solution:

• Use a signal conditioner or amplifier to scale the load cell's output to match the PLC's input range. This ensures the PLC interprets the signal correctly.
• Many PLCs offer built-in scaling functions within their programming software. You can configure these to convert the raw input value to the desired engineering units (e.g., kilograms).

2. Noise on the Signal Line:

• Issue: Electrical noise from nearby cables or machinery can interfere with the weak load cell signal, causing fluctuations in the PLC reading.
• Solution:

• Use shielded cables specifically designed for analog signals to minimize noise pickup.
• Ground the load cell and PLC properly to provide a common reference point and reduce noise.
• Some PLCs offer filtering options within their analog input modules to further reduce noise.

3. Calibration Issues:

• Issue: The load cell or PLC might be miscalibrated, leading to inaccurate readings.
• Solution:

• Regularly calibrate the load cell according to the manufacturer's specifications. This ensures it maintains its accuracy over time.
• Consult the PLC manual for calibration procedures for its analog input modules. Some allow for calibration adjustments within the software.

4. Faulty Hardware:

• Issue: In rare cases, the load cell itself, the cabling, or the PLC's analog input module might be faulty.
• Solution:

• Perform continuity checks on the cables to ensure proper connection.
• If troubleshooting the above steps doesn't resolve the issue, consider replacing the suspected faulty component.