Evaluate the AutomationView desktop suite free for 30 days. No credit card required. Claim trial key →
person_outline shopping_cart
arrow_back Back to Calculators

Load Cell mV/V to Weight Scaling

Scale raw load cell millivolt outputs into precise kilogram or pound weights.

Load Cell Parameters

V
mV
0 mV 20 mV (Max)

Calculated Weight

50.00 kg
info
Disclaimer of Liability: This calculator is provided for educational and estimation purposes only. By using this tool, you agree to discharge AutomationView of any liability for direct, indirect, or consequential damages resulting from its use. It is your strict responsibility to independently verify all calculations, validate the results against official manufacturer documentation, and ensure compliance with all applicable safety and engineering standards before implementing any parameters in a production environment.

Understanding the Millivolt/Volt (mV/V) Output

A load cell is essentially a Wheatstone bridge composed of strain gauges. When an excitation voltage (typically 5VDC or 10VDC) is applied, the load cell produces an output signal proportional to the applied force. This output is measured in millivolts per volt of excitation (mV/V). For example, a 2 mV/V load cell excited by 10VDC will output a maximum of 20mV at its full rated capacity.

This low-level millivolt signal is highly susceptible to electromagnetic interference (EMI) and voltage drops over long cable runs. Furthermore, standard Programmable Logic Controller (PLC) analog input modules cannot directly read such minute signals with sufficient resolution or accuracy.

Signal Conditioning: The Necessity of a Load Cell Amplifier

To bridge the gap between the raw load cell output and the PLC, an intermediary signal conditioner, or load cell amplifier, is mandatory. The amplifier serves two primary functions:

  • Excitation Provider: It supplies a stable, regulated excitation voltage to the load cell bridge.
  • Signal Amplification and Conversion: It amplifies the weak mV signal and converts it into a robust, high-level industrial standard signal, most commonly 4-20 mA or 0-10 VDC.

For harsh industrial environments, a 4-20 mA current loop is overwhelmingly preferred due to its inherent immunity to electrical noise and its capability to traverse long distances without signal degradation.

PLC Analog Input Integration and A/D Conversion

Once the signal is converted to 4-20 mA, it is wired into the PLC’s analog input module. The module’s Analog-to-Digital (A/D) converter translates this continuous analog signal into a discrete integer value. For instance, a 16-bit Siemens S7-1200/1500 analog module scales a 4-20 mA signal to a raw integer range of 0 to 27648, whereas other platforms might use 0 to 32767 or 0 to 4095 depending on the converter’s bit resolution.

The Linear Scaling Mathematics

Because the relationship between the applied weight and the load cell’s output is highly linear, mapping the raw PLC integer to a physical engineering unit (e.g., kilograms or pounds) relies on the classic linear equation:

y = mx + b

  • y: The calculated physical weight.
  • x: The raw digital integer value from the A/D converter.
  • m: The slope of the line, defined as the ratio of the weight range to the signal range.
  • b: The offset or zero-intercept, representing the tare weight of the mechanical scale assembly when no product is present.

Field Calibration: Two-Point Method (Zero and Span)

Theoretical scaling based on datasheet values is insufficient for industrial accuracy due to mechanical dead loads, cable resistance, and amplifier tolerances. A physical two-point field calibration is required:

  1. Zero Calibration (Tare): Ensure the scale is completely empty. Record the raw A/D integer value in the PLC. Assign this raw value to correspond to 0.0 kg.
  2. Span Calibration: Place a precisely known test weight (the closer to the scale’s maximum capacity, the better) onto the load cell. Record the new raw A/D integer value. Assign this value to correspond to the test weight’s mass (e.g., 500.0 kg).

Implementing Scaling in PLC Logic

Modern PLCs offer dedicated function blocks to abstract the linear equation. In Siemens TIA Portal, this is typically handled by cascading a NORM_X (normalize) block into a SCALE_X (scale) block. In Rockwell Studio 5000, the SCL (Scale) instruction achieves the same result.

Engineers should also implement software low-pass filtering or moving average logic immediately after the A/D conversion to dampen mechanical vibrations and provide a stable weight reading to the HMI or control sequence.