Load Cell Scaling

To have any useful meaning, the mV/V signal from a load cell input must be scaled to force units. The Force Scale, Force Offset, Millivolts/Volt Offset, and Negative Correction Factor parameters are used to define force units as a function of the load cell mV/V signal. This topic describes how to manually calculate these parameters for a load cell input.

Delta Recommends using the Scale/Offset Wizard for scaling the load cell force. If you need to do it manually, read this topic.

Scaling Load Cell Force

The RMC calculates the Actual Force every control loop using the following formula:

If (Millivolts/Volt + Millivolts/Volt Offset) < 0 Then

Force = (Millivolts/Volt + Millivolts/Volt Offset) x Negative Correction Factor x Force Scale + Force Offset

Else

Force = (Millivolts/Volt + Millivolts/Volt Offset) x Force Scale + Force Offset

EndIf

To correctly scale the axis for your particular application, use one of the methods below to find the scaling parameters.

Method 1: Use Transducer Specifications

This method uses the calibration data for the load cell.

Unidirectional Load Cells (Tension-Only or Compression-Only)

From the transducer data sheet:

Find the mV/V output of your load cell at the rated load. Call these Max mV/V and Max Force.
Find the mV/V value at zero force. This may be called Zero Balance.

Calculate the Force Scale with the following formula:

Force Scale = Max Force / (Max mV/V - Zero Balance)

Enter the negative of the Zero Balance value in the Millivolts/Volts Offset parameter.

Bidirectional Load Cells (Tension and Compression)

From the transducer data sheet:

Find the mV/V output of your load cell at the rated positive load. Call these Max Positive mV/V and Max Positive Force.
Find the mV/V output of your load cell at the negative rated load. Call these Max Negative mV/V and Max Negative Force.
Find the mV/V value at zero force. This may be called Zero Balance.

Calculate the Force Scale with the following formula:

Force Scale = Max Positive Force / (Max Positive mV/V - Zero Balance)

Calculate the Negative Correction Factor with the following formula:

Negative Correction Factor = (Max Negative Force / (Max Negative mV/V - Zero Balance)) / (Max Positive Force / (Max Positive mV/V - Zero Balance))

Enter the negative of the Zero Balance value in the Millivolts/Volts Offset parameter.

Method 2: Force / Millivolts Per Volt

The accuracy of this method depends on how accurately you can measure the force of the axis with some external device such as a reference load cell or some known mass.

Unidirectional Load Cells (Tension-Only or Compression-Only)

For two different force values (Force0 and Force1), apply a known force, and record the mV/V values (mVV0 and mVV1) at those forces.
Calculate the Force Scale and Force Offset with the following formula:

Force Scale = (Force0 - Force1) / (mVV0 - mVV1)

Force Offset = Force0 - Force Scale x mVV0

Bidirectional Load Cells (Tension and Compression)

For three different force values, apply a known force. The force values should be a large positive value (ForcePos), a large negative value (ForceNeg), and one close to zero (ForceZero). For each force value, record the mV/V values at those forces (mVVPos, mVVNeg, and mVVZero).
Calculate the Force Scale, Force Offset, and Negative Correction Factor with the following formulas:

Force Scale = (ForcePos - ForceZero) / (mVVPos - mVVZero)

Force Offset = ForcePos - Force Scale x mVVPos

Negative Correction Factor = (ForceNeg / mVVNeg ) / (ForcePos / mVVPos )

A more accurate negative correction factor would be obtained from measuring force at four points, two positive and two negative. Generate two scale values from these and use the ratio of the scales as the correction factor.