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Calibration of 6A Force/Torque Sensors

Calibrating a 6A force/torque sensor under partial load demonstrates the accuracy of the sensor in its actual operating range.

Calibration with a maximum force of only 10% of the rated force is perfectly possible and advisable. Likewise, it may be recommended to perform calibration with a maximum of only 10% of the rated torque.

Since a calibration “SL/4” with 4 load levels is performed, for example with (0%), 10%, 50%, 100% of the maximum calibration load, the load level “10%” of the maximum calibration load corresponds to only 1% of the nominal load of the sensor.

Results of the Calibration with Partial Load

Advantages of Partial Load Calibration

When calibrating with a load level of 1% of the rated load of the force/torque sensor, the deviation of the sensor is not necessarily 100 times greater compared to a calibration with rated loads.

The result of calibration under partial load can even be better than calibration under nominal load.

The reasons for this finding are:

Due to the high resolution, the noise amplitude only minimally affects the reproducibility of the results.

Crosstalk is also dependent on the amplitude of the load. Applying partial loads reduces crosstalk.

By applying linear adjustment to a smaller section of the sensor characteristic curve, deviations and crosstalk are also reduced. Every non-linear characteristic curve can be better approximated by several linear segments than would be possible with an adjustment calculation for the entire characteristic curve up to the rated load.

Disadvantages of Partial Load Calibration

The temperature-related drift of the zero signal has a correspondingly greater effect with regard to the partial load than would be the case with a relative reference to the nominal load.

Accordingly, the measurement should be of short duration, e.g., a few minutes. The zero signal should be checked at the beginning and end of the measurement to estimate the influence of zero-signal drift.

Example 1: 6A40 50N/5Nm

  • Rated load: [50N, 50N, 200N, 5Nm, 5Nm, 5Nm]
  • Partial load [0.15 N, 0.15 N, 0.1 N, 0.5 Nm, 0.5 Nm, 0.2 Nm]

The sensor was calibrated at 10%, 50%, and 100% of the partial load. The images show an excerpt from the calibration certificate. The partial load of 0.1 N out of 200 N corresponds to 0.05% of the nominal load; the calibration at the first load stage of 10% therefore corresponds to 0.005% of the nominal load, or one twenty-thousandth of the nominal load. In this case, the noise amplitude of the measurement signal already has a significant influence on the reproducibility of the results.

Evaluation of the Results

Due to the high resolution of the BX8 measuring amplifier, calibration with a calibration step of 1/20000 of the nominal load is just barely feasible. The noise amplitude and the stability of the measured values (temperature-related drift) already have a significant influence on the result. The measurement uncertainty for component Fz is 10% of the partial load and for component Fx approximately 7% of the partial load.

For measuring very small forces and moments, the 6ADF45 sensor should be selected, for example. Overload protection is achieved here with fixed stops. 6ADF45 6-Axis DIN Flange-Type Load Cells

Example 2: 6A80 500N/20Nm

  • Rated load: [500N, 500N, 2000N, 20Nm, 20Nm, 20Nm]
  • Partial load: [100 N, 100 N, 50 N, 8 Nm, 7 Nm, 7 Nm]

Evaluation of the Results

When calibrating one or more load components at 20% of the rated load, the measurement uncertainty typically remains below 1% (of the partial load). Even for the load component Fz, which was calibrated at a partial load of 2.5% of the rated load, the measurement uncertainty remains below 1% of the partial load.