Calibration of "K6D" force/torque sensors

Calibrating a "K6D" force/torque sensor under partial load demonstrates the sensor's accuracy in its actual application range.

Calibration with a maximum force of only 10% of the nominal force is entirely possible and sensible. It may also be advisable to perform a calibration with a maximum of only 10% of the nominal torque.

Since an "SL/4" calibration is performed with 4 load levels, for example, (0%), 10%, 50%, and 100% of the maximum calibration load, the load level "10%" of the maximum calibration load also corresponds to only 1% of the sensor's nominal load.

Partial load calibration results

Advantages of partial load calibration

When calibrating the force/torque sensor at a load level of 1% of its rated load, the sensor's deviation is not necessarily 100 times higher than when calibrated at rated loads.

The results of calibration under partial load can even be better than calibration under rated load.

The reasons for this finding are:

• the extremely high resolution of the GSV-8 measuring amplifier
• the excellent linearity of the K6D sensors

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

Crosstalk also depends on the load amplitude. Applying partial loads also reduces crosstalk.

By applying the linear compensation calculation to a smaller section of the sensor characteristic curve, deviations and crosstalk are also reduced: Any nonlinear characteristic curve can be better approximated using multiple linear segments than would be possible with a compensation 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 greater impact relative to partial load than relative to nominal load.

Accordingly, the measurement should be short, 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.

Exmple 1: K6D40 50N/5Nm

• Nominal Load: [50 N, 50 N, 200 N, 5 Nm, 5 Nm, 5 Nm]
• 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 of 200 N corresponds to 0.05% of the nominal load; calibration at the first load level of 10% thus 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 impact on the reproducibility of the results.

Evaluation of the results

Due to the high resolution of the GSV-8 measuring amplifier, calibration is just about feasible with a calibration level of 1/20,000 of the nominal load. The noise amplitude and the stability of the measured values ​​(temperature-related drift) already have a significant impact on the result. The measurement uncertainty for the Fz component is 10% of the partial load, and for the Fx component, approximately 7% of the partial load.

For measuring the smallest forces and moments, the F6D45 sensor, for example, should be selected. Overload protection is provided by fixed stops. F6D45 20N/1Nm

Example 2: K6D80 500N/20Nm

• Nominal Load: [500 N, 500 N, 2000 N, 20 Nm, 20 Nm, 20 Nm]
• 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 nominal 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 nominal load, the measurement uncertainty remains below 1% of the partial load.