To define the accuracy of a multidimensional sensor, we first need to distinguish between two important terms:
- the sensor accuracy and
- the crosstalk.
The term "accuracy class", which appears in all data sheets and product descriptions, refers to the sensor accuracy.
The document Sensors::Accuracy provides a detailed explanation of this term and a summary of the most important criteria for the accuracy class:
For force and torque sensors, the following properties are used to classify them into an accuracy class:
- relative standard measurement uncertainty
- relative linearity deviation and hysteresis
- temperature-related drift of the zero signal
- temperature-related drift of the slope of the characteristic curve
With multi-axis and multi-component sensors, another effect occurs, the so-called crosstalk. Here is an explanation from the document Multi-component sensors:
Crosstalk
The introduction of a force or a moment in a measuring axis also results in a reading in the axes perpendicular to it. This effect is called crosstalk.
Bei Drei-Achsen Kraftsensoren und bei Mehrkomponenten Sensoren beträgt das Übersprechen bei Einwirkung der Nennlast ca. 1% der Nennlast der übrigen Achsen.
The crosstalk is proportional to the level of the load. With increasing lever arms or larger moments, the deformation of the sensor and the crosstalk increase.
Calibration takes place in the plane of the front surface of the sensors. The origin of the multi-axis force sensor is also located in this plane. In actual use, the force application point (operating point) is usually not located at the origin of the multi-axis force sensor. This creates additional moments.
When selecting the measuring range, these additional moments must be taken into account to avoid overloading the sensor.
If the moments around the force application point are to be calculated, an additional calculation or alternatively a transformation of the calibration matrix is necessary. The calculation is described in the instructions ba-k6d.pdf. The GSVmulti software allows the distance of the force application point from the coordinate origin to be entered.
The crosstalk at the respective operating point can be minimized by calibrating at the operating point. The additional moments are then taken into account during the calibration at the operating point and included in the error compensation.
Methods of calibration
Calibration for special load vectors
With the "Matrix Plus" option, the crosstalk at this operating point can be reduced to 0.2% to 0.5%. The Matrix Plus option includes
- a quadratic approach to error compensation and/or
- a calibration at the operating point of the intended application of the sensor and/or
- a calibration with the load vector of the planned application at the origin of the sensor.
The quadratic approach to error compensation minimizes crosstalk exactly at the operating point. At higher loads, the quadratic approach can lead to larger errors than error compensation with a linear approach.
Calibration with the load vector of the intended application also minimizes crosstalk for that load vector.
The selection of the respective procedure depends on the availability of calibration accessories and the individual requirements of the application.
Universal Calibration
For applications where the load vector is unknown, calibration with 100% of the nominal load is recommended. To minimize crosstalk, a quadratic approach "Matrix Plus" can also be used with a mathematical optimization for the load cases 20%, 40%, 60% and 80% of the nominal load.
The crosstalk value is specified separately from the accuracy class in each data sheet of multi-axis and multi-component sensors.
You can order "Matrix Plus" directly here.
1-axis force sensors
Transverse forces and moments can also be introduced when using 1-axis force sensors. This also leads to a measurement value being displayed as a result of crosstalk. With a constant transverse force or constant moment, this leads to a shift in the zero point. This can be adjusted using the zero setting function or can be taken into account mathematically as an offset.
Summary
Accuracy and crosstalk are shown separately for 6-axis (force/torque) sensors. The accuracy class is typically given as 0.2% and the crosstalk as 1%. As a rule, the temperature-related drift of 6-axis sensors leads to a reduction in the accuracy class to 0.2%. Reproducibility (standard measurement uncertainty) and linearity are on the order of magnitude of 1-axis force sensors with accuracy class 0.1%.
By using special calibration procedures, crosstalk can be minimized to a magnitude of 0.2%...0.5%.
