# Selection of force/torque sensors

The first step in selecting the appropriate force-torque sensor is based on the measuring ranges for the forces and torques in the three coordinate axes.

The other boundary conditions are geometry, installation conditions, accuracy, weight, rigidity.

This article is intended to help you make the right decision when choosing a sensor. Our sales staff will be happy to help you with the selection based on the respective operating conditions for the sensor.

## Selection of the measuring range

First and foremost, the measuring range is decisive for the selection: The force / torque sensor (6D sensor) must not be used above its so-called operating forces and torques. Above these forces or torques, the sensor can be destroyed by plastic deformation. The measurement signals are then outside the range that can still be evaluated by the measurement electronics.

A special feature of force/torque sensors, unlike 1D force or torque sensors, is that the operating forces and torques are often not in the order of 150% to 200% of the measuring range, but usually up to 300% of the measuring range.

The document provides a very good overview of the operating forces and torques k6d-comparisontable-de-en.pdf

With 6D sensors, there is usually no need to plan for additional safety against overload. On the contrary, a deliberate exceedance of individual force or torque components can be taken into account.

In addition to the operating forces and torques, the resolutions are also indicated. The resolution here is understood to be the noise amplitude at 10Hz measuring frequency.

## Consideration of the operating point

The forces are usually applied at a certain distance from the front face of the sensor (from the "origin of the force sensor"). This distance (usually in the positive z-direction) can be e.g. 50 mm to 1000 mm. If a force of e.g. 1kN acts on the sensor at a distance of e.g. 100 mm from the end face, then a torque of 100 Nm must be taken into account in the selection.

## Selection of the sensor

Another selection criterion for force-torque sensors is the diameter of the sensor: The diameter should be as large as possible.

So if you have to choose between K6D27 and K6D40, then you should definitely choose the sensor model with the larger dimensions.

Accordingly, this rule can be applied to all other models K6D80, 110, 130, 150, 175, 225, 300.

The measuring ranges of the 6D sensor for the torques are essentially determined by the diameter of the 6D sensor. With the selection of a larger diameter, the crosstalk of torques can usually be reduced to the display of force signals.

## Consideration of the installation situation

The highest accuracy with a 6D sensor is achieved when the forces are applied in the area of the front surface of the sensor up to a distance of approx. 1x diameter from the front surface.

As the distance between the force application and the front surface increases, the pattern of the signals becomes "blurred", because there is always a superposition of forces with torques in the 6D sensor.

Particularly high demands are placed on the mounting of the sensor: Local deformations of the force application flanges inevitably lead to a measurement error. The thickness of the mating flanges must be chosen in such a way as to avoid local deformations as much as possible. On the page k6d-montag recommendations for the minimum thickness of the flange plates are given. If even one of the usually 6 fastening screws is not tightened or even not used, a measurement error occurs. In particular, local deformations of the flange plates can occur during the introduction of torques. Care must be taken to ensure that the flanges are as symmetrical as possible

If this is not possible, calibration may be necessary under the specific installation situation.

## Calibration at the operating point

From a distance of 1 x diameter, calibration "at the operating point" can be useful. In this case, the calculation of the calibration matrix is carried out in the specific application, i.e. with superimposed moments. Crosstalk in this case can be reduced. Calibration at the operating point either requires specially adapted devices, or it can be carried out on an automatic calibration machine, which can simultaneously introduce all forces and torques.

## Consideration of the load vector

The calibration matrix of the 6D sensor shows the relationship between the 6 (or 12) output signals of the sensor and the acting forces and torques. From approx. 100 to 300 different load combinations, the best possible calibration matrix is determined in an equalization calculation. The closer the load vectors during calibration correspond to the later use, the smaller the error becomes.

Often, individual forces or torques are only applied to a fraction of the nominal forces and torques of the 6D sensor in the later application. Calibration with e.g. 10% of the nominal load of the 6D sensor is easily possible.

Due to the high resolution / low noise amplitude of the GSV-8 amplifier, there is no reduction in accuracy due to the partial load.

However, the relative, temperature-induced drift of the sensor is related to the nominal loads of the sensor. If the drift is related to the partial load of e.g. 10%, the relative drift increases accordingly by a factor of 10.