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23.06. - 25.06 2020 in Nürnberg.
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These sensors K6D and F6D are suitable for measuring forces and moments in the 3 directions of the coordinate system. They consist of a total of 6 strain gauges full bridges. The 6 measuring signals are provided via a 24-pin connector for further processing with a measuring amplifier.
To calculate the forces and moments from the 6 measuring signals, a calibration matrix is required. The calibration matrix establishes the relationship between the 6 measurement signals and the forces Fx, Fy, Fz, and the torques Mx, My, and Mz (manual k6d_en.pdf).
By using special calibration matrices (Matrix Plus), you can optimize accuracy and minimize crosstalk for a specific load case.
The application of the calibration matrix is e.g. in the measuring amplifier GSV-8DS, so that each measuring channel corresponds to one force or one torque via the USB interface or via the analogue outputs.
(Channel 1: Fx, Channel 2: Fy, Channel 3: Fy, Channel 4: Mx, Channel 5: My, Channel 6: Mz).
To calculate the forces and torques, all 6 channels must be evaluated. The vector with the 6 DMS signals is multiplied by a 6x6 matrix to obtain a vector (Fx, Fy, Fz, Mx, My, Mz).
The sensors K3D, K3R and K3A are suitable for measuring forces in the 3 directions of the coordinate system. They consist of a total of 3 strain gauges full bridges.
In contrast to the multi-component sensor, the signal of each channel already corresponds to a force Fx, Fy, Fz.
The separation of the axes is done by the construction and the arrangement of the strain gauges. An amplifier is needed to amplify the signals to voltages or to digitize the signals for e.g. the USB interface or the CAN bus.
A multi-component sensor replaces up to 6 single-axis sensors at the same time. A three-axis force sensor combines three uniaxial force sensors in one component.
The accuracy achieved with a three-axis force sensor or a six-component sensor can not be realized by combining 3 or 6 single-axis sensors: conventional force sensors are sensitive to the "skewed" introduction of forces: the result is a measurement error due to the introduction of a force across the actual measuring direction, which can often not be precisely quantified. When combining three force sensors in a series connection, a force sensor absorbs the load in three dimensions, although it is designed for only one load direction. In addition, the series connection of sensors also means a series connection (and thus a reduction) of spring stiffnesses. On the other hand, if one attempts to separate the individual axes by means of linear guides, errors of the order of magnitude of 10% and more are caused by the static friction in connection with the short measuring paths of force sensors.
Three-axis force sensors and multi-component sensors provide the highest accuracy and highest rigidity with a simple and compact design.
The introduction of a force or a torque in a measuring axis also results in the display in the axes perpendicular thereto. This effect is referred to as crosstalk.
In case of three-axis force sensors and multi-component sensors, the crosstalk when is approx. 1% of the rated load of the other axes.
Crosstalk is proportional to the amount of stress. With increasing lever arms or with larger moments, the deformation of the sensor and the crosstalk increase.
The calibration takes place in the level of the front face of the sensors.
In contrast to three-axis force sensors, in the case of multi-component sensors, the crosstalk in the respective operating point can be minimized by calibration at the operating point. By using a second calibration matrix (Matrix Plus), the crosstalk at this operating point can be reduced to 0.2% to 0.5%.
For multi-component sensors, the measuring ranges are in a fixed ratio. This is due to the measurement principle (Hexapod framework) and the geometry:
the cross-section of the rods in the multi-component sensor determines the mechanical stress at nominal force, the diameter of the sensor determines the nominal torque. The nominal force for Fz is usually two to three times the rated force for Fx and Fy. The reason for this is that in load case Fz all 6 bars of the Hexapod framework are loaded equally, while in Fx and Fy only three to four bars are loaded.
Many applications require that only one axis of the force / moment sensor is used to 50% to 100%, while the other axes of the sensor are only used to 10% or even only up to 1% of the measuring range. An example is shown in Figure 1.
In the example of Figure 1, the friction force Fy with 10N is only 1% of the compression force Fz with 1kN. The aim of the measurement is to reduce the frictional force Fy to e.g. 2N to determine exactly.
The maximum force Fy in the application should be resolved to 1N. Due to the calibration in the operating point and the application of the additional error compensation "Matrix Plus" for this operating point, this is possible.