Crosstalk in Multicomponent Sensors Crosstalk in Multicomponent Sensors | HBM

How It Comes About

The need for a single sensor to measure forces in all three spatial directions or simultaneously measure torque and force can arise in a wide range of applications - be it in experiments, robotics, assembly or production. HBK’s tried and tested MCS10 multi-component sensor has been a reliable tool for such measurement tasks for many years.

The operating principle of a multi-axis transducer is based on an important property that all strain gauge sensors have in common: reliance on the Wheatstone bridge circuit.

How do strain gauge-based sensors work?

A strain gauge is a sensor element that converts a strain – i.e. an elongation or a compression – into a change in resistance. In practice, at least four such elements are connected to form a Wheatstone bridge circuit. When a force or a bending moment or torque to be measured is applied, two of the strain gauges in the bridge circuit are elongated, while the other two are compressed. If this circuit is supplied with an excitation voltage, the output is a well-measurable voltage signal. Due to the linearity of all correlations, highly precise sensors can be built based on strain gauges.

Whether the strain gauges experience positive or negative strain depends on their positions in the Wheatstone bridge circuit: at two points in the circuit, a positive strain results in a positive output voltage, at the two others, a negative strain leads to a positive output signal.

Since the two strain gauges experience a positive strain when increasing the output signal in response to tension and a negative strain when increasing the output signal in response to compression, a sensor can ensure responding with an output signal to only one type of mechanical stress if the strain gauges are cleverly arranged.

An example is given below:

Fig. 1: Bending beam with four installed strain gauges. The strain gauges on the bottom experience positive strain in response to compression and the ones on top in response to tension.

Fig. 2: Bending of the sensor: The strain gauges on the bottom become shorter and the ones on top turn longer. A measuring signal is generated due to the different effects.

Fig. 3: Application of a tensile force: All the strain gauges experience the same strain in terms of sign and amount. The output signal is zero.

If the strain gauges are installed on the beam as shown, the ones on top become longer and the bottom ones become shorter when the beam experiences bending. If the circuit is designed so that the strain gauges on the bottom experience a negative and the ones on top a positive strain, the bending results in a signal being measured (see Fig. 2). If, on the other hand, a tensile or a compressive force is applied to the beam, all four strain gauges become longer or shorter and the output signal is zero. We, thus, obtain a transducer that only responds to bending moments (see Fig. 3).

Unlike force transducers or torque sensors that measure only one component, a multi-component sensor has up to six measuring bridges, the strain gauges of wich are arranged in such to ensure that the output signal represents only one moment or the force in one of the three spatial directions.

An overview of the layout and wiring and the positioning of the strain gauges for different load cases can be found in the article on the Wheatstone bridge circuit.

Crosstalk: How does it come about?

In the example above, a small signal will most likely be measured even if a tensile force is applied to the transducer because zero will only be measured when all four strain gauges experience exactly the same strain and sensitivity. In practice, the measuring bodies have tolerances and the strain gauges differ slightly in their sensitivity as well.

For this reason, every multi-component sensor has so-called crosstalk. Even if the force or a bending moment is only applied in one direction, a very small measuring signal can be detected on the other channels.
By carefully choosing the geometry of the spring element, the crosstalk can be reduced to a minimum.