Utilization of strain transducers to measure forces
Strain transducers are sensors that can be installed on an existing structure
. These transducers are based on a spring element
onto which a strain gauge full bridge
has been installed.
As shown in the picture, these strain transducers come with a silicone coating
(white area on the transducer) which, in addition to moisture protection offers some degree of mechanical protection
Strain transducers are based on the principle of strain transformers
. Strain in the area of the installed strain gauges is bigger than the strain value between the two screwed connections.
shows the SLB spring element
. Strain applied to the transducer is centered on the zone where the strain gauge has been installed. The reason: Here, a significantly reduced stiffness is used. The approximate excessive increase in strain can be calculated as follows:
|εSG ||Strain present under the strain gauge
|εObject ||Strain between the screwed connections
|lStrain sensor ||Distance between the screwed connections
|lStrain zone ||Length of the area of structural weakening
This view includes some idealizations.
It is assumed that the zone of strain application is strain-free. Of course, this is not true - strictly speaking. It becomes apparent that the strain transducer's sensitivity can be adjusted by means of the length ratio of strain zone and distance between the two screwed connections. In principle, a very high sensitivity can be attained, however, in practical use, an output signal of 1.5 mV/V at 500 µm/m has proved favorable. This results in a sensitivity increase of 230 % compared with a strain gauge full bridge as described above.
The temperature dilatation of components has been compensated for by taking appropriate circuitry measures.
In addition, strain transducers with integrated electronics
are available that can be calibrated in the application, therefore providing an extremely efficient measuring chain.
The sensors without electronics
have a high bridge resistance
of 700 Ω. This enables several strain transducers to be connected in parallel
without requiring an excessively high amplifier supply current.
This measure enables strain effects
that need to be suppressed
to be compensated for. For example, for monitoring press forces on a column the only proportion of strain that is relevant is the one resulting from tensile/compressive loading.
Let us assume that two strain transducers are connected in parallel and mounted on a column at the same height, opposite from each other. Under a bending load, one transducer will experience higher strain, the other one will be loaded with strain that is lower by the same amount. On the whole, only the proportion of strain resulting from tensile or compressive loading is measured. Bending is compensated for.
SLB strain transducers
from HBM can be mounted on structures using 4 M6 screws
. This only requires a plane surface, free from paint or other coatings. Upon screw-mounting of the strain transducer with the recommended torque, we recommend applying corrosion protection, for example ABM75 foil. Then, the sensor is immediately ready for use.