HBK U9C Montageanleitung Seite 13

5 DESIGN AND MODE OF OPERATION
5.1 Transducer
The measuring body is a steel loaded member on which strain gages (SG) are installed.
The influence of a force deforms the measuring body, so there is deformation in places
where the strain gages are installed. The SG are attached so that two are stretched and
two are compressed when a force is applied. The strain gages are wired to form a
Wheatstone bridge circuit. They change their ohmic resistance in proportion to their
change in length and so unbalance the Wheatstone bridge. If there is an excitation
voltage, the circuit produces an output signal proportional to the change in resistance and
thus also proportional to the applied force. The strain gage arrangement is chosen to
compensate, as much as possible, for parasitic forces and moments (e.g. lateral forces
and eccentricity influences), as well as the effects of temperature.
5.2 Strain gage covering agent
To protect the SG, the force transducers have thin cover plates that are welded on the
bottom and, in versions with a nominal (rated) force of up to 200 N, on the top. This
method offers very good protection against environmental conditions so that the U9C
reaches the protection class IP67. In order to retain the protective effect, these plates
must not be removed or damaged in any way.
Fig. 5.1 The thin plate on the top side must not be damaged in the variants with
nominal (rated) forces 50 N, 100 N and 200 N; the same applies for the plate on
the bottom of all force transducers.
5.3 Permanently connected amplifier module option
The sensors can optionally be ordered with an inline amplifier. This amplifier module
supplies the bridge circuit of the sensors with a suitable supply voltage, and converts the
small output signal of the force transducers with low noise into a 0 ... 10 V voltage signal
(VA1) or a 4 ... 20 mA current signal (VA2). The delivery is then carried out as a measure­
ment chain, and the test record describes the correlation between the force input quantity
and the output signal in V or mA.
U9C
DESIGN AND MODE OF OPERATION
11