Basic function principles
ELX3351
27
Version: 1.3.1
Measurement of signals
The change in resistance of an individual strain gauge can be determined in principle by resistance
measurement (current/voltage measurement) using a 2/3/4-wire measurement technique.
Usually 1/2/4 strain gauges are arranged in a Wheatstone bridge (-> quarter/half/full bridge); the nominal
resistance/impedance R
0
of all strain gauges (and the auxiliary resistors used if necessary) is usually
equivalent to R1=R2=R3=R4=R
0
. Typical values in the non-loaded state are R
0
= 120 Ω, 350 Ω, 700 Ω and
1 kΩ.
The full bridge possesses the best characteristics such as linearity in the feeding of current/voltage, four
times the sensitivity of the quarter-bridge as well as systematic compensation of disturbing influences such
as temperature drift and creeping. In order to achieve high sensitivity, the 4 individual strain gauges are
arranged on the carrier in such a way that 2 are elongated and 2 are compressed in each case.
Fig. 20: quarter, half, and full bridge
The measuring bridges can be operated with constant current, constant voltage, or also with AC voltage
using the carrier frequency method.
Full bridge strain gauge at constant voltage (ratiometric measurement)
Since the relative resistance change ΔR is low in relation to the nominal resistance R
0
, a simplified equation
is given for the strain gauge in the Wheatstone bridge arrangement:
U
D
/U
V
= ¼ * (ΔR1-ΔR2+ΔR3-ΔR4)/R
0
.
ΔR usually has a positive sign in the case of elongation and a minus sign in the case of compression.
A suitable measuring instrument measures the bridge supply voltage U
V
and the resulting bridge voltage U
D
,
and forms the quotients from both voltages, i.e. the ratio. After further calculation and scaling the measured
value is output, e.g. in kg.
If the voltages U
V
and U
D
are measured simultaneously, i.e. at the same moment, and placed in relation to
each other, then this is referred to as a ratiometric measurement.
The advantage of this is that (with simultaneous measurement!) brief changes in the supply voltage (e.g.
EMC effects) or a generally inaccurate or unstable supply voltage likewise have no effect on the
measurement.
A change in U
V
by e.g. 1% creates the same percentage change in U
D
according to the above equation. Due
to the simultaneous measurement of U
D
and U
V
the error cancels itself out completely during the division.
4-wire vs. 6-wire connection
With a constant voltage supply, the magnitude of the current can be quite considerable, e.g. 12 V
/
350
Ω = 34.3 mA. This leads not only to dissipated heat, wherein the specification of the strain gauge employed
must not be exceeded, but possibly also to measuring errors in the case of inadequate wiring due to line
losses not being taken into account or compensated.
In principle a full bridge can be operated with a 4-conductor connection (2 conductors for the supply U
V
and 2
for the measurement of the bridge voltage U
D
).
