Beckhoff EP3356-0022, Документация

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Basic principles of strain gauge technology
EP3356-0022 12 Version: 1.4
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. 3: 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.
Measuring procedure
The Beckhoff EL/KL335x Terminals and the EP3356 Box support only the constant excitation
• 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
(or U
Supply
) and the resulting bridge
voltage U
D
(or U
Bridge
), 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. Due to the division of U
D
and U
V
the measurement is in
principle independent of changes in the supply voltage.
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-conductor vs. 6-conductor connection
If supplied with a constant voltage of 5 to 12 V a not insignificant current flows of 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
).
If, for example, a 25 m copper cable (feed + return = 50 m) with a cross section q of 0.25 mm² is used, this
results in a line resistance of
RL = l/ (κ * q) = 50 m / (58 S*m/mm² * 0.25 mm²) = 3.5 Ω