Fig. 7 Parallel Connection Of Strain Gauges - Beckhoff EP3356-0022 Documentation

The used measuring range of the load cell must be at least 4 kg. The minimum application range can lie in
any range between E
min
structural reasons. A relationship between n
There are further important characteristic values, which are for the most part self-explanatory and need not
be discussed further here, such as nominal characteristic value tolerance, input/output resistance,
recommended supply voltage, nominal temperature range etc.
Parallel connection of strain gauges
It is usual to distribute a load mechanically to several strain gauge load cells at the same time. Hence, for
example, the 3-point bearing of a silo container on 3 load cells can be realized. Taking into account wind
loads and loading dynamics, the total loading of the silo including the dead weight of the container can thus
be measured. The mechanically parallel-connected load cells are usually also electrically connected in
parallel and connect at two arbitrarily M12 sockets of EP3356-0022 (see figure below). To this end the
following must be observed:
• the load cells must be matched to each other and approved by the manufacturer for this mode of
operation
• the impedance of the load cells must be such that the current feed capability of the transducer
electronics (max. 350 mA) is not overloaded.
Fig. 7: Parallel connection of strain gauges
Sources of error/disturbance variables
Inherent electrical noise of the load cell
Electrical conductors exhibit so-called thermal noise (thermal/Johnson noise), which is caused by irregular
temperature-dependent movements of the electrons in the conductor material. The resolution of the bridge
signal is already limited by this physical effect. The rms value e
√4kTRB
In the case of a load cell with R
the measuring transducer of 50 Hz (and Boltzmann constant k = 1.38 * 10
peak-peak noise e is thus approx. e
pp
Example:
In relation to the maximum output voltage U
U = 5 V * 2 mV/V = 10 mV. (For the nominal load) this results in a maximum resolution of
out-max
10 mV/67.3 nV = 148588 digits. Converted into bit resolution: ln(148588)/ln(2) = 17 bits. Interpretation: a
higher digital measuring resolution than 17 bits is thus inappropriate for such an analog signal in the first
EP3356-0022
and E , e.g. between 2 kg and 6 kg if a tare mass of 2 kg already exists for
max
and V
max
= 350 Ω at an ambient temperature T = 20 °C (= 293 K) and a bandwidth of
0
~ 4* e = 67.3 nV.
pp n
out-max
Version: 1.1.0
is thereby likewise apparent: 4000 * 1 g = 4 kg .
min
of the noise can be calculated by e
n
of a bridge with 2 mV/V and Us = 5 V, this corresponds to
Product overview
-23
J/K), the rms e = 16.8 nV. The
n
=
n
15