Beckhoff ELX3351, Operating Manual

Page: 31 / 71

Basic function principles
ELX3351 31 Version: 1.3.1
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 to one measuring transducer, e.g. the ELX3351. 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 is not overloaded.
• For parallel connection, the minimum permissible connection resistance to the ELX3351 must be
observed (bridge input resistance according to chapter Technical data [
}
12] ). To ensure explosion
protection, the parallel connection of the individual bridge resistors must not fall below the minimum
value of 300 Ω. The individual strain gage bridges do not require safe isolation in accordance with the
requirements of IEC 60079-11 and must be regarded as an intrinsically safe circuit (e.g. for the design
of connection cables in accordance with IEC 60079-25).
Fig. 24: Parallel strain gauge
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
n
of the noise can be calculated by
e
n
= √4kTRB.
In the case of a load cell with R
0
= 350 Ω at an ambient temperature T = 20°C (= 293K) and a bandwidth of
the measuring transducer of 50 Hz (and Boltzmann constant k = 1.38 * 10
-23
J/K), the rms e
n
= 16.8 nV. The
peak-peak noise e
pp
is thus approx. e
pp
~ 6.6* e
n
= 111 nV.
Example
In relation to the maximum output voltage U
out_max
of a bridge with 2 mV/V and U
s
= 5 V, this corresponds to
U
out_max
= 5 V * 2 mV/V = 10 mV. (For the nominal load) this results in a maximum resolution of
10 mV / 111 nV = 90090 digits. Converted into bit resolution: ln(90090)/ln(2) ~ 16 bits.