Fema M Series, Руководство пользователя, Страница: 27 / 36

FEMA ELECTRÓNICA . SERIES M . M60-LC
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1.16.4 Connections with 3 or 4 load cells
Using 3 load cells is the optimal way to distribute the
weight on a plane, although it is common to work with 4
load cells, in applications with tanks, hoppers and similar.
When working with multiple load cells, the optimal con
-
nection is the one that makes the wires of the cell con -
verge in the same central area, so that all the cells are at
the same ‘electrical distance’ from the meter.
Use the same type load cell (for example, load cells with
nominal load of 100 Kg and sensitivity of 2 mV/V) and
connect the wires to the central area as indicated below.
Configure the instrument as indicates in this manual, as -
suming that :
• the sensitivity of the system remains the same (2 mV/V)
• the nominal weight of the system is the addition of
the nominal weight of each cell (3 x 100 Kg = 300 Kg for 3
cells or 4 x 100 Kg = 400 Kg for 4 cells)
• the ‘sense’ wires are carried to the central zone to -
gether with the Vexc wires, but are not propagated to
each individual cell. If you do not want to use the ‘sense’
wires, see section
1.8.1
.
1 2 3 4 5 6 7
Vexc+
Signal-
Sense-
Vexc-
Sense+
Figure 11 - Connection example with 3 load cells.
1 2 3 4 5 6 7
Vexc+
Signal-
Sense-
Vexc-
Sense+
Figure 12 - Connection example with 4 load cells.
1.16 Practical cases (cont.)
1.16.5 Measuring mV at the laboratory
If you wish to configure the instrument to measure a mil -
livolt generated signal at the laboratory, there is a special
configuration to apply. Different from the load cell, the
millivolt generator is not a differential system, and does
not need excitation voltage.
• Connect the instrument to the millivolt meter
(see
Figure 13)
. Add 2 resistances of 10 KOhm to connect the
‘Vexc’ terminals to the common of the signal generator.
• Inside the ‘configuration menu’, select the ‘Vexc’ pa -
rameter to ‘Lab’
(see section 1.12.2)
. This value deac -
tivates the ratiometric measurement and activates the
direct millivolt measurement.
• At the additional parameters at the ‘theoretical con -
figuration of the load cell’
(see section 1.12.2)
assign
the desired reading and the mV/V parameter. Note that
values of 1 mV/V, 2 mV/V and 3 mV/V will activate a full
scale range of 10 mV, 20 mV and 30 mV respectively.
• Do not apply the ‘empirical configuration of the load
cell’
(see section 1.12.2) .
Applying this characterization
to the signal generator will only generate a reduction in
the measurement accuracy.
• Configure the ‘system zero’
(see section 1.12.2) .
This
step is needed for a correct mV measurement. Gener -
ate 0 mV and ‘assign the system zero’.
1 2 3 4 5 6 7
Vexc+
Signal-
Sense-
Vexc-
Sense+
mV Generator
10 KOhm
10 KOhm
Figure 13 - Connections for ‘laboratory’ mode
To reduce leak currents that can affect the labo -
ratory measurement:
1. if the millivolt generator is powered from the mains
network, use an isolator transformer to power the gen -
erator
2. if the instrument is powered from the mains net -
work, use a separate isolator transformer to power the
instrument
!