Fema I4 Series, User Manual

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F E MA · MANUFACTURING FOR INDUSTRIAL AUTOMATION 4
The instrument can be configured to read signals from load cells which
are externally powered up to 10 Vdc, and not use the power provided by
the instrument.
Configure the instrument to read in ‘ load cell ’ mode and set the excitation
voltage parameter to ‘ off ’. Connect the ‘ sense ’ wires to the excitation
voltage terminals of the load cell. With the ‘ sense ’ wires, the instrument
will compensate for variations of the power supply.
With this configuration, values indicated in mV’ units (see section 10.1),
are scaled to a theoretical power value of 5 Vdc, therefor values may not
be directly interpretable.
The instrument accepts up to 4 standard 350 Ohms load cells. The
instrument provides 5 Vdc excitation voltage. For load cells with different
impedance, calculate the current consumption for each cell, and the
total must not exceed the maximum current the instrument can provide
(see section 11).
In case of problems with the signal provided by the load cell, the
instrument provides information for troubleshooting purposes. Configure
the ‘ messages ’ function (see section 13.8) to access the actual values
for the input signal (expressed in mV), the excitation voltage measured
at the ‘ sense ’ terminals (expressed in Vdc) and the current provided to
the cell (expressed in mA). The operator can use these values to identify
the cause of the problem. See section 6 for more information on how to
access these values in real time.
The instrument is designed to measure load cell signals. The instrument
provides 5 Vdc excitation voltage to power the load cell, and reads the
millivolt signal generated by the load cell. The instrument also reads the
actual excitation voltage connected to the load cell, and compensates
the read signal for changes at the excitation voltage.
The actual value of the excitation voltage is detected by using the
‘ sense ’ wires. Connect the ‘ sense + ’ and ‘ sense - ’ (terminals 5 and 2) to
the load cell, to provide the instrument with an accurate value of the
excitation voltage received by the cell. Deviations and errors from the
standard excitation value (5 Vdc) are automatically compensated by the
instrument, increasing the accuracy and reliability of the measure.
If you can not connect the ‘
sense ’ wires to the load cell, place a
shortcircuit between terminals ‘ sense + ’ and ‘ Vexc + ’ (terminals 5 and 4),
and between terminals ‘ sense - ’ and ‘ Vexc - ’ (terminals 2 and 1).
For applications with multiple load cells (2, 3 or 4 cells) connect the
‘ sense ’ wires to the ‘ electrical middle point ’ of the power wires of all the
cells (see section 7.8).
The ‘ sense ’ terminals must be always connected. If you do not
use the ‘ sense ’ wires, shortcircuit with ‘ Vexc ’ terminals
Table 2 | T ypical load cell connection
Vexc +
signal -
sense -
Vexc -
sense +
1 2 3 4 5 6 I4L terminals
The instrument can be configured to measure millivolts in differential
mode. Activating any millivolt measurement mode, disables de
excitation voltage and disables the ‘ sense ’ compensation for changes
at the excitation voltage. The instrument works as a pure differential
millivolt signal converter.
7. Practical load cell information
Table 3 | Millivolt mode connection
mV - mV +
1 2 3 4 5 6 I4L terminals
7.1 Number and type of cells accepted
7.2 Load cell and ‘ sense ’ wires
7.3 Millivolt mode
7.4 Load cell with external power
Measuring with load cells requires an electrically clean installation.
When connecting the ground to the cell system, assure that the load
cell connection to ground is performed in such a way that the current to
ground does not flow through the cell.
7.5 Connecting the cell to the ground
Table 4 | Load cell connection with external power
signal -
sense -
sense +
1 2 3 4 5 6
Vexc -
Vexc +
I4L terminals