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Summing cards
Typically located
where all cells cables will reach providing a termination point between
the analog load cells and
the instrument.
Most applications use a 4 channel summing card.
Load cell
terminations:
(+/-) Excitation, (+/-)
Signal and shield.
Instrument
Connection:
(+/-) Excitation, (+/-) Sense,
(+/-) Signal and shield.
The instrument +/- sense compensates for
distance between cells and Instrument.
There are 2 main types of cards, SIGNAL or EXCITATION trim.
Excitation trim
The + excitation from
the instrument goes through a trim pot to each load cell
terminal.
By adjusting the DC
supply to each cell its output is also increased or decreased.
Cells are matched by
placing the same weight at each cell and adjusting the pots.
Excitation trim can
compensate for cells that
vary
as much as 10% in output.
The +/- signals
are connected on a common buss so each cell must be disconnected to be able to measure
its output when testing or balancing the dead load.
Your
scale must be shut down as this procedure will affect your process.
Excitation cards are less expensive but more time is needed to
test each cell for problems.
Time is money when calling in
service people to check your scale.
Signal trim
Isolation resistors
separate each cell from the common instrument signal.
A Pot is placed
across the load cell signal side of the terminations to adjust
each cell.
The adjustment range
to trim unmatched load cells is limited.
The isolation
resistors do not stop the
cells from affecting each other but do separate the individual
cells enough to measure in circuit without interrupting the
process.
This is a valuable asset when
finding a cell
problem or when balancing load distribution.
For
factory precision load cells the pots may be removed or
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Industrial Weighing
Systems
