LINK Systems System 1100 Tonnage Monitor - Manual (Page 46)

THEORY OF OPERATION
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3-9
distinguished from the strains induced by machine loads and be
compensated if tonnage monitors are to provide accurate reading.
The temperature induced strains are sensed by tonnage monitor
strain links and will shift the "zero" signal (baseline) level from
which strains induced by loads are measured unless automatic
zeroing of the baseline is provided.
3.5.1 Rate of Change Automatic Zeroing
Two alternative methods of automatic zeroing are provided with
System 1100 Tonnage Monitors. The first method distinguishes
between thermal and load induced signals by the rate of change of
sensed strain. Temperature induced strains occur extremely slowly,
while load induced strains occur extremely rapidly (in a few
milliseconds). The slow changes are "zeroed out" of the system by
a special circuit. Load signals reach a threshold level before the
special zero circuit can react and turn off the zero circuit for
the relatively brief time that the signal is above the threshold
level.
After the primary load signal falls below the threshold level, an
internal timer delays turn on of the automatic zero circuit for up
to 200 milliseconds to allow peak circuits to capture reverse loads
due to snapthrough or stripping forces and to avoid integrating
snapthrough forces into the baseline.
The threshold level and internal delay timer associated with rate
of change zeroing are also used to determine when a completed
signal is displayed on the tonnage displays. A load signal is
displayed only after the signal has decreased below the threshold
level and remained below the threshold for the internal delay time.
This allows the highest peak only of several that might result in
a single stroke from progressive or staggered tooling to be
displayed, instead of the last peak. This is illustrated in Figure
9. The amount of delay time is set according to the machine speed
setting located in the Config menu (section 2.8.8). For proper
operation of the System 1100 the machine speed must be set
correctly.