LINK Systems System 1100 Tonnage Monitor - Manual (Page 39)

THEORY OF OPERATION
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3-2
machine structural members can indicate the force applied by the
machine (machine load) as well as the load distribution to
structural members in production processes.
3.2 Measurement of Load
System 1100 Tonnage Monitors use strain gauge transducers (strain
links) mounted to appropriate load bearing structural members of
power presses and other machines to measure load. These strain
links, consisting of a Wheatstone bridge arrangement of strain
gauges attached to an intermediate fixture, provide electrical
signals proportional to the amount the intermediate fixture is
stretched or compressed. When mounted to a machine structural
member, the intermediate fixture follows dimensional changes of the
machine structure, stretching and compressing with the section of
machine structure between fixture mounting points.
A very stable voltage is used to excite the strain gauges on the
intermediate fixture. When the fixture is stretched or compressed,
the strain gauges change resistance with dimensional change,
increasing or decreasing resistance due to tension or compression,
respectively. A change in output voltage proportional to this
resistance change and the excitation voltage is generated,
providing a signal to the tonnage monitor that can be used to
determine machine load.
The dimensional changes of machine structure between strain link
mounting points are usually in the microinch range, with
corresponding strain link signal outputs of millivolts. The strain
links are connected to the tonnage monitor with shielded wire. The
strain link signals are amplified by the System 1100 channel cards
to larger voltage levels and the maximum (peak) signal is stored in
a peak hold circuit each cycle until read by the microprocessor and
displayed on the digital readouts as tonnage. Calibration of the
tonnage monitor occurs when electrical signals from all strain
links due to known load are amplified to a voltage level that
causes the known load to be displayed on the digital readouts.
The selection of machine structural members whose strain is to be
measured depends on machine structural configuration and load
information desired. For example, on a single point gap or OBI
power press, the total tonnage exerted by the press passes through
the single connection from crankshaft to slide (ram). Therefore,
a single strain link mounted on this connection can be used to