pressure rise, shocks, and vibrations of typical firings.
d. Load Cells.
Errors resulting from the use of piezo-type gages are the
(1) The load cell uses a high-strength load
result of friction of the gage piston and the possible
carrying member as its sensing element
pressure gradient within the device. With the addition of
to which four strain-gage windings
errors due to the nonlinearity of amplifiers,
connected in the form of a Wheatstone
oscilloscopes, or oscillographs and film shrinkage, the
bridge are bonded.
The load cell,
data. may be in error by 3 percent under the best
mounted opposite the stroking end of the
possible field conditions.
propellant actuated device, measures
(2) Strain Resistance Gages.
reactive thrust.
(a) The active element of a typical
(2) A schematic diagram of a coupling
strain resistance pressure gage is a
network which may be used with any
thin-walled ferrule. Bonded to the
bridge type gage, such as the pressure
exterior of the ferrule is a winding of
pickup, accelerometer, load cell, etc., is
fine resistance wire.
As the
shown in figure 89. This circuit, when
pressure increases, the ferrule
used with the load cell, produces a thrust-
expands, straining the wire winding
time plot on the recorder.
and
increasing
its
electrical
(3) Figure 90 is a reproduction of a catapult
resistance.
performance (thrust-time) curve obtained
with a load cell. The shift in the base line
pressure gage used in testing
is explained by the fact that at the start of
propellant actuated devices is the
the record, the cell was loaded with the
pressure pickup. At one end, the
weight of the carriage and device while at
pressure pickup has a diaphragm
the end of the record the weight of the
which is acted upon by the pressure
carriage and part of the device were
in the chamber of the propellant
absent. The "noise" appearing at the start
actuated device. Inside the gage, a
of the curve probably is caused by a
steel strain tube contacts the
shock load on the cell at the start of
diaphragm. Two 1,000-ohm single
motion. The "noise" on the rising portion
layer strain-gage windings are
of the curve probably results from
cemented to the tube in such a way
mechanical imperfections of the carriage
that straining the tube increases the
track system.
The disturbance which
resistance of one winding and
appears after the curve peak is caused by
decreases the resistance of the
the sudden stopping of the telescoping
other.
tube of the device at the end of its travel.
The second curve shown in figure 90 is a
(c) Strain-type pressure gages are low
stroke-time curve. Each peak on the
impedance devices. They are not
stroke-time curve represents one inch of
so susceptible to moisture as are
travel. The lines to the left of the curves
piezo-type transducers and do not
are record calibration lines. Each space
require high-input
between the calibration lines represents
They have the
1,000 pounds of thrust.
"Calibration"
disadvantage, however, of not
marks are placed customarily on each
standing up as well as piezo gages
photographic record at the time the record
under repeated firings. They also
is made by applying appropriate signals
have
much
lower
through the coupling network.
responses.
(4) Load cells are used to produce thrust-time
curves. This data may also be converted
pressure gages have all four of the
to pressure-time curves, since the area of
strain gage windings mounted within
the stroking member of the device is
the gage (as in the load cell to be
known. Thrust-time data also are used in
discussed next).
They may be
determining acceleration or velocity at
bonded or unbonded.
The
unbonded type have a higher
frequency response but are very
susceptible to damage when used
on propellant actuated devices.
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