SM0486
(3) The reciprocating movement of the driver coil is transferred to the
signal generator coil by the connecting rod. The signal generator coil is forced
to move in and out of the fixed magnetic field of the signal generator permanent
magnet at the same rate as that produced at the driver coil.
The cutting of
magnetic lines of force by the wires of the signal generator coil produces a
voltage at the terminals of the coil.
(4) The third component of the moving element assembly (the exciter table)
is attached to the same rod as the driver coil and the signal generator coil. All
three moving element components are thus forced to move in the same manner and at
the same rate.
Since the table and the signal generator coil are moving at the
coil through the air gap of the signal generator permanent magnet is proportional
to the rate of coil movement, the voltage output of the signal generator coil can
be used as an indication of table velocity.
(5) Examine the input circuits in Figure 5, you can see that the output
voltage of either the calibrator signal generator or the vibration pickup under
test can be fed to the amplifier circuit depending on the selected position of a
two-pole, double-throw switch mounted on the panel. In one of the selector switch
positions, the value of the output of the amplified calibrator signal generator or
vibration pickup is controlled by two variable resistors arranged in series. This
hookup permits vernier adjustment to the desired value.
Amplitude of vibration
produced at the calibrator table is controlled by a panel-mounted rheostat, which
controls the current in the moving element driver coil.
(6) The signal generator output across the null indicator meter is adjusted
by a calibrated potentiometer.
The potentiometer is equipped with a calibrated
dial that reads directly in sensitivity units of millivolts per inch per second.
An AC output jack is tied in between the calibrated potentiometer and the amplifier
to provide a means of reading actual calibrator signal generator voltage output
with an accessory vacuum tube voltmeter during testing of the calibrator, after
repair or overhaul.
Any meter or other device plugged into the output jack must
have a high input impedance (2 megohms or larger) to prevent loading the signal
generator or pickup coils.
d. Calibrator Application. Before we proceed to the calibration of a pickup,
let's examine the instrument panel and the controls which you use in the
calibration of a pickup. The controls and panel components are shown in Figure 6.
(1) The model PC1-A calibrator meter panel provides the means of comparing
vibration pickup voltages with the output of the signal generator.
The panel is
equipped with an on-off power switch which controls the operating voltage for the
circuits represented on the panel and to the voltage needed for the exciter's
driver coil.
The amplitude of the exciter table's movement is limited by the
position of the amplitude control.
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