Page 305 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 15 Sensors 267

Voltmeter Clamp Screw
.3 .4 .5 .6 .7
.1 .2 .8 .9
1
DC VOLTS
Potentiometer Terminals
Full Wave Bridge
79°: 3.00 G
− +
Lever Arm
67°: 2.00 G
− 56°: 1.50 G
Dead Weight
+
45°: 1.00 G
33.75°: 0.75 G
22.5°: 0.50 G
Vibration Sensor Filter Capacitor 11.5°: 0.25 G
0°: 0.00 G
Figure 15-75 Averaged Vibration Reading
Figure 15-77 Pendulum Accelerometer

If a higher order analysis is desired, then the vibration sen- 22.5 , then a / 2 -G acceleration is indicated. If the pendulum
1
sor can be set up with an oscilloscope and frequency counter, swings 45 , then a 1-G acceleration is indicated.
as shown in Figure 5-76. The oscilloscope provides a clear An accelerometer can be configured with a dead weight,
indication of the waveform that the equipment is generating two centering springs, and a linear potentiometer, as shown in
while the frequency counter indicates the resonant frequency. Figure 15-78. The tension on the springs can be tuned for sen-
Either of these two instruments can be used independently sitivity and the position of the springs is adjusted to center the
with a vibration sensor. potentiometer. The sensitivity of this type of sensor can also
be increased by increasing the size of the dead weight.
The output of the potentiometer in an accelerometer is
Accelerometers generally used as the moving element of a slide wire bridge,
as shown in Figure 15-79. Sensitivity adjustment is achieved
When an object is moving in a circular arc, a certain amount by configuring a potentiometer in a voltage divider arrange-
of centrifugal force is generated. In the case of an automobile ment. This circuit is appropriate for either a pendulum or
traversing a corner, this force has a tendency to through the spring accelerometer.
driver toward the outside of the curve. This force is generally
described in G, for gravity. One G equals the equivalent force
of gravity. Therefore, an object weighing 10 pounds and Linear Potentiometer Dead Weight
Left Hand Spring
accelerated at 20 G will have the equivalent weight of 200 Right Hand Spring
Centering
pounds. Screws
Figure 15-77 shows a pendulum accelerometer. A dead
weight is placed at the end of a lever arm and allowed to hang Base
down. If a force is applied that makes the pendulum swing Figure 15-78 Spring Centered Accelerometer

100K 1Meg
1K 20 Meg On
Frequency Counter
INPUT Off
RANGE

TRIGGER
Oscilloscope AC DC EXT
POSITION POSITION
VOLTS/DIV SEC/DIV
COUPLING
2C 5C .1 .2 .5 5 2 1 ms .5 .1 20 10 5 INT LINE EXT
50 1C CAL 2 1 10 CAL us 2
10 20 5 2 1 20 s 1
AC DC .5 .2
XY 5 .1
SOURCE
GND
0
+
500 V HOR EXT _
INTENSITY FOCUS BEAM FIND POWER INPUT INPUT LEVEL
Vibration Sensor
Figure 15-76 Vibration Analysis Setup