Page 10 - Combined_41_OCR

cable losses and minimizing impedance-matching The reference value in this table was 1 g; it could
problems. as easily have been another value, such as 1 [ig
(10-6g).
Peripheral Instrumentation In some instruments used for power analysis,
readout is in terms of power rather than voltage.
Selecting Readout Parameter: Sometimes a user Because power is proportional to E2, then a dif­
of vibration information prefers one of the param­ ferent equation is required:
eters (displacement, velocity, or acceleration) while
using a pickup that senses another. For instance, I
aircraft flight crews may prefer a displacement dB — 10 logio a?
readout but use velocity-sensing pickups on en­ Systems Calibration: Calibration is an orderly
gines. The signals are electronically converted (in­ procedure for finding out how sensitivity varies
tegrated) to represent displacement. Increasingly, over a given range of frequency and amplitude.
accelerometers are used in such a manner; their Best accuracy is achieved if all elements are cali­
signals can be integrated once, to represent ve­ brated as a system. Several general rules for cali­
locity, or twice, to represent displacement. bration are suggested here; literature obtainable
from the sensor manufacturer provides specific,
Selecting Readout Format: If it is necessary to detailed calibration instructions.
examine the details of a motion, an oscilloscope Displacement pickups are easy to calibrate
or recording oscillograph is required. Meters give statically, since relative motion between fixed and
statistical properties of vibration such as peak or moving parts can be generated by a lead screw.
rms values, but measuring short, severe spikes However, resonance in the pickup or its mounting
(representing impact superimposed upon a vibra­ might change sensitivity in the frequency range
tion) cannot be done with meters. of interest. The only way to demonstrate flat fre­
Only in the rare condition of sinusoidal motion, quency response is to attach the moving portion
with all energy at one frequency, can electronic to a motion generator.
meters be trusted. The one exception to this rule Strain gage, PR, and servo accelerometers can
arises when using true-rms (trms) meters. All be statically calibrated very simply. Hold the pick­
other a-c-sensing electronic meters err in the com­ up on a flat, horizontal surface, and balance any
mon situation of nonsinusoidal vibration where electrical output to zero. Then invert the pickup
energy exists at several frequencies. The extra so that gravity acts in the opposite direction and
cost of trms meters is justified, since these meters balance the same output, again to zero. Divide
give accurate rms indications no matter what the the difference in output by 2 to get output/g.
waveform. Note: A scale indicating that readout Static accelerations on the order of ±1 g can now
is in “volts rms” is no guarantee that the meter is be measured. Before static accelerations signifi­
trms. cantly larger than ±1 g are measured, the accel­
erometer should be calibrated on a centrifuge.
Linear vs. Log Scale: Normal variations of fre­ Static calibration, unfortunately, will not give
quency or amplitude are often large enough to dynamic sensitivity. Most crystal accelerometers
drive the reading off of the scale of a linear-scale (and all velocity pickups) can only be calibrated
meter. Many meters, in short, lack sufficient dy­ dynamically on a shaker, since their output can­
namic range. This range can be increased several not be measured at zero frequency. For these
orders of magnitude by using a logarithmic con­ sensors, the mechanical input should be purely
verter. Readout of a log converter is usually in
sinusoidal so the electrical output is also sinusoidal.
decibels, calculated from: This simplifies measurement of the output voltage.
E
dB = 20 logio------
Handle With Care: The finest measurement sys­
where E is a voltage representing displacement, tem does not guarantee accurate measurements,
velocity, or acceleration. E() represents an arbitrary even when sensitivity is determined accurately by
reference or standard value (zero dB) of that volt­ calibration. The system must be used correctly and
age. Accelerometer output can thus be readily con­ handled carefully. The pickup must be attached
verted from linear to logarithmic (decibel) form: firmly to the vibrating structure, and the signal
cable protected from undue stresses. In summary,
Linear Output Log Output the instruction manuals must be read and followed.
(g units) (dB)
Erroneous data caused by carelessness may
0.001 -60 mean that the whole data-gathering job must be
0.01 -40
0.1 -20 done again—if the engineer is lucky. If he is not
1 0 lucky, he may continue with design, fabrication,
2 + 6 and installation, never knowing until too late that
4 + 12 his data are faulty.
10 + 20
100 + 40 Next article in this series discusses data gath­
1,000 + 60
ering and analysis.
124 Machine Design