Measurement Computing IOtech 640 Series - Manual (Page 51)

640 & 650 Series 878893 Analog Signals 6-9
Sound & Vibration Sensors – Supplemental Information
A system with an extremely high Q excited near its resonant frequency produces large oscillations which can mask
the true acceleration signal, produce inaccurate measurements, and damage the sensor. Consequently oil, or a
similar material, is frequently added to control the damping.
Figures 1 and 2 show the frequency response of harmonic resonators with Q = 1 and 100,000. For Q = 100,000, the
accelerometer is accurate within 10% to 1/3 of its resonant frequency. In contrast, for Q = 1, the accelerometer is
accurate within 10% to half of its resonant frequency.
Most strain gages in accelerometers are wired in Wheatstone bridge circuits. Signal conditioners for a common
strain-gage bridge also apply to this type of transducer since it resembles a strain-gage circuit in a load cell.
10 5
0.0 0.5 1.0 1.5 2.0
Q=1
Amplitude
ω / ω
0
10 4
10 3
10 2
10 1
10 0
10 -1
Q=10 5
0.0 0.1 0.2 0.3 0.4 0.5
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
Amplitude
ω / ω
0
Q=10 5
Q=1
Fig. 1. An undamped accelerometer can have a high Q at
its resonant frequency, which produces large oscillations
and completely overrides the acceleration signal. Also, if
large enough, resonant oscillations can destroy the
accelerometer.
Fig. 2. This figure shows the low frequency
portion of Figure 1 magnified.
Low-Impedance Transducers
Most modern piezoelectric transducers contain integrated signal conditioning amplifiers. Also known as integrated-
circuit piezoelectric transducers, these units have a low impedance output and require an external power supply. The
manufacturer usually specifies their sensitivity and frequency range. Users need only connect a supply to the power
terminals and the output to a voltmeter circuit. The voltage is then scaled to the measured engineering units.
The most common application for piezoelectric-type accelerometers is in measuring a wide range of accelerations
and mechanical vibrations. They monitor automobile deceleration (and deploy the air bag at the correct millisecond)
in safety systems, lift-off acceleration and motion during space-shuttle missions, and mechanical vibration in
numerous machines. Piezoelectric sensors, however, cannot measure constant acceleration as do strain-gage sensors.
Low-impedance piezoelectric transducers also measure pressure or force. The accelerometer circuit requires only
two wires to handle both power and signals.
Piezoelectric sensors have resonant frequencies as high as 120 kHz giving them a usable frequency range of
<1 Hz to >40 kHz.