Polytec PDV-100, User Manual

Page: 12 / 63

2-2
2 Introduction
2.2 Measurement Principle
The PDV use the principle of the heterodyne interferometer to acquire the
characteristics of mechanical vibrations or transient motion processes (refer
to
APPENDIX
B). With this type of interferometer, a high-frequency carrier
signal is generated on the photo detector with the aid of a Bragg cell. To make
the vibration measurement, the beam of a helium neon laser is pointed at the
vibrating object and scattered back from it. The velocity amplitude of a
vibrating object generates a frequency modulation of the laser light due to the
Doppler effect. This modulation is recovered in the signal processing unit with
the aid of suitable demodulations (or decoders). A schematic layout of the
signal path is shown as a diagram in
FIGURE
2.2. The velocity information is
recovered from the frequency modulation of the Doppler signal.
Figure 2.2: Signals in the vibrometer
In the PDV, the optics and the signal processing unit are inside the housing.
The demodulation of the Doppler signal is purely based on a digital process.
In contrast to the conventional vibrometers which use analog, this then means
that the measurement accuracy is virtually independent of aging and
environmental influences. State-of-the-art DSP technology makes it possible
to realize excellent measurement system characteristics, despite the compact
design and minimal energy consumption.
By using a serial interface proven in digital audio technology, the output signal
ca be fed into the digital inputs of modern recording devices or signal
analyzers without any loss of accuracy. For the conventional type of signal
processing, an analog signal output with 24 bit amplitude resolution is also
available.
Velocity
decoder
Object
s(t)
v(t) = ds/dt
Doppler
signal
f
D
v(t)
D
f(t)
Filter
DSP
IVS-400
INDUSTRIALVIBRA TIONS ENSOR
u(t) ~ v(t)
LP
HP
A
D
Audio
transmitter
S/P-DIF