Rosemount TankRadar Rex DAU 2100, Руководство по обслуживанию

Страница: 19 / 154

3-5
Rosemount Tank Radar Rex
Chapter 3 Device Description
Service Manual
308012En, Ed.2
May 2013
3.1.3 Measurement principle
By using
FMCW
(Frequency Modulated Continuous Wave) method, Rex
transmits a microwave signal with a continuously varying frequency
towards the liquid surface. When the reflected signal returns to the
antenna, it is mixed with the outgoing signal.
Figure 3-5. The FMCW method: The distance (d) is calculated my mea-
suring the frequency distance (Df) between the transmitted
frequency (f1) and the reflected frequency (f0).
Since the transmitter continuously changes the frequency of the
transmitted signal, there will be a difference in frequency between the
transmitted and the reflected signals.
The gauge mixes the two signals, resulting in a low frequency signal
which is proportional to the distance to the liquid surface. This signal can
be measured very accurately allowing fast, reliable and accurate level
measurements.
Rex uses an optimum microwave frequency, which reduces sensitivity to
vapor, foam and contamination of the antenna, and keeps the radar
beam narrow in order to minimize influence from walls and disturbing
objects.
Fast Fourier Transformation - FFT
Rex uses
FFT
, which is a well established signal processing technique,
to obtain a frequency spectrum of all echoes in the tank. From this
frequency spectrum the surface level is extracted. In combination with
Rosemount’s Echofixer, FFT
allows measurements in tanks with
agitators, mixers and other disturbing objects.
Echofixer
Rosemount’s Echofixer provides a technique to adapt measurements to
various situations, by using information from previous measurements.
Frequency, f, (GHz)
t0

f~d
Time, t
f1
f0

f {