Radar measuring devices for fill level measurement are produced and sold by Endress +Hauser in a number of different embodiments. A freely radiating radar measuring device is offered, for example, under the mark “Micropilot”. Microwaves are radiated toward the medium via an antenna. The EM-waves reflected on the surface of the medium are received back by the measuring device and an echo function formed, which shows echo amplitude as a function of distance. From this function, the wanted echo is ascertained and the associated travel time is determined. Based on the travel time, the distance between the antenna of the radar measuring device and the medium is determined.
There are different measuring methods known, of which pulse radar and frequency modulation continuous wave radar (FMCW radar) represent the most common. In the case of pulse radar, short microwave pulses are periodically transmitted and the pulses reflected on the surface of the medium received back. The received signal amplitude as a function of, time represents the echo function, wherein each value of a reflected echo corresponds to a certain distance from the antenna.
In the case of the FMCW method, a continuous microwave is transmitted, which is periodically frequency modulated. The frequency of the received signal has at the point in time of receipt a certain difference compared with the frequency of the transmission signal, and this difference depends on the travel time of the echo. Determinable from the frequency difference is, thus, the distance between antenna and medium. The echo function is represented by a Fourier spectrum of the mixed signal of the transmission signal and the received signal.
Frequently used for calibrating distance measuring devices, especially the mentioned radar measuring devices, is a measuring track, whose length corresponds to the measuring range of the distance measuring device, and on which an areal reflector, also called a strike plate, is shiftably arranged. The distance measuring device to be calibrated is positioned, for example, at the starting point of the measuring track, the measurement signal is transmitted in the direction of the reflector and the reflected measurement signal is received back. In the case of a radar measuring device, the distance between radar measuring device and reflector is determined based on the signal travel time. The measured distance is compared with a reference value, which was determined by means of a calibration standard. The calibration standard is, for example, a laser interferometer with an accuracy higher by a defined factor than the accuracy of the distance measuring device to be calibrated. As a rule, a number of measurement points are used for determining the linearity.
Distance measuring devices include, as a rule, a flange, on which a directional antenna is arranged. As a result of welding or screwing procedures, the directional antenna can become tilted on the flange, whereby the accuracy of the distance measuring device is lessened.