Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Reexamination Certificate
2002-04-19
2003-07-01
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
C342S118000, C342S195000, C342S196000
Reexamination Certificate
active
06587073
ABSTRACT:
This invention relates to a method for the processing of a frequency signal, for use in particular in the evaluation of distance measurements by means of pulsed electromagnetic waves or of continuous-mode frequency-modulated electromagnetic waves, employing the radar principle. This invention further relates to a distance measuring device incorporating a transmitter, a receiver, a measuring path extending from the transmitter to the receiver, and at least one reference path extending from the transmitter to the receiver.
The term frequency signal in this case refers to a signal R(&ohgr;) which describes a frequency spectrum defined by the amplitude R as a function of the frequency &ohgr;. Accordingly, the term time signal in this case signifies a signal r(t) defined by the amplitude r as a function of time.
BACKGROUND OF INVENTION
Non-contact gap-scanning distance and fill-level measurements by a variety of methods utilizing acoustic or electromagnetic waves have been described in the prior art. A transmitter sends sound or electromagnetic waves toward a target where they are reflected and then collected by a receiver. In a fill-level gauge, for example, electromagnetic waves travel from a transmitter vertically into a tank where they are reflected by the surface of the substance in the tank and sent back to a receiver. The run time of the transmitted and reflected signal permits a direct or indirect determination of the distance between the transmitter and/or receiver and the surface of the substance in the tank. Direct distance determinations employ, for instance, a pulse-count process in which the distance-measuring signal is composed of short pulses. Given the short run time of the signal, any direct time measurement is virtually impossible which is why a sampling method is used. One approach to indirect distance measurements involves a process that employs a frequency-modulated, continuous-mode high-frequency time signal, or FMCW, short for Frequency Modulated Continuous Wave. In this case, consecutive frequency sweeps serve to expand the frequency of the signal for instance in linear fashion, permitting the determination of the run time of a back-reflected signal by way of the differential frequency relative to the frequency attained by the sweep as of the time of the back-reflection. A corresponding time signal with the low-pass differential frequency is typically generated via a mixer to which both the sweep signal and the retroreflected signal are fed.
The accuracy and reliability of such distance measurements by means of wave reflection can be increased by employing a reference signal that travels along a predefined, known reference path. This reference signal is used for calibrating the effective measuring signal that traverses the actual measuring path from the transmitter via the reflecting surface back to the receiver. U.S. Pat. No. 4,665,403 describes, for instance, a microwave-based fill-level gauge whose reference path is in the form of a reference circuit into which the transmitted signal is fed and at whose end it is reflected, thus generating a reference signal for a predefined, known propagation path. It is also possible, however, to integrate the entire reference path as part of the measuring path by providing in the measuring path, for instance, a semireflective element which reflects part of the transmitted signal before the latter impinges on the surface which will reflect it back essentially in its entirety. An example thereof is described in the German publication DE 42 40 491 C2.
However, these prior-art approaches are afflicted with a variety of problems. In many cases, a simple design without a reference path is not good enough for highly precise distance measurements. On the other hand, in the existing concepts which do employ a reference path, it is often difficult to include the reference signal as an integral factor in the evaluation. Most of all, less than ideal pulse patterns in the pulse-mode process or dispersions and amplitude characteristics in the FMCW approach complicate the evaluation, so that at times the accuracy and the reliability of the distance measurements are inadequate.
SUMMARY OF THE INVENTION
It is therefore the objective of this invention to introduce a method for the processing of a frequency signal and a corresponding distance measuring device by means of which the aforementioned problems can be avoided or neutralized.
The frequency-signal processing method which achieves this objective is characterized in that the frequency signal is divided into at least two frequency ranges corresponding to two main components of the frequency signal and the frequency signal is subjected in each of the two frequency ranges to a separate Fourier transform, whereby the Fourier-transformed component of the frequency signal in one frequency range is generated resulting in a first complex time signal while the Fourier-transformed component of the frequency signal in the other frequency range is generated resulting in a second complex time signal, the second time signal is complex-divided by the first time signal which generates a third time signal, and the third time signal is subjected to a Fourier transform the product of which is a processed frequency signal.
Employing the pulse-mode process described further above, the processing method according to this invention permits direct application in the evaluation of a distance measurement. If the distance measurement is to be based on the FMCW process, also described further above, one additional step will be necessary. The reason is that in the FMCW process, the first signal available is a time signal, that being the low-pass signal generated in the mixer, from which by means of a Fourier transform, a frequency signal must be derived first.
While the frequency-signal processing method according to this invention offers versatile applicability, a preferred conceptual embodiment of the invention provides for the process to be used in the evaluation of a distance measurement employing pulsed electromagnetic waves and frequency-modulated continuous-mode electromagnetic waves based on the radar principle. As another preferred feature in this context, the frequency signal encompasses an effective measuring signal corresponding to the run time along a measuring path and a reference signal corresponding to the run time along a reference path and the two main components of the frequency signal are representative of the effective measuring signal and, respectively, the reference signal. It is the effective measuring signal which, sent by a transmitter, reflected off a surface and collected by a receiver, serves to measure the actual distance. The calibration of this effective measuring signal is performed by means of the reference signal which is established by its passage along a known, predefined reference path.
It has been found that the method according to this invention delivers particularly accurate and reliable results when the maximum peak of the effective measuring signal and the maximum peak of the reference signal are spaced apart by at least half the amplitude width of either signal. Indeed, the maxima of the effective signal and the reference signal are preferably spaced apart by at least the full width of the measuring signal and the reference signal at the 10% level of the amplitude height of either signal. And most desirably, the maxima of the effective measuring signal and the reference signal are spaced apart by several times, preferably at least five times, the width of the measuring signal and the reference signal at the 10% amplitude height level of either signal.
A preferred embodiment of the frequency-signal processing method employs a large signal bandwidth. The preferred modulation bandwidth of the frequency signal is at least 500 MHz. In the case of a pulse-mode process, this requires short pulse lengths while in the case of an FMCW process, a suitably large frequency deviation must be applied.
The basic concept of the frequency-signal processing method according to this i
Brumbi Detlef
Küppers Michael
Much Thomas
Schiek Burkhard
Wegemann Uwe
Alsomiri Isam
Cesari and McKenna LLP
Krohne Messtechnik GmbH & Co. KG
Tarcza Thomas H.
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