Abstract:
The present invention pertains to a radar range finder for high-precision, contactless range measurement, which is based on the FMCW principle and operates with digital signal processing at a limited frequency shift. One exemplary embodiment is described.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention pertains to a high-precision radar range finder, whose design corresponds to the FMCW principle, the radar range finder operating in a frequency range of 24.125 GHz±125 MHz, the range finder being particularly useful for closed containers. 
     Besides other fields of use, this device is preferably used in the level measuring technique. A great variety of designs of level meters have been known. The contactless level metering methods are based, besides radar methods, preferably on the travel time measurement in the ultrasound range or in the laser range. The ultrasound sensors have the drawback that the velocity of propagation of sound changes with the medium whose level is to be measured, as a result of which very substantial errors in measurement occur. The laser sensors have the drawback that, aside from the high purchase price, the level cannot usually be recognized in the case of clear liquids, because the laser beam is reflected from the bottom of the container rather than at the level. The radar sensors are free of these drawbacks. 
     The requirements imposed on a radar sensor are, aside from the high degree of reliability required, the measurement of the level to an accuracy in the cm range. Compliance with various regulations is required including specified frequency bands (FCC in the U.S.A. and FTZ-German Federal Central Bureau of Telecommunications) and others (e.g., approval by the German postal administration, as well as compliance with the protection requirements of the German Federal Institute for Physics and Technology, Group 3.5 &#34;Explosion Protection of Electrical Equipment&#34;). 
     SUMMARY AND OBJECTS OF THE INVENTION 
     To reach an accuracy of measurement in the cm range, the transmitted signal is modulated either in the time range or in the frequency range. In the case of pulsed systems, extremely short pulses corresponding to the equation 
     
         accuracy of measurement=velocity of light·pulse length/2 
    
     are required for this. At an accuracy of measurement of, e.g., 10 cm, this would correspond to a pulse length of 0.66 nanosec. This cannot be economically achieved for industrial applications with the technology currently available. 
     In the case of frequency modulation of the transmitted signal, the range resolution is determined by the modulation bandwidth, (deflection from the center frequency) according to the equation 
     
         accuracy of measurement=velocity of light/(2·modulation bandwidth). 
    
     At an accuracy of measurement of, e.g., 10 cm, this would correspond to a modulation bandwidth of 1.5 GHz. This can be accomplished economically for higher frequencies, and the frequency approved for this application is 24.125 GHz, but the requirement of the German postal administration in terms of a band limitation of 24.125±125 MHz for the general FTZ approval is not met. The modulation bandwidth is reduced step by step, from one measurement to the next, beginning with the maximum allowable bandwidth (250 MHz). 
     The modulation method is shown schematically in FIG. 2. To determine the range between the level and the measuring apparatus, a spectral analysis of the received signal is performed by means of a discrete Fourier transformation. The relationships are represented in FIG. 3. 
     However, if general approval by the postal administration is not required, e.g., in closed containers, accuracies of a few mm can be reached according to the method described at a bandwidth of, e.g., 1.5 GHz. 
     The primary object of the present invention is to provide a high-precision radar level meter, which operates according to the FMCW principle with digital signal processing for high-precision range measurement with limited frequency shift. 
     According to the invention, a radar range finder is provided which is designed using the FMCW (frequency modulated continuous wave) principle and which operates in the frequency range of 24,125 GHz±125 MHz. High precision range determination is performed using means for determining the optimal or adapted frequency shift in a stepwise manner. Structure is provided for approximately determining the allowable shift, after which the shift is successively broken down until an integer multiple of range gates generated corresponds to the exact distance between the measuring apparatus and the reflection surface. The maximum amplitude of the spectrum or the minimum of the sidelobes of the measurement sequence is used as the indicator of the optimal or adapted shift. A control loop is provided including an oscillator, a directional coupler, a divider chain, a counter as well as a microprocessor and a digital to analog converter with a frequency which is formed having an integer multiple which is the actual transmitted frequency. 
     A video device is provided with a device for level processing and is coupled with a controllable amplifier, controlled by the microprocessor and with means for band filtration. The oscillator (preferably a voltage controlled oscillator) is followed by an amplifier which is optimized for generating a fifth harmonic of the oscillator frequency. The amplifier is associated with the band filter for the fifth harmonic of the oscillator frequency and with an additional amplifier for raising the level and for decoupling. The transmitted signal is separated from the receive signal by a directional Coupler. The receiver has an LO buffer amplifier and a mixer which are manufactured according to the highly integrated technology (monolithic microwave integrated circuit). To convert the voltage controlled oscillator frequency into the MHz range, a divider chain with a plurality of divider steps is arranged in the control loop. The divider chain is part of a control loop, which performs an auto calibration cycle on the oscillator. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic view of a level measurement by a radar level meter; 
     FIG. 2 is a modulation/bandwidth-time diagram of the modulation method, 
     FIG. 3 is a diagram for range determination from the spectrum of the received signal; 
     FIG. 4 is a schematic diagram of a discrete measurement of the range by distance gates; 
     FIG. 5 is a diagram for superimposing different spectra at different frequency shifts (B x ); 
     FIGS. 6a-6d are diagrams of spectrum/echo profiles with different frequency shifts (B); and FIG. 7 shows a block diagram of an exemplary embodiment of a radar level meter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The discrete Fourier transformation of a time signal quasi breaks down the detected range into so-called distance gates, also called range gates, as is illustrated in FIG. 4, and each range gate corresponds to one spectral line, as is shown in FIG. 3. 
     In the case of level measurement, it is necessary to deal with a single, dominant &#34;reflector,&#34; namely, the surface of the filling material, as can be determined from FIG. 1. All other reflectors, e.g., container wall, etc., are not located in the major lobe of the antenna and therefore they are only very weakly noticeable, if at all, in the received signal. 
     The following method of high-precision range determination is proposed for this special case: 
     The echo amplitude of the received signal reaches its maximum precisely when a suitably large range gate is found, so that the range gate can be placed exactly n times between the measuring apparatus and the reflector surface. 
     The detection method is carried out as follows: 
     The process is begun with the maximum allowable shift of 250 MHz corresponding to the extension of the range gate, equaling 0.6 m. The apparatus first approximately determines the range. Depending on the approximate distance determined, the range of reduction of the shift as well as the step size from one measurement cycle to the next are subsequently set. The shift is then reduced stepwise, from one measurement to the next, until the extension of the range gate fits exactly n times into the range between the measuring apparatus and the level. This is determined by determining the maximum of the spectrum of the measurement series (cf. FIG. 5), after which the range is calculated from the simple equation 
     
         R=Fb·c/(2·B·Fm) 
    
      in which: 
     R=range, 
     Fb=beat frequency (corresponding to the position of the echo amplitude in the spectrum), 
     c=velocity of light, 
     B=&#34;optimized&#34; or adapted frequency shift, and 
     Fm=modulation frequency (cf. FIG. 2). 
     The sequence of FIGS. 6a through 6d shows how the secondary lines in the spectrum decrease in the course of the stepwise optimization or adaption of the shift. Consequently, the signal energy is concentrated onto the principal line, so that it is &#34;maximal&#34; in the case of the optimal shift bandwidth. 
     Consequently, the accuracy of the range is no longer determined by the maximum possible shift alone in this process, but mainly by the accuracy with which the optimal shift is determined. This depends, above all, on the selected step size, i.e., the maximum allowable measurement time. 
     It should also be noted that the shift can be set with high accuracy, and that the frequency modulation should be performed as linearly as possible in order to minimize the side lines represented in FIGS. 6a through 6d, which is made possible by the frequency generation method described below. 
     As is illustrated in FIG. 7, the voltage-controlled oscillator (VCO) 1 generates the basic frequency of 4.825 GHz. This oscillator signal is amplified in the buffer amplifier 2 following it, and the oscillator 1 is at the same time decoupled from additional steps in order to avoid the so-called pulling effect, which would lead to a deviation from the linearity of the frequency shift and consequently to an inaccuracy in range determination. 
     The buffer amplifier 2 operates in the saturation mode, thus generating frequency components at multiples of the basic frequency. It is designed to be such that the fifth harmonic (24.125 GHz) is particularly salient. The directional coupler 3 (20 dB) is dimensioned for the fundamental wave, and it decouples the VCO signal to the divider chain 4, 4a with a suitable level. The divider chain 4, 4a, which consists of a plurality of divider stages, converts the oscillator frequency into the MHz range. The control loop is closed via a counter 5, a microprocessor 6, and a D/A converter 7 for the required linearization of this frequency. This control loop makes it possible to eliminate both aging effects of the components and temperature-dependent long-term drifts of the frequency of the oscillator 1 via an autocalibration cycle. The fifth harmonic of the oscillator basic frequency of 24.125 GHz, which is already preferably generated, is selected with the band filter 8, and all other multiples as well as the fundamental wave are suppressed. 
     The level is processed once again in a connected power amplifier 9 and additional decoupling of the oscillator unit from loaded impedances is achieved at the same time. 
     The transmitting/receiving unit of the radar level meter proposed here has a directional coupler 11 (10 dB), which decouples part of the transmitted signal for the receiver. Its transmitting branch main arm leads directly to the 6-dB directional coupler 12, which separates the transmitted signal and the received signal. The level of the decoupled part of the transmitted signal is raised in the LO buffer amplifier 13 in order to achieve optimal level control of the mixer 14. The received signal enters the mixer 14 from the antenna via the 6-dB coupler 12, and the video signal is formed in the basic band (beat signal) due to so-called homodyne frequency conversion at the IF (intermediate frequency) output of the mixer 14. This beat signal is subjected to low-noise amplification in the subsequent IF amplifier 15, and decoupling of the mixer 14 from the subsequent filters and amplifier stages of the video part 16 is also achieved at the same time. This video section is used essentially for level adaption and frequency filtering for the A/D converter 17 in the microcomputer 6, in which the control of the overall system, as well as the signal processing, and the range determination are performed. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.