Radar tank gauge

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radar tank gauge wherein a virtual modulated carrier 50 storage bin sampled periods results in a frequency do

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tained, applicant's invention utilizes a novel technique to in essence, remove the modulation, thereby making the carrier available for distance measurement

In keeping with the further aspects of applicants' invention, the power spectral density of the carrier signal as determined, is utilized to determine a centroid of the carrier fundamental. A similar centroid of the calibration signal is also determined. It will be readily understood by those skilled in the art that the quotient of the carrier centroid over the calibration centroid contains the information which allows correction based on the delay line length, thus providing a true measurement of the distance between tank antenna and tank content surface. Further corrections to the true length due to physical locations of radio frequency components are also provided by central processing unit operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent on reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a semi-schematic depiction of the radar tank gauge system.

FIG. 2 is a detailed semi-schematic depiction of the RF unit of FIG. 1, particularly showing the functions of RF generation, signal coupling, RF transmitted signal * control, and difference signal detection

FIG. 3 is a functional block diagram of the audio amplifier and signal conditioner.

FIG. 3a is a graphic representation of the gain characteristic of the signal conditioner of FIG. 3.

FIG. 4 is a graphical representation of the signal processing and conversion of the disclosed invention, particularly showing the relationship in time and frequency domain of transmitted, reflected, and difference signals.

FIG. 4a shows four cycles of a typical swept frequency signal as transmitted by the tank gauge antenna. Also shown is a representation of a reflected signal.

FIG. 4b is a graphic representation of the sampled window or "bin" as used in the invention.

FIG. 4c is a graphic representation of the reflected and mixed difference signal stored in the bins of 46.

FIG. 4d is a conceptual representation of "typical" signals obtained from mixing the transmitted and reflected signals of 4a.

FIG. 4e is a graphic representation of the time based reflected difference signal, stored in the bins of FIGS. 4b and 4c now transformed to the frequency domain.

FIG. 5 shows in graphic 'display the amplitude/frequency relationship of the difference signal, particularly showing determination of the centroid or "carrier" frequency of the reflected difference signal.

While the invention will be described in connection with a preferred method and typical embodiment, it will be understood that the radar gauge disclosed does not limit the invention to the methods and embodiments disclosed. On the contrary, applicants intend to cover 60 all alternatives, modifications, and equivalents as may be included within the spirit and scope of the radar gauge as defined by the appended claims.

DETAILED DESCRIPTION OF THE
INVENTION

Turning first to FIG. 1, there is shown the radar gauge and signal processing system 1 incorporating an

antenna 15 mounted internal of a tank or other containing structure 17, having contents 18 contained by the tank to a level such that the difference between the tank content surface 20 and height of the antenna 15 results 5 in a distance or measure range 19. The RF and tank assembly 3 is separated from the signal processing assembly 5 by safety barriers 6. The function of the safety barrier is to allow use of the RF assembly 3 in hazardous areas by limiting electrical power available to a value 10 which does not provide the opportunity for ignition of the volatile tank contents.

The RF and tank assembly 3 further utilizes an RF unit 11 (reference FIG. 2), supplying continuous wave. . radio frequency power, typically in the 9.5 to 10.5 giga15 hertz range. Also in the RF and tank assembly is a control and signal processing unit 12 for establishing time sequence of transmitted antenna signals, calibration signals, and preliminary operation on tank difference signals. The control and signal processing unit 12 performs the operations necessary to process the complete range of return signals. Included in this range is the predetermined maximum distance as represented by reflection of the tank bottom. Bottom reflections are recognized and distinguished for large ullage measurements, i.e. levels of fluent materials in the tank. This provides positive identification of a known distance, resulting in improved range measurement

The safety barrier 6 also acts as an interface between the RF and tank assembly, and the above mentioned signal processing and display assembly 5. The signal processing assembly further comprises a central pro- • cessing unit 7 having a frequency analysis function for performing discrete Fourier transforms of difference signals. Also contained in the signal processing and display assembly is a display 25 comprising a cathode ray tube, and d'Arsonval meters providing continuous indication of the measured tank range 19 and other quantities in signal processing as may be required. Simultaneous digital display of tank ullage, time and frequency domain representations of the reflected signals, and other specialized displays essential for operation of a specific gauge application are readily provided. Those skilled in the tank level gauging arts will readily see that many forms and variations of tank display can be used with the signals provided by the invention disclosed herein.

Transmission of the radio frequency signal generated by microwave oscillator 27 is routed between the RF switch 33, and detector 31, by microwave circulator 35. An additional circulator is utilized to route calibration signals from the control switch 33 through the calibra-. tion delay line 13 in order to generate a reference signal. The significance of this signal will be described in more detail.

As indicated above, the return or received signal 42 differing in time phase from the transmitted signal 40 by a time/frequency difference 41 (reference FIG. 4a) turns from the antenna 15 through the RF switch control 33 and circulator 35 to the detector or mixer 31.

With further reference to FIG. 4, an RF signal 40 having a swept frequency as shown between 9.5 and 10.5 gigahertz is transmitted by an antenna 15 arranged to direct a concentrated signal to the surface 20 of tank contents lSi In keeping with conventional radar gaug65 ing techniques, the signal 40 returns to the RF unit via the antenna 15 as the signal 42 delayed by an amount 41 in both frequency and time relative to the transmitted signal 40 due to travel to and from the fluent material

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55

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surface. As shown, the received signal 42 is mixed with plished through generating a compensating control the originally transmitted signal 4 in the detector 31, voltage wave form utilizing digital information stored generating a difference signal 48 (ref. FIG. 4c). Differ- in the central processor. Under control of the central ence signals are stored in sample windows 43, having a processor, linearizing signals are applied to the voltage period 44 identical to and synchronized with that of the 5 controlled oscillator portion of the variable frequency frequency sweep portion of the transmitted signal 40. oscillator 27.

Operation of the circulator 35 will be well known to Novel aspects of the discovery disclosed herein can those skilled in the microwave arts and therefore not best be described through the following exemplary and discussed in further detail. In operation, portions of the nonlimiting example. For the case typically disclosed transmitted signal are also directly sent to the detector. 10 where the tank ullage range or distance of 40 meters is Detector inputs now consist of the initially transmitted desired, the following well known relationship between signal and the received signal delayed in time as indi- return difference frequency, and other system

cated above. The mixing or detecting operation, also parameters is representative;
well known to those skilled in the radio frequency arts,

provides an output frequency equal to the difference of 15 /^the return difference frequency)-=45/!/rc
the transmitted and returned signals. In the case of the
invention disclosed herein, the mixed or difference sig- where;

nal 48 is in the audio frequency range characteristically B=the return signal frequency band width, i.e. the
0.250 to 9.6 kilohertz for reflected signals traveling the sweep frequency range of the oscillator 27 under sweep
distance 19 of 1 to 40 meters. A graphic representation 20 control 36 from the sweep linearizer 10.
of these return signals 48 are shown in FIG. 4d R=40 meters, the desired measurement range.

In keeping with the novel and inventive aspects of the T=0.025 seconds corresponding to the sweep freapplicants' invention, applicants have discovered that quency period of oscillator 27 at 19 sweeps per second, samples of the difference signal 48, taken in synchroni- it shou]d ^ noted that the swecp freqUency is essenzation with the period of the transmitted sweep signal 25 tially an arbitrary quantity chosen to provide adequate 41, are stored for periods identical, i.e. having the same measurement taking into account the maximum rate of initial and terminal times, to that of the sweep portion of change of the ullage 19.

the transmitted signal. This synchronous processing of c=3 X 10» meters/seconds, the speed of light and the difference signals, when transformed into the frequency transmitted signal.

domain, eliminates the phenomena known as frequency 30 If the above relationship is rearranged to determine leakage. Non-synchronous sampling would result in ^e range R; introduction of additional frequency components, introducing frequency leakage errors into the signal repre- R^Tcfj/AB sentative of the tank ullage 19, in the mixed signal 48.

Applicants have further discovered that application 35 Substituting the above constant provides the followof on-line direct Fourier transforms of the stored differ- jng; ence signals during the intervals Ts', indicated as 46 on FIG. 4c, provides a frequency domain representation 50 R=fd/2so (reference FIG. 4e) of the measured and stored time

domain return frequency 48, containing essentially all of 40 Since the difference frequency is measured to the the frequency components necessary to accurately de- accuracy of the quantity Sf, then the corresponding termine the tank ullage 19 contained in the returned accuracy of the range measurement SR is; signal 42.

Further signal processing in the frequency domain in BR**6fj/iso keeping with applicants' inventive discovery, proceeds 45

with determination of a centroid of the synchronous Since, in the system disclosed, the sweep or sample frequency distribution of the return signal 48. With frequency is approximately 18.75 hertz, the observaparticular reference to FIG. 5, the now transformed tional period is approximated by; signal 55 of the returned mixed signal 48 is shown in a

SINx/x form. As those skilled in the art of transform 50 , - -L 1 = 53 3 x 10-3 seconds

mathematics will readily see, the SINx/x representation 1 ~ fi '8-75 hz ~ ■

55 shows as vertical components along the frequency

axis here typified as 57 and 58, frequency components of In the frequency domain then, the return difference the returned and mixed signal 48. As transformed, the frequency represents a measured range increment 6R components of the returned signal include extraneous 55 equal to 1/250.X53.3X 10-3=0.075 meters, frequency components returned to the antenna 15 The above figure, i.e. 0.075 meters represents the through oblique reflection, noise, and non-direct radio basic distance which can be resolved through convenfrequency signals from other unknown sources. As tional conversion of the returned time domain signal fd these are not of interest in measuring the tank ullage, in the frequency domain. Also, return signal errors in applicants have discovered that the frequency domain 60 the above measurement are introduced by conventional representation establishes a dominant frequency shown instrumentation and measurement inaccuracies, in FIG. 5 as component 56. An additional and substan- Most importantly, errors due to corruption of the tial error is also introduced due to the relative non- return signal spectrum in the frequency domain as linearity of the frequency sweep over its range of 9.5 to would be transformed by prior art approaches, by inclu10.5 gigahertz. In keeping with the invention disclosed, 65 sion of energy in the form of extraneous signals due to applicants have utilized techniques for linearizing noise, uncontrollable, spurious reflections from inside sweep frequency voltages as are typically available the measuring tank or container, and other random from state-of-the-art equipment. Linearization is accom- energy measured as signals, are incorporated in the