Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
2000-04-17
2002-07-09
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Reexamination Certificate
active
06415660
ABSTRACT:
The invention relates to a method for the highly accurate determination of the filling level of a product in a container, measuring signals being transmitted in the direction of the surface of the filled product and reflected at the surface as echo signals and the filling level in the container being determined by evaluation of the amplitude and phase values of the reflected echo signals by means of a pulse delay-time method. Both the pulse-radar method and the FMCW method, in which continuous waves are frequency-modulated in a periodically linear manner, for example with a sawtooth voltage, may be used. Furthermore, the invention relates to an apparatus for the highly accurate determination of the filling level of a product in a container.
Pulse delay-time methods use the physical law according to which the transit distance is equal to the product of the delay time and the propagation velocity. In the case of filling level measurement, the transit distance corresponds to twice the distance between the antenna and the surface of the filled product. The actual useful echo signal and its delay time are determined on the basis of the so-called echo function or the digital envelope curve, the envelope curve reproducing the amplitudes of the echo signals as a function of the ‘antenna-surface of the filled product’ distance. The filling level itself is then obtained from the difference between the known distance of the antenna from the bottom of the container and the distance of the surface of the filled product from the antenna, determined by the measurement.
DE 31 07 444 A1 provides a description of a high-resolution pulsed radar method. A generator generates first microwave pulses and transmits them via an antenna at a predetermined transmission repetition rate in the direction of the surface of the filled product. A further generator generates reference microwave pulses, which are identical to the first microwave pulses but differ slightly from them in the repetition rate. The echo signal and the reference signal are mixed. At the output of the mixer there is an intermediate-frequency signal. The intermediate-frequency signal has the same waveform as the echo signal, but is stretched in comparison with the latter by a time dilation factor which is equal to a quotient of the transmission repetition rate and the difference in frequency between the first microwave pulses and the reference microwave pulses. At a transmission repetition rate of several megahertz, a difference in frequency of a few hertz and a microwave frequency of several gigahertz, the frequency of the intermediate-frequency signal is below 100 kHz. The advantage of the transformation to the intermediate frequency is that relatively slow, and consequently low-cost, electronic components can be used for signal acquisition and/or signal evaluation.
The signal evaluation takes place by means of the so-called envelope-curve evaluation. The envelope curve itself is the result of a rectification, optionally a logarithmization and a digitization of the intermediate frequency. The distance is determined by determining the distance from a reference signal of the useful echo signal, which represents the filling level. The number of sampling points between the two maxima with a constant sampling time is directly proportional to the ‘antenna-surface of the filled product’ distance.
To increase the measuring accuracy, it has already become known to use for the evaluation not only the maxima (peaks), which supply amplitude information, but also their phase relationships. For this purpose, the amplitude-modulated intermediate frequency is demodulated and broken down into its complex elements. This is achieved, for example, by so-called quadrature demodulation, i.e. the intermediate frequency is multiplied once by a sine oscillation (Q) and once by a cosine oscillation (I), both oscillations having a frequency similar to the intermediate frequency. The high frequencies produced by the multiplication are filtered out with a low-pass filter. The envelope-curve signal HK is obtained from the root of the sum of the squares of I (in-phase component) and Q (quadrature component): HK=·{square root over (I
2
+L +Q
2
+L )}. Subsequently, the customary amplitude evaluation is carried out; at the found locations of the maxima, the respective phase relationship and the difference of the two phase relationships are additionally determined. The distance of the antenna from the surface of the filled product is made up of a component comprising integral wavelengths, resulting from the amplitude evaluation, and a phase remainder.
The known methods produce reliable results as long as it is ensured that the signals received by the antenna are reflected exclusively at the surface of the filled product (incidence and reflection take place in the direction of the normal). As soon as so-called multipath propagation occurs, the measuring accuracy deteriorates. Multipath propagation means that the echo signals contain not only the actual useful signal but also an interference signal component, which is attributable to retroreflections of the measuring signals at the container wall or at other internal elements located in the space inside the container.
Previous proposals for solving this problem are confined to excluding the occurrence of interference signals from the outset. According to a first configuration, undesired retroreflections can be prevented by the antenna being positioned so far away from the container wall (or some other interfering retroreflector) that the impingement of measuring signals on these ‘interference retroreflectors’ can be reliably excluded.
The disadvantages of this possibility of avoidance are obvious; particularly with the considerable dimensions of the storage containers of the kind used in petrochemistry, for example, the retrofitting, repair or exchange of a filling-level measuring device in the central region of the container cover is much more difficult to accomplish than in the region of the edge. In addition to this there is a further source of error in the measurement of the filling level, the significance of which is all the greater the further the measuring device is away from the edge region of the container cover: the covers of large storage containers, such as storage tanks or silos, usually have an outwardly curved or tapered shape. If the side walls of the container bulge, which is always to be found in the region of the maximum filling level of the containers, the lowering of the cover, and consequently the changing of the ‘antenna-container bottom’ reference distance, has a greater effect in the central region than in the edge region.
Furthermore, it has previously been attempted to tackle the problem of multipath propagation by using an antenna with an optimized directional pattern. Such antennas, adjusted specifically for the individual case, are of course very expensive, which drives up the costs of the filling-level measuring device.
In certain applications it is necessary to replace the free-field measurement for determining the filling level by a measurement using a stilling well. Stilling wells are always used, for example, if the measurement results would be falsified by wave formation of the filled product within the container.
As already mentioned above, the filling level is determined by means of the delay time of the electromagnetic waves over twice the ‘antenna-surface of the filled product’ distance. In order that the measurement result is correct, the propagation velocity of the electromagnetic waves in the intermediate space between the antenna and the filled product must therefore be known exactly. In many applications, the propagation velocity is equated by approximation with the propagation velocity in air. However, this assumption is reliable only if the transverse dimensions of the space in which the electromagnetic waves propagate are large in comparison with the wavelength.
If the measurement of the filling level is performed by means of a stilling well, the assumption men
Lalla Robert
Möller Roland
Sinz Michael
Spanke Dietmar
Endress + Hauser GmbH + Co.
Rose McKinney & Evans LLP
Williams Hezron
Wilson Katina
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