CA2423781C - Method for sensing a predetermined level of a material, and device therefor - Google Patents
Method for sensing a predetermined level of a material, and device therefor Download PDFInfo
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- CA2423781C CA2423781C CA002423781A CA2423781A CA2423781C CA 2423781 C CA2423781 C CA 2423781C CA 002423781 A CA002423781 A CA 002423781A CA 2423781 A CA2423781 A CA 2423781A CA 2423781 C CA2423781 C CA 2423781C
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- time domain
- rods
- domain reflectometer
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/284—Electromagnetic waves
- G01F23/2845—Electromagnetic waves for discrete levels
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- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
- Measurement Of Resistance Or Impedance (AREA)
- Radar Systems Or Details Thereof (AREA)
- Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)
Abstract
The invention relates to a method and device for detecting the limit state o f a material having a given dielectric constant. To this end, the invention us es a holding device, inside of which two electrically conductive rods are arranged. When a limit state is attained, said rods are immersed into the material to be monitored and are connected to an electric circuit. Said circuit generates high-frequency transmit pulses, which are fed to the rods via the line according to the principle of time-domain reflectometry. The signals reflected into the air by the boundary layer of the material are evaluated on the basis of their waveform.
Description
Method for Sensinga Predetermined Level of a Material, and Device Therefor This invention relates to a method for sensing a predetermined level of a material having a given dielectric constant, using a holding means as a process bushing which contain one and of at least one electrically conductive rod whose othex end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over thd line tv the rod as a guided microwave on the principle of time domaln reflectometry, TDR, wherein the signals reflected at the interfaae between the material and air are returned to the echo amplifier far evaluation and the reflected signal=is atretched in time, and wherein three sections following each other succeaeively in time, namely transmit pulse (Section I), transit time (section II), and temporal sampling window (Section III), axe distinguished, with the temporal sampling window beginning at a starting instant The invention also relates to a time domain reflectometer for carrying out the above method.
To determine the level of a medium in a vessel, sensors based on time domain reflectometry are known. An overview of such sensors is given in U.S. Patent 5,609,059. Such sensars operate as continuous systems and are based on a transit-time measurement of electromagnetic signals propagating along an open waveguide, namely on the evaluation of the transit time and the reflection of a pulse on the waveguide. Depending on the level of the medium, the waveguide extends or does not extend into the medium; in the former case, it signals a limit value. The waveguide is, for instance, a Sommerfeld line, a Goubau line, a coaxial cable, a microstrip, or a coaxial or parallel arrangement of two conductors, for example two probe rods. When these come into contact with the medium, the characteristic impedance changes because of the different dielectric constants of the medium and air. At the interface to the external medium, or within the medium in case of stratification, because of the step change in its dielectric properties, causes a discontinuity in the transmission characteristics of the immersed waveguide, so that pulaes propagating along or within the waveguide will be at least partially reflected at these points. From the reflected signal, the distance or height of an interface can be determined by comparing the time of reception of the reflected pulse with the time of transmission. A transmit-time measurement takes place via an evaluation of the echo amplitude. At small dielectric constants, amplitude evaluation is not possible.
An overview of the behaviour of pulses on lines is given in Wolfgang Hilberg, "Impulse auf Leitungen", Oldenbourg Verlag 1981. A wave will propagate on a line unchanged as long as the line characteristics and particularly the cross section of the line remain unchanged. If this changes abruptly, the outgoing wave will be split into a reflected, returning partial wave and a refracted, ongoing partial wave. The wave reflected at the reflection point has the same form as the outgoing wave;
only the direction of propagation of the returning wave as well as the amplitude have changed. Tf a surge is applied to the open end of a line, i.e., at the transition from a given characteristic impedance to the characteristic impedance co and with matched conditions at the input, the voltage of the returning wave will double and the current will reverse. In the event of a short circuit of the line ends, the voltage will be reflected with oppo9ite signs, and the current will double with unchanged sign.
During operation of a TDR sensor, a transmit pulse is generated and transmitted with each period of a transmit trigger signal. The reflected signal is fed to a signal sampling circuit in order to make the short-time process repre' entable and evaluatable in stretched form. The signal sampling circuit is triggered with the trigger signal of the sampling frequency, the periodic signal being sampled at the sampling trigger instants. By introducing a time-proportional delay of the sampling trigger signal with respect to the transmit trigger signal, the sampling device produces an output signal whose amplitude characteristic is determined by the corresponding instantaneous values of the probe signal.
The output signal thus represents a time-stretched image of the probe signal. After being amplified and filtered, this output signal, or a temporal section of the same, forms the reflection profile from which the transit time of the reflected signal and, hence, the distance of the interface can be determined.
A problem encountered with such sensors is the high sensitivity to radio-frequency interference signala. An interference signal coupled into the waveguide will be superimposed on the reflected signal and will also be captured by the broadband sampling circuit. In electromagnetic compatibility (EMC) tests, a typical narrow-band interference signal is simulated by a carrier wave with a fundamental frequency of 80 MHz to 1 GHz with a low-frequency amplitude modulation (e.g. 1 kHz). If the carrier frequency is close to an integral multiple of the sampling frequency, i.e., if it i,s within aso-called frequency receiving window, this interference cannot be suppressed by low-pass filtering after the sampling device. Since the interforence signal is sampled at the sampling frequency in the manner of a bandpass filter sampling operation, the reflection profile, compared with the interference-free case, has a wave superimposed on it which complicates and possibly invalidates its evaluation. Because of the measurement principle employed, which involves the use of a broadband receiving circuit and a probe acting as a rod antenna, the interference factor is very high. As a result, if an interference lies within a frequency receiving window, the useful signal is, as a rule, difficult to evaluate.
DE 298 15 069 Ul discloses a TDR level sensor which comprisee a waveguide immersed in a material and having a sampling circuit connected to it which contains a transmit pulse generator for generating a pulsed radio-frequency signal, a receiver for receiving the radio-frequency signal, a transmit/receive isolation network for separating the transmitted and received radio-frequency signals, a sampler for sampling the received radio-frequency signal, a sampling pulse generator for controlling the sampler, and a buffer for temporarily storing the received radio-frequency signal. The sampling circuit has two oscillators, at least one of which is variable in frequency, and one of which controls the transmit pulse generator, while the other controls the sampling pulse generator. A frequency mixer determines the difference of the two frequencies, which is used to set the time stretch factor to a desired value. The reflected signal of such a device, however, is difficult to evaluate, because the signal and the reflected signal are nearly superposed and can be sufficiently separated only with a large amount of circuitry.
The object of the invention is to provide a method for sensing a predetermined level of a material and for determining the dielectric conatant of the material as well as a time domain reflectometer for use as a limit switch for sensing the predetermined level to carry out the method which provides increased immunity to interference, is universally applicable, namely regardless of temperature, pre9sure, or particularly the nature of the medium, i.e., liquid or bulk material, and is also suitable for materials having a small dielectric constant (from 1.8 to 5).
This object is attained by a method for sensing a predetermined level of a material having a given dielectric constant, using holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the interface between the material and air are returned to the echo amplifier for evaluation and the reflected signal is stretched in time, and wherein three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), are distinguished, with the temporal sampling window beginning at a starting instant, characterized by the following features:
a) in both operational states of the material to be detected, namely coverage, short circuit or near short circuit, and no coverage, open circuit, a reflected signal is produced at the rod-medium or rod-air interface by the change in characteristic impedance at the rod-medium or rod-air interface;
b) the waveform of the stretched reflected signal obtained at the echo amplifier serves to sense the predetermined level, with at least three significant points of the reflected signal within the temporal sampling window being evaluated numerically or by waveform analysis, and a reference voltage being determined from at least one waveform during Section II, with c) no coverage, open circuit, being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there is only one low point which lies below a predetermined first threshold, which differs from the reference voltage by an offset, d) a first covered state being detected if the reflected signal has the following characteriatics within the temporal sampling window:
= there ie a high point which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset, e) a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there are two low points, = the second low point in time lies below the first low point by a predetermined amount, f) a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there is a low point which lies below a predetermined first threshold, which differs from the reference voltage by an offset, = between the starting instant of the temporal sampling window and the low point is a point of inflection which lies between a local high point and a local low point, with the local low point and the local high point exceeding a predetermined minimum distance.
The object is further attained by a time domain reflectometer for use aa a limit switch for aenaing a predetermined level of a material having a given dielectric constant, the time domain reflectometer comprising holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reachedt wherein the end of the rod fitted in the holding means is connectod via an lectric lin0 to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes, wherein the radio-frequency transmit pulsea can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the matsrial-air interface are returned to the echo amplifier and stretched in time for evaluation, wherein the characteristic impedances of the rad and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section TII), can be distingui-shed, and wherein the waveforms of the reflected signals determined within the temporal sampling window 9erve to sense the predetermined level.
An essential advantage of the invention over the prior arz consists in the fact that it permits reliable evaluation even at small dielectric constants.
A time domain reflectometer for use as a limit switch for sensing a predetermined level of a material having a given dielectrie constant compriaes holding means a9 a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry (TDR), wherein the signalo reflected at the material-ai.r interface are returned to the electric circuit for evaluation, wherein the charaeteristic impedance of the rod is chosen to differ from the characteristic impedance of the material, wherein the waveform of the reflected signal serves to sense the predetermined level, and wherein up to three significant points of the waveform are evaluated. Preferably, the material haa a dielectric constant greater than 1.8.
The invention starts from the fact that the wave reflected at an interface has the same form as the outgoing wave; only the direction of propagation of the returning wave and the ampl.itude have changed. If two parallel rods are used in the process bushing, the characteristic impedance betwea.n the rods is changed by the material present between them. The characteristic impedance of such an arrangement is calculated from:
zZ 120 =ilnCd where Z ~ characteristic impedance/ohma sr a relative dielectric constant a distance between the centers of the rods/mm d ffi diameter of the rod9/mm By "open-circuit measurement", that reflected signal is understood which is reflect d at the rod ends in the open-circuit condition, i.e., without the rod ends being in contact with the material. At the predetermined level, the intensity of the reflection depends on the dielectric constant, so that at high dielectric constants, the greater part will be r flected at the air-medium interface and the rod ends immersed in the material will have hardly any effect on the signal waveform.
According to the invention, the shape of the reflected pulse ia evaluated, because at different characteristic impedances, not only reflections with differently high amplitudes and different polaritiea occur, but also deformations of the reflected eignal as a function of the dielectric constant of the material and a9 a function of the wetting of the rods with the material. If the dielectric constant of the material is greater than 10, a nearly complete reversal of the pulse will occur at the ends of the rods, because a near short circuit exists.
Typical media with a high dielectric constant are water, with E= :v 80, or Pril, with Er ms 40.
Medium dielectric constants lie in the range of 5-101 here, typical media are vinegar, honey, and ethanol. In this range, high reflections occur at the rods only conditionally, but these are far higher than in the case of inedia with dielectric constanta less than 5. Low dielectric constanta lie in the range of >1-5, 1 being the dielectric constant of air. Typical media in this range are coffee powder, plaster of paris, rice, salt, and sugar. At those dielectric constants, only a small reflection occurs at the rods, since the dielectric constants do not differ appreciably from the dielectric constant of air, so that this is nearly the case of an open-end line. However, the detection of materials with dielectric constants >1.8 already covers a spectrum of 95% of all materials used in the area of procesa automation.
Feature d) is detected at a high dielectric constant of the material, namely at a dielectric constant >10;
feature a) is det cted at a medium dielectric constant of the material, namely at a dielectric constant in the range from 5 to 10; and feature f) is detected at a small dielectric constant of the material, namely at a dielectric constant <5.
The result9 obtained with the invention show that the invention is ideally suited for sensing predetermined levels of all sorts of media with adhering behavior, particularly of bulk materials or liquids or viscous media, such as honey, because the method according to the invention and the time domain reflectometer can tolerate a certain range of adherence without falsifications and still recognize that no material or medium is on the roda. The time domain reflectometer according to the invention detects considerably more material than prior-art sensors, is insensitive to adherance of the medium to the rods at small dielectric constants of the medium, and permits reliable evaluation even at small dielectric constants.
The characteristic impedances and dimensions of the process bushing are preferably chosen so that a reflected signal is obtained which has up to six siqniticant points for reliable evaluation. Thus, up to six significant points of the waveform are preferably evaluated. The waveform of the reflected signal is preferably sampled after A/D conversion by means of the electronic circuit, which determinea significant points of the waveform falling into the temporal sampling window, particularly high point, low points, local high point, and local low point, whose locations are evaluated. The inventive evaluation of the characteristic waveform makes it possible to use short rod lengths even with relatively slow rise times of the transmit pulse of approx. 300-600 ps. The usability of short rod lengths is a further advantage over amplitude evaluation, for which considerably longer rods must be used.
The process bushing may advantageously be a threaded connection. In a preferred embodiment of the invention, the process bushing is a tubular bushing with an external metal thread which contains at least one insulating body as an insulating holder for the rods as well as the rods.
The temporal aampling window may be variable, and the starting instant of the sam may be defined by the fact that the reflected signal differs from the reference value by a predetermined amount and particularly is less than thi5 reference value by a predetermined amount.
Preferably, the holding means support two parallel rods, in which case the line is a coaxial line whose selectable length serves to obtain a predeterminable transit-time extension between the outgoing, transmitted pulses and the returning, reflected signals, and thus to make these separable in time, with the inner conductor of the coaxial line connected to one of the rods, and the other rod connected to ground of the electric circuit through the outer conductor or capacitively coupled to ground.
The electric circuit preferably comprises a delay circuit which generates a square-wave voltage for the transmit pulse which is applied to two branches and delayed, the delay of the first branch providing th transmit pulse and being greater than the delay of the second branch, which provides the sampling pulse, the time stretching being effected by means of a sequential sampling circuit.
In that case, the stretch factor need not be known.
In a preferred embodiment of the invention, the reflected signal is sampled by a four-diode sampling circuit and applied through the echo amplifier and an A/D converter to a microprocessor or microcontroller which evaluates the reflected signal and outputs the result, "coverage detected" or "no coverage detected", to a display unit or converts it into a switching signal.
The starting instant of the temporal sampling window can always be determined from the reflections which occur at the interface between the delay line and the proceas bushing due to a difference in characteristic impedance.
The determination of the starting instant in this manner has the advantage that the time stretch factor of the electronic circuit must only be present with an accuracy ls of approx. d10% to d20t, so that the circuit can be implemented at low cost.
Erom a plurality of waveforms during Section II, a baseline can be determined, e.g. by averaging over a plurality of waveforms, which acts as a reference voltage, with the starting instant of the temporal sampling window being defined by the fact that the reflected signal differs from the baseline by a predetermined value, and a determination being made as to whether the stretched signal derived from the reflected signal has within the temporal sampling window a high point, a first low point, a second low point, and/or a local low point and a local high point, and thus a point of inflection.
The stretched aignal derived from the reflected signal can be converted from analog to digital form and evaluated several times in a cycle, with a plurality of values being determined and an average voltage value being formed therefrom which serves as the baseline for the evaluation of the high point, Then, a determination is made as to whether the value of the stretched signal lies below the baseline by more than a predetermined amount, whereby the starting instant of the reflection is determined. Thereafter, in further cyclas, beginning with thia determined starting instant, the stretched signal is determined with the maximum sampling rate, and it is ascertained whether a high point, a second low point, or a local low point and a local high point is contained in the stretched signal.
For the level sensing, either filters, e.g. FIR filters, or two counters may be used, namely one counter for "coverage detected" and one counter for "no coverage detected".
Preferably, two parallel rods are arranged in the holding means. The line is preferably a coaxial line whose selectable length serves to introduce a predeterminable transit-time extension between the outgoing, transmitted pulees and the returning, reflected signals and thus to make them distinguishable by the electronic circuit. The coaxial line thus represents a delay line at the process bushing, with the inner conductor of the coaxial line connected to one of the rods, and the other rod grounded through the outer conductor. The delay line is thus coupled to the process bushing.
The characteristic impedance of the coaxial line may be chosen to be matched to that of the process bushing. In a preferred embodiment of the invention, however, the characteristic impedance of the coaxial line is chosen to be mismatched to that of the process bushing.
In one embodiment of the invention, the insulating body in the process bushing consists of layers of different materials with different dielectric constants, such as PEEK and Teflon, so that it is a laminated dielectric, with the materials, on the one hand, sealing off the procese bushing and, on the other hand, having the minimum thickness required to produce the reflected signal for determining the starting instant of the temporal sampling window. The process bushing is preferably cylindrical and is preferably made of electrically insulating material, such as Teflon (PTFE) or PEEK, within which the rods are located. This material may also provide protection for the rods during use in chemically aggres9ive media.
In a preferred embodiment of the invention, the rods are provided with a coating, such as a coating of Teflon, ceramic, or PEEK. If Teflon or PEEK is used, the thickness of the coating preferably ranges from 0.1 to 1 mm. In one embodiment of the invenZion, the length of the rods protruding from the process bushing is 2 to 15 cm, preferably 5 to 7 cm.
In another embodiment of the invention, the length of the delay line from the electric circuit to the connection to the rod ends fitted in the process bushing is at least 30 cm, preferably 30 to 60 cm, in order to simplify the temporal separation between transmit pulse and reflected signal. The distance bctween the rods preferably ranges between 10 mm and 30 mm. The characteriatic impedance can be selected via the ratio of this distance to the diameter of the rods. The height of the process bushing is preferably between 2 and 5 cm. In a further embodiment of the invention, the process bushing is made pressure-tight, preferably up to pressures of 30 bars.
17a According to one aspect of the present invention, there is provided a method for sensing a predetermined level of a material having a given dielectric constant, using holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over the electric line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the Interface between the material and air are returned to the echo amplifier for evaluation and the reflected signal is stretched in time, and wherein three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), are distinguished, with the temporal sampling window beginning at a starting instant, the method compressing the steps of producing at the rod-medium or rod-air interface by the change in characteristic impedance at the rod-medium or rod-air interface a reflected signal in both operational states of the material to be detected, namely coverage, short circuit or near short circuit, and no coverage, open circuit; and sensing the predetermined level by the waveform of the stretched reflected signal obtained at the echo amplifier, with at least three significant points of the reflected signal within the temporal sampling window being evaluated numerically or by waveform analysis, and a reference voltage being determined from at least one waveform during Section II, with: no coverage, open circuit, being detected 17b if the reflected signal has the following characteristics within the temporal sampling windows: there is only one low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset; a first covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a high point (HP) which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset; a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there are two low points (TP1, TP2), and the second low point (TP2) in time lies below the first low point (TP1) by a predetermined amount; a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset, and between the starting instant of the temporal sampling window and the low point (TP) is a point of inflection which lies between a local high point (LHP) and a local low point (LTP), with the local low point (LTP) and the local high point (LHP) exceeding a predetermined minimum distance.
According to another aspect of the present invention, there is provided a time domain reflectometer for use as a Limit switch for sensing a predetermined level of a material having a given dielectric constant, comprising:
holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein: the end of the rod fitted in the holding means is connected via an 17c electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes; the radio-frequency transmit pulses can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR; the signals reflected at the material-air interface are returned to the echo amplifier and stretched in time for evaluation; the characteristic impedances of the rod and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), can be distinguished; and the waveforms of the reflected signals are determined within the temporal sampling window serve to sense the predetermined level.
In the accompanying drawings:
1$
Fig. 1 is a block diagram of a measuring circuit with associated process bushing;
Figs. 2a and 2b show an equivalent circuit diagram of Lhe process bushing (a) and a aociated voltages (b);
Fig_ 3 shows measured echo curves of different materials;
Fig. 4 shows a schematic cross section through a process bushing;
Fig. 5 is a flowchart of an evaluation algorithm for the level sensing method using two counters for "detection"
and "no detection"i and Figs_ 6a-d show individual echo curvea with the extremC
values used for evaluation.
Fig. 1 shows a schematic block diagram of a measuring circuit with an associated cylindrical process bushing 12, which extends into a vessel 10 containing a material, the medium 11. A coaxial cable 13 is connected to Lhe rear ends of rods 3, 4 and serves aa a delay line.
Coaxial cable 13 ends in a TDR circuit 14 having two branches 18, 19.
During operation of the TDR sensor or the TDR sensor electronics 14, a transmit pulse XS is generated and transmitted by a transmitting etage 14 with each period of a transmit trigger signal XTS which is produced by a Lrigger generazor 23 and delayed by a constant time in a first delay stage 20, and which has a pulse repetition frequency fPRE. A typical pulse repetition frequency lies between a few 100 kHz and a few MHz.
In a signal sampling circuit, here a four-diode sampling circuit 22, of TDR circuit 14, the transmit pulse XS from transmitting stage 17 and the reflected signal Xprobe are sampled and stretched in time, so that the signal is easier to evaluate, e.g. in a microcontroller or microprocessor 16.
The periodically reflected signal Xprobe is fed to signal sampling circuit 22 to make the short-time process representable in stretched form and suitable for evaluation. Signal sampling circuit 22 is triggered with the trigger signal XTA of sampling frequency fA, the trigger signal XTA being delayed by a variable time in a second delay stage 21, and the periodic signal Xprobe being sampled at the sampling trigger instants_ This variable delay can be influenced by microprocessor 16. By introducing a time-proportional delay of the sampling trigger signal with respect to the transmit trigger signal, for instance by meane of a frequency of the sampling trigger signal XTA which is slightly lower than that of the transmit trigger signal XTS, or by a phase modulation of the sampling trigger signal XTA with respect to the transmit trigger signal XTS, signal sampling circuit 22 produces an output signal whose amplitude characteristic is determined by the corresponding instantaneous values of the probe signal.
The output signal thus represents a time-stretched image of the probe signal Xprobe.
After being amplified in an echo amplifier 15 and filtered, this output signal, or a temporal section thereof, forms the reflection profile XVideo, from which the transit time of the reflected signal, and thus the distance to the interface, can be determined. The reflection profile XVideo is fed via an A/D converter 24 to microprocessor 16, which evaluatea it in accordance with the invention and outputs the result, "coverage detected" or "no coverage detected", to a display unit 25 or converts it into a switching signal, for example.
The measured curves of the reflected signals are evaluated as described above by means of software, and maxima and/or minima and/or points of inflection are determined. From these characteristic curve points it follows that the reflected siqnal varies with varying dielectric constants, so that the invention is also suited for approximately determining the dielectric constant of a material. The waveforms, which are always similar in principle, differ significantly with respect to the dielectric constant of the material to be measured. As can be seen, the higher the dielectric constant of a material, the higher the overshoot between transmit pulse and reflected signal will be.
Difficulties may only be encountered with low dielectric constants of materials if these are below approximately 2.2 to 3, but materials with a small dielectric constant on the order of 2.2...3 and below can still be reliably discriminated, particularly if two parallel rods are used, in which case the process bushing according to the invention is well suited for evaluating both materials having a high dielectric constant and materials having a low dielectric constant.
Figs. 2a and 2b show an equivalent circuit diagram of the process bushing (Fig. 2a) and the associated voltages (Fig. 2b). To illustrate the invention, Fig, 2a shows an equivalent circuit of the process bushing, beginning on the left with a TDR circuit which is followed by a delay line that is connected to the rods in the process bushing. TDR circuit and delay line each have a characteristic impedance of, e.g., 75 ohms. The process bushing is, for instance, a tubular metal bushing incorporating aeveral insulating materials with different dielectric constants in which metal rods serving as probes are embedded at one end, the rods being wettable or uncoverable by a rising or falling level of the material. The insulating materials have a characteristic impedance of 140 and 170 ohms, respectively, and the metallic process bushing has a characteristic impedance of -245 ohms, for example. The rods have a characteristic impedance of, e.g., 250 ohms; the characteristic impedances of the material and of the rod ends are assumed to be unknown.
The reflected-signal voltages corresponding to this sequence in case of excitation with a positive voltage step are shown in Fig. 2b. What is important is that in the open-circuit condition of the two rods, the reflected signal shows an overshoot which has the same sign as the transmit pulse. In case of a short circuit, the waveform of the reflected signal shows a dip which is opposite in aign to the transmit pulse.
Fig. 3 shows measured echo curves of different materials which were obtained with a process bushing according to Fig. 4 in case of excitation with a pulse 30. 8hown at the left in the diagram is the transmit pulae which is applied to the rods. To the right, different reflections from different materials, including an open-circuit curve Lopen eircuit- are shown, namely reflections from Pril, honey, and coffee. Between the transmit pulse and the reflected signal is a relatively straight curve portion, which reflects the delay line and permits sugfiuient temporal separation of the transmit pulse from the reflected signal.
The resulting waveform of the stretched reflected signal at the echo amplifier serves to detect the predetermined level. To accomplish this, e.g. three significant points of the reflected signal located within a predetermined temporal sampling window are evaluated numerically or by waveform analysis-It can be seen that the open-circuit curve corresponds to a reflected signal having the same sign as the transmit pulse. When the voltage value of the reflected signal or of the stretched signal exceeds a predetermined value, the free end(s) of the rod(s) is(are) not wetted; the rods are in the open-circuit condition. If the rods have just changed to the open-circuit condition, a switching signal is obtained.
The predetermined level of the material is detected if only one high point, or one low point, depending on the sign of the transmit pulse, is detected which lies above a predetermined voltage threShold, and if the high point is opposite in sign to the tranamit pulse (near short circuit). In that case, the material has a dielectric constant >10. If two low points, or two high points, depending on the sign of the transmit pulae, are detected which are relatively far apart in time and have the same sign as the transmit pulse, and if the voltage difference measured between the two low points exceeds a predetermined threshold, a predetermined level of a material is detected which has a dielectric constant between 5 and 10.
A predetermined level of the material is also detected if a low point having the same aign as the transmit pulse, or a high point, depending on th sign of the transmit pulse, and a subsequent high point opposite in sign to the transmit pulse are detected which lie closely together in time, thus forming a quasi-point of inflection, and if the voltage difference measured between the low point and the high point exceeds a predetermined threshold. The quasi-point of inflection of the curve for coffee is defined here by two turning points lying closely together in time, minimum and maximum according to Fig. 3.
If the material has a high di lectric constant above 10, the feature will be detected that there is only one high point which lies above a predetermined voltage threshold and is opposite in sign to the transmit pulse (near short circuit), If the material has a medium dielectric constant in the range from S to 10, the feature will be detected that there are two low points which are relatively far apart in time and have the same sign as the tranamit pulse, with the voltage difference measured between the two low points exceeding a predetermined threshold.
If the material ha5 a small dielectric constant <5, the feature will be detected that there are a low point having the same sign as the transmit pulse and a subsequent high point opposite in sign to the transmit pulse which are close together in time and thus form a quasi-point of inflection, with the voltage difference measured between the low point and the high point exceeding a predetermined threshold.
The two low points of the reflected signal, which are relatively far apart in time, are 9eparated by an interval between 3 and 10 ms, for example. By contrast, in the case of a material with a small dielectric constant, namely between 1.5-5, the low point is separated from the sub9equent high point of the reflected signal by an interval of typically only 0.1 to 3 m9.
According to the basic principles of a TDR sensor, the stretched signal derived from the rerlected signal is converted from analog to digital form and evaluated several times in a cycle, with a plurality of values being determined and an average voltage value being formed therefrom which serves as the baseline for triggering the starting point of the temporal sampling window and for the evaluation of the high point. Then, a determination is made as to whether the value of the stretched signal lies below the baseline by more than a predetermined amount, whereby the starting instant of the reflection is determined. After that, in further cycles, beginning with this determined starting instant, the atretched signal is determined with the high sampling rate, and it i9 ascertained whether a high point, a second low point, or quasi-point of inflection is contained in the stretched 9ignal.
To sense the predetermined level, use is preferably made of two counters, namely one counter for "detection" and one for "no detection", and of an evaluation algorithm according to the flowchart of Fig. 5, for example. The detection of the state "covered" or "not covered" 1s preferably filtered, e.g. by means of an FIR filter, before being output. The repetition frequency may be increased for the purpose of improving the immunity to interference, gor example.
Fig. 4 shows a achematic cross section through a process bushing. The process bushing, which is mounted on a pressure tank, for example, is a cylindrical bushing 1 with a metal thread, containing holding means 8, 9 of insulating material as well as rods 3 and 4, whose ends are connected to conductora 7 and 6, respectively, of a coaxial line 5, which con9titutes a delay line. The characteristic impedance of the coaxial line may be matched to that of the electric circuit, but it is not matched to the characteristic impedance of the process bushing, so that discontinuities exist between the characteristic impedances, whereby a desired reflection occurs at the process bushing which serves to unambiguously determine the beginning of the reflected signal. The characteristic impedance of the coaxial line and of the electric circuit may be between 65 and 85 ohms, preferably 75 ohms.
The electrically insulating material 8 may be disk of Teflon, with the ends of the rods 3, 4 additionally passed through a disk 9 of PEEK (polyether ether ketone) which is placed on the Teflon disk. The cylindrical process bushing 1 has a height of approximately 4 cm. The rods 3, 4, which extend through the Teflon cylinder 1, are arranged symmetrically within the cylinder 1. Their free length ranges from 2 to 15 cm, preferably from 5 to 7 cm.
The process bushing may also be a cylindrical bushing of only one insulating material, such as Teflon (PTFE) or PEEK (polyether ether ketone), which is a partially crystalline thermoplastic, in which the rods are embedded. In that case, too, the characteristic impedance vf the process bushing is not or not precisely matched to the characteristic impedance of the delay line or coaxial line.
The time domain reflectometer according to the invention haa the advantage that, particularly as a result of the uae of two parallel rods, a good reflection of the reflected pulses is achieved, which, because of the delay line, are sufficiently separated in time from the transmit pulses, so that the reflective characterietics, namely the resulting waveforms of the reflected 9ignals, are well suited for evaluation. In a further variant of the time domain reflectometer, the rods are coated with Teflon or ceramic, the thickness of the Teflon coating preferably ranging from 0.1 to 1 mm. In a further embodiment of the invention, the distance (d) between the rods ranges from 10 to 30 mm, and the height of the process bushing ranges between 2 and 5 cm.
Figs. 6a-d show individual echo curves with the extreme values used for evaluation. Fig. 6a shows an open-circuit echo curve. An open circuit, no coverage, is detected if the reflected signal has the following characteristics in the temporal sampling window: There ia only one low point TP which is below a predetermined first threahold (Threshold 1). Threshold 1 is determined from the baseline and a predetermined offset.
Fig. 6b show9 an echo curve for Pril. A firat covered state is detected if the reflected signal has the following characteristics within the temporal sampling window: There is a high point HP which exceeds a predetermined second threshold (Threshold 2). Threshold 2 is determined from the baseline and the predetermined offset Fig. 6c shows an echo curve for honey. The second covered state is detected if the reflected signal has the following characteristics within the temporal sampling window:
- There are two low points TP1, TF2 which have the same aense as the transmit pulse.
- The secvnd low point TP2 lies below the low point Tel by a predetermined amount As.
Fig. 6d ahows an echo curve for coffee. The third covered atate is detected if the reflected signal has the following characteri9tics within the temporal sampling window:
- There is only one low point TP which lies below a predetermined first threshold (threshold 1). Threshold 1 is determined from the baseline and a predetermined offset.
- aetween the starting instant of the tamporal sampling window and the low point TP is a point of inflection which lies between a local low point LTP and a local high point LHP. The local low point LTP and the local high point exceed a predetermined minimum distance.
The starting instant of the temporal sampling window is determined as follows;
- A baseline is determined in Section II.
- The reflected signal falls below the baseline in Section III by a predetermined amount.
The starting instant of the temporal sampling window can generally be determined from the reflections which occur at the interface between the delay line and the process bushing due to differances in characteristic impedance.
The determination of the starting instant in this manner ofters the advantage that the time stretch factor of the electronic circuit 14 must only be present with an accuracy of approx. 10 to 20%, so that the electronic circuit 14 can be implemented at low cost.
To determine the level of a medium in a vessel, sensors based on time domain reflectometry are known. An overview of such sensors is given in U.S. Patent 5,609,059. Such sensars operate as continuous systems and are based on a transit-time measurement of electromagnetic signals propagating along an open waveguide, namely on the evaluation of the transit time and the reflection of a pulse on the waveguide. Depending on the level of the medium, the waveguide extends or does not extend into the medium; in the former case, it signals a limit value. The waveguide is, for instance, a Sommerfeld line, a Goubau line, a coaxial cable, a microstrip, or a coaxial or parallel arrangement of two conductors, for example two probe rods. When these come into contact with the medium, the characteristic impedance changes because of the different dielectric constants of the medium and air. At the interface to the external medium, or within the medium in case of stratification, because of the step change in its dielectric properties, causes a discontinuity in the transmission characteristics of the immersed waveguide, so that pulaes propagating along or within the waveguide will be at least partially reflected at these points. From the reflected signal, the distance or height of an interface can be determined by comparing the time of reception of the reflected pulse with the time of transmission. A transmit-time measurement takes place via an evaluation of the echo amplitude. At small dielectric constants, amplitude evaluation is not possible.
An overview of the behaviour of pulses on lines is given in Wolfgang Hilberg, "Impulse auf Leitungen", Oldenbourg Verlag 1981. A wave will propagate on a line unchanged as long as the line characteristics and particularly the cross section of the line remain unchanged. If this changes abruptly, the outgoing wave will be split into a reflected, returning partial wave and a refracted, ongoing partial wave. The wave reflected at the reflection point has the same form as the outgoing wave;
only the direction of propagation of the returning wave as well as the amplitude have changed. Tf a surge is applied to the open end of a line, i.e., at the transition from a given characteristic impedance to the characteristic impedance co and with matched conditions at the input, the voltage of the returning wave will double and the current will reverse. In the event of a short circuit of the line ends, the voltage will be reflected with oppo9ite signs, and the current will double with unchanged sign.
During operation of a TDR sensor, a transmit pulse is generated and transmitted with each period of a transmit trigger signal. The reflected signal is fed to a signal sampling circuit in order to make the short-time process repre' entable and evaluatable in stretched form. The signal sampling circuit is triggered with the trigger signal of the sampling frequency, the periodic signal being sampled at the sampling trigger instants. By introducing a time-proportional delay of the sampling trigger signal with respect to the transmit trigger signal, the sampling device produces an output signal whose amplitude characteristic is determined by the corresponding instantaneous values of the probe signal.
The output signal thus represents a time-stretched image of the probe signal. After being amplified and filtered, this output signal, or a temporal section of the same, forms the reflection profile from which the transit time of the reflected signal and, hence, the distance of the interface can be determined.
A problem encountered with such sensors is the high sensitivity to radio-frequency interference signala. An interference signal coupled into the waveguide will be superimposed on the reflected signal and will also be captured by the broadband sampling circuit. In electromagnetic compatibility (EMC) tests, a typical narrow-band interference signal is simulated by a carrier wave with a fundamental frequency of 80 MHz to 1 GHz with a low-frequency amplitude modulation (e.g. 1 kHz). If the carrier frequency is close to an integral multiple of the sampling frequency, i.e., if it i,s within aso-called frequency receiving window, this interference cannot be suppressed by low-pass filtering after the sampling device. Since the interforence signal is sampled at the sampling frequency in the manner of a bandpass filter sampling operation, the reflection profile, compared with the interference-free case, has a wave superimposed on it which complicates and possibly invalidates its evaluation. Because of the measurement principle employed, which involves the use of a broadband receiving circuit and a probe acting as a rod antenna, the interference factor is very high. As a result, if an interference lies within a frequency receiving window, the useful signal is, as a rule, difficult to evaluate.
DE 298 15 069 Ul discloses a TDR level sensor which comprisee a waveguide immersed in a material and having a sampling circuit connected to it which contains a transmit pulse generator for generating a pulsed radio-frequency signal, a receiver for receiving the radio-frequency signal, a transmit/receive isolation network for separating the transmitted and received radio-frequency signals, a sampler for sampling the received radio-frequency signal, a sampling pulse generator for controlling the sampler, and a buffer for temporarily storing the received radio-frequency signal. The sampling circuit has two oscillators, at least one of which is variable in frequency, and one of which controls the transmit pulse generator, while the other controls the sampling pulse generator. A frequency mixer determines the difference of the two frequencies, which is used to set the time stretch factor to a desired value. The reflected signal of such a device, however, is difficult to evaluate, because the signal and the reflected signal are nearly superposed and can be sufficiently separated only with a large amount of circuitry.
The object of the invention is to provide a method for sensing a predetermined level of a material and for determining the dielectric conatant of the material as well as a time domain reflectometer for use as a limit switch for sensing the predetermined level to carry out the method which provides increased immunity to interference, is universally applicable, namely regardless of temperature, pre9sure, or particularly the nature of the medium, i.e., liquid or bulk material, and is also suitable for materials having a small dielectric constant (from 1.8 to 5).
This object is attained by a method for sensing a predetermined level of a material having a given dielectric constant, using holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the interface between the material and air are returned to the echo amplifier for evaluation and the reflected signal is stretched in time, and wherein three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), are distinguished, with the temporal sampling window beginning at a starting instant, characterized by the following features:
a) in both operational states of the material to be detected, namely coverage, short circuit or near short circuit, and no coverage, open circuit, a reflected signal is produced at the rod-medium or rod-air interface by the change in characteristic impedance at the rod-medium or rod-air interface;
b) the waveform of the stretched reflected signal obtained at the echo amplifier serves to sense the predetermined level, with at least three significant points of the reflected signal within the temporal sampling window being evaluated numerically or by waveform analysis, and a reference voltage being determined from at least one waveform during Section II, with c) no coverage, open circuit, being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there is only one low point which lies below a predetermined first threshold, which differs from the reference voltage by an offset, d) a first covered state being detected if the reflected signal has the following characteriatics within the temporal sampling window:
= there ie a high point which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset, e) a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there are two low points, = the second low point in time lies below the first low point by a predetermined amount, f) a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window:
= there is a low point which lies below a predetermined first threshold, which differs from the reference voltage by an offset, = between the starting instant of the temporal sampling window and the low point is a point of inflection which lies between a local high point and a local low point, with the local low point and the local high point exceeding a predetermined minimum distance.
The object is further attained by a time domain reflectometer for use aa a limit switch for aenaing a predetermined level of a material having a given dielectric constant, the time domain reflectometer comprising holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reachedt wherein the end of the rod fitted in the holding means is connectod via an lectric lin0 to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes, wherein the radio-frequency transmit pulsea can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the matsrial-air interface are returned to the echo amplifier and stretched in time for evaluation, wherein the characteristic impedances of the rad and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section TII), can be distingui-shed, and wherein the waveforms of the reflected signals determined within the temporal sampling window 9erve to sense the predetermined level.
An essential advantage of the invention over the prior arz consists in the fact that it permits reliable evaluation even at small dielectric constants.
A time domain reflectometer for use as a limit switch for sensing a predetermined level of a material having a given dielectrie constant compriaes holding means a9 a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry (TDR), wherein the signalo reflected at the material-ai.r interface are returned to the electric circuit for evaluation, wherein the charaeteristic impedance of the rod is chosen to differ from the characteristic impedance of the material, wherein the waveform of the reflected signal serves to sense the predetermined level, and wherein up to three significant points of the waveform are evaluated. Preferably, the material haa a dielectric constant greater than 1.8.
The invention starts from the fact that the wave reflected at an interface has the same form as the outgoing wave; only the direction of propagation of the returning wave and the ampl.itude have changed. If two parallel rods are used in the process bushing, the characteristic impedance betwea.n the rods is changed by the material present between them. The characteristic impedance of such an arrangement is calculated from:
zZ 120 =ilnCd where Z ~ characteristic impedance/ohma sr a relative dielectric constant a distance between the centers of the rods/mm d ffi diameter of the rod9/mm By "open-circuit measurement", that reflected signal is understood which is reflect d at the rod ends in the open-circuit condition, i.e., without the rod ends being in contact with the material. At the predetermined level, the intensity of the reflection depends on the dielectric constant, so that at high dielectric constants, the greater part will be r flected at the air-medium interface and the rod ends immersed in the material will have hardly any effect on the signal waveform.
According to the invention, the shape of the reflected pulse ia evaluated, because at different characteristic impedances, not only reflections with differently high amplitudes and different polaritiea occur, but also deformations of the reflected eignal as a function of the dielectric constant of the material and a9 a function of the wetting of the rods with the material. If the dielectric constant of the material is greater than 10, a nearly complete reversal of the pulse will occur at the ends of the rods, because a near short circuit exists.
Typical media with a high dielectric constant are water, with E= :v 80, or Pril, with Er ms 40.
Medium dielectric constants lie in the range of 5-101 here, typical media are vinegar, honey, and ethanol. In this range, high reflections occur at the rods only conditionally, but these are far higher than in the case of inedia with dielectric constanta less than 5. Low dielectric constanta lie in the range of >1-5, 1 being the dielectric constant of air. Typical media in this range are coffee powder, plaster of paris, rice, salt, and sugar. At those dielectric constants, only a small reflection occurs at the rods, since the dielectric constants do not differ appreciably from the dielectric constant of air, so that this is nearly the case of an open-end line. However, the detection of materials with dielectric constants >1.8 already covers a spectrum of 95% of all materials used in the area of procesa automation.
Feature d) is detected at a high dielectric constant of the material, namely at a dielectric constant >10;
feature a) is det cted at a medium dielectric constant of the material, namely at a dielectric constant in the range from 5 to 10; and feature f) is detected at a small dielectric constant of the material, namely at a dielectric constant <5.
The result9 obtained with the invention show that the invention is ideally suited for sensing predetermined levels of all sorts of media with adhering behavior, particularly of bulk materials or liquids or viscous media, such as honey, because the method according to the invention and the time domain reflectometer can tolerate a certain range of adherence without falsifications and still recognize that no material or medium is on the roda. The time domain reflectometer according to the invention detects considerably more material than prior-art sensors, is insensitive to adherance of the medium to the rods at small dielectric constants of the medium, and permits reliable evaluation even at small dielectric constants.
The characteristic impedances and dimensions of the process bushing are preferably chosen so that a reflected signal is obtained which has up to six siqniticant points for reliable evaluation. Thus, up to six significant points of the waveform are preferably evaluated. The waveform of the reflected signal is preferably sampled after A/D conversion by means of the electronic circuit, which determinea significant points of the waveform falling into the temporal sampling window, particularly high point, low points, local high point, and local low point, whose locations are evaluated. The inventive evaluation of the characteristic waveform makes it possible to use short rod lengths even with relatively slow rise times of the transmit pulse of approx. 300-600 ps. The usability of short rod lengths is a further advantage over amplitude evaluation, for which considerably longer rods must be used.
The process bushing may advantageously be a threaded connection. In a preferred embodiment of the invention, the process bushing is a tubular bushing with an external metal thread which contains at least one insulating body as an insulating holder for the rods as well as the rods.
The temporal aampling window may be variable, and the starting instant of the sam may be defined by the fact that the reflected signal differs from the reference value by a predetermined amount and particularly is less than thi5 reference value by a predetermined amount.
Preferably, the holding means support two parallel rods, in which case the line is a coaxial line whose selectable length serves to obtain a predeterminable transit-time extension between the outgoing, transmitted pulses and the returning, reflected signals, and thus to make these separable in time, with the inner conductor of the coaxial line connected to one of the rods, and the other rod connected to ground of the electric circuit through the outer conductor or capacitively coupled to ground.
The electric circuit preferably comprises a delay circuit which generates a square-wave voltage for the transmit pulse which is applied to two branches and delayed, the delay of the first branch providing th transmit pulse and being greater than the delay of the second branch, which provides the sampling pulse, the time stretching being effected by means of a sequential sampling circuit.
In that case, the stretch factor need not be known.
In a preferred embodiment of the invention, the reflected signal is sampled by a four-diode sampling circuit and applied through the echo amplifier and an A/D converter to a microprocessor or microcontroller which evaluates the reflected signal and outputs the result, "coverage detected" or "no coverage detected", to a display unit or converts it into a switching signal.
The starting instant of the temporal sampling window can always be determined from the reflections which occur at the interface between the delay line and the proceas bushing due to a difference in characteristic impedance.
The determination of the starting instant in this manner has the advantage that the time stretch factor of the electronic circuit must only be present with an accuracy ls of approx. d10% to d20t, so that the circuit can be implemented at low cost.
Erom a plurality of waveforms during Section II, a baseline can be determined, e.g. by averaging over a plurality of waveforms, which acts as a reference voltage, with the starting instant of the temporal sampling window being defined by the fact that the reflected signal differs from the baseline by a predetermined value, and a determination being made as to whether the stretched signal derived from the reflected signal has within the temporal sampling window a high point, a first low point, a second low point, and/or a local low point and a local high point, and thus a point of inflection.
The stretched aignal derived from the reflected signal can be converted from analog to digital form and evaluated several times in a cycle, with a plurality of values being determined and an average voltage value being formed therefrom which serves as the baseline for the evaluation of the high point, Then, a determination is made as to whether the value of the stretched signal lies below the baseline by more than a predetermined amount, whereby the starting instant of the reflection is determined. Thereafter, in further cyclas, beginning with thia determined starting instant, the stretched signal is determined with the maximum sampling rate, and it is ascertained whether a high point, a second low point, or a local low point and a local high point is contained in the stretched signal.
For the level sensing, either filters, e.g. FIR filters, or two counters may be used, namely one counter for "coverage detected" and one counter for "no coverage detected".
Preferably, two parallel rods are arranged in the holding means. The line is preferably a coaxial line whose selectable length serves to introduce a predeterminable transit-time extension between the outgoing, transmitted pulees and the returning, reflected signals and thus to make them distinguishable by the electronic circuit. The coaxial line thus represents a delay line at the process bushing, with the inner conductor of the coaxial line connected to one of the rods, and the other rod grounded through the outer conductor. The delay line is thus coupled to the process bushing.
The characteristic impedance of the coaxial line may be chosen to be matched to that of the process bushing. In a preferred embodiment of the invention, however, the characteristic impedance of the coaxial line is chosen to be mismatched to that of the process bushing.
In one embodiment of the invention, the insulating body in the process bushing consists of layers of different materials with different dielectric constants, such as PEEK and Teflon, so that it is a laminated dielectric, with the materials, on the one hand, sealing off the procese bushing and, on the other hand, having the minimum thickness required to produce the reflected signal for determining the starting instant of the temporal sampling window. The process bushing is preferably cylindrical and is preferably made of electrically insulating material, such as Teflon (PTFE) or PEEK, within which the rods are located. This material may also provide protection for the rods during use in chemically aggres9ive media.
In a preferred embodiment of the invention, the rods are provided with a coating, such as a coating of Teflon, ceramic, or PEEK. If Teflon or PEEK is used, the thickness of the coating preferably ranges from 0.1 to 1 mm. In one embodiment of the invenZion, the length of the rods protruding from the process bushing is 2 to 15 cm, preferably 5 to 7 cm.
In another embodiment of the invention, the length of the delay line from the electric circuit to the connection to the rod ends fitted in the process bushing is at least 30 cm, preferably 30 to 60 cm, in order to simplify the temporal separation between transmit pulse and reflected signal. The distance bctween the rods preferably ranges between 10 mm and 30 mm. The characteriatic impedance can be selected via the ratio of this distance to the diameter of the rods. The height of the process bushing is preferably between 2 and 5 cm. In a further embodiment of the invention, the process bushing is made pressure-tight, preferably up to pressures of 30 bars.
17a According to one aspect of the present invention, there is provided a method for sensing a predetermined level of a material having a given dielectric constant, using holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over the electric line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the Interface between the material and air are returned to the echo amplifier for evaluation and the reflected signal is stretched in time, and wherein three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), are distinguished, with the temporal sampling window beginning at a starting instant, the method compressing the steps of producing at the rod-medium or rod-air interface by the change in characteristic impedance at the rod-medium or rod-air interface a reflected signal in both operational states of the material to be detected, namely coverage, short circuit or near short circuit, and no coverage, open circuit; and sensing the predetermined level by the waveform of the stretched reflected signal obtained at the echo amplifier, with at least three significant points of the reflected signal within the temporal sampling window being evaluated numerically or by waveform analysis, and a reference voltage being determined from at least one waveform during Section II, with: no coverage, open circuit, being detected 17b if the reflected signal has the following characteristics within the temporal sampling windows: there is only one low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset; a first covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a high point (HP) which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset; a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there are two low points (TP1, TP2), and the second low point (TP2) in time lies below the first low point (TP1) by a predetermined amount; a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset, and between the starting instant of the temporal sampling window and the low point (TP) is a point of inflection which lies between a local high point (LHP) and a local low point (LTP), with the local low point (LTP) and the local high point (LHP) exceeding a predetermined minimum distance.
According to another aspect of the present invention, there is provided a time domain reflectometer for use as a Limit switch for sensing a predetermined level of a material having a given dielectric constant, comprising:
holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein: the end of the rod fitted in the holding means is connected via an 17c electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes; the radio-frequency transmit pulses can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR; the signals reflected at the material-air interface are returned to the echo amplifier and stretched in time for evaluation; the characteristic impedances of the rod and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), can be distinguished; and the waveforms of the reflected signals are determined within the temporal sampling window serve to sense the predetermined level.
In the accompanying drawings:
1$
Fig. 1 is a block diagram of a measuring circuit with associated process bushing;
Figs. 2a and 2b show an equivalent circuit diagram of Lhe process bushing (a) and a aociated voltages (b);
Fig_ 3 shows measured echo curves of different materials;
Fig. 4 shows a schematic cross section through a process bushing;
Fig. 5 is a flowchart of an evaluation algorithm for the level sensing method using two counters for "detection"
and "no detection"i and Figs_ 6a-d show individual echo curvea with the extremC
values used for evaluation.
Fig. 1 shows a schematic block diagram of a measuring circuit with an associated cylindrical process bushing 12, which extends into a vessel 10 containing a material, the medium 11. A coaxial cable 13 is connected to Lhe rear ends of rods 3, 4 and serves aa a delay line.
Coaxial cable 13 ends in a TDR circuit 14 having two branches 18, 19.
During operation of the TDR sensor or the TDR sensor electronics 14, a transmit pulse XS is generated and transmitted by a transmitting etage 14 with each period of a transmit trigger signal XTS which is produced by a Lrigger generazor 23 and delayed by a constant time in a first delay stage 20, and which has a pulse repetition frequency fPRE. A typical pulse repetition frequency lies between a few 100 kHz and a few MHz.
In a signal sampling circuit, here a four-diode sampling circuit 22, of TDR circuit 14, the transmit pulse XS from transmitting stage 17 and the reflected signal Xprobe are sampled and stretched in time, so that the signal is easier to evaluate, e.g. in a microcontroller or microprocessor 16.
The periodically reflected signal Xprobe is fed to signal sampling circuit 22 to make the short-time process representable in stretched form and suitable for evaluation. Signal sampling circuit 22 is triggered with the trigger signal XTA of sampling frequency fA, the trigger signal XTA being delayed by a variable time in a second delay stage 21, and the periodic signal Xprobe being sampled at the sampling trigger instants_ This variable delay can be influenced by microprocessor 16. By introducing a time-proportional delay of the sampling trigger signal with respect to the transmit trigger signal, for instance by meane of a frequency of the sampling trigger signal XTA which is slightly lower than that of the transmit trigger signal XTS, or by a phase modulation of the sampling trigger signal XTA with respect to the transmit trigger signal XTS, signal sampling circuit 22 produces an output signal whose amplitude characteristic is determined by the corresponding instantaneous values of the probe signal.
The output signal thus represents a time-stretched image of the probe signal Xprobe.
After being amplified in an echo amplifier 15 and filtered, this output signal, or a temporal section thereof, forms the reflection profile XVideo, from which the transit time of the reflected signal, and thus the distance to the interface, can be determined. The reflection profile XVideo is fed via an A/D converter 24 to microprocessor 16, which evaluatea it in accordance with the invention and outputs the result, "coverage detected" or "no coverage detected", to a display unit 25 or converts it into a switching signal, for example.
The measured curves of the reflected signals are evaluated as described above by means of software, and maxima and/or minima and/or points of inflection are determined. From these characteristic curve points it follows that the reflected siqnal varies with varying dielectric constants, so that the invention is also suited for approximately determining the dielectric constant of a material. The waveforms, which are always similar in principle, differ significantly with respect to the dielectric constant of the material to be measured. As can be seen, the higher the dielectric constant of a material, the higher the overshoot between transmit pulse and reflected signal will be.
Difficulties may only be encountered with low dielectric constants of materials if these are below approximately 2.2 to 3, but materials with a small dielectric constant on the order of 2.2...3 and below can still be reliably discriminated, particularly if two parallel rods are used, in which case the process bushing according to the invention is well suited for evaluating both materials having a high dielectric constant and materials having a low dielectric constant.
Figs. 2a and 2b show an equivalent circuit diagram of the process bushing (Fig. 2a) and the associated voltages (Fig. 2b). To illustrate the invention, Fig, 2a shows an equivalent circuit of the process bushing, beginning on the left with a TDR circuit which is followed by a delay line that is connected to the rods in the process bushing. TDR circuit and delay line each have a characteristic impedance of, e.g., 75 ohms. The process bushing is, for instance, a tubular metal bushing incorporating aeveral insulating materials with different dielectric constants in which metal rods serving as probes are embedded at one end, the rods being wettable or uncoverable by a rising or falling level of the material. The insulating materials have a characteristic impedance of 140 and 170 ohms, respectively, and the metallic process bushing has a characteristic impedance of -245 ohms, for example. The rods have a characteristic impedance of, e.g., 250 ohms; the characteristic impedances of the material and of the rod ends are assumed to be unknown.
The reflected-signal voltages corresponding to this sequence in case of excitation with a positive voltage step are shown in Fig. 2b. What is important is that in the open-circuit condition of the two rods, the reflected signal shows an overshoot which has the same sign as the transmit pulse. In case of a short circuit, the waveform of the reflected signal shows a dip which is opposite in aign to the transmit pulse.
Fig. 3 shows measured echo curves of different materials which were obtained with a process bushing according to Fig. 4 in case of excitation with a pulse 30. 8hown at the left in the diagram is the transmit pulae which is applied to the rods. To the right, different reflections from different materials, including an open-circuit curve Lopen eircuit- are shown, namely reflections from Pril, honey, and coffee. Between the transmit pulse and the reflected signal is a relatively straight curve portion, which reflects the delay line and permits sugfiuient temporal separation of the transmit pulse from the reflected signal.
The resulting waveform of the stretched reflected signal at the echo amplifier serves to detect the predetermined level. To accomplish this, e.g. three significant points of the reflected signal located within a predetermined temporal sampling window are evaluated numerically or by waveform analysis-It can be seen that the open-circuit curve corresponds to a reflected signal having the same sign as the transmit pulse. When the voltage value of the reflected signal or of the stretched signal exceeds a predetermined value, the free end(s) of the rod(s) is(are) not wetted; the rods are in the open-circuit condition. If the rods have just changed to the open-circuit condition, a switching signal is obtained.
The predetermined level of the material is detected if only one high point, or one low point, depending on the sign of the transmit pulse, is detected which lies above a predetermined voltage threShold, and if the high point is opposite in sign to the tranamit pulse (near short circuit). In that case, the material has a dielectric constant >10. If two low points, or two high points, depending on the sign of the transmit pulae, are detected which are relatively far apart in time and have the same sign as the transmit pulse, and if the voltage difference measured between the two low points exceeds a predetermined threshold, a predetermined level of a material is detected which has a dielectric constant between 5 and 10.
A predetermined level of the material is also detected if a low point having the same aign as the transmit pulse, or a high point, depending on th sign of the transmit pulse, and a subsequent high point opposite in sign to the transmit pulse are detected which lie closely together in time, thus forming a quasi-point of inflection, and if the voltage difference measured between the low point and the high point exceeds a predetermined threshold. The quasi-point of inflection of the curve for coffee is defined here by two turning points lying closely together in time, minimum and maximum according to Fig. 3.
If the material has a high di lectric constant above 10, the feature will be detected that there is only one high point which lies above a predetermined voltage threshold and is opposite in sign to the transmit pulse (near short circuit), If the material has a medium dielectric constant in the range from S to 10, the feature will be detected that there are two low points which are relatively far apart in time and have the same sign as the tranamit pulse, with the voltage difference measured between the two low points exceeding a predetermined threshold.
If the material ha5 a small dielectric constant <5, the feature will be detected that there are a low point having the same sign as the transmit pulse and a subsequent high point opposite in sign to the transmit pulse which are close together in time and thus form a quasi-point of inflection, with the voltage difference measured between the low point and the high point exceeding a predetermined threshold.
The two low points of the reflected signal, which are relatively far apart in time, are 9eparated by an interval between 3 and 10 ms, for example. By contrast, in the case of a material with a small dielectric constant, namely between 1.5-5, the low point is separated from the sub9equent high point of the reflected signal by an interval of typically only 0.1 to 3 m9.
According to the basic principles of a TDR sensor, the stretched signal derived from the rerlected signal is converted from analog to digital form and evaluated several times in a cycle, with a plurality of values being determined and an average voltage value being formed therefrom which serves as the baseline for triggering the starting point of the temporal sampling window and for the evaluation of the high point. Then, a determination is made as to whether the value of the stretched signal lies below the baseline by more than a predetermined amount, whereby the starting instant of the reflection is determined. After that, in further cycles, beginning with this determined starting instant, the atretched signal is determined with the high sampling rate, and it i9 ascertained whether a high point, a second low point, or quasi-point of inflection is contained in the stretched 9ignal.
To sense the predetermined level, use is preferably made of two counters, namely one counter for "detection" and one for "no detection", and of an evaluation algorithm according to the flowchart of Fig. 5, for example. The detection of the state "covered" or "not covered" 1s preferably filtered, e.g. by means of an FIR filter, before being output. The repetition frequency may be increased for the purpose of improving the immunity to interference, gor example.
Fig. 4 shows a achematic cross section through a process bushing. The process bushing, which is mounted on a pressure tank, for example, is a cylindrical bushing 1 with a metal thread, containing holding means 8, 9 of insulating material as well as rods 3 and 4, whose ends are connected to conductora 7 and 6, respectively, of a coaxial line 5, which con9titutes a delay line. The characteristic impedance of the coaxial line may be matched to that of the electric circuit, but it is not matched to the characteristic impedance of the process bushing, so that discontinuities exist between the characteristic impedances, whereby a desired reflection occurs at the process bushing which serves to unambiguously determine the beginning of the reflected signal. The characteristic impedance of the coaxial line and of the electric circuit may be between 65 and 85 ohms, preferably 75 ohms.
The electrically insulating material 8 may be disk of Teflon, with the ends of the rods 3, 4 additionally passed through a disk 9 of PEEK (polyether ether ketone) which is placed on the Teflon disk. The cylindrical process bushing 1 has a height of approximately 4 cm. The rods 3, 4, which extend through the Teflon cylinder 1, are arranged symmetrically within the cylinder 1. Their free length ranges from 2 to 15 cm, preferably from 5 to 7 cm.
The process bushing may also be a cylindrical bushing of only one insulating material, such as Teflon (PTFE) or PEEK (polyether ether ketone), which is a partially crystalline thermoplastic, in which the rods are embedded. In that case, too, the characteristic impedance vf the process bushing is not or not precisely matched to the characteristic impedance of the delay line or coaxial line.
The time domain reflectometer according to the invention haa the advantage that, particularly as a result of the uae of two parallel rods, a good reflection of the reflected pulses is achieved, which, because of the delay line, are sufficiently separated in time from the transmit pulses, so that the reflective characterietics, namely the resulting waveforms of the reflected 9ignals, are well suited for evaluation. In a further variant of the time domain reflectometer, the rods are coated with Teflon or ceramic, the thickness of the Teflon coating preferably ranging from 0.1 to 1 mm. In a further embodiment of the invention, the distance (d) between the rods ranges from 10 to 30 mm, and the height of the process bushing ranges between 2 and 5 cm.
Figs. 6a-d show individual echo curves with the extreme values used for evaluation. Fig. 6a shows an open-circuit echo curve. An open circuit, no coverage, is detected if the reflected signal has the following characteristics in the temporal sampling window: There ia only one low point TP which is below a predetermined first threahold (Threshold 1). Threshold 1 is determined from the baseline and a predetermined offset.
Fig. 6b show9 an echo curve for Pril. A firat covered state is detected if the reflected signal has the following characteristics within the temporal sampling window: There is a high point HP which exceeds a predetermined second threshold (Threshold 2). Threshold 2 is determined from the baseline and the predetermined offset Fig. 6c shows an echo curve for honey. The second covered state is detected if the reflected signal has the following characteristics within the temporal sampling window:
- There are two low points TP1, TF2 which have the same aense as the transmit pulse.
- The secvnd low point TP2 lies below the low point Tel by a predetermined amount As.
Fig. 6d ahows an echo curve for coffee. The third covered atate is detected if the reflected signal has the following characteri9tics within the temporal sampling window:
- There is only one low point TP which lies below a predetermined first threshold (threshold 1). Threshold 1 is determined from the baseline and a predetermined offset.
- aetween the starting instant of the tamporal sampling window and the low point TP is a point of inflection which lies between a local low point LTP and a local high point LHP. The local low point LTP and the local high point exceed a predetermined minimum distance.
The starting instant of the temporal sampling window is determined as follows;
- A baseline is determined in Section II.
- The reflected signal falls below the baseline in Section III by a predetermined amount.
The starting instant of the temporal sampling window can generally be determined from the reflections which occur at the interface between the delay line and the process bushing due to differances in characteristic impedance.
The determination of the starting instant in this manner ofters the advantage that the time stretch factor of the electronic circuit 14 must only be present with an accuracy of approx. 10 to 20%, so that the electronic circuit 14 can be implemented at low cost.
Claims (28)
1. A method for sensing a predetermined level of a material having a given dielectric constant, using holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the echoes, wherein the radio-frequency transmit pulses are applied over the electric line to the rod as a guided microwave on the principle of time domain reflectometry, TDR, wherein the signals reflected at the Interface between the material and air are returned to the echo amplifier for evaluation and the reflected signal is stretched in time, and wherein three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), are distinguished, with the temporal sampling window beginning at a starting instant, the method compressing the steps of producing at the rod-medium or rod-air interface by the change in characteristic impedance at the rod-medium or rod-air interface a reflected signal in both operational states of the material to be detected, namely coverage, short circuit or near short circuit, and no coverage, open circuit; and sensing the predetermined level by the waveform of the stretched reflected signal obtained at the echo amplifier, with at least three significant points of the reflected signal within the temporal sampling window being evaluated numerically or by waveform analysis, and a reference voltage being determined from at least one waveform during Section II, with:
no coverage, open circuit, being detected if the reflected signal has the following characteristics within the temporal sampling windows: there is only one low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset;
a first covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a high point (HP) which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset;
a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there are two low points (TP1, TP2), and the second low point (TP2) in time lies below the first low point (TP1) by a predetermined amount;
a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset, and between the starting instant of the temporal sampling window and the low point (TP) is a point of inflection which lies between a local high point (LHP) and a local low point (LTP), with the local low point (LTP) and the local high point (LHP) exceeding a predetermined minimum distance.
no coverage, open circuit, being detected if the reflected signal has the following characteristics within the temporal sampling windows: there is only one low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset;
a first covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a high point (HP) which lies above a predetermined second threshold, this second threshold being also determined from the reference voltage and the offset;
a second, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there are two low points (TP1, TP2), and the second low point (TP2) in time lies below the first low point (TP1) by a predetermined amount;
a third, different covered state being detected if the reflected signal has the following characteristics within the temporal sampling window: there is a low point (TP) which lies below a predetermined first threshold, which differs from the reference voltage by an offset, and between the starting instant of the temporal sampling window and the low point (TP) is a point of inflection which lies between a local high point (LHP) and a local low point (LTP), with the local low point (LTP) and the local high point (LHP) exceeding a predetermined minimum distance.
2. The method as claimed in claim 1, wherein the starting instant of the temporal sampling window is defined by the fact that the reflected signal differs from the reference value by a predetermined value.
3. The method as claimed in claim 1 or 2, wherein two rods disposed in the holding means in parallel are used, in which case the line is a coaxial line whose selectable length serves to introduce a predeterminable transit-time extension between the outgoing, transmitted pulses and the returning, reflected signals, and thus to make these separable in time, with the inner conductor of the coaxial line connected to one of the rods, and the other rod connected to ground of the electric circuit through the outer conductor or capacitively coupled to ground.
4. The method as claimed in claim 1, wherein said first covered state is detected at a high dielectric constant of the material, namely at a dielectric constant > 10, wherein said second, different covered state is detected at a medium dielectric instant of the material, namely at a dielectric instant between 5 and 10, and wherein said third, different covered state is detected at a small dielectric constant of the material, namely at a dielectric constant < 5.
5. The method as claimed in claim 2, further comprising the steps of determining from a plurality of waveforms during Section II, a baseline as a reference voltage, defining the starting instant of the temporal sampling window by the fact that the reflected signal differs from the baseline by a predetermined value; and making a determination as to whether within the temporal sampling window, the stretched signal derived from the reflected signal has a high point, a first low point, a second low point, or a local low point and a local high point, and thus a point of inflection.
6. The method as claimed in any one of claims 1 to 5, wherein for the level sensing, either filters or two counters are used, namely one counter for "coverage detected" and one for "no coverage detected", the detection then being applied to one of the counters.
7. The method as claimed in claim 6, wherein the filters are FIR filters.
8. The method as claimed in any one of claims 1 to 7, wherein up to six significant points (TP, TP1, TP2, HP, LTP, LHP) of the waveform are evaluated.
9. A time domain reflectometer for use as a Limit switch for sensing a predetermined level of a material having a given dielectric constant, comprising: holding means as a process bushing which contain one end of at least one electrically conductive rod whose other end is immersed in the material to be monitored when the predetermined level has been reached, wherein:
the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes;
the radio-frequency transmit pulses can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR;
the signals reflected at the material-air interface are returned to the echo amplifier and stretched in time for evaluation;
the characteristic impedances of the rod and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), can be distinguished; and the waveforms of the reflected signals are determined within the temporal sampling window serve to sense the predetermined level.
the end of the rod fitted in the holding means is connected via an electric line to an electric circuit for generating radio-frequency transmit pulses which comprises an echo amplifier for receiving the reflected signals, echoes;
the radio-frequency transmit pulses can be applied over the line to the rod as a guided microwave on the principle of time domain reflectometry, TDR;
the signals reflected at the material-air interface are returned to the echo amplifier and stretched in time for evaluation;
the characteristic impedances of the rod and the process bushing are chosen so that during evaluation, three sections following each other successively in time, namely transmit pulse (Section I), transit time (Section II), and temporal sampling window (Section III), can be distinguished; and the waveforms of the reflected signals are determined within the temporal sampling window serve to sense the predetermined level.
10. The time domain reflectometer as claimed in claim 9, wherein up to six significant points (TP, TP1, TP2, HP, LTP, LHP) of the waveform are evaluated.
11. The time domain reflectometer as claimed in claim 9, wherein two parallel rods are disposed in the holding means, and wherein the line is a coaxial line whose selectable length serves to introduce a predeterminable transit-time extension between the outgoing, transmitted pulses and the returning, reflected signals, and thus to make these distinguishable by the electric circuit, and thus represents a delay line at the process bushing, with the inner conductor of the coaxial line connected to one of the rods, and the other rod connected through the outer conductor to ground of the electric circuits.
12. The time domain reflectometer as claimed in claim 11, wherein the characteristic impedance of the coaxial line is chosen to be mismatched to that of the process bushing.
13. The time domain reflectometer as claimed in claim 9, wherein the process bushing is a tubular bushing with an external metal thread within which at least one insulating body as an insulating holding means for the rods as well as the rods are located.
14. The time domain reflectometer as claimed in any one of claims 9 to 13, wherein the insulating body in the process bushing consists of layers of different materials, such as PEEK and Teflon, having different dielectric constants, so that it is a laminated dielectric, with the materials, on the one hand, sealing off the process bushing and, on the other hand, having the minimum thickness required to produce the reflected signal for determining the starting instant of the temporal sampling window.
15. The time domain reflectometer as claimed in claim 13, wherein the process bushing is cylindrical and is made of electrically insulating material within which the rods are located.
16. The time domain reflectometer as claimed in claim 15, wherein the electrically insulating material is any one of Teflon (PTFE) and PEEK.
17. The time domain reflectometer as claimed in any one of claims 13 to 16, wherein the rods are provided with a coating.
18. The time domain reflectometer as claimed in claim 17, wherein the coating is any one of Teflon, ceramic, and PEEK coating.
19. The time domain reflectometer as claimed in claim 18, wherein a thickness of the coating ranges from 0.1 mm to 1 mm if Teflon or PEEK is used.
20. The time domain reflectometer as claimed in any one of claims 13 to 19, wherein the length of the rods protruding from the process bushing ranges from 2 cm to 15 cm.
21. The time domain reflectometer as claimed in any one of claims 13 to 19, wherein the length of the rods protruding from the process bushing ranges from 5 cm to 7 cm.
22. The time domain reflectometer as claimed in any one of claims 13 to 21, wherein the length of the delay line from the electric circuit to the connection to the ends of the rods fitted in the process bushing is at least 30 cm.
23. The time domain reflectometer as claimed in claim 22, wherein the length of the delay line from the electric circuit to the connection to the ends of the rods fitted in the process bushing is 30 cm to 60 cm.
24. The time domain reflectometer as claimed in any one of claims 13 to 23, wherein the distance between the rods is between 10 mm and 30 mm.
25. The time domain reflectometer as claimed in any one of claims 13 to 24, wherein the height of the process bushing is between 2 cm and 5 cm.
26. The time domain reflectometer as claimed in any one of claims 13 to 25, wherein the process bushing is made pressure-tight.
27. The time domain reflectometer as claimed in claim 26, wherein the process bushing is made pressure-tight with up to pressures of 30 bars.
28. The time domain reflectometer as claimed in any one of claims 9 to 27, wherein the reflected signal is sampled by a four-diode sampling circuit and fed through the echo amplifier and an A/D converter to a microprocessor which evaluates the reflected signal and outputs the result, "coverage detected" or "no coverage detected", to a display unit or converts it into a switching signal.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE20016962U DE20016962U1 (en) | 2000-09-27 | 2000-09-27 | Time-domain reflectometer for use as a limit switch to record the limit level of a good |
| DE20016962.9 | 2000-09-27 | ||
| DE10115150A DE10115150A1 (en) | 2000-09-27 | 2001-03-27 | Process for detecting the limit level of a good and device therefor |
| DE10115150.0 | 2001-03-27 | ||
| PCT/EP2001/010813 WO2002027349A2 (en) | 2000-09-27 | 2001-09-19 | Method for detecting the limit state of a material, and device therefor |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2423781A1 CA2423781A1 (en) | 2003-03-26 |
| CA2423781C true CA2423781C (en) | 2009-02-24 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002423781A Expired - Fee Related CA2423781C (en) | 2000-09-27 | 2001-09-19 | Method for sensing a predetermined level of a material, and device therefor |
Country Status (6)
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| EP (1) | EP1322922A2 (en) |
| JP (1) | JP2004519661A (en) |
| CN (1) | CN1250944C (en) |
| AU (1) | AU2002218183A1 (en) |
| CA (1) | CA2423781C (en) |
| WO (1) | WO2002027349A2 (en) |
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| DE10360711A1 (en) | 2003-12-19 | 2005-07-14 | Endress + Hauser Gmbh + Co. Kg | Level measuring device and method for level measurement and monitoring |
| DE102005057053A1 (en) | 2005-11-30 | 2007-05-31 | Vega Grieshaber Kg | High frequency module for e.g. liquid level radar device, has tap for decoupling reference signal from transmission line, and delay unit arranged after tap for delaying transmission signal on its path to antenna or sensor |
| DE102007007024A1 (en) | 2007-02-08 | 2008-08-21 | KROHNE Meßtechnik GmbH & Co. KG | Use of a working according to the radar level gauge |
| GB0822283D0 (en) * | 2008-12-06 | 2009-01-14 | Mobrey Ltd | Improvements in or relating to level sensors |
| CN102735313B (en) * | 2012-06-19 | 2014-07-30 | 郭云昌 | Method for determining middle material level curve of continuous passive nuclear material level gage |
| DE102014103212A1 (en) * | 2014-03-11 | 2015-09-17 | Sick Ag | Sensor and method for detecting an object located on a roller conveyor |
| DE102015100417A1 (en) * | 2015-01-13 | 2016-07-14 | Krohne Messtechnik Gmbh | Method for determining the level of a medium in a container |
| DE102015100415A1 (en) * | 2015-01-13 | 2016-07-14 | Krohne Messtechnik Gmbh | Device for determining the level of a medium |
| HUE036364T2 (en) * | 2015-02-03 | 2018-07-30 | Grieshaber Vega Kg | Limit level switch with integrated position sensor |
| DE102015202448A1 (en) * | 2015-02-11 | 2016-08-11 | Vega Grieshaber Kg | Evaluation procedure for a TDR limit switch |
| DK3258296T3 (en) * | 2016-06-14 | 2023-09-04 | Grieshaber Vega Kg | REFLECTION MICROWAVE COUNTER |
| US11041899B2 (en) | 2016-11-11 | 2021-06-22 | Leoni Kabel Gmbh | Method and measuring assembly for monitoring a line |
| DK3527959T3 (en) * | 2018-02-14 | 2024-01-15 | Grieshaber Vega Kg | FILLING LEVEL AURA RADAR WITH ADHESION DETECTOR |
| DE102021201364A1 (en) * | 2021-02-12 | 2022-08-18 | Vega Grieshaber Kg | Measuring device with position sensor |
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| DE4407369C2 (en) * | 1994-03-05 | 1999-09-30 | Grieshaber Vega Kg | Method and circuit arrangement for measuring the transit time and their use |
| US5884231A (en) * | 1995-12-21 | 1999-03-16 | Endress & Hauser Gmbh & Co. | Processor apparatus and method for a process measurement signal |
| US6085589A (en) * | 1996-12-23 | 2000-07-11 | Venture Measurement Company Llc | Material level sensing system calibration |
| US5943908A (en) * | 1997-09-08 | 1999-08-31 | Teleflex Incorporated | Probe for sensing fluid level |
| EP1128169A1 (en) * | 1999-12-29 | 2001-08-29 | Endress + Hauser GmbH + Co. | Method and apparatus for determining a limit filling level of a product in a container |
-
2001
- 2001-09-19 AU AU2002218183A patent/AU2002218183A1/en not_active Abandoned
- 2001-09-19 CN CN01816386.6A patent/CN1250944C/en not_active Expired - Fee Related
- 2001-09-19 EP EP01985756A patent/EP1322922A2/en not_active Withdrawn
- 2001-09-19 JP JP2002530876A patent/JP2004519661A/en active Pending
- 2001-09-19 WO PCT/EP2001/010813 patent/WO2002027349A2/en active Application Filing
- 2001-09-19 CA CA002423781A patent/CA2423781C/en not_active Expired - Fee Related
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| JP2004519661A (en) | 2004-07-02 |
| CN1250944C (en) | 2006-04-12 |
| WO2002027349A2 (en) | 2002-04-04 |
| AU2002218183A1 (en) | 2002-04-08 |
| CA2423781A1 (en) | 2003-03-26 |
| CN1466674A (en) | 2004-01-07 |
| EP1322922A2 (en) | 2003-07-02 |
| WO2002027349A3 (en) | 2002-10-24 |
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