CA2310764A1 - Method for level measurement on containers, and an apparatus for carrying out the method - Google Patents
Method for level measurement on containers, and an apparatus for carrying out the method Download PDFInfo
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- CA2310764A1 CA2310764A1 CA 2310764 CA2310764A CA2310764A1 CA 2310764 A1 CA2310764 A1 CA 2310764A1 CA 2310764 CA2310764 CA 2310764 CA 2310764 A CA2310764 A CA 2310764A CA 2310764 A1 CA2310764 A1 CA 2310764A1
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Abstract
The invention proposes a method for continuous level measurement on containers, according to which a sequence of a~ mode Lamb wave pulses is initiated in the container wall (1) by means of a Lamb wave excitor (6) arranged on it, and the propagation time difference (.DELTA.T) between the propagation time (T2) of these a~ mode Lamb wave pulses between the Lamb wave excitor (6) and a Lamb wave receiver (7) (which is arranged at a distance from it on the container wall (1)) for the current level in the container (4), and the propagation time (T1) when the container (4) is empty, is determined, and this is used to derive the current level, which is directly proportional to this propagation time difference (.DELTA.T).
Since a~ mode Lamb waves which are initiated in a container wall experience a change in their propagation time between two defined points when a different medium comes into contact with the container wall, the method according to the invention allows continuous monitoring of the level on containers, to be precise with high accuracy.
Since a~ mode Lamb waves which are initiated in a container wall experience a change in their propagation time between two defined points when a different medium comes into contact with the container wall, the method according to the invention allows continuous monitoring of the level on containers, to be precise with high accuracy.
Description
Eh 361 CA
May 3, 2000 Method for level measurement on containers, and an apparatus for carrying out the method FIELD OF THE INVENTION
The invention relates to a method for level measurement on containers, and to an apparatus for carrying out the method.
BACKGROUND OF THE INVENTION
For level measurement or level monitoring on containers, it is known for Lamb waves to be produced in the container wall by means of a Lamb wave excitor in the form of an electroacoustic transducer fitted to the outside of the container wall. The Lamb waves propagate as periodically sinusoidal deformation of both surfaces of the container wall, are received by a Lamb wave receiver fitted at a distance from the Lamb wave excitor on the same container wall, and are converted back into an electrical signal. Waves of different order can be excited depending on the thickness of the container wall and on the excitation frequency. However, only the zero order or fundamental mode (zeroth mode) reach the surface and cause deformations of the two surfaces. The deformations may be symmetrical or antisymmetrical with respect to the center plane (ULTRASONIC TECHNOLOGY, A Series of Monographs, General Editor Lewis Balamuth, Cavitron Corporation, New York, N.Y., RAYLEIGH AND LAMB WAVES, Physical Theory and Applications, I.A. Viktorov, pages 69, 70 and page 82, paragraph 3).
Until now only zero-order symmetrical Lamb waves have been of any practical importance for the use of Lamb waves in level measurement devices. The zero-order symmetrical Lamb waves are called the so mode (zeroth - 2 - Eh 361 CA
May 3, 2000 symmetrical mode) or the symmetrical fundamental mode.
Specifically, as soon as one side of the container wall comes into contact with a different medium, the amplitude of these waves is significantly attenuated.
Thus, if the Lamb wave excitor and the Lamb wave receiver of a corresponding level measurement apparatus are fitted to the container wall at a distance from one another in the region of a level which is to be monitored, a signal received by the Lamb receiver can be used as an indication that none of the material in the container is located in the measurement range, while the lack of the signal or a signal whose amplitude is greatly attenuated indicates that the level has reached the measurement range. Such an apparatus can thus be used as a limit switch for monitoring a maximum and/or minimum level.It is advantageous that neither the measurement apparatus or parts of it come into contact with the material in the container, so that such use is possible even in conjunction with, for example, explosive or irritant materials.
However, continuous level measurement and monitoring are not possible with these known apparatuses.
SLIMMPaRY OF THE INVENTION
An object of the invention is to provide a method and an apparatus for continuous measurement and monitoring of the level of material in widely different types of containers including pipelines.
This is achieved according to the invention by a method according to which a sequence of ao mode Lamb wave pulses (zeroth antisymmetrical mode) is initiated in the container wall by means of a Lamb wave excitor arranged on it, and the propagation time difference OT
between the propagation time T2 of these ao mode Lamb - 3 - Eh 361 CA
May 3, 2000 wave pulses between the Lamb wave excitor and a Lamb wave receiver (which is arranged at a distance from it on the container wall) for the current level in the container, and the propagation time T1 of this Lamb wave pulse over the same path when the container is empty, is determined, and this is used to derive the current level, which is directly proportional to this propagation time difference 0T.
Since ao mode Lamb waves initiated in a container wall do not experience any change in their amplitude when a different medium comes into contact with the container wall but, in fact, experience a change in their propagation time between two defined points, the method according to the invention allows continuous monitoring of the level in containers, to be precise with high precision; discrepancies in the measurement accuracy of less than 1 cm have been found.
In this case, the propagation time T1 (which applies to a specific container or application) of the ao mode Lamb wave between the Lamb wave excitor and the Lamb wave receiver when the container is empty can be stored as a reference variable in order to determine the propagation time difference 0T.
Since the propagation time of the ao mode Lamb waves has been found to be temperature-dependent, it may be advantageous to compensate for a change (caused by temperature fluctuations) in the propagation time of the ao mode Lamb wave in the container wall using a measured value recorded by a temperature measurement sensor.
Another option according to the invention for compensating for the temperature dependency of the propagation time of the ao mode Lamb waves is to measure continuously the propagation time T2 (which _ q - Eh 361 CA
May 3, 2000 varies with the level in the container) of the Lamb wave over a measurement path which extends over the filling range of the container, and the propagation time T1 (which applies to the empty container) of the Lamb wave via a reference path which extends beyond the filling range. Fluctuations in the propagation time which are not due to a change in the level are thus automatically compensated for in each measurement process and each evaluation of the measured variables.
In a first embodiment of an apparatus for carrying out the method according to the invention, the ao mode Lamb wave receiver or the ao Lamb wave transmitter can be arranged on the outer wall of the container at the maximum permissible filling level, and the ao mode Lamb wave receiver or the ao mode Lamb wave transmitter are preferably arranged closely above the container base.
This allows continuous monitoring over the entire permissible filling level of the container. All the parts of the measurement apparatus are arranged outside the container, and do not come into contact with the material in the container.
The ao mode Lamb wave transmitter can be connected to a controller via a pulse generator and clock transmitter, and the ao mode Lamb wave receiver can be connected to controller via a bandpass filter and an amplifier, with the propagation time of the ao mode Lamb wave for the empty container being stored by the controller.
The propagation time of the ao mode Lamb wave when the container is filled to the maximum permissible level can also be stored by the contoller.
In one embodiment of the apparatus for carrying out the method according to the invention and having temperature compensation, the ao mode Lamb wave transmitter can be arranged on the outer wall of the - 5 - Eh 361 CA
May 3, 2000 container at the maximum permissible filling level, a first ao mode Lamb wave receiver can be arranged closely above the container base, and a second ao mode Lamb wave receiver can be arranged outside the filling range, at a distance d from the ao mode Lamb wave transmitter, in which case this distance d is used as a reference measurement path for continuous determination of the propagation time T1 of the ao mode Lamb wave when the container is empty. Thus, for each ao mode Lamb wave pulse initiated in the container wall, the propagation time which varies with the filling level is on the one hand measured in the one direction and, at the same time, the propagation time which applies to the empty container is measured via the reference path in the other direction, and temperature-dependent fluctuations in the propagation time are thus automatically compensated for when forming the difference.
The method according to the invention and the apparatus for carrying it out are highly economic and at the same time reliable. No contact paste is required between the Lamb wave receivers or the Lamb wave sensor and the container wall. Further, the temperature compensation is feasible without major complexity.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in more detail by way of example in the following text with reference to the attached drawings, in which:
Fig. 1 shows, schematically, the deformation of a container wall by propagating so mode and ao mode Lamb waves, - 6 - Eh 361 CA
May 3, 2000 Fig. 2 shows a diagram to illustrate the different propagation times of an ao mode Lamb wave, as a function of the level, Fig. 2a shows a diagram illustrating the direct relationship between the propagation time of an ao mode Lamb wave in a container wall and the level in the container, Fig. 3 shows a sketch illustrating the method according to the invention, Fig. 4 shows, schematically, the arrangement of the major parts of a first embodiment of a level measurement apparatus, according to the invention, on a container, Fig. 5 shows schematically, the arrangement of the major parts of a second embodiment of a level measurement apparatus according to the invention, having temperature compensation, on a container, Fig. 6 shows the block diagram of a level measurement apparatus according to the invention, as shown in Fig. 4.
Fig. 7 shows a flowchart for measuring the level in a container. With a measuring apparatus as shown in Fig. 4 Fig. 8 shows a block diagram of a level measurement apparatus according to the invention, a shown in Fig. 5 Fig. 9 shows a flowchart for measuring the level in a container. With a measuring apparatus as shown in Fig. 5 - 7 - Eh 361 CA
May 3, 2000 DETAILED DESCRITPTION OF EXEMPLARY EMBODIMENTS
While the invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail.
It should be undertstood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary,the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Fig. 1 shows, schematically, the deformation of a container wall resulting from a zeroth symmetrical mode so Lamb wave on the one hand and an antisymmetrical zeroth mode ao Lamb wave on the other hand, propagating as surface acoustic waves. These Lamb waves can be produced in a manner known per se by an electroacoustic transducer which is fitted to the container wall. In order to obtain the desired wroth mode so or ao, the excitation frequency must be matched, as is known per se, to the material and to the thickness of the container wall. It is known that, in the case of the symmetrical so mode, the surface acoustic waves reach their minima and maxima at the same time on both surfaces of the container wall, and the two surface acoustic waves thus run symmetrically with respect to the center line of the container wall. In the case of the antisymmetrical ao mode, on the other hand, the minima of the one surface acoustic wave coincide with the maxima of the other surface acoustic wave, and vice versa. The particle displacements which occur are indicated by arrows.
- g - Eh 361 CA
May 3, 2000 It has been found by experience that, as soon as the one surface of the container wall comes into contact with the material in the container, the ao mode Lamb waves, in contrast to the so mode Lamb waves, do not experience any pronounced attenuation in their amplitude in the contact region, but their propagation time is considerably lengthened. This delay in their propagation is directly proportional to the contact area between the container wall and the material in the container, preferably a liquid. This is shown in the diagrams in Figs 2 and 2a.
Figure 2 shows, horizontally one above the other and starting from the same point, the propagation times tl, t2, t3 (by way of example) of an ao mode Lamb wave in a container wall for three different levels in the relevant container, in which case the short propagation time tl represents the state when the container is empty, the propagation time t2 represents an intermediate state of the material in the container, and the longest propagation time t3 represents the state when the container is full. In Fig. 2a, the propagation time t of the ao mode Lamb wave in the container wall is plotted on the horizontal, and the level in this container is plotted as a percentage on the vertical. It can be seen that the propagation time of the ao mode Lamb wave rises in direct proportion to the level.
This linear relationship between the propagation time of the ao mode Lamb waves and the level in the container is used by the invention for continuous level measurement in containers. The principle is shown in simplified form in Fig. 3. One (1) denotes a section from a container wall, the arrow Pl points to the point on the container outer wall at which an ao mode Lamb wave pulse is initiated by a Lamb wave excitor in the form of an electroacoustic transducer, and this Lamb - 9 - Eh 361 CA
May 3, 2000 wave pulse propagates, as is indicated at 2, in a circular shape in all directions. At the point denoted by the arrow P3, the Lamb wave pulse which is indicated at 3 and propagates in the container wall 1 in the direction denoted by the arrow PZ is received by a Lamb wave receiver and is converted back into an electrical signal while, at the same time, its propagation time along this shortest path (which is denoted by the arrow Pz on the container wall 1) in the container wall 1 between P1 and P3 is determined. The propagation time is defined as the quotient of this (shortest) propagation path between P1 and P3 and the group velocity C of the Lamb wave, or of the Lamb wave pulse.
Figure 4 shows a section through the wall 1 and a part of the base 8 of a container 4, which is filled to the level H with a material 5, preferably a liquid. A Lamb wave excitor 6 and a Lamb wave receiver 7 are arranged in fixed positions, at a vertical distance L from one another, on the outer wall la of this container 4. By way of example, it is assumed that the Lamb wave excitor 6 is located at the maximum permissible filling level, and that the Lamb wave receiver 7 is arranged closely above the container base 8. However, it is also possible for the arrangement to be fitted the opposite way round.
It follows from what has been said above that an ao mode Lamb wave pulse initiated by the Lamb wave excitor 6 in the container wall 1 propagates in the region H
between the (assumed) current level of the material 5 in the container 4 and the Lamb wave receiver 7, that is to say where the material 5 in the container is in contact with the container inner wall lb, at a different (and to be precise lower) group velocity C
than in the region L - H between the Lamb wave excitor 6 and the current level where there is no material in the container; in other words, the Lamb wave pulse has - 10 - Eh 361 CA
May 3, 2000 a different propagation time in the two said regions, this time being shorter in the region L - H than in the region H. It further follows that, when the filling level changes, the total propagation time TZ of the Lamb wave between the Lamb wave excitor 6 and the Lamb wave receiver 7 likewise varies in a corresponding manner, that is to say it increases as the level rises.
When the container is empty or when there is no material in the container in the region L between the Lamb wave excitor 6 and the Lamb wave receiver 7, the propagation time of the Lamb wave pulse is L
T1 = -Ci If there is material in this region L of the container, then the propagation time changes (increases) to T2;
this is composed jointly of the propagation time Tl° in the region L - H without any material in the container, and T2° in the region H where there is material in the container, resulting in L- H H
T2 = T1° + T2°= +-C~ CZ
The difference in time ~T between T1 and T2 is then directly proportional to the absolute filling level, so that 4T = T 2 - T 1 = H (-_ -) C~ Cl If the propagation time T1 of the Lamb wave pulse when the container 4 is empty is fixed and is used as a calibration factor in a measurement apparatus, the level in a container 4 can thus be monitored continuously by continuously measuring the current propagation time T2 of an ao mode Lamb wave pulse which - 11 - Eh 361 CA
May 3, 2000 is applied to the respective container 4 and is repeated continuously.
However, since the propagation time T of the ao mode Lamb waves has also been found to be temperature-dependent, it is recommended that the measurement apparatus be calibrated, for example by means of a temperature sensor. Another measure to compensate for temperature fluctuations and to make the measurement result temperature-independent may comprise the provision of a reference measurement path in the region of the container which remains free of material.
Figure 5 shows the principle of such a development of the measurement apparatus shown in Fig. 4. According to this proposal, a second Lamb wave receiver 7' is fitted to the outer wall 1a of the container 4 above the maximum permissible filling level L and at a specific distance d from the Lamb wave excitor 6. The distance d thus defines a reference measurement path d. Since there is never any material 5 in this region of the container 4, the propagation time Tref which can be measured between the Lamb wave excitor 6 and the second Lamb wave receiver 7' in this case always corresponds to the propagation time T1 when the container 4 is empty, and can thus be used as a reference variable in order to derive from it the respectively applicable propagation time T1 when the container 4 is empty, when temperature fluctuations occur and the group velocity C1 changes in consequence, in which case:
d L L
Tref = C and T1 = C ; that is to say Tl = a Tref This factor L/d must therefore be taken into account in the calibration of the measurement apparatus in order, in the end, to obtain temperature-independent measurement results. The propagation time T2 and the - 12 - Eh 361 CA
May 3, 2000 reference propagation time Tref Of the Lamb wave pulse transmitted by the Lamb wave transmitter 6 are determined in both directions to the Lamb wave receivers 7 and 7', the applicable propagation time T1 in the empty container in the instantaneous conditions is derived from the reference propagation time Tref using the factor L/d, and the difference 0T = TZ - T1 is formed as a measurement variable which is directly proportional to the current level and which, in this embodiment of the measurement apparatus, is independent of temperature. In this embodiment of the measurement apparatus with temperature compensation, it is essential that the Lamb wave receiver 6 be arranged on the container wall 1 at the maximum permissible filling level (or above this level by a tolerance amount), in order to allow the propagation time measurement to be carried out in both directions.
Figure 6 shows the block diagram of a level measurement apparatus shown in Fig. 4. The Lamb wave transmitter 6 is arranged on the outer wall la of the container 4 (which is partially filled with the material 5) at the maximum permissible filling level, and the Lamb wave receiver 7 is arranged on the outer wall 1a of the container 4, closely above the container base 8. The lamb wave transmitter 6 is connected to a pulse generator 9, for example in the form of an electroacoustic transducer, whose pulse output or pulse repetition rate is controlled by controller 11 via a clock transmitter 10. This software is used, inter alia, to store the data used for the excitation of an ao mode Lamb wave in the container wall 1 and which, under some circumstances (for example because they are dependent on the material) are determined empirically for each application.For example, the frequency f of the ultrasound signal which is to be applied to the container wall by the Lamb wave excitor 6 may be determined empirically. The ao mode Lamb wave can be - 13 - Eh 361 CA
May 3, 2000 set optimally by varying the frequency of the ultrasound signal and by searching for the Lamb wave whose amplitude change 4A during filling of the container 4 is a minimum, and whose propagation time change ~T is at the same time a maximum. The Lamb wave receiver 7 records the Lamb wave pulses propagating on the direct path in the container wall 1 and converts them back into an electrical signal. Secondary interference waves can advantageously be removed from this signal in a bandpass filter 12, before this signal is passed via an amplifier 13 to the controller 11, for evaluation and for determining the propagation time difference OT. To do this, it is necessary to store the propagation time Tl (which applies to the relevant application) of the Lamb wave pulse when the container 4 is empty, as well as the propagation time T for the container 4 when it is filled with material 5 up to the maximum permissible level. The instantaneous level can be read directly from the propagation time difference 4T.
It is recommended that the pulse be picked up from the electroacoustic transducer at half the wavelength ~/2, in order to make the measurement apparatus insensitive to external effects, such as shaking and external vibration.
Fig. 7 shows a flowchart for measuring the level in a container.With a measuring apparatus as shown in Fig. 4 Fig. 8 shows a block diagram of a level measurement apparatus according to the invention, as shown in Fig.
May 3, 2000 Method for level measurement on containers, and an apparatus for carrying out the method FIELD OF THE INVENTION
The invention relates to a method for level measurement on containers, and to an apparatus for carrying out the method.
BACKGROUND OF THE INVENTION
For level measurement or level monitoring on containers, it is known for Lamb waves to be produced in the container wall by means of a Lamb wave excitor in the form of an electroacoustic transducer fitted to the outside of the container wall. The Lamb waves propagate as periodically sinusoidal deformation of both surfaces of the container wall, are received by a Lamb wave receiver fitted at a distance from the Lamb wave excitor on the same container wall, and are converted back into an electrical signal. Waves of different order can be excited depending on the thickness of the container wall and on the excitation frequency. However, only the zero order or fundamental mode (zeroth mode) reach the surface and cause deformations of the two surfaces. The deformations may be symmetrical or antisymmetrical with respect to the center plane (ULTRASONIC TECHNOLOGY, A Series of Monographs, General Editor Lewis Balamuth, Cavitron Corporation, New York, N.Y., RAYLEIGH AND LAMB WAVES, Physical Theory and Applications, I.A. Viktorov, pages 69, 70 and page 82, paragraph 3).
Until now only zero-order symmetrical Lamb waves have been of any practical importance for the use of Lamb waves in level measurement devices. The zero-order symmetrical Lamb waves are called the so mode (zeroth - 2 - Eh 361 CA
May 3, 2000 symmetrical mode) or the symmetrical fundamental mode.
Specifically, as soon as one side of the container wall comes into contact with a different medium, the amplitude of these waves is significantly attenuated.
Thus, if the Lamb wave excitor and the Lamb wave receiver of a corresponding level measurement apparatus are fitted to the container wall at a distance from one another in the region of a level which is to be monitored, a signal received by the Lamb receiver can be used as an indication that none of the material in the container is located in the measurement range, while the lack of the signal or a signal whose amplitude is greatly attenuated indicates that the level has reached the measurement range. Such an apparatus can thus be used as a limit switch for monitoring a maximum and/or minimum level.It is advantageous that neither the measurement apparatus or parts of it come into contact with the material in the container, so that such use is possible even in conjunction with, for example, explosive or irritant materials.
However, continuous level measurement and monitoring are not possible with these known apparatuses.
SLIMMPaRY OF THE INVENTION
An object of the invention is to provide a method and an apparatus for continuous measurement and monitoring of the level of material in widely different types of containers including pipelines.
This is achieved according to the invention by a method according to which a sequence of ao mode Lamb wave pulses (zeroth antisymmetrical mode) is initiated in the container wall by means of a Lamb wave excitor arranged on it, and the propagation time difference OT
between the propagation time T2 of these ao mode Lamb - 3 - Eh 361 CA
May 3, 2000 wave pulses between the Lamb wave excitor and a Lamb wave receiver (which is arranged at a distance from it on the container wall) for the current level in the container, and the propagation time T1 of this Lamb wave pulse over the same path when the container is empty, is determined, and this is used to derive the current level, which is directly proportional to this propagation time difference 0T.
Since ao mode Lamb waves initiated in a container wall do not experience any change in their amplitude when a different medium comes into contact with the container wall but, in fact, experience a change in their propagation time between two defined points, the method according to the invention allows continuous monitoring of the level in containers, to be precise with high precision; discrepancies in the measurement accuracy of less than 1 cm have been found.
In this case, the propagation time T1 (which applies to a specific container or application) of the ao mode Lamb wave between the Lamb wave excitor and the Lamb wave receiver when the container is empty can be stored as a reference variable in order to determine the propagation time difference 0T.
Since the propagation time of the ao mode Lamb waves has been found to be temperature-dependent, it may be advantageous to compensate for a change (caused by temperature fluctuations) in the propagation time of the ao mode Lamb wave in the container wall using a measured value recorded by a temperature measurement sensor.
Another option according to the invention for compensating for the temperature dependency of the propagation time of the ao mode Lamb waves is to measure continuously the propagation time T2 (which _ q - Eh 361 CA
May 3, 2000 varies with the level in the container) of the Lamb wave over a measurement path which extends over the filling range of the container, and the propagation time T1 (which applies to the empty container) of the Lamb wave via a reference path which extends beyond the filling range. Fluctuations in the propagation time which are not due to a change in the level are thus automatically compensated for in each measurement process and each evaluation of the measured variables.
In a first embodiment of an apparatus for carrying out the method according to the invention, the ao mode Lamb wave receiver or the ao Lamb wave transmitter can be arranged on the outer wall of the container at the maximum permissible filling level, and the ao mode Lamb wave receiver or the ao mode Lamb wave transmitter are preferably arranged closely above the container base.
This allows continuous monitoring over the entire permissible filling level of the container. All the parts of the measurement apparatus are arranged outside the container, and do not come into contact with the material in the container.
The ao mode Lamb wave transmitter can be connected to a controller via a pulse generator and clock transmitter, and the ao mode Lamb wave receiver can be connected to controller via a bandpass filter and an amplifier, with the propagation time of the ao mode Lamb wave for the empty container being stored by the controller.
The propagation time of the ao mode Lamb wave when the container is filled to the maximum permissible level can also be stored by the contoller.
In one embodiment of the apparatus for carrying out the method according to the invention and having temperature compensation, the ao mode Lamb wave transmitter can be arranged on the outer wall of the - 5 - Eh 361 CA
May 3, 2000 container at the maximum permissible filling level, a first ao mode Lamb wave receiver can be arranged closely above the container base, and a second ao mode Lamb wave receiver can be arranged outside the filling range, at a distance d from the ao mode Lamb wave transmitter, in which case this distance d is used as a reference measurement path for continuous determination of the propagation time T1 of the ao mode Lamb wave when the container is empty. Thus, for each ao mode Lamb wave pulse initiated in the container wall, the propagation time which varies with the filling level is on the one hand measured in the one direction and, at the same time, the propagation time which applies to the empty container is measured via the reference path in the other direction, and temperature-dependent fluctuations in the propagation time are thus automatically compensated for when forming the difference.
The method according to the invention and the apparatus for carrying it out are highly economic and at the same time reliable. No contact paste is required between the Lamb wave receivers or the Lamb wave sensor and the container wall. Further, the temperature compensation is feasible without major complexity.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in more detail by way of example in the following text with reference to the attached drawings, in which:
Fig. 1 shows, schematically, the deformation of a container wall by propagating so mode and ao mode Lamb waves, - 6 - Eh 361 CA
May 3, 2000 Fig. 2 shows a diagram to illustrate the different propagation times of an ao mode Lamb wave, as a function of the level, Fig. 2a shows a diagram illustrating the direct relationship between the propagation time of an ao mode Lamb wave in a container wall and the level in the container, Fig. 3 shows a sketch illustrating the method according to the invention, Fig. 4 shows, schematically, the arrangement of the major parts of a first embodiment of a level measurement apparatus, according to the invention, on a container, Fig. 5 shows schematically, the arrangement of the major parts of a second embodiment of a level measurement apparatus according to the invention, having temperature compensation, on a container, Fig. 6 shows the block diagram of a level measurement apparatus according to the invention, as shown in Fig. 4.
Fig. 7 shows a flowchart for measuring the level in a container. With a measuring apparatus as shown in Fig. 4 Fig. 8 shows a block diagram of a level measurement apparatus according to the invention, a shown in Fig. 5 Fig. 9 shows a flowchart for measuring the level in a container. With a measuring apparatus as shown in Fig. 5 - 7 - Eh 361 CA
May 3, 2000 DETAILED DESCRITPTION OF EXEMPLARY EMBODIMENTS
While the invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail.
It should be undertstood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary,the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Fig. 1 shows, schematically, the deformation of a container wall resulting from a zeroth symmetrical mode so Lamb wave on the one hand and an antisymmetrical zeroth mode ao Lamb wave on the other hand, propagating as surface acoustic waves. These Lamb waves can be produced in a manner known per se by an electroacoustic transducer which is fitted to the container wall. In order to obtain the desired wroth mode so or ao, the excitation frequency must be matched, as is known per se, to the material and to the thickness of the container wall. It is known that, in the case of the symmetrical so mode, the surface acoustic waves reach their minima and maxima at the same time on both surfaces of the container wall, and the two surface acoustic waves thus run symmetrically with respect to the center line of the container wall. In the case of the antisymmetrical ao mode, on the other hand, the minima of the one surface acoustic wave coincide with the maxima of the other surface acoustic wave, and vice versa. The particle displacements which occur are indicated by arrows.
- g - Eh 361 CA
May 3, 2000 It has been found by experience that, as soon as the one surface of the container wall comes into contact with the material in the container, the ao mode Lamb waves, in contrast to the so mode Lamb waves, do not experience any pronounced attenuation in their amplitude in the contact region, but their propagation time is considerably lengthened. This delay in their propagation is directly proportional to the contact area between the container wall and the material in the container, preferably a liquid. This is shown in the diagrams in Figs 2 and 2a.
Figure 2 shows, horizontally one above the other and starting from the same point, the propagation times tl, t2, t3 (by way of example) of an ao mode Lamb wave in a container wall for three different levels in the relevant container, in which case the short propagation time tl represents the state when the container is empty, the propagation time t2 represents an intermediate state of the material in the container, and the longest propagation time t3 represents the state when the container is full. In Fig. 2a, the propagation time t of the ao mode Lamb wave in the container wall is plotted on the horizontal, and the level in this container is plotted as a percentage on the vertical. It can be seen that the propagation time of the ao mode Lamb wave rises in direct proportion to the level.
This linear relationship between the propagation time of the ao mode Lamb waves and the level in the container is used by the invention for continuous level measurement in containers. The principle is shown in simplified form in Fig. 3. One (1) denotes a section from a container wall, the arrow Pl points to the point on the container outer wall at which an ao mode Lamb wave pulse is initiated by a Lamb wave excitor in the form of an electroacoustic transducer, and this Lamb - 9 - Eh 361 CA
May 3, 2000 wave pulse propagates, as is indicated at 2, in a circular shape in all directions. At the point denoted by the arrow P3, the Lamb wave pulse which is indicated at 3 and propagates in the container wall 1 in the direction denoted by the arrow PZ is received by a Lamb wave receiver and is converted back into an electrical signal while, at the same time, its propagation time along this shortest path (which is denoted by the arrow Pz on the container wall 1) in the container wall 1 between P1 and P3 is determined. The propagation time is defined as the quotient of this (shortest) propagation path between P1 and P3 and the group velocity C of the Lamb wave, or of the Lamb wave pulse.
Figure 4 shows a section through the wall 1 and a part of the base 8 of a container 4, which is filled to the level H with a material 5, preferably a liquid. A Lamb wave excitor 6 and a Lamb wave receiver 7 are arranged in fixed positions, at a vertical distance L from one another, on the outer wall la of this container 4. By way of example, it is assumed that the Lamb wave excitor 6 is located at the maximum permissible filling level, and that the Lamb wave receiver 7 is arranged closely above the container base 8. However, it is also possible for the arrangement to be fitted the opposite way round.
It follows from what has been said above that an ao mode Lamb wave pulse initiated by the Lamb wave excitor 6 in the container wall 1 propagates in the region H
between the (assumed) current level of the material 5 in the container 4 and the Lamb wave receiver 7, that is to say where the material 5 in the container is in contact with the container inner wall lb, at a different (and to be precise lower) group velocity C
than in the region L - H between the Lamb wave excitor 6 and the current level where there is no material in the container; in other words, the Lamb wave pulse has - 10 - Eh 361 CA
May 3, 2000 a different propagation time in the two said regions, this time being shorter in the region L - H than in the region H. It further follows that, when the filling level changes, the total propagation time TZ of the Lamb wave between the Lamb wave excitor 6 and the Lamb wave receiver 7 likewise varies in a corresponding manner, that is to say it increases as the level rises.
When the container is empty or when there is no material in the container in the region L between the Lamb wave excitor 6 and the Lamb wave receiver 7, the propagation time of the Lamb wave pulse is L
T1 = -Ci If there is material in this region L of the container, then the propagation time changes (increases) to T2;
this is composed jointly of the propagation time Tl° in the region L - H without any material in the container, and T2° in the region H where there is material in the container, resulting in L- H H
T2 = T1° + T2°= +-C~ CZ
The difference in time ~T between T1 and T2 is then directly proportional to the absolute filling level, so that 4T = T 2 - T 1 = H (-_ -) C~ Cl If the propagation time T1 of the Lamb wave pulse when the container 4 is empty is fixed and is used as a calibration factor in a measurement apparatus, the level in a container 4 can thus be monitored continuously by continuously measuring the current propagation time T2 of an ao mode Lamb wave pulse which - 11 - Eh 361 CA
May 3, 2000 is applied to the respective container 4 and is repeated continuously.
However, since the propagation time T of the ao mode Lamb waves has also been found to be temperature-dependent, it is recommended that the measurement apparatus be calibrated, for example by means of a temperature sensor. Another measure to compensate for temperature fluctuations and to make the measurement result temperature-independent may comprise the provision of a reference measurement path in the region of the container which remains free of material.
Figure 5 shows the principle of such a development of the measurement apparatus shown in Fig. 4. According to this proposal, a second Lamb wave receiver 7' is fitted to the outer wall 1a of the container 4 above the maximum permissible filling level L and at a specific distance d from the Lamb wave excitor 6. The distance d thus defines a reference measurement path d. Since there is never any material 5 in this region of the container 4, the propagation time Tref which can be measured between the Lamb wave excitor 6 and the second Lamb wave receiver 7' in this case always corresponds to the propagation time T1 when the container 4 is empty, and can thus be used as a reference variable in order to derive from it the respectively applicable propagation time T1 when the container 4 is empty, when temperature fluctuations occur and the group velocity C1 changes in consequence, in which case:
d L L
Tref = C and T1 = C ; that is to say Tl = a Tref This factor L/d must therefore be taken into account in the calibration of the measurement apparatus in order, in the end, to obtain temperature-independent measurement results. The propagation time T2 and the - 12 - Eh 361 CA
May 3, 2000 reference propagation time Tref Of the Lamb wave pulse transmitted by the Lamb wave transmitter 6 are determined in both directions to the Lamb wave receivers 7 and 7', the applicable propagation time T1 in the empty container in the instantaneous conditions is derived from the reference propagation time Tref using the factor L/d, and the difference 0T = TZ - T1 is formed as a measurement variable which is directly proportional to the current level and which, in this embodiment of the measurement apparatus, is independent of temperature. In this embodiment of the measurement apparatus with temperature compensation, it is essential that the Lamb wave receiver 6 be arranged on the container wall 1 at the maximum permissible filling level (or above this level by a tolerance amount), in order to allow the propagation time measurement to be carried out in both directions.
Figure 6 shows the block diagram of a level measurement apparatus shown in Fig. 4. The Lamb wave transmitter 6 is arranged on the outer wall la of the container 4 (which is partially filled with the material 5) at the maximum permissible filling level, and the Lamb wave receiver 7 is arranged on the outer wall 1a of the container 4, closely above the container base 8. The lamb wave transmitter 6 is connected to a pulse generator 9, for example in the form of an electroacoustic transducer, whose pulse output or pulse repetition rate is controlled by controller 11 via a clock transmitter 10. This software is used, inter alia, to store the data used for the excitation of an ao mode Lamb wave in the container wall 1 and which, under some circumstances (for example because they are dependent on the material) are determined empirically for each application.For example, the frequency f of the ultrasound signal which is to be applied to the container wall by the Lamb wave excitor 6 may be determined empirically. The ao mode Lamb wave can be - 13 - Eh 361 CA
May 3, 2000 set optimally by varying the frequency of the ultrasound signal and by searching for the Lamb wave whose amplitude change 4A during filling of the container 4 is a minimum, and whose propagation time change ~T is at the same time a maximum. The Lamb wave receiver 7 records the Lamb wave pulses propagating on the direct path in the container wall 1 and converts them back into an electrical signal. Secondary interference waves can advantageously be removed from this signal in a bandpass filter 12, before this signal is passed via an amplifier 13 to the controller 11, for evaluation and for determining the propagation time difference OT. To do this, it is necessary to store the propagation time Tl (which applies to the relevant application) of the Lamb wave pulse when the container 4 is empty, as well as the propagation time T for the container 4 when it is filled with material 5 up to the maximum permissible level. The instantaneous level can be read directly from the propagation time difference 4T.
It is recommended that the pulse be picked up from the electroacoustic transducer at half the wavelength ~/2, in order to make the measurement apparatus insensitive to external effects, such as shaking and external vibration.
Fig. 7 shows a flowchart for measuring the level in a container.With a measuring apparatus as shown in Fig. 4 Fig. 8 shows a block diagram of a level measurement apparatus according to the invention, as shown in Fig.
Fig. 9 shows a flowchart for measuring the level in a container. With a measuring apparatus as shown in Fig.
5.
- 14 - Eh 361 CA
May 3, 2000 While the invention has been illustrated and described in detail in the drawing and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the invetnion are desired to be protected.
5.
- 14 - Eh 361 CA
May 3, 2000 While the invention has been illustrated and described in detail in the drawing and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the invetnion are desired to be protected.
Claims (8)
1. A method for level measurement on containers, according to which a sequence of a0 mode Lamb wave pulses is initiated in the container wall (1) by means of a Lamb wave excitor (6) arranged on it, and the propagation time difference (.DELTA.T) between the propagation time (T2) of these mode Lamb wave pulses between the Lamb wave excitor (6) and a Lamb wave receiver (7) (which is arranged at a distance from it on the container wall (1)) for the current level in the container (4), and the propagation time (T1) over the same path when the container (4) is empty, is determined, and this is used to derive the current level, which is directly proportional to this propagation time difference (.DELTA.T).
2. The method as claimed in claim 1, according to which the propagation time (T1) of the a0 mode Lamb wave between the Lamb wave excitor (6) and the Lamb wave receiver (7) when the container is empty is stored as a reference variable in software (11), in order to determine the propagation time difference (.DELTA.T).
3. The method as claimed in claim 2, according to which a change (caused by temperature fluctuations) in the propagation time of the a0 mode Lamb wave in the container wall (1) is compensated for as a function of the value measured by a temperature measurement sensor.
4. The method as claimed in claim 1, according to which the propogation time (T2) (which varies with the level in the container (4)) of the Lamb wave over a measurement path (L) which extends over the filling range of the container (4), and the propagation time (T1) ( which applies to the empty container (4)) of the Lamb wave via a reference path (d) which extends beyond the filling range are measured continuously.
5. An apparatus for carrying out the method as claimed in one of claims 1 to 3, in which the a0 mode Lamb wave transmitter (6) or the a0 mode Lamb wave receiver (7) is arranged on the outer wall (1a) of the container (4) at the maximum permissible filling level (L), and the a0 mode Lamb wave receiver (7) or the a0 mode Lamb wave transmitter (6) is arranged closely above the container base (8).
6. The apparatus as claimed in claim 5, in which the a0 mode Lamb wave transmitter (6) is connected to software (11) via a pulse generator (9) and a clock transmitter (10), and the a0 mode Lamb wave receiver (7) is connected to software (11) via a bandpass filter (12) and an amplifier (13) with the propagation time (T1) of the a0 mode Lamb wave for the empty container (4) being stored in this software.
7. The apparatus as claimed in claim 6, in which the propagation time (T3) of the a0 mode Lamb wave when the container (4) is filled to the maximum permissible level is also stored in the software (11).
8. The apparatus for carrying out the method as claimed in claim 4, in which the a0 mode Lamb wave transmitter (6) is arranged on the outer wall (1a) of the container (4) at the maximum permissible filling level (L), a first a0 mode Lamb wave receiver (7) is arranged closely above the container base (8), and a second a~ mode Lamb wave receiver (7') is arranged outside the filling range, at a distance (d) from the a~ mode Lamb wave transmitter (6), in which case this distance (d) is used as a reference measurement path for continuous determination of the propagation time (T1) of the a~ mode Lamb wave when the container (4) is empty.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP99110858A EP1059516A1 (en) | 1999-06-07 | 1999-06-07 | Apparatus and method for measuring filling level of containers |
| EP99110858.0 | 1999-06-07 | ||
| US14767199P | 1999-08-06 | 1999-08-06 | |
| US60/147,671 | 1999-08-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2310764A1 true CA2310764A1 (en) | 2000-12-07 |
Family
ID=26153019
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2310764 Abandoned CA2310764A1 (en) | 1999-06-07 | 2000-06-06 | Method for level measurement on containers, and an apparatus for carrying out the method |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2310764A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016131547A1 (en) * | 2015-02-20 | 2016-08-25 | Ergolines Lab S.R.L. | Measuring method, system and sensor for a continuous casting machine |
| IT202000020620A1 (en) | 2020-08-28 | 2022-02-28 | Ergolines Lab S R L | SENSOR, SYSTEM AND METHOD OF MEASUREMENT AND CASTING MACHINE |
-
2000
- 2000-06-06 CA CA 2310764 patent/CA2310764A1/en not_active Abandoned
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016131547A1 (en) * | 2015-02-20 | 2016-08-25 | Ergolines Lab S.R.L. | Measuring method, system and sensor for a continuous casting machine |
| US20180021849A1 (en) * | 2015-02-20 | 2018-01-25 | Ergolines Lab S.R.L. | Measuring method, system and sensor for a continuous casting machine |
| US11020793B2 (en) | 2015-02-20 | 2021-06-01 | Ergolines Lab S.R.L. | Measuring method, system and sensor for a continuous casting machine |
| IT202000020620A1 (en) | 2020-08-28 | 2022-02-28 | Ergolines Lab S R L | SENSOR, SYSTEM AND METHOD OF MEASUREMENT AND CASTING MACHINE |
| WO2022042869A1 (en) | 2020-08-28 | 2022-03-03 | Ergolines Lab S.R.L. | Measuring sensor, system and method and casting machine |
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