WO2004031704A2

WO2004031704A2 - Method and device for modifying the oscillating elements of a vibration detector

Info

Publication number: WO2004031704A2
Application number: PCT/EP2003/010008
Authority: WO
Inventors: Martin Brändlin; Sergej Lopatin; Alexander Müller
Original assignee: Endress + Hauser Gmbh + Co. Kg
Priority date: 2002-09-18
Filing date: 2003-09-09
Publication date: 2004-04-15
Also published as: WO2004031704A3; DE10243430B4; DE10243430A1; AU2003293301A8; AU2003293301A1

Abstract

The invention relates to a method and a device for modifying the oscillating elements of a vibration detector (1), said oscillating elements (2, 3) being fastened to a membrane (5) in the form of a vibrating fork (4). The inventive device comprises a fixing part (7) and a guiding part (6) between which the final area of an oscillating element (2, 3), which faces the membrane (5), is fixed. Also provided is a movable plunger (8) which is guided within a holding device (9) and is disposed relative to the fixed oscillating element (2; 3) such that said plunger (8) exercises a bending force upon the final area of the oscillating element (2, 3), which lies opposite the membrane (5), when being advanced.

Description

Method and device for modifying the vibratable elements of a vibration detector

The invention relates to a method and a device for modifying the vibratable elements of a vibration detector. The oscillatable elements are preferably attached to a membrane in the form of an oscillating fork. The modification is preferably a bending apart of the free end regions of the vibratable elements. Furthermore, the invention relates to a particularly advantageous embodiment of a vibration detector.

The applicant will use the name LIQUIPHANT

Vibration detectors are offered and sold that are just as suitable for determining and monitoring the level of a medium in a container as for determining the density or viscosity of the medium to be monitored.

Vibration detectors designed as level limit switches take advantage of the effect that the oscillation frequency and the oscillation amplitude of an oscillatable unit depend on the respective degree of coverage of the oscillatable unit: while the oscillatable unit can carry out its oscillations freely and undamped in air, it experiences a change in frequency and amplitude, as soon as it is partially or completely immersed in the medium. On the basis of a predetermined change in frequency (usually the frequency is measured), it is consequently possible to draw a clear conclusion that the predetermined fill level of the medium in the container has been reached.

In addition, the damping of the vibration of an oscillatable unit is also influenced by the density and viscosity of the medium. Therefore, with a constant degree of coverage, there is a functional relationship to the density or to the viscosity of the medium, so that vibration detectors are ideally suited for determining the fill level as well as for density and viscosity. In practice, the purpose is Monitoring and detection of the fill level or the density of the medium in the container, the vibrations of the membrane are recorded and converted into electrical reception signals by means of at least one piezo element.

The electrical received signals are then evaluated by evaluation electronics. In the case of level determination, the evaluation electronics monitor the oscillation frequency and / or the oscillation amplitude of the oscillatable element and signals the status 'sensor covered' or 'sensor uncovered' as soon as the measured values fall below or exceed a predetermined reference value. A corresponding message to the operating personnel can take place optically and / or acoustically. Alternatively or additionally, a switching process is triggered; for example, an inlet or outlet valve on the container is opened or closed.

LIQUIPHANTS are used to determine the desired process size in liquid media. The trend is towards minimizing the dimensions of the vibration detectors. As a result of miniaturization, problems sometimes arise when the vibration detectors are used to monitor viscous media which have a high solids content. One example is the use of a vibration detector for filling level monitoring in fruit preparation. Specifically, the medium can be yogurt or jam with pieces of fruit. Since the vibratable elements of the tuning fork are relatively close together, there is an increased risk that fruit pieces get stuck between the two vibratable elements, so that the vibration detector then only outputs the faulty signal "sensor covered". One solution to this problem would be to completely redesign a vibration detector with correspondingly widely spaced vibratable elements. It goes without saying that such a solution is associated with a high outlay.

The invention is based on the object of proposing a method and a device which make it possible to subsequently mechanically change the standard fork geometry in a vibration detector without influencing its functionality. Furthermore, the invention is the The task is to propose a vibration detector that can be used in highly viscous media with a high solids content.

With regard to the method, the object is achieved in that, in a first step, the two oscillatable elements are positioned in or on or on a guide part; in a second step, the first vibratable element to be bent is fixed in its end region facing the membrane; in a third step, a force is applied to the end region of the fixed oscillatable element facing away from the membrane, as a result of which this end region is bent; then steps 1 to 3 are carried out for the second vibratable element.

According to an advantageous development of the method according to the invention, a force is applied to the end region facing away from the membrane, which leads to a defined bending of the end region. Defined bending is very important so that the symmetry of the tuning fork is not destroyed. A symmetrically constructed tuning fork and the excitation of modes with opposite vibrations has the advantage that the forces transmitted from each vibratable element to the membrane cancel each other out. This compensation minimizes the mechanical stress on the clamping of the membrane, so that approximately no vibration energy is transmitted to the housing or to the fastening part of the vibration detector. Ultimately, this effectively prevents the fastening part of the vibration detector from being excited to resonate vibrations, which in turn interfere with the vibrations of the oscillatable unit and can thus reduce the measuring accuracy.

The two end regions of the oscillatable elements facing away from the membrane are advantageously maximally bent apart so far that the distance between the outer surfaces of the two oscillatable elements is smaller than the inner diameter of an opening in a container in which the vibration detector is fastened. An alternative embodiment suggests that the previously defined distance is smaller than the outer diameter of the fastening part of the vibration detector. This measure will allows easy installation and removal of the vibration detector in a container opening.

On the basis of series of measurements, it has been found that the sensitivity to water of vibratable elements modified according to the invention is slightly reduced in contrast to corresponding unmodified vibratable elements of a vibration detector. This is not critical insofar as a bending of the vibratable elements of a vibration detector according to the invention is just superfluous when used in aqueous solutions. The series of measurements further show that a change in the sensor sensitivity or the sensor dynamics of a vibration detector with oscillatable elements modified according to the invention cannot be measured when used in highly viscous media. Consequently, vibration detectors modified according to the invention can be handled in a completely analogous manner to the non-modified vibration detectors.

With regard to the device according to the invention, the object is achieved in that a fixing part and a guide part are provided, between which the end region facing the membrane is fixed by an oscillatable element. Furthermore, a movable stamp is provided which is guided in a holder and which is arranged with respect to the fixed oscillatable element in such a way that it exerts a bending force on the end region of the oscillatable element facing away from the membrane. With the bending device according to the invention it is, for. B. possible to increase the distance of the vibratable elements of the tuning fork in the sensitive area of the fork tips from 10mm to 22mm without any kind of stress on the membrane or the piezo drive.

In addition, the device according to the invention and the method according to the invention can be used universally. It is possible to carry out the modification, in particular the defined spreading of the vibratable elements, without changing the sensor characteristics, both at the end of the manufacturing process of the vibration detector and, if necessary, at the current place of use. An advantageous development of the device according to the invention provides that the longitudinal axis of the guide part and the longitudinal axis of the stamp or the holder for the stamp are arranged at an angle to one another which is greater than 90 °.

A preferred embodiment of the device according to the invention provides that an adjustable stop is assigned to the advance of the stamp. This enables a defined bending of the vibratable element. The stamp is preferably a screw which is rotatably mounted in a bore in a holder. The stamp is preferably made of a material that is softer than the material from which the vibratable elements or the coating of the vibratable elements consist or consists. For example, the stamp is made of brass. This configuration makes it possible to use the device according to the invention to also bend vibratable elements with a relatively sensitive surface. In this context, vibration detectors with vibrating elements that are highly polished are worth mentioning.

In some applications it is necessary to provide the vibrating elements of the vibration detector that come into contact with the medium with a special coating. The coating can be an enamel coating, for example. Such an enamel coating must be applied prior to the bending / modification of the oscillatable elements according to the invention, since the enamel coating would break through a subsequent bending process. If the vibratable elements are to be bent / modified after coating, a plastic coating must be selected instead of the enamel coating. For example, ECTFE or PFA can be used as coating materials.

According to an advantageous development, the stop is a lock nut or a knurled screw. Of course, any other person who is familiar with a professionally qualified person can also do so Design of a stop can be used in connection with the device according to the invention.

In the vibration detector according to the invention, the vibratable elements of the tuning fork are bent up. The maximum distance between the outer surfaces of the two bent-up oscillatable elements of the tuning fork is preferably smaller than the inner diameter of an opening in the container in which the vibration detector is fastened. As a result, the assembly can be carried out analogously to a vibration detector known from the prior art.

The vibration detector according to the invention has the advantage that it still works reliably even in highly viscous media with a high solids content. As a result of the constantly increasing distance between the vibratable elements in the direction of the fork tip, the risk of solid parts jamming between the vibratable elements is considerably reduced.

According to an advantageous development of the device according to the invention, the vibratable elements which come into contact with the medium are provided with an inert and corrosion-resistant coating. If this coating is applied to the vibratable elements before the modification or bending process, it must consist of a flexible material so that it does not break when bent. A plastic coating is preferably used in this context. If the coating is applied to the modified or already curved vibratable elements, it can also consist of enamel, for example.

The invention is illustrated by the following drawings. It shows:

1: a plan view of a preferred embodiment of the device according to the invention with a vibration detector to be processed,

2: a top view of the embodiment of the device according to the invention shown in FIG. 1 without vibration detector, 3: a cross section according to the identification AA in Fig. 1,

4: a side view according to the marking B in Fig. 1,

5: a side view according to the marking C in Fig. 1,

6 is a side view of an unmodified vibration detector and

7: a side view of an embodiment of a vibration detector according to the invention.

An embodiment of a non-modified vibration detector 1 is shown in side view in FIG. 6. The vibration detector 1 is used for

Detection and / or monitoring of the level of a medium in a container. The medium is not shown separately in FIG. 6. In addition to the level detection, the vibration detector 1 can also be used to determine the density or the viscosity of the medium in the container. While in the case of level detection the vibratable elements 2, 3 are immersed in the medium (- overfill protection) or not in the medium (- dry running protection) only when the detected limit fill level is reached, they have to be continuously used for monitoring or for determining the density or viscosity be in contact with the medium up to a predetermined immersion depth. In which

The container can be, for example, a tank, but also a pipe through which the medium flows.

The vibration detector 1 has an essentially cylindrical housing 28. A thread 17 is provided on the lateral surface of the housing 28. The thread 17 is used to fasten the vibration detector 1 in a corresponding opening 27 in a container 26. It goes without saying that other types of fastening, for example by means of a flange, can replace screwing. The housing 28 of the vibration detector 1 is closed off by the membrane 5 at its end region projecting into the container 26, the membrane 5 being clamped into the housing in its edge region. Two oscillatable elements 2, 3 in the form of an oscillating fork 4 are attached to the membrane 5.

The membrane 5 is driven by a. Receiver unit 16 set in vibrations with a predetermined excitation frequency. The drive / receiver unit 16 is - as sketched in FIG. 6 - a stack drive or a bimorph drive. Both types of piezoelectric drive / reception units 16 are sufficiently known from the prior art, so that a corresponding description can be dispensed with here.

Due to the vibrations of the membrane 5, the vibrating fork 4 also carries out vibrations, the vibrating frequencies being different when the vibrating fork 4 is in contact with the medium and is coupled to the mass of the medium, or when the vibrating fork 4 is free and without contact with can swing the medium. The data exchange between the vibration detector 1 and an evaluation and display unit (not shown separately in FIG. 6) takes place via the data lines 21, 22.

As already mentioned above, the trend is towards miniaturization of the vibration detectors 1. In addition to material and cost saving reasons, an expansion of the possible uses also plays a role here.

If standard vibration detectors 1 with relatively close together oscillatable elements 2, 3 are used in highly viscous media with a high solids content, there is a risk that solid parts get jammed between the oscillatable elements 2, 3; the vibration detector 1 therefore usually no longer functions or no longer functions correctly.

The method according to the invention and the device according to the invention as well as the vibration detector 1 according to the invention provide solutions to this problem. 1 to 5 are different To see views of an advantageous embodiment of the device according to the invention.

The essential parts of the bending or modification unit according to the invention are: a guide part 6 with an elevation 20:

a fixing part 7, the longitudinal axis 23 of which is arranged essentially at right angles to the longitudinal axis 10 of the guide part 6;

- A movable stamp 8, which is arranged in the bore 14 of a holder 9, the longitudinal axis 11 of the stamp 8 or the holder 9 and the longitudinal axis of the guide part 6 forming an angle greater than 90 °. The individual components are preferably integral or attached parts of a base plate 25.

As shown in FIG. 1, the vibration detector 1 is positioned in the bending unit according to the invention. The elevation 20 on the guide part 6 is adapted to the shape of the tuning fork 4 and is designed such that the two vibratable elements 2, 3 can be positioned on both sides of the elevation 20.

After the vibration detector 1 has been positioned, one of the two vibratable elements - in the case shown the vibratable element 3 - is fixed in the region of the root by means of the fixing part 7. The fixing part 7 is a screw 18 which is guided in the bore 19 of the holder 24. Fixing the vibratable element 3 in the region of the root ensures that no forces act on the membrane 5 and the piezo drive 16 when the vibratable elements 2, 3 are bent open.

The vibratable element 3 is bent open by the propulsion of the movable stamp 8. The stamp 8 is guided in the bore 14 of the holder 9. With regard to the free end region of the oscillatable element 3, the tip of the plunger 8 is oriented such that a force F is applied to the end region of the oscillatable element 3 facing away from the membrane 5 such that it is bent outwards. In the case of FIG. 1 shown vibration detector 1, the vibratable element 2 is already bent.

So that the bending of the oscillatable elements 2, 3 can take place in a defined manner, the screw 13 can only be screwed into the bore 14 up to the stop 12. In the case shown, the stop 12 is a lock nut. The stop 12 is preferably adjustable, as a result of which the defined bending of the oscillatable elements 2, 3 can be adapted to the respective application.

A standard vibration detector can be seen in FIG. Here, the oscillatable elements 2, 3 are aligned essentially parallel to one another (i.e. with the exception of the area of the fork root). If the diameter of the solid parts contained in a medium is comparable to the distance between the inner surfaces of the two vibratable elements 2, 3, there is a risk that the solid parts get jammed between the elements 2, 3. As a result, the vibration detector 1 permanently outputs the status "sensor in contact with the medium", with which the vibration detector 1 no longer functions correctly.

7 shows a preferred embodiment of the vibration detector 1 according to the invention. The structure of the vibration fork 4 according to the invention reliably prevents a malfunction of the vibration detector 1 which is attributable to solid parts clamped between the vibratable elements 2, 3. According to the invention, the two oscillatable elements 2, 3 are bent so far that the distance between their inner surfaces, at least outside the region of the fork root, is greater than the maximum diameter of the solid parts. It should be noted that the modified tuning fork 4 is constructed to a high degree symmetrically. This is the only way to effectively avoid stress on the membrane 5 and the piezo drive 16. This has already been described in detail in the previous section.

It goes without saying that the manufacture, modification or bending of the vibratable elements either during the manufacture of the vibratable elements 2, 3, or before or after the installation of the vibration detector 1 can be done. If the modification is carried out before the vibratable elements 2, 3 are attached to the membrane 5, then they can be produced in any manner. If the bending or modification takes place after attachment to the membrane, care must be taken that the membrane or the piezo drive is not damaged by the necessary manipulations. If the vibratable elements 2, 3 are bent on the mounted vibration detector 1, the method according to the invention already described is preferably used, since the membrane 5 is not stressed at all by the successive modification of the two vibratable elements 2, 3.

LIST OF REFERENCE NUMBERS

Vibration detector Vibratable element

Vibratory element

tuning fork

membrane

guide part

fixing part

stamp

bracket

Longitudinal axis of the guide part

Longitudinal axis of the stamp or the holder

Stop / locknut

screw

drilling

coating

Piezo drive or drive / receiver unit

attachment portion

screw

drilling

survey

signal line

Longitudinal axis of the fixation part

bracket

baseplate

container

opening

casing

Claims

claims

1. A method for modifying the vibratable elements (2, 3) of a vibration detector (1), the vibratable elements (2, 3) being attached to a membrane (5) in the form of a vibrating fork (4), the first step being the two oscillatable elements (2, 3) are positioned with respect to a guide part (6), the first oscillatable element (2; 3) to be bent being fixed in its end region facing the membrane (5) in a second step, in a third step a force is applied to the end region of the fixed oscillatable element (2; 3) facing away from the membrane (5), whereby this end region is bent, and steps 1 to 3 are then carried out for the second oscillatable element (3; 2).

2. The method according to claim 1, wherein a force is applied to the end region facing away from the membrane (5), which leads to a defined bending of the end region.

3. The method according to claim 2, wherein the two end regions facing away from the membrane (5) are maximally bent so that the distance between the outer surfaces of the two oscillatable elements (2, 3) is smaller than the inner diameter of an opening in a container, in which the vibration detector (1) is attached.

4. Device for modifying the vibratable elements 2, 3 of a vibration detector (1), the vibratable elements (2, 3) in the form of a vibrating fork (4) being fastened to a membrane (5), a guide part (6) and a Fixing parts (7) are provided, between which the end region facing the membrane (5) is fixed by an oscillatable element (2; 3), a movable plunger (8) being provided, which is guided in a holder (9), and which is arranged with respect to the fixed oscillatable element (2; 3) such that it bends a bending force on the Exerts diaphragm (5) facing away from the oscillatable element (2; 3).

5. The device according to claim 4, wherein the longitudinal axis (10) of the guide part (6) and the longitudinal axis (11) of the stamp (8) or the holder (9) for the stamp (8) are arranged at an angle to each other is greater than 90 °.

6. The device according to claim, wherein the propulsion of the stamp (8) has a preferably adjustable stop (12).

7. The device according to claim, wherein the stamp (8) is a screw (13) which is rotatably mounted in a bore (14) of the guide part (6).

8. The device according to claim wherein the stop (12) is a lock nut or a knurled screw.

9. The device according to claim, wherein the stamp (8) is made of a material which is softer than the material from which the vibratable elements (2, 3) or the coating (15) of the vibratable elements (2, 3) is made is.

10. Device for determining and / or monitoring the fill level of a medium in a container (26) or for determining the density or the viscosity of a medium in the container (26), wherein two oscillatable elements (2, 3) in the form of a tuning fork (4) are attached to the level of the predetermined level or the vibrating fork (4) is attached in such a way that it dips into the medium up to a defined immersion depth, with a drive. Receiving unit (16) is provided which excites the vibrating fork (4) to vibrate at a predetermined excitation frequency and which receives the vibrations of the vibrating fork (4), and a control / evaluation unit is provided which detects when the predetermined fill level is reached as soon as a given one Frequency change occurs or which determines the density or viscosity of the medium on the basis of the vibration frequency of the tuning fork, characterized in that the vibratable elements (2, 3) of the tuning fork (4) are bent.

11. The device according to claim 10, characterized in that the maximum distance (a) of the outer surfaces of the two bent oscillatable elements (2, 3) of the tuning fork (4) is smaller than the inner diameter (b) of an opening (27) in the container (26) in which the vibration detector (1) is attached.

12. The apparatus of claim 10 or 11, characterized in that the vibratable elements coming into contact with the medium (2, 3) have an inert coating.

13. The apparatus according to claim 12, characterized in that the coating is plastic or enamel.