WO2006005660A1 - Device for determining and/or monitoring a process variable of a medium - Google Patents

Abstract

The invention relates to a device (1) for determining and/or monitoring a process variable of a medium (5), said device comprising a mechanically oscillatable unit (10), a cupular housing (15), and at least one excitation/receiving unit (30) that excites the oscillatable unit (10) in such a way that it oscillates, and receives the oscillations. According to the invention, the excitation/receiving unit (30) comprises a rod (35) that is defined by a stamp (40), said stamp (40) has a minimal bearing surface, the excitation/receiving unit (30) comprises at least one piezoelectric element (45) which is penetrated by the rod (35), the excitation/receiving unit (30) comprises an abutment disk (50) arranged above the piezoelectric element (45), and the excitation/receiving unit (30) is clamped between a pressure screw (60) and the membrane (16).

Description

 description

Device for determining and / or monitoring a process variable of a medium

The invention relates to a device for determining and / or

Monitoring at least one process variable of a medium in a container, with at least one mechanically oscillatable unit, with at least one be¬ cherförmigen end closed by a membrane housing, the mechanically oscillatable unit is mechanically coupled at least with the membrane of the housing, with at least one excitation / Receiving unit, which stimulates the mechanically oscillating unit to generate mechanical vibrations and which receives the mechanical vibrations of the mechanically oscillating unit, with at least one control unit which controls the mechanical vibrations of the mechanically oscillatable unit and the process variable of the mechanically oscillatable unit Medium determined and / or monitored. The process variable is, for example, the fill level, the density or the viscosity of a medium, for example a liquid in a tank. In particular, the process variable is the fill level of a bulk material.

Such aforementioned devices use the interaction between the

Medium and the mechanically oscillatable unit and the effect on the vibrations of the oscillatory unit. The level can be monitored, for example, by the fact that with liquids the frequency of the oscillations depends on whether the medium is in contact with the sensor or not. Thus, it is possible to conclude from a frequency change that the level has changed. In connection with the phase between the excitation of the vibrations and the vibrations received, for example, the viscosity or the density of the medium can be determined. Such measuring devices are described in more detail in the patent documents and applications of the applicant: DE 33 36991 C2, EP 1 134038 A1 and WO 92/21945. The mechanically oscillatable units shown there are so-called vibration forks. These vibration forks are attached to a membrane, which complete a cup-shaped housing. On the inside of the membrane is the excitation / reception unit, which are formed of different arrangements of stacks of piezoelectric elements. In the piezoelectric elements, use is made of the fact that an electrical alternating voltage leads to a mechanical oscillation and conversely that a mechanical oscillation is transmitted to an alternating electrical voltage. In the document DE 33 36 991 C2 is a stack of piezoelectric elements

(Piezo stack), which is connected on one side with the membrane and which is held on the other side by a yoke under bias. The problem with this design is that the membrane must be sufficiently large so that the yoke can be placed, and that the assembly is very expensive.

In the EP 1 134 038 Al, the vibration excitation and detection is carried out using a piezo stack, which is pressed by a pressure screw against the membrane. The piezo stack consists of piezoelectric elements glued together, which are enclosed at the top and bottom of each of a stamp. For this embodiment, a temperature-resistant adhesive and a complex adhesive process are particularly required.

The piezoelectric drive, which can be found in the document WO 92/21945, consists of a stack of piezoelectric elements which are mounted on a clamping screw and which are biased by a nut. However, this embodiment is not suitable for bulk materials, since the lateral action of the bulk material on the tines and an associated approach of the tines, the bias is canceled and the piezoelectric elements have no positive contact with the membrane more.

[006] It is therefore the object of the invention to provide a measuring device with a starting

Specify / receiving unit, a membrane and a mechanically oscillatable unit whose production is simple, and in which the non-positive contact between the pickup / receiving unit and membrane is maintained even when laterally acting on the oscillatory unit forces.

The invention solves the problem in that the exciting / receiving unit has at least one rod, that the rod is bounded at one of the membrane end facing by a punch that the stamp is designed such that a side facing the membrane a has minimal support surface, that the exciting / receiving unit has at least one piezoelectric element, which is penetrated by the rod, that the exciting / receiving unit has a Drackscheibe, which is arranged in the direction away from the membrane above the piezo¬ electrical element on the rod , And that the pickup / receiving unit is clamped between a pressure screw and the membrane. The inventive starting / receiving unit thus consists of at least one piezoelectric element which is inserted on a rod. The rod is bounded in the direction of the membrane by a stamp, wherein the stamp on the membrane side has the smallest possible or minimum contact surface, so that the bias acts specifically only on a small area. At the same time, however, the contact surface should be so large that optimum and above all sufficient force Transmission between membrane and pickup / receiving unit can take place. Under this criterion, the support surface is thus to minimize, while it is possible to prevent that the support surface is connected to areas of the membrane, which are exposed to different forces, especially with respect to the direction of the forces. Above the piezoelectric element is a thrust washer. This has a double task. On the one hand, this completes the pickup / receiving unit and results in a compact unit which facilitates assembly, since, for example, no piezoelectric element can slip off the rod during transport or during assembly. As a result, no sticking of the piezoelectric elements is required. On the other hand, this thrust washer absorbs the pressure that is exerted by the pressure screw in the installed state. The pressure screw is preferably screwed into the inner wall of the housing, so that an automatic guide and a centering of the pickup / receiving unit results, and so that the piezo unit is non-positively connected to the membrane. The piezoelectric element or possibly also a multiplicity of such elements are thus arranged on a rod. Therefore, gluing is not necessary. The rod ends on a punch, which serves the power transmission between the pickup / receiving unit and membrane and which is further designed so that the force transmission occurs only at the point of the membrane, attach to the forces acting from outside through the bulk material, so that the Preload can not be canceled. The rod, the punch, the piezoelectric element or the piezoelectric elements and the pressure plate form a unit that allows easy installation.

An embodiment provides that the support surface of the punch is located substantially on the extension of the longitudinal axis of the rod. Thus, the stability of the structure of the pick-up / receiving unit is increased by the point on which primarily the bias voltage is supported by the rod as possible in its full length.

An advantageous embodiment includes that the pressure screw, the rod and the pressure disk are designed and matched to one another such that the pressure screw essentially exerts pressure only on the outer edge of the pressure disk. The pressure screw, the rod and the thrust washer, so generally the pickup / receiving unit and the means for their attachment in the housing are designed aus¬ such that the pressure of the pressure screw acts essentially only on the edge of the pressure plate. Due to the structure of this embodiment, the pressure is thus transmitted from the side edges of the rod on the small contact area of the punch on the membrane. Thus, this device is also suitable for bulk materials, since in a flowing bulk material and thus an approach of the forks to each other, the bias is not canceled and thus there is the pickup / receiving unit always in contact with the membrane. Thus, the device does not fail due to lateral load, especially due to the moving bulk material.

[010] An embodiment includes that the pressure screw is connected to the housing. Preferably, the pressure screw is screwed or screwed into a corresponding thread of the housing. This serves - as stated above - the centering and guiding the pickup / receiving unit. Furthermore, there is the advantage that thus the bias also depends on the attachment in the housing and thus the bias is generated by more area and also by symmetrically distributed surface. This is in contrast to the drive of WO 92/21945, in which the bias at the upper end is generated only by the mother.

[011] An embodiment provides that the container, the pickup / receiving unit, the

Pressure screw and the mechanically oscillatable unit are designed and matched to one another such that the bearing surface is located on the center axis of the mechanically oscillatable unit. Under center axis here is e.g. the longitudinal or symmetry axis understood with respect to reflection or the axis of the center of gravity. The aim of this embodiment is that the bearing surface of the punch and thus the lower point of the bias is opposite to the point on which attach the laterally acting by a bulk material forces. By way of example, the mechanically oscillatable unit is a tuning fork with two fork teeth. The bearing surface on the membrane is thus in this case opposite the region of the membrane which lies in the (geometric or center of gravity) center between the two fork tines. If the tines are pressed together laterally, a force acts in the direction of the interior of the housing at this point of the membrane. However, since the contact point of the pickup / receiver unit is exactly on the opposite side, this force is already canceled by the bias voltage and it is not possible for the pickup / receiver unit to lose contact with the diaphragm. Thus, the device is optimal and reliable suitable for bulk materials.

An embodiment provides that the punch on the membrane facing

Side at least partially has a spherical surface. By such a spherical surface is realized that the support surface is low and that when screwing the drive, the sensitive ceramic parts are under the lowest possible torque. Furthermore, this is also a very easy to implement design. At Auf¬ location point, it is also possible that the spherical surface has a corresponding flattening.

[013] An embodiment provides that within the housing at least one

Inner tube is mounted so that the inner tube surrounding the pickup / receiving unit, and that the pressure screw is connected to the inner tube. Such a tubular Inner part is described for example in the published patent application DE 101 29 556 A1 of the applicant. The pressure screw is thus advantageously screwed to the inner tube, which is connected to the housing and / or the membrane, and thus generates the bias voltage for the pickup / receiving unit. The inner tube causes a thermal decoupling of the area of the pickup / receiving unit. This is important in temperature shocks. If the temperature changes very quickly, usually the outer and inner components are at different temperatures, since the temperature adjustment does not take place at the same speed. In conjunction with different expansion coefficients, there is a risk that the frictional connection is canceled. By the inner tube an un¬ uniform temperature distribution is avoided and even with a temperature shock, the optimum bias remains guaranteed.

[014] An embodiment includes that the rod and the punch are made in one piece. If the rod and the punch are a fitting, then difficulties can be avoided that can occur at the connection or contact point.

An embodiment provides that at least the rod and / or the punch consist of a metal or a ceramic. This is also to be seen in connection with the one-piece design. The advantage of a ceramic is that no insulating elements between the rod or stamp and the piezoelectric element or elements must be provided, as is required for a metal.

An embodiment includes that the pressure screw has a recess for the end remote from the membrane of the rod. Thus, the easier Zu¬ sammenbau the pickup / receiver unit is possible because the rod does not have to be flush with the pressure plate.

[017] An embodiment provides that the pickup / receiving unit has at least two piezoelectric elements. The elements may be a transmitting unit and a receiving unit, between which an insulating disk is located, depending on the electrical control. This means that a piezoelectric element is acted upon by the regulating / control unit with an alternating electrical voltage and the membrane vibrates the mechanically oscillatable unit, and that the other element receives the mechanical vibrations via the membrane and converts them into an electric current AC voltage - as a measuring signal or feedback signal for the fundamental excitation of the mechanically oscillatable unit thus - converts. In general, therefore, the mechanically oscillatable unit is excited by the piezoelectric element or the piezoelectric elements to resonance and the vibrations are conversely converted into electrical signals. [018] An embodiment includes that the mechanically oscillatable unit at the

Membrane is attached. Preferably, the mechanically oscillatable unit is attached by welding to the membrane or it is integrally connected to the membrane by the same manufacturing process.

[019] An embodiment provides that the mechanically oscillatable unit is a

Tuning fork is. Such a tuning fork usually has two spaced-apart tines, which oscillate in opposite directions to one another, so that as far as possible no resulting forces act on the clamping in the region of the membrane. However, the invention is not limited to a number of subunits of the oscillatable unit. Between the tines there is also a middle or symmetrical line. In order that no energy is lost as a result of an imbalance, preferably both tines are essentially of the same design at least with regard to their mass and their moment of inertia. In the middle between the two tines force components also act on the membrane, which result from the forces acting laterally on the tines by bulk materials. It can also be a single rod or a variety of prongs.

[020] One embodiment provides that the mechanical oscillations of the mechanically oscillatable unit are bending oscillations. The pickup / receiver unit first bends the membrane and via this the oscillatory unit is excited to vibrate.

[021] One embodiment provides that it is the density or the viscosity of the medium is the process variable to the filling level, _Λ. These process variables and their measurement can be found in the writings mentioned in the introduction.

[022] The invention will be explained in more detail with reference to the following drawings. It shows:

[023] FIG. 1 shows the schematic structure of a measurement with the invention

Contraption,

[024] FIG. 2: a cross-section through the pickup / receiving unit of the invention,

[025] FIG. 3 shows a cross section through an inventive starting / receiving unit installed in the housing,

[026] FIG. 4: a schematic representation of the forces acting on the device according to the invention by a bulk material, and

[027] FIG. 5 shows a schematic representation of an additional variant of the installation of the inventive starting / receiving unit into the housing.

Fig. 1 shows the basic structure of an application of an inventive

Device 1. A medium 5 is located in a container 6. On the wall of the container 6, the device 1 is mounted, so that the mechanically oscillatable unit 10 can come into contact with the medium 5. The mechanical oscillatable unit 10 is excited by the pickup / receiving unit 30 to vibrate and the vibrations of the unit 10 are received by the pickup / receiving unit 30 again, converted into an electrical alternating voltage and fed to the control / evaluation unit 20. If the medium 5 reaches the mechanically oscillatable unit 10, this has an effect on the vibrations and thus it can be seen that the filling height predetermined by the installation height and orientation of the device during installation has been reached. The detection of falling below this level is also possible accordingly. If the mechanically oscillatable unit 10, which is a tuning fork in the illustrated case, is covered by the medium 5, the density or the viscosity of the medium 5 can also be determined in the case of a liquid as medium from the oscillations, eg amplitude, frequency and phase be determined.

FIG. 2 shows the pickup / receiving unit 30 according to the invention. The unit 30 itself is preferably designed to be rotationally symmetrical. A rod 35 is bounded on a side 35.1 by a punch 40. The punch 40 has on its end face 40.1 a spherical surface, so that the contact surface between the membrane (see Fig. 3) and the pickup / receiving unit 30 is as small as possible. Possibly. can also be provided a flattening, so that the contact area between the punch 40 and diaphragm 16 is slightly increased. This surface is also - as can be seen - on the extension of the longitudinal axis of the rod 35. Are the rod 35 and the punch 40 integrally formed, so it simplifies the production and there are above all no joints that are exposed to mechanical stress and thus can become sources of error. On the rod 35 some piezoelectric elements 45 are attached. Here, the elements separately take over the task of transmitter and receiver, so that between these insulating discs are attached. The piezoelectric elements 45 are likewise electrically insulated from the punch 40 and the pressure disk 50. The insulation with respect to the rod 35 is effected by a - not shown here - plastic tube which is slipped over the rod 35. Furthermore, electrical contacts for contacting the piezoelectric elements 45 are also provided. The stack of piezoelectric elements 45 is bounded upwardly by the aforesaid thrust washer 50. This ensures that the pickup / receiving unit 30 can be manufactured and transported as a single piece, without individual piezoelectric elements 45 could fall off. For this purpose, a spring or a snap ring is provided at the top, so that the pressure plate 50 weakly presses against the piezoelectric elements 45. Furthermore, the thrust washer 50 is used to install the excitation / reception unit 30 by applying pressure to it, or more precisely by applying the pressure on its edge region 50.1, which leads to the biasing of the excitation / reception unit 30 on the diaphragm. Due to its design is the Pickup / receiving unit 30 an independent component, which is manufactured in a suitable number of pieces and then installed.

[030] In FIG. 3, the pick-up unit 30 is shown in the installed state.

The housing 15 can be seen, which is completed by the membrane 16 on the process side. On the membrane 16, the two prongs of the mechanically oscillatable unit 10 are attached. The excitation receiving unit 30, which is inserted into the housing 15, rests on the support area 40.2 of the punch 40 on the inside of the membrane 16. The pressure screw 60 is screwed by a thread into the housing 15. The screw 60 has a recess 60.1 in which just the upper end 35.2 of the rod 35 fits. The pressure screw 60 is further configured such that its pressure acts only on the outer edge 50.1 of the thrust washer 50. Furthermore, a slight edge on the pressure screw 60 can be seen, so that a centering of the pickup / receiving unit 30 is achieved during assembly. As a result, the contact surface 40.2 on the membrane 16 opposes the region which lies on the longitudinal axis and here also on the axis of symmetry of the piston mechanically oscillatable unit 10 is located. Thus, the device 1 is also suitable for bulk materials. If the medium 5 pushes the forks together and thus together, this does not reduce the prestressing in the starting / receiving unit 30. According to the invention, the pickup / receiving unit 30 is an independent unit that is introduced into the finished housing 15. This is e.g. important if the housing 15 is provided with a special coating, which may need to be applied with such temperatures, which are above the temperatures at which the piezoelectric elements lose their polarization. At the same time the design is so space-saving that the meter can be made very small.

In FIG. 4, the acting forces (indicated by arrows) are shown, which can occur through a bulk material. If the bulk material comes into contact with the mechanically oscillatable unit 10, this causes in most cases that the distance between the fork tines becomes smaller, ie the tines are compressed together. The force of the bulk material is indicated by the lateral arrows. These forces cause a force component, which acts in the direction of the pickup / receiving unit 30, to occur between the fork prongs on the membrane 16. However, this component is just compensated by the design of the pickup / receiving unit 30. The resulting component acts at the (center of gravity) center between the two prongs. By the bias on the Auf¬ the bearing surface of the punch 40 acts exactly on the opposite side of the membrane 16, to which the resulting force component attaches, it comes through the bulk material not to cancel the bias. This purpose is also served by the pressure screw 60 acting on the edge region 50.1 of the thrust washer 50 and not, for example, directly along the rod 35. FIG. 5 shows a further possible embodiment. In this case, the pickup / receiving unit 30 is mounted in an inner tube 65. This can improve the behavior of the sensor during a temperature shock. At a rapid temperature change, the inner and outer parts of the sensor / the device 1 are not at the same temperatures, ie there is a temperature difference between inside and outside. Since the materials of the components may have different coefficients of expansion, there is the risk that the frictional connection between the excitation pump unit 30 and the membrane 16 may be lost for a short time. The inner tube 65 shown here prevents an uneven temperature distribution of the sensor parts, so that the pickup / receiving unit 30 remains under optimal bias even during a Temperaturschicks.

Claims

claims Device (1) for determining and / or monitoring at least one process variable of a medium (5) in a container (6), with at least one mechanically oscillatable unit (10), with at least one cup-shaped end of a membrane (16 ), wherein the mechanically oscillatable unit (10) is mechanically coupled at least to the membrane (16) of the housing (15), with at least one pickup / receiving unit (30) which holds the mechanically oscillatable unit (10). excites me¬ chanical vibrations and receives the mechanical vibrations of the mechanically oscillatable unit (10), with at least one control / evaluation unit (20) which controls the mechanical vibrations of the mechanically oscillatable unit (10) and based on the mechanical vibrations of the mechanically oscillatable unit (10) determines and / or monitors the process variable of the medium (5), characterized in that the excitation / reception unit t (30) has at least one rod (35), that the rod (35) at one of the membrane (16) facing end (35.1) of a punch (40) is limited, that the punch (40) is designed such that one of the membrane (16) facing side (40.1) has a minimal bearing surface (40.2) that the exciting / receiving unit (30) at least one piezo¬ electrical element (45), which is penetrated by the rod (35) that the Pickup / receiving unit (30) has a thrust washer (50) which is arranged in the direction away from the membrane (16) above the piezoelectric element (45) on the rod (35), and that the pickup / receiving unit (30) between a pressure screw (60) and the membrane (16) is clamped. Device (1) according to claim 1, characterized in that the contact surface (40.2) of the punch (40) lies substantially on the extension of the longitudinal axis of the rod (35). Device (1) according to claim 1, characterized in that the Pressure screw (60), the rod (35) and the pressure plate (50) designed aus¬ and matched to one another that the pressure screw (60) Essentially only on the outer edge (50.1) of the thrust washer (50) exerts pressure. Device (1) according to claim 1, characterized in that the Compression screw (60) is connected to the housing (15). Device (1) according to claim 1, characterized in that the container (6), the pickup / receiving unit (30), the pressure screw (60) and the mechanically oscillatable unit (10) configured and successive are matched, that the support surface (40.2) on the center axis of the mechanically oscillatable unit (10). Device (1) according to claim 1, characterized in that the punch (40) on the membrane (16) facing side (40.1) at least partially has a spherical surface. Device (1) according to claim 1, characterized in that within the Housing (15) at least one inner tube (65) is mounted, that the inner tube (65) surrounds the energizing receiving unit (30), and that the pressure screw (60) is connected to the inner tube (65). Device (1) according to claim 1, characterized in that the rod (35) and the punch (40) are made in one piece. Device (1) according to claim 1 or 8, characterized in that at least the rod (35) and / or the punch (40) consist of a metal or of a ceramic. Device (1) according to claim 1, characterized in that the Compression screw (60) has a recess (60.1) for the end remote from the membrane (16) of the rod (35.2). [011] Device (1) according to claim 1, characterized in that the mechanically oscillatable unit (10) is a tuning fork.