Patent Application
20050183514
The invention relates to a magnetic-inductive measuring device for flowing substances—also referred to hereafter as MID for short, according to the precharacterizing clause of claim 1, and to a method for producing a magnetic-inductive measuring device for flowing substances according to the precharacterizing clause of claim 33.
Magnetic-inductive measuring devices for flowing substances have long been known in the prior art. The basic construction and the operating principle are described for example in the Lexikon der Mess-und Automatisierungstechnik [Lexicon of measuring and automation technology], by Elmar Schrufer, VDI-Verlag Dusseldorf 1992, pages 262-263. On account of the operating principle, magnetic-inductive measuring devices can be used only for measuring the through-flow of electrically conductive fluid substances. However, there are also known devices today in which a detection as to whether the measuring tube is fully or partly filled or not filled takes place in addition to the flow measurement.
The simple and robust mechanical construction, in particular the straight measuring tube without built-in obstacles or moving parts, makes them preferred measuring devices for use in a series of industrial process installations, for example in the area of water management (flow measurement in drinking water preparation and sewage treatment), in the area of the chemical and petrochemical industry (flow measurement of water, acids, alkaline solutions, etc.), in the area of the pharmaceutical industry and in the area of the food industry (flow measurement of water, juices, beer, milk-products, etc.).
The wide range of possible applications is reflected in the necessity there is today for manufacturers to stock a large number of product variants.
Magnetic-inductive measuring devices of the generic type comprise a measuring tube, which is flowed through by the substance being measured, and further subsystems attached to the measuring tube, in particular a subsystem for signal pickup, a subsystem for generating a magnetic field, known as a magnetic system for short, a housing for protection from environmental influences and a connection device for receiving the electrical interface of the measuring device.
Today, the measuring tube is generally a steel tube, with or without end flanges, with connecting elements welded onto the outer circumference of the measuring tube for the mounting of other device components and subsystems on the measuring tube. Installation in the process pipeline system may take place either by flange mounting, or else by intermediate-flange mounting, known as wafer mounting.
Magnetic-inductive measuring devices are today used in applications with a process pressure of up to 40 bar, which imposes very high requirements on the compressive strength and resistance to deformation of the hydraulic system. Pressure-induced deformations, vibrations and other loads in the hydraulic system can lead to distortions and geometrical displacements in signal pickup and magnetic systems that are attached to the measuring tube and impair the measuring accuracy and reproducibility of the measurement.
Today there are also known measuring tubes made of ceramic for niche applications, some with measuring and shielding electrodes embedded in them, and measuring tubes made of thermoplastic. However, ceramic measuring tubes are expensive and brittle, and the fastening of other subsystems on them presents difficulties. Thermoplastic tubes can only be used for low process pressures.
As the subsystem for signal pickup, today there are known conductive and capacitive signal pickups. In the case of the conductive signal pickup, the electrodes are galvanically in connection with the substance being measured. Conductive signal pickups are usually two electrodes which are introduced into the measuring tube through bores. Since the steel tube and the electrodes have to be electrically insulated from one another, it is necessary today to line the steel tube on the inside with an electrically nonconducting layer, known as the liner. This makes mounting of the measuring device very complex. Furthermore, there is the disadvantage that, because of the direct contact between the electrode and the substance being measured, and the resultant interfacial effects, different electrodes have to be used according to the measuring task, which leads to a considerable variety of the variants of the measuring devices.
In the case of the capacitive signal pickup, the electrodes are galvanically separated from the substance being measured. Extensive electrodes which are located in or behind the electrically nonconductive tube lining are used. Shielding electrodes are also provided between the measuring electrode and the measuring tube to prevent capactitive feedthrough to the outside.
It is difficult to realize capacitive signal tapping on measuring tubes made of steel, because of the capacitive interaction of the steel tube with the electrical field of the electrodes. Although a capacitive signal pickup can be realized more easily in combination with measuring tubes made of ceramic or thermoplastic, this is with the disadvantages mentioned above of the measuring tubes.
As an extension of the signal pickup system, in some magnetic-inductive measuring devices today at least one reference electrode is used for forming an electrical “earth” with the substance being measured.
The subsystem for generating a magnetic field, known as the magnetic system for short, is today generally realized by two coils with poles and a magnetic return, which are mounted around the measuring tube. The geometrical tolerances to be maintained during mounting are very small, which makes mounting complex and expensive.
The magnetic return is usually realized by a number of sheet metal strips stacked one on top of the other and riveted together. Several production steps are necessary to produce them in this form.
The housing represents a considerable cost factor in today's customary magnetic-inductive flow measuring devices (MID), because it is realized either from separate half-shells, produced for example by the diecasting method, or else as a welded sheet-metal construction.
One technical disadvantage is that the elements within the housing, for example cables, cannot be positionally fixed by the attachment of the housing alone. Vibrations of the connection lines can lead to considerable signal disturbances and consequently measuring inaccuracies, in particular in the case of capacitive signal pickups.
The connection device for receiving the electrical interface of the measuring device makes it possible for the measuring device to be connected to the evaluation electronics, which are accommodated in a subassembly that is often also referred to as a transmitter or measuring transducer. The transmitter may be spatially separate from the measuring device, or else integrated in the connection device.
Today's customary production method is characterized by relatively small sizes of the batches of each variant of the device on the one hand, and a high number of variants of the device on the other hand. The variants of the device differ for example by different measuring tube cross sections and measuring tube wall thicknesses, in order to achieve an adaptation to the flow and pressure ranges of the respective application. Since the subsystems for signal pickup and magnetic field generation are mechanically built onto the hydraulic system, each variant in the hydraulic system also requires different mounting kits for the built-on subsystems.
It is therefore the object of the present invention to provide a magnetically inductive measuring device which can be produced at low cost and covers the entire range of applications with only a very small variety of variants.
The object is achieved with regard to the measuring device by the characterizing features of claim 1, and with regard to the method for its production by the characterizing features of claim 33.
Therefore, according to the invention, at least one of the subsystems of a magnetic-inductive measuring device is entirely or partly formed by layers of fibre composite material and/or is embedded in layers of fibre composite material.
According to a preferred embodiment of the invention, the measuring tube is entirely or partly formed by layers of a fibre composite material, in particular an electrically nonconductive fibre composite material.
A glassfibre reinforced plastic (GRP) comes into consideration in particular as the fibre composite material. The measuring tube may advantageously be produced by means of a fibre winding technique known from composite material technology for the production of other components. However, another layer building-up technique, known from composite material technology, may also be used.
A measuring tube according to the invention, with a measuring tube formed by the fibre winding technique, combines mechanical stability and compressive load-bearing capacity with chemical resistance and electrical insulating properties. It has been found that, if it is of a suitable diameter, a measuring tube produced by the fibre winding technique from GRP material already withstands an internal tube pressure of 40 bar for a long period without inadmissible mechanical deformation of the tube when it has a wall thickness of a few millimetres.
Since a measuring tube formed from GRP is an electrical insulator, no additional insulating lining (liner) is required, which makes production of the measuring device much easier and much less expensive. If capacitive signal tapping is realized in connection with a GRP measuring tube, no capacitive interaction occurs between the GRP measuring tube and the measuring electrodes.
Alternatively, other production methods known in composite material technology may also be used, such as for example the Resin Transfer Moulding (RTM) method, which is based on a suitable fibre preform and subsequent impregnation with matrix resin, or the prepreg/autoclave method, which is based on preimpregnated materials.
As an alternative to glass fibres, other fibre materials, preferably materials which are electrically nonconducting but mechanically reinforce the matrix resin, such as aramid fibres, PE fibres or Kevlar for example, may also be used as the filler.
If further subsystems, such as for example a signal pickup or a magnetic system, are to be fastened on the measuring tube made of fibre composite material, this may either take place conventionally by screwing or clamping, or the components are fixed by being partially or completely incorporated during winding at certain locations by fibre composite material in strip form.
An advantageous embodiment may also be, however, that the electrode arrangement is entirely or partly formed by layers of fibre composite material and/or is embedded in layers of fibre composite material, or that the magnetic system is completely or partly formed from fibre composite material and/or is embedded in layers of fibre composite material.
If the electrode arrangement is embedded in layers of fibre composite material, the electrodes can be incorporated during winding in a simple way when winding the layers. In particular in the case of capacitive signal tapping, where extensive electrodes are usually used, a significant simplification of the production process can be brought about in this way.
The electrodes may in this case consist of bendable metal sheet, metal foil or of conductive fibre composite material, for example carbon-fibre-reinforced fibre composite material.
The dielectric properties of the resin (low dielectric losses, high dielectric constant, suitable frequency response of the dielectric constant, etc.) can in this case be adapted to the requirements of capacitive signal pickup, whereby the range variation of the possible types of product is reduced to a fraction of that which is customary and required today.
Apart from embedding the electrodes, embedding the electrical leads in layers of fibre composite material is also possible. The leads are then kept free from vibration without any great effort, which results in increased measuring accuracy and interference immunity.
Embedding in fibre composite material makes it possible to choose the distance of the measuring electrode from the medium being measured to be very small, down to the thickness of a single wound layer of fibre constant material. With the GRP materials that are usually used today, this single layer thickness lies in ranges from 0.1 to 3 mm. The proximity to the medium being measured results in a high coupling capacitance and consequently low-noise signal tapping.
If the electrodes are wound in fibre composite material, they may also be formed from perforated materials, such as, for example from perforated metal foil, metal gauze or conductive synthetic fabric. The advantage of using such materials is that the eddy currents caused in the electrodes by the alternating magnetic field of the magnetic system are reduced and constantly higher excitation frequencies can be used.
The impedance converter for signal processing, necessary for operating on the capacitive principle, may be embedded in the layers of fibre composite material in the direct vicinity of the measuring electrodes together with the latter. This ensures undisturbed signal transmission between the electrodes and the impedance converter.
The magnetic return or even the complete magnetic system may also be embedded in layers of fibre composite material, in that flexible semifinished ferromagnetic material is applied ‘from a roll’ together with the fibre composite material in a suitable number of repetitions. A winding method is advantageously also use here.
This makes particularly economical production possible, because, unlike in the case of the known production methods for magnetic systems, preformed parts with a specific nominal width are no longer required, but instead a standard semifinished material can be used.
In addition, technical advantages are achieved: better operating conditions can be achieved, since the eddy currents which always occur when there are changes in magnetic flux, on account of the flux distribution between many small partial areas, cannot form as well as in the case of conventional, more massive formations. This makes it possible to realize higher excitation frequencies of the pickups, and this in turn shortens the response time, producing faster systems. Furthermore, the signal-to-noise ratio with respect to the normal fluid noise is increased, so that undisturbed signals are obtained as a result.
At the same time, there is a decrease in core losses. Either operation is carried out at lower power—in comparison with conventional devices—producing an energy saving, or operation is carried out at a higher signal level—with unchanged power—again resulting in an improved signal-to-noise ratio.
A further advantage is the very good positional fixing of the ferromagnetic core if it is embedded in layers of fibre composite material. While in the case of conventional systems a great effort has to be expended in respect of constructing a system that is insensitive to vibration and resistant to shock (by mechanical means or additional adhesive bonds), this is obtained automatically and without additional effort in the case of the magnetic system embedded in fibre composite material.
The temperature distribution is likewise more favourable than in the case of conventionally produced magnetic systems, and a more uniform temperature distribution is obtained.
What is more, no additional corrosion protection has to be provided on the magnetic system, since the magnetic system is completely protected by being embedded.
It is also possible for parts of the magnetic system to be formed from fibre composite material. For example, the coil may be built up from thin layers of copper wires embedded in a fibre reinforced epoxy matrix.
An embodiment according to which the housing is formed from fibre composite material is particularly advantageous. This permits the very low cost formation of the housing as one of a number of logically successive production steps, or else as a low-cost separate production step.
After preassembly of the electrical and magnetic components of the signal-pickup subsystem and of the magnetic system, these components are surrounded by the layer realizing the housing function by applying layers of fibre composite material. The cost advantage is obtained by only little material and production time being required for this production step.
The technical advantage is obtained by all the components being permanently fixed in place in this way in an outstanding manner, since the semifinished material used adapts itself to almost any desired surface geometry.
A shielding layer against electromagnetic interference fields (EMF shielding) may be integrated in the winding of the housing by being incorporated during winding. This layer may be produced for example by a semifinished material capable of being wound, which consists of a metallic gauze or mesh, for example made of copper or the like or of conductive fibre composite materials, for example carbon-fibre-reinforced plastic.
Finally, a particularly advantageous embodiment of the invention is one in which the signal pickup and the magnetic system are embedded in the measuring tube formed by layers of fibre composite material. In the case of this embodiment, the measuring tube is firstly wound from fibre composite material, then the signal pickup and subsequently the magnetic system and the housing are wound onto it one after the other, respectively as described above. The advantage of this embodiment is that a measuring device which can be produced at very low cost but combines altogether in it all the advantages mentioned above in the description of embedding the individual systems is obtained by the consistent integration of all subsystems in layers of fibre composite material.
Further advantageous refinements and improvements of the invention and further advantages can be taken from the further subclaims.
The invention and further advantageous refinements and improvements of the invention and also further advantages are to be explained and described in more detail on the basis of the drawings, in which 13 exemplary embodiments of the invention are represented and in which:
For producing the measuring tube from fibre composite material, every method known in composite material technology can be used in an adapted form. Typical thicknesses of individual layers of fibre composite material are of the order of magnitude of 0.12 mm-3 mm.
In the MID there is a capacitive signal pickup subsystem, comprising two measuring electrodes 20, 20a, lying opposite perpendicularly in relation to the tube axis 3, and two shielding electrodes 22, 22a, respectively shielding a measuring electrode towards the outside and divided here (22, 22′; 22a, 22a′) in order to keep a passage free for measuring lead 26.
The electrodes 20, 20a, 22, 22a are made of extensive metal sheets or foils with an essentially rectangular peripheral contour—in the state in which they are unwound into the plane—and arranged parallel to the measuring tube axis 3 in the form of segments of the lateral surface of a cylinder.
The electrodes 20, 20a, 22, 22a could also be formed from perforated materials, such as for example metal gauze or conductive synthetic fabric.
In
Deformations are inadmissible in particular whenever they cause damage to the device or cause the electrodes or other parts that are attached to the tube to be displaced as a result of the deformation in such a way that the measuring accuracy is reduced.
In the vicinity of the electrodes 20, 20a, 22, 22a, an electronic signal preprocessing subassembly 24, essentially an impedance converter and a signal preamplifier, is also embedded in the layers, in the same way as the measuring signal leads 26 from the electrodes 20, 20a, 22, 22a to the signal preprocessing subassembly 24 and from the signal preprocessing subassembly 24 to the transmitter 8.
The magnetic system of the MID that is shown in
The ferromagnetic core 32 comprises a flexible ferromagnetic metal sheet, which between the two coils 30, 30a follows an inner lateral surface of the cylinder of the measuring tube, and consequently ensures the magnetic return flow, and is embedded in the layers of fibre composite material. The exciting coils are conventionally wound coils. They are firmly embedded together with the leads 26 to the exciting coils 30, 30a in the fibre composite material.
For producing an MID according to the invention that is shown in
Firstly, a first, inner layer is wound onto a cylindrical core made of metal, for example aluminium. This may consist either of resin-impregnated fibres in the form of what is known as a roving or of a semifinished fibre material in the form of for example what is known as a scrim, which has suitable individual fibre layers and is cut to size for the total width of the measuring tube.
A number of further layers are wound onto this first layer, and the measuring electrodes are fixed on top and wound around by a number of further layers of semifinished fibre material.
The measuring electrodes could also already be applied to the first layer of semifinished fibre material. In this case, the measuring electrodes would be even closer to the substance being measured, which means even greater sensitivity of the measurement.
In the case of the scrim technique, the fixing may take place for example by means of spots of adhesive. If appropriate care is taken when subsequently incorporating them during winding, a very high positioning accuracy of the electrodes is achieved.
A corresponging procedure is subsequently followed with the shielding electrodes 22, 22a, the signal leads 26 and the signal converter subassembly 24. The wall is subsequently made thicker by a number of further layers.
As the next step, the parts of the magnetic system, the ferromagnetic core and the exciting coils, are in turn initially fixed temporarily and subsequently incorporated during winding and, in this way, are definitively fastened. The exciting coils are fitted in such a way that the magnetic field in the interior of the tube runs perpendicularly in relation to the tube centre axis 3 and perpendicularly in relation to the joining line between the measuring electrodes 20, 20a, as indicated in
In the case of the magnetic system, very high positioning accuracy is important, in particular little twisting, if a high measuring accuracy is to be achieved. With appropriately careful winding, the achievable geometrical precision is very high. For instance, twisting of the coils and the core of less than 1° can be achieved.
Then, a number of further layers are wound on, to achieve protection of the magnetic system from environmental influences. Subsequently, a shielding layer 40 of conductive material, for example of a semifinished fibre material of electrically conductive material, such as for example of conductive carbon fibres, is also wound on, and this is again wound around with a number of final outer protective layers. The outer protective layers provide protection in particular from external influences, so that the measuring device as a whole conforms to the corresponding protection class, for example IP 68. They may in this case also consist of some other semifinished material, for example of aramid fibre reinforced material, if the measuring tube is made up of semifinished GRP material.
An additional shielding layer—not represented here—may also be embedded in the space between the electrodes and the magnetic system.
During the winding, the measuring signal leads 26 and the leads to the coils 30 and to the signal converter subassembly 24 must be led through between the individual winding layers.
Then, a connection device 8 is fastened on the finished-wound measuring tube. This may take place either by screwing or adhesive bonding, or else again by incorporation during winding with semifinished material in strip form, so that the connection bushes and any operator control and display elements remain free.
The connection device 8 may in this case either serve merely for contacting and itself contain no electronic subassemblies but only connection elements with which the measuring device is electrically connected to its system surroundings (energy supply and signal derivation). It may, however, also already contain various functional subassemblies for signal processing, filtering, storage and transmission—either via bus cables or else wirelessly by means of radio transmitters, Bluetooth or other common signal transmission protocols—and is then usually referred to as a transmitter.
Finally, as the last step, the core is removed again, in that it is pulled out from the finished-cured tube. Known techniques, such as for example warming up the core, may be used to assist this.
One variant of the production method is that a cylindrical hose made of liner material, for example thermoplastic, is used as the core. The hose is brought into the required cylindrical form by for example being subjected to compressed gas (compressed air for example). After applying the fibre composite material with all the embedded components and subsystems to the liner, the liner core can be removed again by pulling it out.
In a further embodiment, not represented here in a figure, the liner remains inside the measuring device after the layers of fibre composite material have been applied.
As a result, in this variant a measuring device of the generic type, made of fibre composite material with a conventional liner, is obtained in this way.
FIGS. 2 to 13 show a number of variants, not to be considered exhaustive, of the embodiment that is shown in
Otherwise, the construction of the measuring device that is shown in
The reference electrode 23 is produced from a metal sheet, a piece of metal foil or from electrically conductive semifinished fibre composite material, in that it is placed directly on the core and subsequently enclosed by the first winding layers. After removal of the core, the reference electrode then has galvanic contact with the substance being measured.
The surface contour of the measuring tube 2 can be configured within wide limits on the basis of the winding technique. For instance,
The integration of the measuring device that is shown in
Since, however, in the case of a construction of the MID according to the invention that is shown in
The embodiment of an MID according to the invention shown in
The nanostructure on the device bounding surface 4 on the substance side may be produced for example by a corresponding matrix being applied to the surface of a cylindrical metal core and the structure then being moulded in the first layer of fibre composite material during the winding thereof. Another possibility would be to apply a thin layer of plastic to the cylindrical metal core as a first layer, for example by spinning or immersion, into which the nanostructure of the matrix is moulded. Subsequently, the first layer of fibre composite material is applied on top. The device bounding surface on the substance side would then be formed by the thin, nanostructured layer of plastic; the mechanical stability and the supporting function for the other subsystems would be ensured by the structural material that is the fibre composite material.
The embodiment shown in
It goes without saying that the measuring tube 202 in the embodiment according
The embodiment according to
In the exemplary embodiment of
In the embodiment that is shown in
Here, the transmitter 8 comprises a wireless signal transmission unit, for wireless signal transmission—indicated by the arrow R—and consequently wireless incorporation of the MID in a higher-level process control system.
In the embodiment represented in
The embodiment that is shown in
In the case of the embodiment of an MID according to the invention shown as a cross-sectional view in
All the customary MIDs that are available today are directed at measuring the through-flow of flowing substances and, what is more, do not have any additional measuring means by which additional information can be derived from the substance being measured or the measuring device itself concerning other measured variables, and be used for example for diagnostic purposes.
Near the device bounding surface 4 on the side of the substance being measured, a first temperature sensor 56 is embedded. In the case of the method of building up layers that is described under
Near the device bounding surface 6 on the ambient side, a second temperature sensor 57 is embedded. With this, the tube wall temperature or the ambient temperature of the measuring device can be recorded. The processing of the sensor signals of the two temperature sensors takes place in the transmitter 8, but could also be already performed in the signal converter subassembly 24.
With knowledge of the geometry and the material properties of the fibre composite material, the difference between, the temperature values determined by the two temperature sensors 56, 57 can be used to calculate the thermal flux, and from it the energy exchanged between the substance being measured and the ambience. In this way it would be possible to monitor maintenance of the permissible operating limits of the device. The remaining service life of the measuring tube or of the entire measuring device could also be estimated on the basis of the tempterature values recorded. For this purpose, additionally integrated in the transmitter 8, or already in the signal converter subassembly 24, is a microprocessor with a memory, in which a corresponding service life model is implemented as software. The measured temperature values are stored there; the historical variation of the temperatures is then supplied to the service life model as an input variable, and the remaining service life to be expected is calculated and passed on via the transmitter for display or further processing in a higher-level process control or plant management system.
The first further measuring pickup 56 may also or additionally be a pressure sensor. It would be embedded on the basis of a method as described under
Furthermore, a strain sensor 58 is embedded approximately in the centre between the measuring tube side and an outer side. This may be, for example, a strain gauge or a strain gauge arrangement in a bridge circuit comprising metal-based or semiconductor-based strain gauges. With the strain sensor 58, the mechanical state of stress in the wound body is determined. A number of strain sensors may also be embedded, distributed at a number of points in the cast body 19. From the information obtained in this way, the multiaxial state of stress of the measuring tube can be calculated, and in this way the permissible loading in the respective installation situation can be monitored.
Components of fibre composite material built up layer by layer can be made transparent if, after winding with layers that are preimpregnated with resin, the curing takes place in a high vacuum. This is so because the glass fibres in the case of GRP materials are transparent; the non-transparency of fibre composite materials is caused by the presence of many small pockets of air trapped in the resin. These are removed in the vacuum treatment, with the result that the fibre composite material becomes transparent in this way.
In the case of a measuring tube of transparent fibre composite material, sensors which are based on optical operating principles can additionally be embedded. In the MID represented in
In this way, a series of optical measuring methods can be realized and consequently additional further information concerning other measured variables can be obtained from the substance being measured.
For example, the light source 53 may be a laser diode tuned to a specific wavelength. By measuring the light intensity arriving at the receiver in relation to the light intensity emitted by the laser diode, the light absorption in the substance being measured at the specific wavelength can be determined, and from this conclusions as to the composition of the substance being measured can be obtained. In this way a small absorption spectrometer can in principle be embedded in the measuring tube of transparent fibre composite material. If the light source 53 emits a wide spectrum, it is possible to obtain information on the turbidity of the substance being measured from the ratio of the total intensity registered at the receiver 52 in relation to the light intensity transmitted. This is very advantageous for example when using the measuring device for process monitoring in breweries.
On the left side of the measuring tube 2 in
Optical image recording chips, for instance the CCD chips used in digital cameras, also come into consideration for example as optical sensor components. With such image recording subassemblies and image processing software stored in the transmitter 8, in a microprocessor with a memory, automatic optical monitoring of the substance being measured can be carried out. For example, flowing transparent foods such as juices could be monitored for the presence of undesired foreign bodies at the same time as the flow measurement.
The exemplary embodiments described above do not constitute all possible embodiments of MIDs according to the invention. All further embodiments that are not mentioned here but arise as a result of combinations of the embodiments described here or parts thereof are therefore intended to be covered by the present invention. In particular, in the case of the construction of the magnetic system it is also possible to proceed in such a way—unlike that described in FIGS. 1 to 4—that the exciting coils are first fixed on the wound body and then the ferromagnetic core is fixed on them.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 47 890.6 | Oct 2003 | DE | national |
