EP1834159A1

EP1834159A1 - Electronic level sensor

Info

Publication number: EP1834159A1
Application number: EP05784780A
Authority: EP
Inventors: Franz Stuhlbacher
Original assignee: Exess Engineering GmbH
Current assignee: Exess Engineering GmbH
Priority date: 2004-09-15
Filing date: 2005-09-12
Publication date: 2007-09-19
Also published as: WO2006029427A1

Abstract

The invention relates to a device (1) for measuring the level (2) of fluids (gases, liquids) in containers. The ratio of a measuring capacity in relation to a reference capacity is measured. Said device uses the different dielectric properties of two different fluids in order to electronically determine the level. As a result, no displaceable parts, such as a float and similar are necessary. A multi-layered, constructed reference capacitor (5) is completely filled with the measuring fluid (3) in the intermediate chamber of the electrode surfaces (13-16) which are arranged in pairs. The capacitor is used to detect the media specific and temperature-dependent dielectricity constants. The intermediate chamber of the measuring capacitor is filled with the measuring fluid but only in proportion to the filling level, the remainder with a freely selectable expanding agent (9) (preferably air) having a dielectric constant determined by known or via a second reference capacitor (26). The filling level over each volume part in the measuring capacitor is ratiometrically determined from the measuring signal and the reference signal by electronically processing with the aid of a microprocessor (31), said volume part being provided by the measuring fluid.

Description

description

The invention relates to the technical fields of metrology and sensor technology, in particular to the level measurement of fluids (liquids or gases). The subject of the invention is an electronic level gauge with capacitive level detection and microprocessor-based evaluation.

The object, which is solved by the invention, is the economic, compact and largely independent of the nature of the filling medium and the temperature detection of a Füllstan¬ or the ratio of the fill contents of two fluids with different Dielektrizitäts¬ constant, without moving parts such as float or like.

Capacitive level meters made of layered and glued together circuit board material are already known as described in patent DE 198.50.245 A1, with at least two capacitor pads wherein the printed circuit board material is at the same time a carrier of protective coverings and of the evaluation circuit. As spacers are preferably used insulating circuit board materials with recesses.

DE 196.44.777 C1 describes a capacitive measuring system with many discrete individual coatings along the measuring path, wherein each individual capacitor represents the state of full or empty. The evaluation is performed by a multiplexer, which controls each electrode individually.

DE 199.16.979 A1 describes a method which evaluates the change of the signals with phase shift of 180 degrees, two adjacent capacitance elements as a function of the fill level.

EP 401.111 B1 describes a capacitive fill level sensor

Potentialreproduzierstufe to eliminate interference or edge effects, wherein the shielding electrodes are switched to the same potential as the measuring electrodes.

DE 101 19 555 A1 describes a capacitive fill level sensor with a reference electrode pair which, relative to a height section, has a larger surface area than the pair of measuring electrodes.

Finally, ITUD20010139 is mentioned, which describes a coaxial capacitive sensor with reference capacitance and determines the filling level via the ratio of the charging times.

The present invention determines the level height by ratiometric measurement of the displacement current of a measuring capacitor compared to a Referenzkondensa¬ gate. The measuring capacitor is preferably a plate capacitor consisting of two elongate planar electrodes (insulating carrier material with metallic coverings). The plates have a distance in which the measuring medium and the supplementary medium can penetrate. The distance is large enough to neglect capillary action. The reference capacitor is arranged at one end of the carrier material for the Messkonden¬ capacitor and also represents a Piattenkondensator of electrode surfaces whose distance is a fraction of the electrode spacing of the measuring capacitor, whereby on the one hand, the capacitance is higher and on the other hand, the capillary becomes so large that the Measuring medium is held completely, even with almost empty tank between the electrode surfaces. An alternative arrangement provides a second reference capacitor at the other end of the carrier material for the measuring capacitor.

Beyond the state of the art, the inventive level is given by the special shape and arrangement of the electrode surfaces to achieve different capillary effects in the measuring capacitor over those in the reference capacitor. Furthermore, it is significant that at the same time the capacitance size of the reference capacitor is adapted to the value of the measuring capacitor filled with the measuring fluid at a much smaller overall length. The ratiometric measurement of the displacement currents allows the use of different fluids or non-mixing fluid combinations. In principle, the supplemental medium and measuring medium can be exchanged by "turning it upside down".

The possibilities of reducing the distance in the reference capacitor section of the level meter are manifold (for example, thicker coverings, gradation of the support material, attachments, deformations). Preferably, the distance is reduced by introducing additional pairs of electrodes in the distance between the measuring electrodes. As a result, the effective capacitor area is additionally increased. The measurement accuracy is improved by lower edge effects.

The main advantages of the invention are therefore given by the solution of the object of the invention. The measuring method in combination of the preferably used Bau¬ elements allows low production costs based on the high reliability. Other features include low drift over the lifetime, low temperature drift. No floats or other moving parts are necessary. Manufacturing in bulk is easy, as is calibration.

As a result, the electro-technical principles for the measuring method used are summarized: The dielectric displacement D with sinusoidal excitation is

D = D ₀ sin ωt and from this the displacement current density is increased

D = D ₀ ωcosωt

I _L = Q = CÜ = C d E = ε _o ∈ _r AE I _L = DA = I _V j _v = D The line current I _L is continued in the plate capacitor as a displacement current I _v and is proportional to the Kondensator¬ area, the frequency and inversely proportional to the distance between the plates and the filling medium when excited by a sinusoidal AC voltage. The equality of I _L and I _v is valid on the assumption that the dielectric between the capacitor plates has no conductivity. Otherwise, a proportionate line current is added to the displacement current. The current densities j _v in the capacitor and J _L in the conductor are generally different, since the line cross-section is not equal to the cross section A of the capacitor.

For the capacitance C of a plate capacitor with the area A and the plate spacing d, neglecting edge effects:

C = ε ^* A / d ε = _εr ε ₀ with ε _o = 8.8542 ^* 10 ¹² *% ", where ε represents the specific dielectric constant for the medium (relative permittivity ε _r multiplied by ε ₀ dielectric constant of the vacuum).

The ratiometric measurement (ratio of the measuring capacity C _meS s to the reference _capacitance C _r e _f gives a clear continuous or piecewise continuous function of the level A _m658 , or the filling level ratio A _mess / A _ref .

Cmess / Cref = ε _m e _SS / ε _re f * GF; GF = Amess / Aref * D _ref / D _meas

In the case of a geometry factor GF, which represents the area-to-area ratio of the capacitors to the reference area (A _m ess / A _ref ) multiplied by the capacitor-to-plate ratio (D _ref / D _mess ) of the reference area to the measuring area, the ratiometric Measurement directly proportional to the change of ε _mes s. In this case, ε _mess represents a fictitious dielectric constant. For rectangular electrode shapes, A = l * b with length I and width b. With I ₁ as the length covered by the measuring medium follows: ε _m ess = li / l * (ε _re f -ε _erg ) + ε _erg or

(εmess-εerg) / (ε _re rεerg) = Ml

By measuring the displacement _currents I _mess , I _ref and by knowing the geometries and the dielectric constant of the supplemental medium (eg calibration measurement, or by second reference electrode and measuring l _erg ), I ₁ /! and thus calculate the filling level. The reference medium = medium has the typical relative permittivity for the following fluids: Oils: 1.8 - 4; Petroleum 2; Gasoline 2.3; Diesel 2-2.2, benzene 2.28; distilled water 80 at 20 ⁰ C, 48 (100 ⁰ C) ethanol 23; Methanol 32.6. As a result, dielectric ratios of 1.8 to over 40 are possible.

Example gasoline air: ε _re rε _erg = 2.3-1 = 1, 3

For ε _mΘS s = 1, h = 0 (empty tank), for ε _mβS s = 1, 65 follows li = 0.5 I (half-immersed sensor) and for ε _mβ ss = 2.3 h = I (full tank ). The object of the invention will be explained with reference to the following figures. Show it

Fig. 1, the components of the subject invention.

Fig. 2 shows the side view of the subject invention according to Fig.1,

Fig. 3 shows an alternative variant in side view.

4 shows a block diagram of the signal processing by the electronics. In Figure 1, the structure of the electronic level gauge is visible. Shown here are alternately the front and the back of the single carrier. The level meter consists of at least two main carriers (6) preferably of electrically insulating printed circuit board material with metallic coverings, which constitute the capacitor pads (7,8), Schirmungsbeläge (18), and connecting lines for the evaluation electronics (17) and in pairs with their main surfaces at a distance from each other are arranged opposite. If more than two main carriers are used, the number of main carrier surface pairs increases and thus the active area of the capacitor plates increases. Depending on one of the main carrier carries the exciter electrode (13) and the opposite carrier, the reference electrode (s) (14), and the measuring signal electrode (15). Additional main carriers can be arranged between two main carriers, wherein these carriers have both types of electrodes whose main surfaces are oriented in the same direction per type and are connected in parallel by electrical connecting lines. This results in plate capacitors or layer capacitors with measuring (3) and supplemental medium (9) as a dielectric.

The measuring capacitor (4) consists of the length component I of the excitation electrode, the measuring electrode and is in the filling level I ₁ (immersion depth) with the measuring medium (3) and in the filling reserve I- Ii with the supplemental medium (9). filled.

The reference capacitor (5) consists of the reference signal electrode (14) and a portion of the excitation electrode (13). The linear expansion of the reference capacitor (5) is small compared to that of the measuring capacitor (4). To increase the capillarity and equalization of the capacity values, suitable measures will increase the capacity coverage. This encompassing: the reduction of the plate distance by

• the shape of the main beams (6) or

The shape of the reference signal electrodes (14),

• Attachments between the main beams or preferably as shown

• Introduction of further covering layers (10) with coverings (7) between the main supports (6), preferably on further supports (19) of non-conductive material, with the excitation electrodes (13) and the reference signal electrodes (14) electrical and mechanical connection means (20) are connected.

This also increases the electrically active area and thus the capacity. The same applies to any second reference capacitor (26) which is used to determine a unknown dielectric constant of a supplemental medium (9) can be used. Depending on the application, there is either completely the measuring medium (3) or completely the supplementary medium (9) between the electrodes (13, 14) or (13, 16) of the reference capacitor (5), or (26). In the special case with two reference capacitors, one (5) of the two is completely submerged in the medium to be measured (or filled with medium by capillarity), the other one (26) is completely filled by the supplementary medium. Special pot-shaped formations of the main beams or insulating barriers help to prevent the penetration of the other medium.

The ratio of measuring surface (8) to measuring capacitor pad spacing compared to the ratio reference signal electrode surface (7) to the reference capacitor pad spacing should be chosen as similar as possible, which can achieve optimal measurement results. Favorable are capacity ratios of 2: 5 to 5: 2 measured with the same filler (dielectric). The consideration of the ratio between the dielectric constant of the measuring medium to the supplementary medium in the geometrical conditions, which is expected according to the field of application, also brings metrological advantages.

For shielding against extraneous field influences Schirmungsbeläge are provided on the backs of the outer main carrier. Shielding pads with preferably constant reference potential around the edge regions of the measuring electrodes can also be present. Likewise, the evaluation electronics is provided on the backs of the outer main carrier, whereby the electronic level meter represents an overall system, which is preferably supplied by two lines, via which at the same time the data exchange of the actual level takes place.

In Fig. 2, the typical vertical arrangement of the electrodes (13,14,15) in side view is clear. The exciter electrode (13) at a distance from the measuring signal electrode (15) and to the separate reference signal electrode (14). Here, secondary supports (19) with further exciter and reference signal electrodes are recognizable, which are held by the conductor connections (20) in distance and electrically connected. Fig. 3 shows the variant with two reference signal electrodes. An oblique arrangement of the level gauge in the medium is possible. To protect the circuit thin insulating coatings such as paints of synthetic resins, or plastics such as parylene or the like are provided. If the coating does not conduct, C changes to C = A / (2d _L ack / ε _La c _k + d _F | _U id / sFiuid) - For protection, to guide the carriers and to fix the distance between the electrodes, guide tubes can be used the inside have attachments, guide tabs, warps and the like. In addition, at the upper end of the guide tube, a joint or a kink can be attached, whereby an inclination in the tank is facilitated, and thereby the resolution of the Füllstands- knife (1) becomes larger because of the longer possible way. An alternative protection can be provided by a heat-shrinkable tube or by plastic casting. Processes are generated, these sheaths must have the necessary recesses for the flow through the fluid.

The block diagram in FIG. 4 shows the signal processing according to the invention by the electronics. The electronic oscillator (21) supplies the drive signal for the excitation electrode (13). The resulting alternating current corresponds to the displacement current through the measuring capacitance or the reference capacitance (s). For the analog-to-digital conversion, a standard component capacitor (25) is additionally connected to the excitation electrode (13). i (t) = C ^* du (t) / dt = C ^* d (U * sin (2πf t)) / dt = C ^* U * 2πf * cos (2πf t)

U = RC ^* Ü * 2πf

These alternating currents are all rectified by the same assemblies (semiconductor rectifying diodes (22), transimpedance amplifiers (23)), and converted into voltages. Their amount is proportional to the dielectric constant of the media or the media pairing as a function of the level (2) I ₁ He is further proportional to the oscillator output voltage and its frequency and the transimpedance gain R. The design of the individual capacitors determines the geometry factor. The analog measurement voltages can be evaluated directly by an analog quotient measuring instrument or preferably by analog-to-digital converter (24) converted into digital values, which are further processed in a microprocessor (27) for calculating the level and output. The reference voltage for the analog-to-digital converter is generated by the standard capacitor current after rectification and current-voltage conversion.

LIST OF REFERENCE NUMBERS

1 Electronic level gauge 15 Measuring signal electrode

2 level, immersion depth 16 Reference signal electrode for the

3 measuring medium supplemental medium

4 Measuring capacitor 17 Electronic components for

5 reference capacitor with measuring medium as signal processing

Dielectric 18 Schirmungsbelage

6 carriers made of non-conductive material 19 secondary carriers for exciter and reference

7 reference capacitor pad Signal electrodes

8 Measuring capacitor pads 20 Electrically conductive connections with

9 supplementary medium distance holding function

10 plaque layers with comb-like 21 electric oscillator

Arrangement 22 semiconductor diodes

11 main carrier of the excitation electrode 23 transimpedance amplifier

12 main carriers of reference and measuring 24 analog-to-digital converters

Signal electrodes 25 Standard capacitor

13 Excitation electrode 26 Optional reference capacitor with

14 reference signal electrode for the supplemental medium as a dielectric

Measuring medium 27 microprocessor

Claims

claims:

1. Device (1) for the ratiometric determination of the level (2) of a liquid or gaseous medium (3) (fluid) by comparison measurement of the displacement currents of at least two capacitors (measuring capacitor (4), reference capacitor (5), optional reference capacitor (26), which is formed from opposing electrically conductive preferably metallic coverings along the measuring length on insulating carrier material (6) preferably printed circuit boards, characterized in that

Reference capacitor pads (7) are present, which have a smaller length and a smaller spacing than the measuring capacitor pads (8) and significantly higher capillarity as a result of the latter,

It has at least one reference capacitor pad gap, which is completely filled with measuring medium (3) with dielectric constant ε _ref or completely with supplementary medium with dielectric constant ε _erg ,

• and at least one measuring capacitor pad spacing, in which along the immersion depth (2) equal filling level I _1, the measuring medium (3) and along the filling reserve 1-I _{1 is} the supplementary medium (9) with dielectric constant ε _er g.

2. Device according to claim 1, characterized in that the Referenzkondensator- deposits (7) of a plurality of layers (10) are constructed, wherein non-conductive layers, and electrically conductive layers and measuring medium receiving layers are provided and the electrically conductive layers preferably with a thin protective coating of electrically insulating basic paint, of polyamide, epoxy, of polymers / poly-b-xylene, parylene or synthetic resins or the like are provided with or without glass fiber fabric.

3. Apparatus according to claim 1 or 2, characterized in that the ratio of the measuring capacitor pad surface A _mess to the distance of the measuring capacitor pad D _mess corresponds to the ratio reference capacitor pad surface A _ref to reference capacitor pad spacing D _ref or in the order 0.4 to 2.5 lies.

4. The device according to at least one of claims 1 to 3, characterized in that at least two main carrier (preferably printed circuit boards) (11,12) for forming the measuring capacitor (4) and parts of the reference capacitors (5,30) serve on surfaces facing each other have capacitor pads (7, 8), the pad of a carrier (11) representing the common excitation electrode (13) and at least two pads of another carrier (12) reference (14, 16) and measuring signal electrodes (15) represent.

5. The device according to claim 4, characterized in that the main carrier (11,12) carry on the opposite sides of the electronic components (17) for forming the evaluation circuit and wear Schirmungsbeläge (18) with constant reference potential.

6. Apparatus according to claim 4, characterized in that between the main carriers (11,12) in the region of the reference capacitor (5) formed therefrom from a portion of the exciter electrode (13) and reference signal electrodes (14,16) side carrier (19) are spaced from each other, with pads for reference signal electrodes on one side and pads for excitation electrodes on the other side with similar electrodes via electrical conductors (20) preferably pins, silver wire or the like are interconnected, and each having an excitation electrode and a reference electrode lie opposite each other.

7. The device according to at least one of claims 1 to 6, characterized in that an electrical oscillator (21) is connected to the exciter electrode (13), the measuring signal electrode (15) and the reference signal electrodes (14,16) with the input of Semiconductor diodes (22) are connected and the output of the diodes connected to the input of transimpedance amplifiers (23).

8. The device according to claim 7, characterized in that the output of the transimpedance amplifier (23) is connected to inputs of analog-to-digital converters (24).

9. Apparatus according to claim 7, characterized in that the outputs of the analog-to-digital converter (24) are connected to a microcontroller device.

10. The device according to at least one of claims 1 to 9, characterized in that a tube (26) is provided for guiding and spacing of the printed circuit boards, which preferably has guide tabs on the inside and / or consists of under heat shrinking material.

11. The device according to at least one of claims 7 to 15, characterized in that at the data output, a switching element such as a field effect transistor, a relay or the like is provided, whereby the level value is output by modulating the current consumption of the circuit directly through the supply line.

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