DE3815800A1 - Arrangement for measuring a gas density - Google Patents

Abstract

An arrangement for measuring a gas density comprises a sensor head having an oscillating body (1), excitation means for exciting the oscillating body (1) and detection means for measuring an amplitude of the oscillating body (1). The excitation means have an excitation light source (9) for generating excitation light (3). The sensor head is connected via at least one optical waveguide (2a, 2b) to an evaluation circuit, both excitation light being fed in and measuring light being guided out via these optical waveguides (2a, 2b). The oscillating body is designed as a tuning fork or a beam clamped on one side. <IMAGE>

Description

TECHNICAL AREA

The invention relates to an arrangement for measuring a gas density, which a sensor head with a vibrating body, excitation mit tel to excite the vibrating body and detection means to Measuring an amplitude of the vibrating body includes.

STATE OF THE ART

In gas-insulated high-voltage switchgear, the gas density of an SF ₆ insulating gas in the individual gas compartments, which is required for reliable operation, must not be undercut. In order to ensure the reliability of such a switching arrangement, the gas density must be monitored periodically. For this purpose, the gas rooms are equipped with density monitors, which must be read regularly.

A gas density can be measured in different ways. It can e.g. by measuring a pressure and a temperature  or determined by measuring a resonance of a vibrating body will.

From the company "Bopp + Reuther", for example, a density Distributor for gases, which is based on a density Resonance frequency of a tuning fork is based. The disadvantage of one such density transmitter is that it has a supply voltage needed. This results in problems with the application in High-voltage switchgear with an integrated, computer controlled control technology. A control center should namely if possible be galvanically isolated from the switchgear, so that they occur due to high potential differences (lightning impact, short circuit etc.) cannot be damaged. Where galvanic isolation is not possible, complex, provide voltage-proof interfaces.

There is therefore a need for a density transmitter, which is electrically passive, accurate and inexpensive.

Attempts have recently been made to achieve gas densities with the help of vibrating crystals. Fundamental is in this respect the publication "Dual crystal gas density sensor "Ronald Frank Vogel, Sensors and Actuators, 5 (1984), p. 21-28, and "Air and gas damping of quartz tu ning forks ", Michel Christen, Sensors and Actuators, 4 (1983), p. 555-564. However, such sensors also need one Supply voltage.

From the publication "Optically activated vibrations in a micromachined silica structure "S. Venkatesh and B. Culshaw, Electronics Letters, 1985, vol. 21, no. 8, is a micro mechanical structure known, which photothermally to the Schwin is brought. A silicon dioxide strip of approx. 200 µm Length on a silicon substrate is under-etched in such a way that he spanned a ditch like a bridge. This silicon Dioxide strips are irradiated with light pulses and vibrate then like a clamped string. The problem of one  Such a structure is the complex measuring technique used to detect animals of the amplitude is necessary.

PRESENTATION OF THE INVENTION

The object of the invention is therefore an arrangement for measuring to specify a gas density, which a sensor head with a Vibrating body, excitation means for exciting the vibrating body and detection means for measuring an amplitude of the vibration has body, in which arrangement the sensor head of the detection means is galvanically isolated and without supply electrical energy works.

According to the invention, the solution is that the excitation medium an excitation light source for generating excitation light include that the sensor head has at least one Optical fiber in connection with an evaluation circuit stands over which excitation light is fed and measuring light is led away and that the vibrating body as a tuning fork or is designed as a beam clamped on one side.

The vibrating body is preferably excited by means of excitation light vibrated thermo-optically and the amplitude of the Vibrating body measured optically.

In another preferred embodiment, the oscillation body to vibrate by means of the excitation light brought. It is then an advantage if there is one in the sensor head A PL diode (photo-luminescence Diode) is arranged, which depends on the excitation light speed of the amplitude of the connected vibrating body Transformed measurement signal and this in the optical fiber couples.

With a view to evaluating the measuring sig as far as possible It is also advantageous if the evaluation circuit has a resonance frequency and a bandwidth of the vibrating body measures and both a gas density and a temperature are determined from this.  

The vibrating body is preferably a micromechanical sili zium or quartz structure.

Further preferred embodiments result from the Subclaims.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the invention is intended to be based on exemplary embodiments and will be explained in connection with the drawing. Show it:

Fig. 1 shows an arrangement for measuring a gas density;

Fig. 2 is a sensor head of an inventive arrangement;

Fig. 3 is a sensor head with a monolithically integrated vibrating body;

Fig. 4 is a micromechanical silicon structure as a sensor head;

Fig. 5 is a vibrating body which serves as quartz micro-mechanical structure is performed;

Fig. 6 shows an embodiment in which the vibrating body is excited photovoltaically and the amplitude is detected by means of a PL-diode, and

Fig. 7 shows a typical resonance curve of a tuning fork.

WAYS OF CARRYING OUT THE INVENTION

Fig. 1 shows an arrangement according to the invention. In a Gasbe container 7 is a gas whose gas density is to be measured. For this purpose, a sensor head 6 is arranged in the gas container 7 . The sensor head 6 is connected to the outside world via two optical fibers 2 a , 2 b . One of the two optical fibers 2 a connects the sensor head 6 to an excitation light source 9 and the other optical fiber 2 b ver connects it to a waveguide branch 12 .

The waveguide branch 12 additionally has two further connections, namely one for an optical waveguide 2 c , which supplies measuring light from a measuring light source 8 and one for an optical waveguide 2 d , which leads a reflected part of the measuring light to a photodetector 10 .

For example, commercially available multimode glass fibers can be used as the optical waveguide. A directional optocoupler is then used as a waveguide branch 12 .

The photodetector 10 supplies an output signal to an evaluation circuit 11 . This in turn controls the excitation light source 9 .

It is both simple and inexpensive to use a well-known LED (Light Emitting Diode) as measuring light source 8 . The same applies to the excitation light source 9 . For example, a photodiode is used as the photodetector 10 .

Fig. 2 shows the principle of a preferred embodiment of a sensor head 6. The optical waveguides 2 a and 2 b correspond to those of the same name in FIG. 1.

A vibrating body 1 is clamped in a carrier 5 . The vibrating body 1 has, for example, the shape of a bar, a plate or a tuning fork. In principle, it is not essential to the invention from which material the vibrating body 1 is made. Consequently, a material can be selected appropriately so that the vibrating body 1 has a desired resonance frequency fr for a given geometric dimension.

For example, the vibrating body 1 can be out as a glass plate. With a length of 12 mm, a width of 1 mm and a thickness of 0.14 mm, the resonance frequency fr is of the order of magnitude of 200 Hz. The length relates to the part of the oscillating body 1 protruding from the carrier 5 .

A bracket 15 also fastened on the carrier 5 serves to hold the two optical fibers 2 a , 2 b in a desired position. Of course, the bracket 15 can be part of the carrier 5 .

The optical waveguide 2 a , which supplies the excitation light 3 , is fixed to the holder 15 in such a way that the excitation light 3 falls on the oscillating body 1 at a point which is close to a clamped end 13 . In contrast to this, the optical waveguide 2 b , which supplies the measuring light 4 , is fixed to the holder 15 in such a way that the measuring light 4 strikes the oscillating body 1 at an oscillating end 14 . The vibrating body 1 is preferably mirrored at the corresponding point.

The gas density in the gas container 7 is measured as follows.

The excitation light source 9 generates excitation light 3 in the form of light pulses with an excitation frequency fa . These light pulses pass through the optical waveguide 2 a to the sensor head 6 , where they meet the vibrating body 1 . In this way, the vibrating body 1 is excited to vibrate photothermally. The principle of such a photothermal excitation is based on the fact that this periodic, one-sided heating (for example a beam) is sufficient to cause it to oscillate due to the periodic local thermal expansion. The vibrating body 1 then swings in the manner of a beam clamped on one side.

In order to be able to determine whether the vibrating body 1 vibrates in resonance, an amplitude A must be observed at a vibrating end 14 . This is preferably done optically. For this purpose, the optical waveguide 2 b is provided. It acts on the vibrating body 1 with measuring light 4 and captures part of the reflected measuring light. According to a preferred embodiment, an intensity modulation of the measuring light 4 is measured, which is caused by the fact that a larger part of the measuring light 4 striking the vibrating body 1 is coupled back into the optical waveguide 2 b , the smaller a distance d between the vibrating body 1 (or the end) and optical fiber 2 b .

The measuring light source 8 can be, for example, an LED operated at constant power. In this case, the photodetector 10 will detect periodic intensity modulation. A signal supplied by the photodetector 10 to the evaluation circuit 11 accordingly reflects the periodic change in a distance d between the oscillating body 1 and the optical waveguide 2 b .

The evaluation circuit 11, which knows fa so the excitation frequency, so can track the oscillating body 1 in itself known, the Anre supply frequency fa a changed resonant frequency fr. Either only the magnitude of the amplitude is determined and the excitation frequency fa is chosen so that this magnitude is a maximum; or a phase response of the intensity modulation is also determined and the excitation frequency fa is selected so that the phase response of the intensity modulation is shifted by 90 ° with respect to the phase response of the light pulses.

Experiments have shown that the relationship between a change in gas density and a shift in the resonance frequency fr is highly linear. In the case of a glass plate with the dimensions already described as a vibrating body, the resonance frequency shifted by approximately 0.330 Hz per kg / m ³ gas density. The resonant frequency fr of the glass plate was 192.2 Hz when filled with SF ₆ gas container having a pressure of 4.5 bar.

Because measuring the gas density in switchgear does not take time is critical, the accuracy of the measurement can be increased, by collecting measured values over a certain period of time and then be statistically evaluated. With the just described The above embodiment can e.g. a change in air pressure of 1 mbar.

The following are the abundance of possibilities, which arise from the invention, some preferred embodiment shapes picked out. The main idea is the most advanced and affordable silicon technology to take advantage of a robust, small and cheap Obtain sensor head.

Fig. 3 shows a sensor head 6 with a monolithic inte grated vibrating body 1st The sensor head 6 is, for example, a U-shaped silicon structure. One arm of this structure forms the oscillating body 1 and the other the holder 15 . In order to fix the optical fiber 2 a, 2 b easily and precisely acting on the tion as a holding second arm, the latter has two recesses 16 a, 16 b on. The optical waveguides 2 a , 2 b are glued, for example, in these recesses.

Such a monolithic silicon structure can e.g. by suitable etching steps or other known methods a wafer.

Fig. 4 shows a further preferred embodiment of the invention. The vibrating body 1 arises from the fact that, for example, a U-shaped trench is etched into a silicon cuboid, in such a way that the area lying between the arms of the U is under-etched. In this way, a beam is placed over a trench and clamped on one side.

For the optical waveguides 2 a , 2 b , two recesses 16 a , 16 b can in turn be etched into the cuboid. In the present example, the optical waveguide 2 a for the excitation light would come into the trench 16 a and the optical waveguide 2 b for the measuring light would lie in the trench 16 b .

Instead of the depressions 16 a , 16 b shown in FIGS. 3 and 4, bores can also be provided, for example. On the other hand, the trenches themselves can be replaced by optical waveguides integrated in silicon.

For the sake of simplicity, the vibrating body 1 is shown in the figures as a plate or bar. However, this in no way means that the invention only relates to such spatial forms. A vibrating body in the form of a tuning fork is particularly advantageous because it is more balanced in terms of strength and has less damping.

To increase the gas density dependent damping effect, the oscillating end of the oscillating body is preferably with one or more openings or mushroom-shaped broadened. Because this damping effect is a surface effect is basically any enlargement of the surface of the vibrating end is an advantage.

Fig. 5 shows an embodiment in which the vibrating body 1 is in the form of a tuning fork. For photothermal excitation, the tuning fork is advantageously irradiated in a rich area 18 that lies between the two fork arms. Of course, it is also possible to excite the tuning fork on one side, in analogy to FIG. 2, for example.

Openings 17 reinforce the damping effect caused by the gas density.

Instead of photothermally, the vibrating body can also be photovolute be excited. That is, the excitation light will in the sensor head transformed into a voltage, which leads to this Purpose made of piezoelectric material (e.g. quartz)  Tuning fork excited in a conventional manner. Vibrating body, which are suitable for this embodiment are e.g. in the Article by Michel Christen quoted at the beginning.

A preferred embodiment of the invention, which is also based on photovoltaic excitation, is shown schematically in FIG. 6. It is particularly simple due to the use of a PL diode (photo-luminescence diode). The PL diode is an optical semiconductor component which receives light of a frequency at an optical input and emits light of a second frequency at precisely this input, depending on the resistance which is at the electrical connections 20 a and 20 b of the PL diode hangs. A tuning fork made of quartz, however, represents a frequency-dependent resp. gas density dependent resistance stood.

A great advantage of this embodiment is that it manages with a minimum of components. In particular, a single optical waveguide 2 b is sufficient for excitation and for measurement. Such a PL diode is described, for example, in US Pat. No. 4,644,379.

Fig. 7 shows a typical resonance curve of a tuning fork. In the illustration of Fig. 7, the Anre is on the abscissa supply frequency f and the amplitude A of the vibrating body applied on the ordinate. It can be characterized by the two parameters resonance frequency fr and bandwidth Δ f or a circular quality Q ≈ fr / Δ f (in the case of a high circular quality). The resonance frequency fr is essentially determined by the gas density. However, the temperature of the gas has a not insignificant influence on the gas-related damping of the tuning fork. This can be determined and eliminated via the band width Δ f .

This influence of the gas temperature can also be used for this with the same arrangement not only the gas density  but also to measure the temperature. Formally express it as follows:

A (f ₀, Q ₀) is a 2 × 2 matrix, the elements of which depend on the resonance frequency f ₀ and the circular quality Q ₀ at the working point of the measurement. d f , d Q , d δ and d T are defined by

d f = ff ₀
d Q = QQ ₀
d δ = δ - δ ₀
d T = TT ₀

where f is a resonance frequency, Q is a quality factor, δ is a gas density and T is a temperature at a measuring point. f ₀, Q ₀, δ ₀, T ₀ represent the values of the corresponding physical quantities at the operating point.

The experiment shows that the matrix A (f ₀, Q ₀) has only weak elements in the secondary diagonals, so d δ is mainly determined by d f and d T by d Q.

A particularly compact arrangement is obtained if the excitation light source is simultaneously used as the measuring light source 8 . For this purpose, the arrangement shown in FIG. 1 would have to be changed as follows, for example. Instead of connecting the optical waveguide 2 c to the measuring light source 8 , it is connected to the optical waveguide 2 a with the help of, for example, an optocoupler.

The amplitude of the vibrating body 1 is then only measured at certain, periodic times. One observes the vibrating body 1 as it were with a stroboscope. At the given excitation frequency fa , the same amplitude is therefore always measured. Due to the fact that the resonance of the amplitude is shifted by a phase angle of 90 ° with respect to the light pulses of the excitation light, the resonance frequency fr can be derived directly from the light intensity of the reflected measurement light delivered by the photodetector 10 . There are then no difficulties in determining whether the excitation frequency fa is too small or too large.

In general, it is advantageous to mirror the oscillating body 1 at the oscillating end in order to provide the photodetector 10 with measuring light of sufficient intensity.

A great advantage of the measuring method according to the invention is its security against overpressure and its accuracy. In particular especially for use in high-voltage switchgear there are obvious advantages. The high accuracy works namely that the gas tanks are no longer for safety reasons with a particularly large overpressure have to be filled. Through the simple and accurate measurement The gas density can fix any leaks early be put.

In conclusion, it can be said that the invention is a Arrangement for measuring a gas density, which creates the An requirements of electromagnetic compatibility and is simple and accurate.

Claims (10)

1. Arrangement for measuring a gas density, which

• a) a sensor head with a vibrating body ( 1 ),
• b) excitation means for exciting the vibrating body ( 1 )
• c) and detection means for measuring an amplitude of the vibrating body, characterized in that
• d) the excitation means comprise an excitation light source ( 9 ) for generating excitation light ( 3 ),
• e) that the sensor head is connected via at least one optical waveguide ( 2 a , 2 b ) to an evaluation circuit via which excitation light is supplied and a measurement signal is carried away, and
• f) that the vibrating body ( 1 ) is designed as a cantilever beam or as a tuning fork.

2. Arrangement according to claim 1, characterized in that the vibrating body ( 1 ) is brought to vibrate optically by means of the excitation light. 3. Arrangement according to claim 1, characterized in that the vibrating body ( 1 ) is caused to vibrate photovoltaically by means of the excitation light. 4. Arrangement according to claim 3, characterized in that in the sensor head at one end of the optical fiber PL diode (photo-luminescence diode) is arranged, which the excitation light depending on the amplitude of the ange closed vibrating body transformed into a measurement signal and couples it into the optical fiber. 5. Arrangement according to one of claims 2 or 3, characterized in that the amplitude of the vibrating body ( 1 ) is measured optically. 6. Arrangement according to claim 5, characterized in that a light in front of a swinging end of the vibrating body Waveguide is arranged, which is the swinging end charged with measuring light, so that depending on the amplitude of the oscillating end is part of the measuring light is fed back into the optical fiber. 7. Arrangement according to claim 3, characterized in that the excitation light by means of a photodiode in an electri cal signal is converted with which the vibrating body is vibrated. 8. Arrangement according to claim 1, characterized in that the evaluation circuit has a resonance frequency and a band measures the width of the vibrating body and from it a temperature and determined a gas density. 9. Arrangement according to claim 2, characterized in that the vibrating body is a glass plate. 10. The arrangement according to claim 2, characterized in that the vibrating body is a micromechanical silicon or Includes quartz structure.