WO1988008520A1

WO1988008520A1 - Oscillating wire transducer for measuring stresses, elongations and forces

Info

Publication number: WO1988008520A1
Application number: PCT/HU1987/000022
Authority: WO
Inventors: István KONCZ; Péter KOLTA; Ferenc Armbruszt
Original assignee: Licencia Találmányokat Értékesito^" És Innovációs
Priority date: 1987-04-27
Filing date: 1987-04-27
Publication date: 1988-11-03

Abstract

A transducer for measuring stresses, elongations and forces, has a wire made of electrically conductive material which is secured under tension at two points electrically insulated from each other. The central region of the wire is arranged in a uniform magnetic field and the two points of attachment are connected electrically to a converter (10) with negative impedance which, together with the wire (1), is connected in an oscillating circuit. The frequency of vibration of the oscillator depends on the quantity to be measured, for example the stress.

Description

Sohwing string transducer for measuring mechanical dislocations

The invention is a Sohwing string transducer for measuring mechanical displacements, which contains an electrically conductive string attached to both ends and tensioned with a given force. Many solutions are known for measuring mechanical dislocations. A comprehensive overview of such solutions is given in HF Grave's book entitled "Electrical Measurement of Non-Electrical Values" / Müszaki Könyvkiadό, Budapest, 1968 /. On pages 220-229 of this book, a solution for measuring the elongation of the strings is explained on the basis of Figures 224 and 225. In this solution, both ends of a steel string were fixed in a metal block, and the middle area of the string is with an electromagnet / iron core coil / in magnetic coupling. The fastening device of one end of the measuring string follows the displacement to be measured, so that the tension of the string depends on the displacement to be measured.

Another string of similar shape was used for the measurement as a reference, the end of which can be loaded with a known and a variable force. Before starting the measurement, the same resonance frequency of the hess and reference strings is set in such a way that an oscillator is alternately connected to one and the other string and the Lossajous curve is observed on the oscillograph. In the course of the measurement, the resonance frequency of the measuring string changes due to the offset, whereby the stressing force of the reference string is changed to such an extent that the frequency is again the same. The change in tension of the reference string is proportional to the displacement to be measured. As can be determined from what has been described above, this measurement is cumbersome and not suitable for dynamic investigations. Fundamental difficulty means that the two ends of the vibrating string are electrically connected to one another by the fastening device, so that a short-circuit loop results in the magnetic field with the string, which dampens the vibrations of the string or deteriorates its quality factor.

Another fundamental problem arises from the fact that the string has to be made of magnetizable material so that the electromagnet can detect its displacement. The string plays a passive role in the generation of vibrations.

The object of the invention is the perfection of r known since a long time, but because of its circumstances and mentioned Disadvantages for an extensive application as a method proven unsuitable and the creation of such a vibrating string transducer, the application of which is also suitable for simple and dynamic measurements. The invention is based on the knowledge that it is not necessary to induce the tensioned string with an electromagnet, but that it - when placed in a homogeneous magnetic field - also represents an electrical resonant circuit which vibrates when replacing losses at the resonance frequency of the string caused. If the two ends of the string are electrically separated from each other, the two ends are the two end points of the resonant circuit, and the previous disturbing effect of the short-circuit loop no longer comes into play. The material of the string of the oscillating circuit created in this way does not have to be magnetizable, and tungsten and Molibden wire can be used in strings to secure the larger quality factor. To replace the losses, a negative resistance converter can be used, which is present together with the loop in an oscillator circuit. The frequency of the oscillator is measured by a frequency-measuring circuit, which is calibrated immediately after the displacement to be measured or after another non-electrical quantity.

In an advantageous embodiment of the vibrating string transducer according to the invention, the string is accommodated in a tube made of non-magnetic material, at one end of which there is a string tensioning device which is insulated from the tube, but at the other end the string is fixed or with a connection which permits longitudinal or threaded displacement attached to the pipe. To increase the measuring accuracy, it is expedient if both the material of the fastening element of the tensioning device / which can advantageously be designed as a holding or closing element / for the string and its length are selected such that the linear thermal expansion of the string is compensated for. The application is made easier if the transducer has a tube with a Cuer profile that can be glued to the surface to be measured. For the fulfillment of several measurement tasks, the construction is advantageous in which the outer holding element is provided with a fastening flange which is arranged on the side opposite the insulated clamping device and on the side of the flange which is opposite the holding element Bearing a tension arm is arranged, which is rigidly connected to the string and a tension element is connected to one end of the tension arm. Especially for measuring the liquid level, it is advantageous if a weight is connected to the side of the tension arm, which is opposite the string, and a linear characteristic can be achieved if the weight in the extended line of the string is suspended on the tension arm and a paraboloid shape in rotation having. The application of the vibrating string transducer according to the invention is really simple, and it is so precise that relative displacements of the order of 10 ^-7 can be measured, and due to its construction, the transducer is capable of carrying out both static and dynamic Suitable measurements. Furthermore, the vibrating string transducer according to the invention is explained in detail on the basis of exemplary embodiments and the drawing. In the drawing is in

1 shows the basic arrangement sketch of the vibrating string transducer according to the invention,

Fig. 2 shows the block diagram of the circuit contained in the vibrating string transducer according to the invention,

3 shows the front view of the first embodiment of the vibrating string transducer according to the invention, FIG. 4 shows the side view of the transducer sketched in FIG. 3 in a cross-sectional representation,

Fig. 5 the cross-section along the line V-V. Fig. 3,

Fig. 6 shows the cross section of a further embodiment of the vibrating string transducer according to the invention and Fig. 7 shows the cross section along the line VI-VI of Fig. 5.

Fig. 1 illustrates the basic arrangement sketch of the vibrating string transducer according to the invention.

The middle of the string 1 tensioned at both ends is in a homogeneous magnetic field which is caused by a permanent magnet 2. The string 1 is rigidly attached to both ends in such a way that a tensioning force of adjustable size acts on it. The two fastening points are electrically insulated from one another, to which electrical lines are connected, the ends of which can be regarded as terminals 3 of the converter. Fig. 1 shows the simplified equivalent circuit diagram of the dealer and its identification. The string 1 suitably consists of a non-magnetizable material of high strength, advantageously z. D. made of tungsten wire with a diameter of 0.1 mm, from which it can be seen that the vibrating string sprays in parallel on a Sohwing circuit loaded with a resistance. FIG. 2 shows the block diagram of an arrangement for sensing the signals of the converter according to FIG. 1. In sketch a, converter 4 is connected via a transformer 5 for impedance matching to a converter with negative impedance, which consists of an operational amplifier 5, feedback resistors 7 and 8 and from an element with a non-linear characteristic, e.g. B. incandescent lamp 9 is formed. As is known, the negative impedance converters provided with non-linear elements function as oscillators if they are connected to a damped resonant circuit, if the size of the negative resistance is able to compensate for the losses of the resonant circuit, FIG. 2 b shows the negative resistor converter 10 in a state connected to the converter 4. This arrangement produces sinusoidal signals of constant amplitude at its output, the oscillation frequency being given by the equation:

is determined. In equation 1, l is the length of the string 1, ε is the relative extension of the string 1, E is the elastic modulus of the string material and is a

Material coefficient.

The relative expansion is caused by the force F acting on the string 1, whereby the equation: exists between the force F acting on the string and the oscillation frequency f.

3-5, which shows an advantageous embodiment of the converter 4. The transducer 4 was realized in an aluminum tube of rectangular cross section, the end of which is closed by a metal closure which is adapted to the interior of the tube 11 and on which the lower end of the string is rigid is attached. At the other end of the tube 11, a plastic holder 13 is connected, which tightly closes the tube 11 and whose internal structure is well shown in the drawing. The holder 13 consists of two parts which have an inner bore, and a threaded spindle 14 is guided in the bore and fastened by a nut 15. The spindle 14 and the nut 15 serve not only to hold the two halves of the holder 13 together, but at the same time to transfer the string 1. In the interior of the holder 13, a groove IS and a pin 17 bridging the groove are used to guide the string 1 the string arranged / Fig. 4 /. The other end of the string 1 can be attached to the spindle 14, or can be led out between the halves of the holder.

In the embodiment in which the string is attached to the spindle 14, the electrical connection is formed by the tube 11 and the spindle 14. In the interior of the tube 11, a magnetic pole 18 is arranged approximately in the central region, which can be seen in the cross-sectional view of FIG. 5, and the pole 18 consists of a pole body 19 of magnetizable material and of two fixed magnets 20, 21, between which an air gap with a suitable width for the string 1 is formed. The magnetic field and the air gap are homogeneous, and the direction of the field is transverse to string 1.

3-5, the transducer 4 according to FIGS. 3-5 has a thermo-compensated structure, the condition of which is that the coefficient of thermal expansion of tube 11 exceeds that of string 1, and that the coefficient of thermal expansion of plastic holder 13 is substantially greater than that of the material of the tube 11 will. Since the holder 13 is firmly connected to the end of the tube 11 by gluing or another non-releasable bond, the thermal expansion in the section between the end of the tube 11 and the pin 14 supporting the upper end of the string 1 reduces the tension of the string 1. With a suitable selection of the dimensions, such sedations can be created, the acting on the string 1 and by the Spindle 14 adjustable clamping force remains unchanged regardless of the temperature. Another embodiment of the converter 4 is shown with reference to FIGS. 6 and 7. This transducer has an outer switch 12 made of a tube of round cross section, the upper end of which is closed with a plastic closure element 23 and a string tensioning device with a threaded spindle 24 is arranged in the middle of the closure element. The device is used for fastening the upper end of the string and for its electrical connection. In the interior of the outer holder 22, a magnetic pole 25 is formed approximately at a medium height, which - as shown in FIG. 7 - consists of a pole body 26, two fixed magnets 27, 28 and a plastic insert 29 supporting these elements. The

String 1 is brought out in the middle in an air gap formed between the fixed magnets 27, 28. At the lower end of the outer tube 22, such a flange 30 is formed, which enables the tight arrangement of the Wandle rs 4 in a circular opening. Under the

Flange 30 is a clamping arm 32 which is held by bearings 31. The string is led out through the tension arm 32 in such a way that it is also rigidly connected to it. On the side of the flange 30, which lies opposite the bearing 31, a clamping element 33 formed as a screw is attached arranges, whose lower end presses the tension arm 32. The tensioning element 33 enables the basic pretensioning of the string to be set.

The transducer 4 shown here can be used primarily for measuring liquid levels. The structural unit holding the outer holder 23 is generally formed from a liquid container, in which the liquid level can change. A weight 34 is suspended on the section of the string 1 which lies under the tension arm 32. It is useful if that

Weight 34 has a rotational paraboloid shape, because this configuration - as will be explained later - enables the output frequency of the transducer 4 to be changed linearly with the liquid level. The upper end of the transducer 4 according to FIGS. 6 and 7 and the same according to FIGS. 3-5 is hermetic / sealed / closed, and the magnetic poles 18 and 25 are arranged in the interior of the tube, damped by silicone rubber, which causes the possible mechanical vibrations of the tube have no detectable effect on string 1.

To illustrate a preferred dimension, it should be mentioned that the length-diameter ratio of the string 1 is expediently between 500 and 1500, which means a length of 110 mm and a diameter of 0.1 mm in the embodiment of the carrion according to FIGS. 6-7 . The measurable force range is between 0.5 N and 100 N, with the frequency between 350 Hz and 3-4 kHz. The length of the magnetic field can be about 10-20% of the string length. The function of the converter 4 according to the invention is explained on the basis of some possible applications. The converter 4 according to FIGS. 3-5 can be designed as a measuring cartridge and is suitable for measuring various deformations, ie buckling, deflections, dislocations and for weight measurements. The application in the form of a measuring cartridge means that the tube 11 of the completely closed transducer 4 must be attached to the body to be measured by gluing or another firm bond. The arrangement of the measuring cartridge can be the same as that of the strain gauge, and several measuring cartridges can be used simultaneously in an arrangement symmetrical to the multi-line of the body to be measured. If several measuring cartridges are arranged, the size of the deformation can be measured with great accuracy in accordance with the differential measurement principle, since the frequency difference of the transducers arranged symmetrically opposite one another in comparison to the neutron line is identical with the magnitude of the accuracy with great accuracy / if the frequency of the unloaded transducers is the same same is /. Meos methods are already known for correct-sign frequency measurement, and each such knife can be connected to the oscillators of the transducers in question.

The measuring accuracy is so high that even a relative displacement of the order of 10 can be measured. This measurement accuracy enables z. B. measuring the load weight of motor vehicles using transducers arranged on the chassis. The deformation of the chassis depends on the weight of the load. The arrangement according to FIGS. 6 and 7 acts on the weight 34 as a driving force leading to the height of the liquid levels is proportional. Depending on the magnitude of the buoyancy, the tension force acting on string 1 changes. In view of the fact that the frequency changes with the square root of the tensioning force, furthermore that the liquid in the container changes linearly with the liquid level, it can be seen that the dependency between the liquid level / the liquid quantity / and the frequency becomes linear, if the buoyancy force acting on the weight 34 changes quadratically with the liquid level, ie the weight 34 has a paraboloid shape in rotation.

The design solution according to FIGS. 5 and 7 is not only suitable for measuring liquid levels, but also for any force or torque measurements. The measurement possibilities are expanded in that the oscillation frequency of the string 1 also changes with its twist, and a twist meter can also be constructed according to the principles described above, e.g. B. when one end of the string is mounted about its axis. For the thermally compensated embodiment of the transducer according to FIGS. 6 and 7, it is necessary that the upper end of the closure element 23 is rigidly attached to the end of the holder 22 and the string tensioning device 24 is connected to the inner end of the closure element 23. The thermal compensation can be ensured by a suitable choice of the material of the closure element 23 and by the correct dimensioning of the distance between the two support points.

The solution according to the invention is suitable for carrying out not only static but also dynamic measurements, since the frequency of the vibrating string 1 immediately follows its displacement. When carrying out the measurements according to the invention, the frequency of the negative impedance converter 10 connected to the converter, which functions as an oscillator, must be measured. Numerous precise analog and digital solutions are already known for frequency measurements. From these solutions, it is advantageous to use a frequency meter, which works on the basis of the frequency number principle, since it delivers a DC signal proportional to the frequency to be measured. For frequency difference measurements, it is advantageous to use the

To connect the signals of the respective oscillators to a multiplication detector, the output voltage of which reflects the frequency difference with the correct sign. The unwanted high-frequency components can be filtered out by a low-pass filter.

The most modern and accurate measurements can be achieved using the solutions of digital measurement technology. With such a solution, predetermined gates are opened in predetermined sampling times and the number of periods of the oscillator signal / or its specific portion / is counted during the opening period of the god. The numerical value obtained in this way can be used as the address of a read-only memory, and it is expedient if the corresponding frequency values or the quantities to be measured are stored in the respective memory addresses.

In the case of such measurements, the measurement arrangement immediately delivers the quantity to be determined, the measurement range being able to be changed by changing the duration of the time segments and / or the partial ratio.

Claims

Patent claims

1. Transducer mum measuring mechanical dislocations, which contains a string of electrically conductive material, which is attached to both ends and tensioned with a pretesting tone force, by means of the fact that the two ends of the

Side / 1 / at one of the electrically insulated points and along a stretch of the string / 1 / - secondly somewhere in the middle - a magnetic pole / 18, 25 / is arranged, which generates a homogeneous magnetic field, and the mentioned mutually insulated fixing points are electrically connected as electrical connections / 3 / to a converter / 10 / negative impedance, which is connected with the string to an oscillator circuit and a frequency meter can be connected to the oscillator.

2. The transducer according to claim 1, characterized in that the string / 1 / is made of a non-magnetizable material.

3. The transducer according to claim 2, characterized in that the string / 1 / is arranged in a tube / 11,22 / made of non-magnetic material, at one end of the tube there is a string tensioning device which is electrically insulated from the tube and the string / 1 / rigid or connected to the other end of the tube via a connection which enables their displacement or rotation.

4. The transducer according to claim 3, characterized in that the material of the string tensioning device is made of plastic and the length and material are selected such that it is suitable for compensating the linear thermal length changes of the string / 1 /

5. The transducer according to claim 3 or 4, characterized in that the tube / 11 / has a rectangular cross-section and can be attached to the surface to be measured by gluing.

6. The transducer according to claim 3 or 4, characterized in that the tube is designed as an outer holder / 22 /, which is provided on the end opposite the string tensioning device / 24 / with a fastening flange / 30 / and on which the holder / 22 / opposite flange site via the interposition of a bearing / 31 / sin tension arm / 32 / attached, the tension arm / 32 / rigidly connected to the string / 1 / and at one end a tension element / 33 / is connected.

7. The transducer according to claim G, characterized in that a tensioning arm / 32 / a weight / 34 / is connected to the side of the tension arm / 32 / opposite the string / 1,

8. The transducer according to claim 7, characterized in that the weight / 34 / is suspended in the extension line of the string / 1 / on the tension arm / 32 / and has a paraboloidal shape.