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Mesure par ultrasons avec Endress+Hauser

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Présentation au sujet: "Mesure par ultrasons avec Endress+Hauser"— Transcription de la présentation:

1 Mesure par ultrasons avec Endress+Hauser
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2 Effet piézo-électrique

3 Technique d'émission Oscillateur Piston Oscillateur Flexural
Oscillateur couplé 4 D A E C 1 2 3 3 2 1 4 5 B 5 A Cristal piézo 1 Cristal piézo B Couche d'adaptation impédance 2 Membrane plate 1 Cristal piézo Ultrasonics drive technologies There are basically only three types of excitation, Piston, Flexural and coulping oscillator. Piston Oscillator With this system the change in volume of the piezocrystal is used to create sound waves. The piezo oscillates at its excitation frequency like a "piston”. With an adaptation layer(/4- layer) an energy is transmitted from the piezo to the air though a diaphragm. Flexural Oscillator This is a direct coupling between the piezo and the metal diaphragm creating the sound wave. It must first pass through special rings in front of the diaphragm functioning as ”acoustic rectifiers”, to enable it to be evaluated. This principle is used mainly with sensors with a wide measuring range. Coupling Oscillator This principle considerably refines the principles of lateral oscillation. It is constructed completely differently from previous sensor systems. The basic core of the sensor is the diaphragm 4. It consists of circular diaphragms and ring diaphragm which are so calibrated to the system that all diaphragms oscillate together in phase. In order to achieve this, diaphragms are excited to oscillate by the studs whereby the studs form the tensioned edges of the diaphragms. The studs are excited by a system consisting of a counter weight 5, the piezo stack 1 and the aluminium body 2, so designed that at its working frequency it is almost piston-shaped (the same amplitude and same phase) in its oscillation. This creates a constant echo amplitude over the entire diaphragm surface. This principle is highly efficient as almost all the energy is transmitted and used for measurement over large ranges. In comparison to lateral oscillation, the coupling oscillation has a smooth surface and is thus insensitive to material build-up. 3 Oscillation de la membrane 2 Convertisseur d'oscillation C Pot de mesure D Couche d'amortissement 4 Grille 3 Stries concentriques 5 Pression sonore 4 Membrane E Attenuation sensor 5 Masselotte

4 Principe de mesure ultrasonique
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5 Effet sur la vitesse du son
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6 Atténuation des ultrasons (dans l'air)

7 Réflexion Réflexion Sur liquides en présence de mousse
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8 Réflexion diffuse sur les solides

9 Réflexion sur les solides
Réflexion diffuse f/kHz /4-Valeurs ,8 mm ,9 mm ,0 mm ,9 mm ,9 mm ,3 mm Mirror-Reflection Reflection from solids Because of its surface profile, the ultrasonics pulse is laterally reflected and does not return to the sensor at full strength. With an almost smooth surface at an angle to the sensor diaphragm, almost no sound energy returns to the sensor diaphragm as it is governed by the laws of reflection just like a mirror. The ultrasonic pulse is completely diverted to the side. It is because of these diffuse reflections that the measuring principle of echo level measurement does not fail with solids. If the wavelength is the size of the grain then part of the wavelength is the size of the grain then part of the ultrasonic pulse returns to the sensor. This will be stronger the smaller the wave is in comparison to grain size. If the roughness is greater than ¼ of the wavelength then the mirror-like reflection becomes diffuse. The wavelength is calculated by:  =v/f (lambda = velocity / frequency).

10 Dispositif d'orientation FAU 40

11 Traitement du signal A D Origine de la courbe enveloppe transmission
réception transmission réception kHz D A Valeur mesurée µP Traitement du signal NTC piézo Ultrasonics signal evaluation A mixture of frequencies between 10 and 70 kHz is transmitted from the piezocrystal of the ultrasonic sensor. The frequency transmitted is dependent on the excitation of the particular ultrasonics sensor. The blocking distance is derived from the decay of the transmission pulse. If the receiving pulse is still reflected in the area of the transmitting pulse, or the blocking distance, then the signal cannot be evaluated. The signal received is amplified in the evaluating unit which filters out secondary frequencies and the signal is then finally rectified and assigned a logarithmic function. The envelope curve thus created is digitised by an analogue/digital converter and is available to the microprocessor for further processing.

12 Courbe enveloppe Interférence Signal du niveau Distance en mètres 120
50 1 2 3 4 5 6 Distance en mètres Atténuation en dB Interférence Envelope curve The ultrasonics signal consists of various frequencies. This frequency spectrum is converted into an envelope curve which can show an actual picture of the vessel. Derived from the envelope curve are the status of the decay (which results from, e.g. nozzle mounting or vessel geometry) the state of the interference echoes (created by, e.g. internals in the vessel and the actual working echo which is created by the surface of the product, identified and evaluated . Signal du niveau

13 Elimination d'échos fixes
120 50 1 2 3 4 5 6 Distance en mètres Atténuation en dB Echos supprimés Fixed target suppression Long nozzles, internals (e.g. a limit switches) agitator blades, welding seams etc. can cause interference reflections which are registered by the ultrasonic sensor but are not to be evaluated. Such reflections can be suppressed by a fixed target suppression. Thus the distance is entered into the measuring system up to which a fixed target suppression should be active. During the suppression, no working signal (level) should be in this area otherwise it wil be suppressd. While registering, an "acoustic photo" is made of the vessel and stored in the instrument. TDT Signal du niveau

14 F R FEF Réflexions multiples acteur econaissance P Echo remier
Solides en vrac Liquides FRPE FRPE Multiple reflections and first echo recognition Multiple reflections can occur with both solids and liquids. With solids multiple reflections are created by various mounds, hopper, etc.. There are thus specific echo amplitudes at various locations. The evaluation jumps between these amplitudes depending on which is the larger. Multiple reflections are diverted to the side and the vessel wall via the mound and then travel to the sensor which then receives the ultrasonic pulse. With measurement in liquids the same comparable symptoms occur depending on the vessel geometry (e.g. curved roofs). As the ultrasonic pulse in this cases is transmitted for a longer distance then the reflection appears further away than it actually is. By reflection at the vessel wall, the echo appears of very high quality. On the other hand, echoes which come back to the sensor without making a detour show the correct distance (as these echoes do not travel along a wall as a detour). This means that the first echo is the correct one but it is often not the strongest one. The correct echo can automatically be evaluated using a software filter. acteur F FEF econaissance P Echo remier R

15 Influence d'un agitateur sur la mesure
VUE Agitator Agitator blades in a vessel give rise to sporadic echoes. These occur if the measurement is carried out at the same time as the agitator blade is directly in front of the ultrasonic sensor . The frequency of this effect depends on the speed of the agitator blade. It can be statistically evaluated, i.e. the level, contrary to the agitator blade, is always under the sensor, is evaluated as it is given greater statistical weight than an agitator echo. A lower value for statistics means a lower statistical evaluation.

16 Conseils de montage des sondes ultrasoniques
Solides en vrac Liquides Mounting an ultrasonic sensor When mounting any ultrasonic sensor the following points must always be observed: The max. height should not protrude into the blocking distance For liquids the sensor should be positioned parallel to the surface of the liquid and for solids the sensor should be directed at the mound or the conical outlet The sensor should protrude into the vessel at least the max. permissible nozzle geometries should be considered Mounting the sensor away from the centre of the vessel is ideal as it causes multiple reflections if mounted in the centre of the vessel The sensor should not be mounted over the inlet nozzle or filling curtain.

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