Level measurement device.

The invention relates to a level measuring device for measuring the level in a container, with a transmitting and receiving arrangement sonic or ultrasonic pulses directed to the product surface and the light reflected from the product surface backscatter pulses and converts into electrical reception signals, and with the transmission and receiving arrangement connected evaluation circuit which generates in a signal processing path a of the envelope of the received signals corresponding envelope signal samples of the envelope signal digitized, stores the digitized samples to create a distance or run-time-dependent amplitude profile of the measurement path in a memory and the amplitude profile for determining the transit time of the most probable useful -Echosignals evaluates.

In known level meters of this type can be obtained by evaluating the stored in the memory digitized amplitude profile unwanted echo signals, in particular those derived from fixed objects in the container or be caused by multiple reflections on the product surface, be distinguished from the useful echo signal, so that measurement errors due to such interference echo signals are largely avoided. A significant advantage of such level measuring devices is that the amplitude profile of the test section is retained. From the amplitude profile information about additional parameters can be obtained, which are relevant for the precise determination of the duration of the sonic or ultrasonic waves in the container. However, it has shown that the transit time measurement can be affected by interference echo signals still coming from obstacles that move relative to the level measuring device, since such interference echo signals completely irregular occur with ever-changing maturities. In the measurements in containers such unwanted echo signals caused by reflections in particular on filling when filling is filled at the same time to measure up in the tray.

In level meters of another kind, it is known to distinguish originating from moving obstacles echo signals due to the Doppler effect caused by the frequency shift of the fixed target echo signals by the received signals are passed through a frequency detection stage. At the output of frequency detection level but no amplitude information, but only one frequency information available. This measure is therefore not applicable where the amplitude profile of the test section to be stored and analyzed.

The object of the invention is the provision of a level at which the in which the transit time measurement, unaffected by interference echo signals originating from moving obstacles, however, formation the Amplitudenin of data originating from immovable obstacles echo signals is retained while the useful echo signals are distinguished from false measurements.

According to the invention this object is achieved in that in the evaluation circuit to the signal processing path for the generation of the envelope signal a frequency detection of the echo signals is performed in parallel, are detected by means of which the echo signals with the transmitting frequency of the sonic or ultrasonic pulses and that in dependence on the result of the frequency detection the signal processing path is opened only for those echo signals, whose frequency corresponds to the transmission frequency.

The level measuring device exported to the invention are all receiving signals whose frequency does not correspond to the transmission frequency, excluded from digitization and further processing. For this purpose, due to the Doppler frequency shift include all received signals caused by reflections from moving obstacles. By contrast, all received signals whose frequency corresponds to the transmission frequency, passed unchanged for digitizing and further signal processing. In the memory, therefore, the full amplitude profile of this same frequency received signals is stored, so that it is available for evaluation in the recognition of the most probable useful echo signal, and in determining the run time.

Advantageous refinements and developments of the invention are characterized in the dependent claims.

Further features and advantages of the invention will become apparent from the following description of an embodiment with reference to the drawing. In the drawings:<ul><li>Fig. 1 is a schematic representation of the measurement in a container with the aid of a filling level,</li><li>Fig. 2 shows the block diagram of the electronics of the fill level measuring device of Fig. 1 according to an embodiment of the invention, and</li><li>Fig. 3 shows time diagrams of signals which may occur in the electronics of FIG. 2.</li></ul>

FIG. 1 shows a container 10 which is filled with a pourable product 12 partially. The filling material 12 may be powdery or granular or consist of still coarser pieces. It is introduced from above into the container 10, for example by means of a conveyor belt 14, from which it falls as the filling 16 in the container, and it can be removed by a controllable discharge opening 18 at the lowest point of the container 10th Moreover, some fixed barriers 19 are shown in the container 10th

For continuously measuring a fill level 20 is mounted on the container 10, the sonic or ultrasonic pulses sent down into the container 10 and which receives the reflected echo pulses 12 on the surface of the filling material. The time interval between the emission of a sonic or ultrasonic pulse and receiving an echo pulse corresponds to the sound propagation time by the level 20 to the product surface and back to the level meter. In known speed of sound can be determined from the distance of the material surface by the level and thus the reservoir level is 10th

The level measuring device 20 is composed in the usual manner of an electro-acoustic transducer 22 and associated electronics 24. The electroacoustic transducer 22 serves alternately as a transmitting transducer for emitting sonic or Ul trasound transmitting pulses and in each pause between two transmitted pulses as a receiving transducer which reflected the echo pulses into electrical receive signals. The time interval between two successive transmission pulses is greater than the greatest occurring duration of a sonic or ultrasonic pulse by the level to the product surface and back again. The electronics 24 includes circuits that energize the electro-acoustic transducer 22 at periodic time intervals for transmission of the transmit pulses, circuits for amplifying and processing of the 22 supplied by the electro-acoustic transducer electrical reception signals, and circuits for the determination of the desired level from the time interval between transmitted pulses and the received echo pulses , In general, it is desirable not only to determine the duration of the sonic or ultrasonic pulses, but to evaluate the shape and amplitude of the reception signals delivered by the electroacoustic transducer 22, because therefrom information about the conditions in the vessel 10 can be obtained, that of the detection probable useful echo signal can be and are used for the precise runtime measurement. This additional information resulting from the envelope or the time-amplitude profile of the received signals supplied by the electroacoustic transducer 22nd

In known manner, the electroacoustic transducer 22, which works alternately as a transmitting transducer and a receiving transducer, also be replaced by two separate transducers, one of which the other exclusively serves exclusively as a transmitting transducer and a receiving transducer.

A significant problem in this level measurement based on the echo sounder principle consists in that, apart from the useful echo pulses reflected at the surface of the material, as indicated in Fig. 1 by the arrows N can also unwanted echo pulses occur of other obstacles are reflected in the reservoir and superimpose the useful echo pulses. While unwanted echo pulses originating from the fixed obstacles 19 in the tank, as indicated by the arrows F, always occur with the same term and therefore easily recognized by evaluating the amplitude profile of the received signals and can be rendered harmless, arising in particular the echo signals, which are reflected at the falling into the container Füllgutteilchen of Befüllstroms 16, as indicated in Fig. 1 by the arrows S, a massive failure of the level measurement. These unwanted echo signals S occur with ever-changing times in the entire range of maturities.

FIG. 2 shows the block diagram of one embodiment of the electronics 24, which makes it possible to make the originating from the filling 16 unwanted echo pulses S harmless without the information contained in the envelope or the time-amplitude profile of the received signals are lost. The diagrams A to D of Fig. 3 show the time course of various signals that may occur at the designated with the same letter circuit points of the circuit of FIG. 2.

In FIG. 2, the electroacoustic transducer 22 is shown again connected to the output of a transmit pulse generator 30th The transmission pulse generator 30 sends at periodic time intervals each comprise a pulse-shaped electric oscillation train with the frequency of the sonic or ultrasonic pulse to be emitted as an excitation pulse to the electro-acoustic transducer 22 is thereby excited for emitting a sonic or ultrasonic transmitting pulse. The duration of the sonic or ultrasonic transmission pulse is small compared to the duration of the transmission periods dur is ch periodic time intervals between the successive transmission pulses determined.

The electroacoustic transducer 22 is also connected to the input of an amplifier 31 which amplifies the transducer 22 from the COM Menden electrical signals. These signals include first pulses corresponding to the sonic or ultrasonic transmission pulses. After the decay of each transmit pulse, the electroacoustic transducer 22 operates as a receiving transducer which converts the incoming sonic or ultrasonic echo pulses into electrical reception signals, which are also supplied to the amplifier 31st Since the level of the received signals is small compared to the level of the transmit pulses, corresponding to the transmission pulses electrical signals are limited by suitable, per se known measures to prevent overdriving of the amplifier 31st

The amplifier 31 thus outputs at its output in each transmission period, a signal from, for example, as shown in diagram A of FIG. 3. The beginning of the measurement period is determined by the transmission pulse E, whose amplitude is limited. While coming from the transmission pulse generator 30 electrical excitation pulse is a square pulse appears at the input of the amplifier 31 a exponentially decaying pulse, since the electroacoustic transducer 22 after the cessation of the excitation pulse still reverberates.

In an interval T_MWhich is determined by the distance of the material surface from the electro-acoustic transducer 22, a useful echo pulse N that corresponds to the light reflected by the material surface sonic or ultrasonic echo pulse occurs after the end of the transmit pulse E. This useful echo pulse N is an oscillation train at the transmission frequency of the sonic or ultrasonic wave with an envelope which is compared with the original rectangular shape of the transmit pulse more or less strongly smoothed and deformed. Between the excitation pulse E and the useful echo pulse N 3 is the diagram A of Fig. Shown an unwanted echo pulse S, which has been reflected from the falling into the container filling 16, and an interference echo pulse F, which at a fixed obstacle 19 has been reflected in the container 10th Of course, even zahlrei che further interference echo pulses S of this type can in this area of ​​the sending period be available, probably with different maturities from different parts of the Befüllstroms 16, and unwanted echo pulses F, which have been reflected at fixed obstacles in the tank. As a result of multiple reflections can also appear behind the useful echo pulse N unwanted echo pulses.

The amplifier 31 is an envelope circuit 32 downstream, which is designed so that it emits at its output a signal corresponding to the envelope of its input signal. Hüllkurvenschaltungen that fulfill this function are well known. In the simplest case, the envelope circuit may be an amplitude demodulator, which rectifies the amplitude modulated carrier wave with the frequency of the sonic or ultrasonic wave and suppressed by low-pass filtering. If the input of the envelope circuit 32 the signal shown in diagram A is supplied, then the low frequency signal of the diagram B which the envelope E 'of the transmit pulse E, the envelope of N' of the useful echo pulse N, and the envelope F appear at its output ' and S 'of the interference echo pulses F or S contains. More generally, the envelope signal represents the time-amplitude profile of the output signal of the electroacoustic transducer 22 is, and it still contains all of the amplitude information of the received signal. Since the envelope signal is a low frequency, it can be easily processed and transmitted with less effort over long distances, for example to an evaluation unit, which is arranged on a container from 10 remote location.

Supplied by the output of the envelope circuit 32 envelope signal is an analog to digital converter 35 is supplied in the circuit of Fig. 2 via a delay stage 33 and a gate circuit 34th In the analog-digital converter 35, the envelope signal is periodically sampled, and each sample is converted into a digital signal in the form of a code group with the desired resolution corresponding number of digits.

The output of the analog-digital converter 35 is connected to a digital signal processing device 36, which can for example be formed by a suitably programmed microcomputer. The digital signal processing device 36 includes a memory in which the data provided by the analog-digital converter 35 code groups, which, however represent the digitized reception signal of at least one transmission period, preferably of several consecutive transmission periods, are stored, so that in the memory a distance or term-dependent amplitude profile of the test section is. This amplitude profile is then statistically smoothed and analyzed according to experience. Based on this analysis of fixed obstacles 19 originating clutter F are detected and the echo pulse determines which represents the reflected by the material surface useful echo pulse with the greatest probability, and finally the duration of this probable useful echo pulse is determined for the determination of the level. This echo evaluation can be adverse effects that occur with certain reception conditions, such as double and multiple reflections, better mastered echo from installations in the container or the like. However, the measurement of massive disturbances can still be very difficult by the problems caused by the filling irregular interference echo pulses S or even thwarted.

To prevent such interference by the originating from the filling unwanted echo pulses, the circuit of FIG. 2 includes a frequency detector 37, of which the envelope circuit is parallel connected 32 containing the signal processing path to the output of the amplifier 31. A second input of the frequency detector 37 is connected to the transmit pulse generator 30, and the output of the frequency detector 37 is connected to the control input of the gate 34th

The frequency detector 37 receives from the transmit pulse generator, a signal with the frequency of the transmitted pulses, and it constantly compares the frequency of the appearing at the output of the amplifier 31 the received signal with this transmission frequency. If the frequency detector 37 detects a coincidence of the frequency of the received signal at the transmission frequency, it returns to the control input of the gate circuit 34 a control signal the gate circuit 34 opens, so that the latter transmits the transferred via the delay stage 33 envelope signal to the analog-to-digital converter 35th In contrast, if there is no match between the frequency of the reception signal and the transmission frequency, the gate 34 is locked by the output signal of the frequency detector 37th

The frequency detector 37, in conjunction with the gate 34 the effect that the envelope signals S 'of posts originating from the filling 16 unwanted echo pulses not enter S for analog-digital converter. The fact is utilized that the filling 16 is moved relative to the echosounder unit 20, so that the reflected on the filling 16 echo pulses suffer a frequency shift with respect to the transmission frequency due to the Doppler effect. The frequency detector 37 must of course be designed such that it responds to this slight Doppler frequency shift and the output at the output control signal changes so that the gate circuit 34 is inhibited.

In contrast, the reflected on fixed obstacles and by the material surface echo pulses F, N have the transmission frequency, since these reflection planes are relative to the filling level measuring device at rest. The corresponding these blips envelope signals F ', N' are therefore transmitted from the gate 34 to the analog-digital converter 35th

The diagram C of FIG. 3 shows the output signal of the gate circuit 34, when it is assumed that the delay stage 33 is not present. In this output the envelope S 'of Störechoimpulses S is completely suppressed. However, the frequency detector 37 requires a certain time until it has determined the frequency matching of its two input signals with sufficient accuracy, since it has to compare several oscillations. The opening of the gate circuit 34 is therefore only a certain time after the start of the useful echo pulse, so that the initial part of the envelope N 'is cut off. This can lead to a distortion of the amplitude information.

This phenomenon is eliminated by the Verzögungsstufe 33rd They left the envelope a delay corresponding to the frequency of the detector 37 required for the frequency detection time. It is thereby achieved that the gate circuit 34 is already open when the beginning of the envelope curve N 'of the useful echo pulse arrives at the N gate. The output signal of the gate circuit 34 then corresponds to the diagram D of Fig. 3. It contains the complete envelope of the useful echo pulse N as well as the envelopes of the unwanted echo pulses F and other components of the received signal having the transmission frequency, while the envelope of all the signal components whose frequency are different from the transmission frequency, is suppressed.

In the memory of the digital signal processing arrangement 36 is not then the time-amplitude profile of the entire reception signal stored, but the time-amplitude profile of the echo signals having the original transmission frequency.

The caused by the delay stage 33 signal delay has no effect on the running time measurement, because the the excitation pulse E corresponding envelope signal and the useful echo pulse N corresponding envelope signal is delayed in the same way, so that the operative for the duration of measurement time interval T_M remains unchanged.

Various modifications of the described circuit will be apparent to the skilled artisan. For example könnnten the delay stage 33 and the gate 34 instead of behind the envelope circuit 32 before these are placed.