FILL LEVEL MEASURING DEVICE

Abstract

Disclosed is a method for a radar-based fill level measurement according to the pulse transit time method. Also disclosed a fill level measuring device for carrying out said method. On the basis of an evaluation signal, the relation between the clock rate and the sampling rate, and a predefined target relation, an evaluation curve is generated. The fill level is thereby determined on the basis of said evaluation curve. The evaluation curve is generated by means of temporal expansion or compression of the evaluation signal, wherein the compression or the expansion is carried out as a function of a ratio between the measured relation and the target relation. Any deviation of the sampling rate from the setpoint value of the sampling rate, for example due to faulty control, is compensated. Thus, the potentially attainable accuracy of the fill level measurement is increased due to the invention.

Claims

1-8. (canceled)

9. A method for radar-based measurement of a fill level of a filling material located in a container, the method comprising: emitting microwave pulses in a direction of the filling material at a defined clock rate; receiving reflected microwave pulses after reflection of the emitted microwave pulses at a surface of the filling material; generating an evaluation signal by sampling the received microwave pulses at a defined sampling rate; measuring a relation between the clock rate and the sampling rate; generating an evaluation curve by using the evaluation signal, the relation between the clock rate and the sampling rate, and a predefined target relation; and determining the fill level by using the evaluation curve.

10. The method according to claim 9, wherein the evaluation signal is generated by means of temporal expansion or compression of the evaluation curve, wherein the compression or the expansion is carried out as a function of a ratio between the measured relation and the target relation.

11. The method according to claim 10, wherein the expansion or compression (Δt) is carried out according to the relationship: $\frac{t_{ges}-\Delta t}{t_{ges}}=\frac{\phi }{{\phi }_{\tau ef}}$ proportionally as a function of the ratio (φ/φref) between the measured relation (φ) and the target relation (φref).

12. The method according to claim 11, wherein the evaluation curve is generated by using the evaluation signal, the measured relation, and the target relation by: digitizing the evaluation signal; determining a mathematical function by at least a regional approximation of the digitized evaluation signal; and generating the evaluation curve by means of temporal expansion or temporal compression of the approximated mathematical function, wherein the compression or the expansion is carried out as a function of the ratio between the measured relation and the target relation.

13. The method according to claim 12, wherein the approximation is carried out as a polynomial approximation.

14. The method according to claim 9, wherein the fill level is determined by using the evaluation curve by: ascertaining a maximum of the evaluation curve; assigning a signal transit time corresponding to the maximum; and determining the fill level by using a measuring distance that corresponds to the signal transit time.

15. A radar-based fill-level measuring device, comprising: a pulse generating unit designed to generate high-frequency electrical pulses having a defined clock rate; a transmitting/receiving unit designed: to emit the high-frequency pulses as microwave pulses in a direction of a filling material, and to receive reflected microwave pulses after reflection at a surface of the filling material; a sampling unit designed to generate electrical sampling pulses at a defined sampling rate; a mixer designed to mix the received microwave pulses with the sampling pulses such that an evaluation signal is generated; a detector designed to measure a relation between the sampling rate of the sampling pulses and the clock rate of the high-frequency pulses; and an evaluation unit designed: to generate an evaluation curve by using the evaluation signal, the measured relation, and a predefined target relation, and to determine the fill level on the basis of the evaluation curve.

16. The fill-level measuring device according to claim 15, wherein the fill-level measuring device is designed to control the sampling rate as a function of the measured relation such that the relation corresponds to the target relation.

Description

[0039] The invention is explained in more detail with reference to the following figures. The following is shown:

[0040] FIG. 1: an arrangement of a radar fill-level measuring device,

[0041] FIG. 2: a circuit technology structure of the fill-level measuring device, and

[0042] FIG. 3: an evaluation curve according to the invention.

[0043] To fundamentally understand the invention, FIG. 1 shows a typical arrangement of a freely radiating radar-based fill-level measuring device 1 on a container 2. In the container 2 is a filling material 3 whose fill level L is to be determined by the fill-level measuring device 1. For this purpose, the fill-level measuring device 1 is mounted on the container 2 above the maximum permissible fill level L. Depending on the field of application, the height h of the container 2 can be up to 75 m.

[0044] As a rule, the fill-level measuring device 1 is connected via a bus system, such as “Ethernet,” “PROFIBUS,” “HART” or “Wireless HART,” to a higher-level unit 4, such as a process control system or a decentralized database. On the one hand, information about the operating status of the fill-level measuring device 1 can thus be communicated. On the other hand, information about the fill level L can also be transmitted in order to control any inflows and/or outflows that may be present at the container 2.

[0045] Since the fill-level measuring device 1 shown in FIG. 1 is designed as freely radiating radar, it comprises a corresponding antenna 121. As indicated, the antenna 121 can be designed as a horn antenna, for example. Especially in the case of radar frequencies above 100 GHz, the antenna 121 can also be realized as a planar antenna. Regardless of the design, the antenna 121 is oriented such that corresponding microwave pulses S.sub.HF are emitted in the direction of the filling material 3 according to the pulse transit time method.

[0046] The microwave pulses E.sub.HF are reflected at the surface of the filling material 3 and, after a corresponding signal transit time, are received as corresponding electrical receive signals e.sub.HF at the antenna 121. In this case, the signal transit time of the microwave pulses S.sub.HF, E.sub.HF is a function of the distance d=h−L of the fill-level measuring device 1 from the filling material surface.

[0047] A circuitry design of the fill-level measuring device 1 with which the microwave pulses S.sub.HF can be generated and with which the fill level L can be determined on the basis of the electrical receive signals e.sub.HF is shown in FIG. 2:

[0048] In order to generate the microwave pulses S.sub.HF, the circuit of the fill-level measuring device 1 shown in FIG. 2 comprises a pulse generating unit. The pulse generating unit is designed to generate high-frequency electrical pulses s.sub.HF at a defined clock rate f.sub.c. For this purpose, the pulse generating unit in the shown exemplary embodiment consists of a first pulse generator 110 which drives a first high-frequency oscillator 111. The frequency of the microwave pulses S.sub.HF, E.sub.HF is established by the oscillation frequency of the high-frequency oscillator 111. In the simplest case, the high-frequency oscillator 111 can be designed as a quartz oscillator. A VCO (voltage-controlled oscillator) can also be used. In this case, the high-frequency oscillator 111 is driven by the pulse generator 110 by means of a DC voltage signal. The pulse generator 110 thereby defines the pulse duration of the individual microwave pulses S.sub.HF and the clock rate f.sub.c at which the microwave pulses S.sub.HF are emitted. Normally, a semiconductor-based digital resonant circuit is used as the high-frequency oscillator 111. In practice, the clock rate is between 100 kHz and 1 MHz.

[0049] The high-frequency pulses s.sub.HF thereby generated by the high-frequency oscillator 111 are supplied to the antenna 121 via a transmitting/receiving switch 120 so that they are correspondingly emitted as microwave pulses S.sub.HF. Since the reflected microwave pulses E.sub.HF are also received via the antenna 121, the transmitting/receiving switch 120 supplies the corresponding electrical receive signal e.sub.HF to a mixer 14.

[0050] In contrast to the shown embodiment variant, instead of the antenna 121, an electrically conductive probe such as a waveguide or a coaxial cable extending toward the container bottom can also be used. In the implementation of this embodiment variant known by the term TDR (time-domain reflectometry), the high-frequency oscillators 111, 131 are not required in contrast to the circuit shown in FIG. 2.

[0051] The undersampling of the receive signal e.sub.HF characteristic for the pulse transit time method is performed by the mixer 14. For this purpose, the receive signal e.sub.HF is mixed with electrical sampling pulses s′.sub.HF by the mixer 14. In this case, the sampling rate f′.sub.c at which the sampling pulses s′.sub.HF are generated deviates by a defined, low relation of far less than 0.1 per thousand from the clock rate f.sub.c of the generated high-frequency pulses s.sub.HF.

[0052] The sampling pulses s′.sub.HF are generated by a sampling unit which, analogously to the pulse generating unit, comprises a second pulse generator 130 and a second high-frequency oscillator 131. Thus, correspondingly to the high-frequency pulses s.sub.HF, the frequency f.sub.HF of the sampling pulses s′.sub.HF is defined by the second high-frequency oscillator 131. The second pulse generator 130 controls the sampling rate f′.sub.c at which the sampling pulses s′.sub.HF are generated.

[0053] By mixing the receive signal e.sub.HF with the electrical sampling pulses s′.sub.HF by means of the mixer 14, an evaluation signal ZF is generated, which represents the receive signal e.sub.HF in a time-expanded manner. The time expansion factor is proportional to the relation φ between the clock rate f.sub.c and the sampling rate f′.sub.c.

[0054] The time expansion is advantageous in that the evaluation signal ZF can be evaluated considerably more easily technically due to the time expansion in comparison with the pure receive signal e.sub.HF: The reason for this is that the receive signal e.sub.HF has a correspondingly short time scale tin the nanosecond range due to the high speed of propagation of the microwave pulses S.sub.HF, E.sub.HF at the speed of light. The time expansion results in the evaluation signal ZF having a time scale in the millisecond range. An evaluation curve ZF is schematically illustrated in FIG. 3: According to the prior art, the fill level L is determined based on the evaluation signal ZF by identifying an amplitude maximum M.sub.−ZF of the evaluation signal ZF that corresponds to the microwave pulse E.sub.HF reflected by the filling material surface. By using the signal transit time t.sub.m assigned to the maximum M.sub.−ZF, an evaluation unit 16 of the fill-level measuring device 1 can determine the distanced to the surface of the filling material 3 in order to derive the fill level L therefrom.

[0055] To correctly determine the fill level L based on the evaluation signal ZF, it is essential that the sampling rate f′.sub.c of the sampling unit corresponds exactly to its target sampling rate: This means that the relation φ to the clock rate f.sub.c of the pulse generating unit corresponds to a required target relation φ.sub.ref. In order that the sampling rate f′.sub.c does not drift therefrom, it is correspondingly controlled. In the exemplary embodiment shown in FIG. 2, the control takes place by a detector 15 which measures the relation φ between the clock rate f.sub.c and the sampling rate f′.sub.c downstream of the first pulse generator 110 or the second pulse generator 130. This can be technically implemented, for example, by the detector 15 measuring, over a plurality of phases, a change in the time shift between the positive flank at the first pulse generator 110 and the positive flank at the second pulse generator 130.

[0056] From the change in the time shift, the evaluation unit 16 calculates the relation φ between the clock rate f.sub.c and the sampling rate f′.sub.c. The evaluation unit 16 also compares the determined relation φ between the clock rate f.sub.c and the sampling rate f′.sub.c with a stored target relation φ.sub.ref. In this way, the evaluation unit 16 can control the sampling rate f′.sub.c at the second pulse generator 130 as a function of the measured relation φ such that the relation φ corresponds to the target relation φ.sub.ref. In the embodiment variant shown in FIG. 2, a signal converter 17 is interposed between the evaluation unit 16 and the second pulse generator 130 in order to provide an analog control signal to the pulse generator 130. For this purpose, the signal converter 17 can be designed, for example, as a DAC driving a capacitance diode.

[0057] Depending on the technical implementation of the control, the control may have an imprecise, overcontrolling or delayed effect. These effects lead to a distorted measurement of the fill level L since the evaluation signal ZF is thereby inadvertently temporally expanded or compressed. The maximum M.sub.ZF corresponding to the filling material surface is thereby shifted in the evaluation signal ZF.

[0058] According to the invention, this distortion is counteracted in that the evaluation unit 16 either temporally expands or temporally compresses the stored, digitized evaluation signal ZF as a function of the ratio φ/φ.sub.ref between the measured relation φ and the target relation φ.sub.ref. Since the measurement is a freely radiating radar measurement, the evaluation signal ZF is initially rectified before the A/D conversion, in contrast to guided radar.

[0059] The evaluation unit 16 can carry out the expansion or compression by the value Δt by generating a mathematical function using the rectified and digitized evaluation signal ZF. For this purpose, the evaluation signal ZF can be fitted by means of polynomial interpolation, for example. The evaluation curve ZF′ is subsequently generated by means of temporal expansion or temporal compression of the approximated mathematical function by the value Δt.

[0060] The temporal expansion or compression Δt is proportional to the ratio φ/φ.sub.ref between the measured relation φ and the target relation φ.sub.ref: The greater the ratio φ/φ.sub.ref is, the more the evaluation signal is expanded. Whether the evaluation signal ZF is expanded or compressed in the evaluation unit 16 depends on whether the resulting ratio φ/φ.sub.ref is greater or less than 1 or greater or less than 100%. In the exemplary illustration in FIG. 3, the evaluation signal ZF is compressed by Δt. The compression Δt is thus based on the relationship

[00001]$\frac{t_{ges}-\Delta t}{t_{ges}}=const*\frac{\phi }{{\phi }_{ref}}$

[0061] The evaluation unit 16 subsequently determines the fill level L by means of the evaluation curve ZF′ thus obtained. The fill level L is determined by the evaluation unit 16 ascertaining the maximum M.sub.−ZF′ of the evaluation curve ZF′ that is caused by the surface of the filling material 3 (in the schematic evaluation curve ZF′ illustrated in FIG. 3, this maximum M.sub.ZF′ is shown exclusively for reasons of illustration). After determining the maximum M.sub.ZF′, the signal transit time t′.sub.m that corresponds to the maximum M.sub.ZF′ is determined. The evaluation unit 16 can thus determine the fill level L according to the relationship L=h−d by using the measuring distance d that corresponds to the signal transit time t′.sub.m.

[0062] As FIG. 3 indicates, the maximum M.sub.ZF′ of the evaluation curve ZF′ is corrected in comparison with the evaluation signal ZF by the compression Δt toward a lower signal transit time t′.sub.m. The invention thus compensates for any unwanted compression or expansion of the evaluation signal ZF, provided that the sampling rate f′.sub.c deviates from its setpoint value due to a faulty control. In addition to an improved accuracy of the fill-level measurement, this also offers the advantage that the fill-level measuring device 1 can be designed to be less complex with regard to the control of the sampling rate f′.sub.c.

LIST OF REFERENCE SIGNS

[0063] 1 Fill-level measuring device [0064] 2 Container [0065] 3 Filling material [0066] 4 Higher-level unit [0067] 14 Mixer [0068] 15 Detector [0069] 16 Evaluation unit [0070] 17 Signal converter [0071] 110 First pulse generator [0072] 111 First high-frequency oscillator [0073] 120 Transmitting/receiving switch [0074] 121 Antenna [0075] 130 Second pulse generator [0076] 131 Second high-frequency oscillator [0077] DK Dielectric value [0078] d Measuring distance [0079] E.sub.HF Reflected microwave pulses [0080] e.sub.HF Receive signal [0081] f.sub.c Clock rate [0082] f′.sub.c Sampling rate [0083] f.sub.HF Frequency of the microwave pulses [0084] h Installation height [0085] L Fill level [0086] M.sub.ZF′ Maximum [0087] R.sub.HF Reference curve [0088] S.sub.HF Microwave pulses [0089] s.sub.HF High-frequency pulses [0090] s′.sub.HF Sampling pulses [0091] t.sub.m, t′.sub.m Signal transit time [0092] ZF Evaluation signal [0093] ZF′ Evaluation curve [0094] Δt Expansion/compression [0095] φ Relation between the clock rate and the sampling rate [0096] φ.sub.ref Target relation