DE102005049500B4 - Method for determining the fill level based on the transit time of a high-frequency measurement signal - Google Patents

Abstract

Method for determining the filling level (l) on the basis of the transit time (t) of a high-frequency measuring signal (S) which is transformed by means of a transformation process with a specific transformation factor (k) into a lower-frequency intermediate-frequency signal (S), - wherein the transformation factor (k) is a difference frequency (f) or a difference time (t) of a difference signal (S) between a pulse repetition signal (S) with a pulse repetition frequency (f) and a sampling signal (S) with a sampling frequency (f) is generated and determined, - wherein the pulse repetition frequency ( f) and / or the sampling frequency (f) on the basis of a control with a controlled variable (c_var) by a corresponding control algorithm is / will be changed so that the difference frequency (f) to a desired value of the difference frequency (f) or the difference time (t) a setpoint value of the difference time (t) is regulated, wherein between at least two values of the difference frequency (f) or between at least two We a gradient (grad) is determined as a function of the controlled variable (c_var), wherein an operating point is determined on the basis of the gradient (grad) and the difference frequency (f) or the difference time (t) when the controlled variable (c_var) is set (OP) of the control is determined, and accordingly the control algorithm is adjusted.

Description

The present invention relates to a method for determining the fill level on the basis of the transit time of a high-frequency measurement signal, which is transformed by means of a transformation process with a certain transformation factor into a low-frequency intermediate frequency signal.

Such methods for determining and monitoring the level in a container are often used in the measuring devices of automation and process control technology. By the applicant, for example, measuring devices under the name Micropilot or Levelflex are produced and distributed, which operate according to the transit time measurement method and serve to determine and / or monitor a level of a medium in a container. According to the method of guided microwave or time domain reflectometry or the Time Domain Reflection (TDR) measuring method, a high-frequency pulse is emitted along a Sommerfeld or Goubauschen waveguide or along a Koaxialwellenleiters, which at a discontinuity of the DK value (dielectric constant) of the waveguide surrounding Part of the medium is reflected back. After the free-radiating transit time measurement method, for example, microwaves are transmitted via an antenna into a free space or process space, and the echo waves reflected at the medium surface are received again by the antenna according to the distance-dependent transit time of the signal. Based on the time duration between the emission of the high-frequency pulses and the reception of the reflected echo signals, the distance of the measuring device to the medium surface can be determined. Taking into account the geometry of the Behälterinnem the level of the medium is then determined as relative or absolute size. The transit time measurement method can essentially be divided into two determination methods: The first determination method is based on a time measurement which requires a pulse train modulated signal for the distance covered; A second widely used method of determination is the determination of the swept frequency difference of the transmitted continuous high frequency signal to the reflected, received high frequency signal (FMCW - Frequency-Modulated Continuous Wave). In general, the following is not limited to a specific investigation.

A common problem with all transit time measurement methods with high-frequency measurement signals in the GHz range (gigahertz) is that high-frequency components that are designed for such high frequency ranges must be used to evaluate the high-frequency measurement signals. These high-frequency components have the disadvantage that their production is complicated and the purchase is very expensive. One way to evaluate the high-frequency measurement signals with cheap low-frequency components is to map the high-frequency measurement signals by means of a sequential sampling in the low frequency range. The method for sequential scanning of high-frequency measurement signals represents a possibility of transformation into the low-frequency range, in which method a time-expanded intermediate-frequency signal is generated from a multiplicity of high-frequency, periodically sampled measurement signals. This additional processing step is performed because there are no corresponding low cost data processing units, e.g. DSPs (Digital Signal Processors), which are capable of reliably processing high-frequency measurement signals.

One approach to generating a time-extended intermediate frequency signal is the mixer principle, where two oscillators produce two oscillations with slightly different frequencies. Due to the slight 'detuning' of the frequencies of the two oscillations, a phase shift increases linearly with each measurement period, which corresponds to a linearly increasing time delay. The mixer principle is used for example in the DE 31 07 444 A1 described by means of a high-resolution pulse radar method. A generator generates first microwave pulses and emits them via an antenna with a predetermined transmission repetition frequency in the direction of the surface of the medium. Another generator generates reference microwave pulses that correspond to the first microwave pulses, but differ slightly from those in the repetition frequency. The echo signal and the reference signal are mixed, for example, by a frequency converter or mixer, whereby an intermediate frequency signal is produced. The intermediate frequency signal has the same course as the echo signal, but is compared to this stretched by a transformation factor which is equal to a quotient of the pulse repetition frequency and the frequency difference between the pulse repetition frequency of the first microwave pulses and sampling frequency of the reference microwave pulses. With a pulse repetition frequency of a few megahertz, a frequency difference of a few hertz and a microwave frequency of a few gigahertz, the frequency of the intermediate frequency signal is well below 200 kHz. The advantage of the transformation to the intermediate frequency is that relatively slow and thus inexpensive electronic components can be used for signal acquisition and / or signal evaluation. Reference is also made in this context to the German utility model DE 29815069 U1 using this known transformation technique in a TDR Level gauge describes. This sampling circuit has two oscillators, of which at least one is variable in frequency, with one oscillator controlling the transmitting generator and the other oscillator controlling the sampling pulse generator. A frequency mixer forms the difference between the two frequencies which is used to set or regulate the transformation factor, which is as constant as possible, to a desired value via a feedback branch.

The disadvantage of adjusting the difference frequency to a setpoint value according to the prior art is that the regulation takes a very long time and under certain circumstances an adjustment to an incorrect setpoint value of the difference frequency occurs.

The invention has for its object to ensure by a control algorithm a fast, safe and accurate adjustment of the difference frequency of the signals from two oscillators to a predetermined desired value of the difference frequency.

This object is achieved by a method for determining the level on the basis of the duration of a high-frequency measurement signal, which is transformed by means of a transformation process with a certain transformation factor in a low-frequency intermediate frequency signal, wherein the transformation factor of a difference frequency or a difference time of a difference signal between a Pulsrepetiersignal with a pulse repetition frequency and a sampling signal having a sampling frequency is generated and determined, the pulse repetition frequency and / or the sampling frequency is changed by a control with a controlled variable by a corresponding control algorithm / is that the difference frequency to a desired value of the difference frequency or the difference time to a Setpoint of the differential time is controlled, wherein between at least two values of the difference frequency or between at least two values of the difference time depending From the controlled variable a gradient is determined, wherein based on the gradient and the difference frequency or the difference time at the set control variable, an operating point of the control is determined, and accordingly the control algorithm is adjusted. By determining the gradient, it is possible to determine the position of the operating point of the control in the safe and critical control range, taking into account the measured value of the difference frequency or the difference time at the operating point. If the operating point is in a critical control range, the controlled variable is set to a defined state with a work point in the safe control range, and from there the control algorithm is started and executed. If, after determination of the gradient, the operating point has been located in the safe control range, then the control algorithm in the control / evaluation unit changes the controlled variable so that the current operating point approaches the setpoint of the operating point.

In a particularly preferred embodiment of the invention it is provided that the polarization or the sign of the difference frequency of the difference signal or the difference time is determined by means of the gradient. By determining the polarization or the sign of the difference frequency or difference time, it is possible to detect whether the first oscillator or sampling clock oscillator is clocked slower or faster relative to the second oscillator or Sendetaktoszillator. This polarization detection is necessary because only the difference frequency between the sampling frequency with respect to the pulse repetition frequency will be determined by the structure of the control loop, no matter which oscillator is clocked faster or slower to generate the sampling frequency or the pulse repetition frequency. The sampling clock oscillator with the sampling frequency normally has a slightly slower clock cycle than the transmit clock oscillator with the pulse repetition frequency. The case that the sampling clock oscillator has a faster clock cycle or a higher frequency than the transmit clock oscillator is not desired in the transformation process, thus preventing the differential frequency from being adjusted to a desired value in this critical control region.

An expedient embodiment of the invention is that a control characteristic of the difference signal is recorded by the difference frequency or the difference time is determined and stored in dependence on the changing control variable. The control characteristic can be recorded by going through the entire range of the controlled variable once and determining and storing the corresponding values of the difference frequency or difference time. These data are displayed in a diagram, whereby according to the difference time a pole or according to the difference frequency a zero with a reversal point in the diagram becomes recognizable. Because of this pole or zero point, at least two identical values of the difference frequency or the difference time can be determined for two different control variables. One of these values or operating points lies in a critical control range, in which the sampling clock oscillator has a faster clock cycle compared with the transmit clock oscillator. The other operating point is a safe control range in which the sampling clock oscillator has a slower clock cycle than the transmit clock oscillator. Based on the gradient and the diagram can be made a statement about the Einregelungsvorgang. In the diagram, a suitable setpoint of the Determined operating point to which regulates the control with the control algorithm. In addition, the control behavior of the control loop can be monitored, as this may change due to external influences or aging phenomena. Thus, with respect to the predictive maintenance of the device, the control loop is monitored for errors and signs of aging.

According to an advantageous embodiment of the method according to the invention it is provided that the control algorithm resets the operating point in a defined safe control range by the control variable is reset to a minimum value when the operating point is determined in a critical control range of the scheme. The operating point is transferred to a safe control range by the controlled variable, e.g. a control voltage is regulated back to a defined minimum value which lies within the safe control range. Starting from this minimum value, the control algorithm in the control / evaluation unit changes the control variable stepwise, so that the current operating point is constantly approaching the setpoint of the operating point.

A very advantageous variant of the method according to the invention is that
the control algorithm adjusts the operating point to the setpoint of the operating point by continuously changing the controlled variable when the operating point is determined in a defined safe control range of the control. The control algorithm adjusts the current difference frequency to the setpoint of the difference frequency or the current difference time to the setpoint of the difference time by the control variable, eg a control voltage is continuously reduced or increased, depending on whether the current operating point located above or below the setpoint of the operating point becomes.

According to an advantageous embodiment of the method according to the invention, it is provided that a fault is detected by the control algorithm if the operating point is permanently determined in the fault range above a maximum limit value of the difference frequency or below a minimum limit value of the difference time. Since a difference time below this minimum limit value or because of the difference frequency above this maximum limit value can not occur in the closed-loop control, a hardware fault must be present. This determination of a disturbance of the control loop can be displayed as a message on a display of the level measuring device or transmitted via the fieldbus to a control center or other field devices. Furthermore, it is possible that by determining a fault in the control loop, the control / evaluation unit or the entire meter re-initializes and thus brings the control in a defined initial state.

In an advantageous embodiment of the method according to the invention, it is proposed that, given a defined limit value of the deviation of the difference frequency from the reference value of the difference time in the control algorithm, an error counter is incremented, if this limit value is exceeded, or decremented, if this limit is met. If the measured values of the difference time or the difference frequency deviate to some extent from the predetermined setpoint values of the difference time or the predetermined setpoint values of the difference frequency, e.g. ± 1 μs (microsecond) or 1 mHz (milli-hertz), an error counter is activated. The error counter is incremented by "one" if the current reading is outside the set limits and decremented by "one" if the current reading meets the set limits.

According to an advantageous embodiment of the inventive method is provided that a maximum count of the error counter is set, in which the error counter is deleted, the control algorithm is interrupted and the operating point is set in a defined safer control range of the control characteristic by returning the controlled variable to a minimum value. If the error counter generates a so-called overflow, if the current count value exceeds the set maximum count value of, for example, 100, a regulation in a safe control range is restarted by reducing the controlled variable to a minimum value. The maximum count of the error counter determines the fault tolerance of the control system or control loop.

An advantageous embodiment of the method according to the invention is to be seen in that as a control variable up to a first tolerance value, a large control deviation of the difference frequency from the setpoint of the difference frequency or the difference time from the setpoint of the difference time, a pulse width modulated control signal is effected by the control algorithm. The first tolerance value determines in which range of the deviations of the current difference time or the current difference frequency from the setpoint values of which a pulse-width-modulated control signal is used for controlling and activating the frequency regulation unit. The pulse width modulated control signal is suitable for large deviations of the current difference time or the current difference frequency of their setpoints, since large changes in the controlled variable or Control voltage can be achieved while fine adjustment of the difference frequency or time difference to the target value for small deviations is difficult possible. This first tolerance value of a large control deviation is, for example, 5 ms (millisecond).

A supplementary advantageous variant of the method according to the invention is to be seen in that as a controlled variable at a second tolerance value of an average deviation of the difference frequency from the setpoint of the difference frequency or the difference time from the setpoint of the difference time, a pulse-sequence-modulated control signal is generated by the control algorithm. From a second tolerance value, a pulse-sequence-modulated control signal is used for the reasons described above, namely that a pulse-width-modulated control signal is not suitable for fine-tuning the actual value to the setpoint value of the difference time or difference frequency. Depending on their operating frequency, the control / evaluation unit or microcontroller generate short pulses which result in the control signal which is modulated by pulse train. The microcontroller generates maximum short pulses which, depending on the required control variable, drive the frequency control unit as a corresponding pulse train. This second tolerance value of a mean control deviation is for example 1 μs (microsecond).

Furthermore, it is proposed that in the case of a third tolerance value of a small or tendency deviation of the difference frequency from the desired value of the difference frequency or the difference time from the target value of the difference time is not immediately countered with a corresponding control variable, but a tendency of the control deviation is determined and a tendency control signal, which is added to the subsequent pulse train modulated control signals or to the pulse width modulated control signals is generated. If the current deviation of the difference time, for example, in the range of a few microseconds microsecond and / or this corresponds to a tendency error, so is not readjusted immediately, but these minimal changes are included in the subsequent pulse train modulated or pulse width modulated control signals. Since such small control deviations below the third tolerance value can only be corrected very difficult by readjustment or tracking of the controlled variable, the computational integration of these tendency deviations into the larger and medium control deviations creates a possibility for making the control more accurate and stable. An overshoot and a subsequent settling of the control can be avoided.

• 1 ein Ausführungsbeispiel einer Vorrichtung zur Ermittlung des Füllstandes in einem Behälter;
• 2 ein Ausführungsbeispiel des Regelkreis der Vorrichtung zur Erzeugung der Zwischenfrequenz mit der Sende-/Empfangseinheit und Regel-/Auswerteeinheit;
• 3 ein Ausführungsbeispiel der Regelkennlinie mit den sicheren und kritischen Regelbereichen und dem Störungsbereich; und
• 4 ein Ausführungsbeispiel eines schematischen Zustandsdiagramms der Signale in des in 2 dargestellten Regelkreises.

The invention will be explained in more detail with reference to the following drawings. For simplicity, identical parts have been given the same reference numerals in the drawings. It shows:

• 1 an embodiment of a device for determining the level in a container;
• 2 an embodiment of the control circuit of the device for generating the intermediate frequency with the transmitting / receiving unit and control / evaluation unit;
• 3 an embodiment of the control characteristic with the safe and critical control areas and the fault area; and
• 4 an embodiment of a schematic state diagram of the signals in the in 2 illustrated control loop.

In 1 is an embodiment of the device according to the invention 1 to determine the distance d or the level l based on the term t of high-frequency measurement signals S _HF demonstrated. For this purpose, the device 1 especially one with the transmitter 2 connected measuring unit 3 on, by means of the high-frequency measurement signal S _HF or transmission signal S _TX in one the contents 5 comprehensive process space 6 of the container 7 is coupled and sent out. The measuring unit 3 is through an opening, eg a nozzle, into the process area 6 of the container 7 introduced. The measuring unit 3 or the transducer element can, as in 1 shown as an antenna 3a and in particular as a Homantenne, a rod antenna, a parabolic antenna or even a planar antenna, configured to be the transmission signal S _TX in the process room 6 of the container 7 radiates freely Instead of in the process room 6 free radiating antennas 3a can as a measurement unit 3 , like in the 2 also illustrated a surface waveguide 3b in the device 1 used for level measurement, the high-frequency measurement signal S _HF due to the skin effect on its surface leads along.

In the transmitter 2 the device 1 become the transmission signals S _TX in a transmitting / receiving unit 17 generated and sent to the sending unit 2 transmitted. Sent transmission signals S _TX , for example, back to the surface 4 of the contents 5 Be reflected by measurement unit 2 received and returned to the transmitting / receiving unit 17 passed, in which the received reflection signals S _RX be preprocessed. The high-frequency measuring signals S _HF , consisting of transmission signals S _TX and from reflection signals S _RX , for example, in the transmitting / receiving unit 17 via a sampling method or a sequential sampling with two slightly frequency-shifted high-frequency pulse sequences into a time-extended, lower-frequency intermediate frequency signal S _ZF transformed. The mixed down in this way intermediate frequency signal S _ZF can then in the low-frequency range of a control / evaluation 15 be evaluated and the term t or the route x of the emitted high-frequency measurement signal S _HF be determined.

Accordingly, in the guided microwave method, the time domain reflectometry or the time domain reflection (TDR) measuring method, a transmission signal S _TX, for example, along a Sommerfeld or Goubauschen surface waveguide 3b or Koaxialwellenleiters emitted, which at a discontinuity of the DK value (dielectric constant) of the surface waveguide 3b surrounding contents 5 is reflected back. By the contents 5 caused impedance jumps within the process space 6 of the container 7 , especially at the boundary layer 4 between free space and contents 5 in the container 7 , the transmission signal S _{TX is} at least partially reflected back. Due to this, a corresponding, usually weaker reflection signal S _RX in the opposite direction to the surface waveguide 3b to the transmitting / receiving unit 17 runs back.

The from the antenna 3a freely radiated and those on the lossless surface waveguide 3b guided high-frequency measurement signals S _HF are spreading in the free process room 6 of the container 7 in the atmosphere of air or inert gas, approximately at the speed of light c ₀ . These transmission signals S _TX be in the radiation cone of the antenna 3a or near the surface waveguide 3b located surfaces 4 of media with a higher DK value, for example the contents 4 than that of the air or shielding gas is partially or completely reflected back. About the measured running time t the transmitted transmission signal S _TX to the reflected reflection signal S _RX is converted by a conversion over the formula of the wave speed the distance traveled x or distance d determined. This difference distance or this distance d corresponds to the height H of the container 7 minus the level l of the contents 5 in the container 7 , Because the height H of the container 7 or the position of the coupling of the transmission signal S _TX is known, thus can the level l and possibly even the volume, or with knowledge of the density of the medium 5 , the mass of the medium 5 in the container 7 determine.

The communication / supply unit 14 has the task of communicating with a remote control center, for example, or another measuring device or field device via a fieldbus 9 to regulate and the measured values, eg the level l , or configuration data of the device 1 to receive and to send. The fieldbus 9 operates according to the usual communication standards, such as Foundation Fieldbus or Profibus-PA, and is designed, for example, in a conventional two-wire technique in process measurement technology. The supply of the device 1 with energy can be added to the power supply of the device 1 over the fieldbus 9 according to the two-wire standard by means of a separate power supply line 8th respectively. The communication / supply unit 14 can be considered an integral part of the control / evaluation unit 15 be designed.

In 2 is an embodiment of a block diagram of the device 1 shown that the transmitter 2 the device for generating the intermediate frequency S _ZF with a transmitting / receiving unit 17 and control / evaluation unit 15 shows. The transmitting / receiving unit 17 can be fundamental in an RF circuit part 21 with transmit pulse generator 18 , Sampling circuit 19 , and transceiver 23 in which RF signals are basically generated and processed, and a LF circuit part 20 with transmit clock oscillator 13 , Sampling clock oscillator 12 , Frequency converter 11 , in which basically LF signals are generated and processed, be divided. The individual circuit elements in the RF circuit part 21 According to experience, they are constructed in analog circuit technology, ie analog measurement signals are generated and processed. On the other hand, the individual circuit elements in the NF circuit part 20 be constructed either on the basis of digital circuit technology and / or analog circuit technology. From the point of view of the rapid progress of digital signal processing, it is also conceivable to use the RF circuit part 21 to execute with digital circuit elements. There are also the most diverse variations of the individual circuit elements in digital and analog circuit technology conceivable, which are not explicitly carried out here. Therefore, the following description of the embodiment is 2 just to look at as an example of many possible execution options. The communication / supply unit 14 was for the sake of reducing the representation to the essentials not in 2 again explicitly shown.

In the transit time measurement of pulsed high-frequency measurement signals S _HF will be the one to the measuring unit 3 coupled transceiver unit 17 used to generate mutually coherent wave packets of predeterminable pulse shape and pulse width, so-called bursts or short wave packets, and to to process. The pulse shape of a single burst or a single short wave packet usually corresponds to acicular or sinusoidal, half-wave pulses of predeterminable pulse width. However, if necessary, other suitable pulse shapes may be used for these bursts. The transmitting / receiving unit 17 for this purpose comprises one of the transmit clock oscillator 13 triggered transmit pulse generator 18 for generating a as a transmission signal S _TX serving first burst sequence. The pulses of the transmission signal S _TX are at a high frequency f _{HF of} the transmit pulse generator 18 supported, which is approximately in the range between 0.5 and 78 GHz, and moreover with a pulse repetition frequency f _PRF or shot rate triggered, which is set to a frequency range of a few megahertz, in particular a frequency range of 1 MHz to 10 MHz. This pulse repetition frequency f _PRF for driving the transmission pulse generator 18 is from a transmit clock oscillator 13 generated. The high frequency f _HF and / or pulse repetition frequency f _PRF but may also, if necessary, be outside the respective specified frequency ranges.

The at the signal output of the transmission pulse generator 14 applied transmission signal S _TX is by means of a transmitting / receiving switch 23 , in particular by means of a directional coupler or a hybrid coupler, the transmitting / receiving unit 17 in the at a first signal output of the transmitting / receiving switch 23 connected measuring unit 3 , eg antenna 3a or surface waveguide 3b , coupled. Practically at the same time lies the transmission signal S _TX also at the second signal output of the transceiver 23 at. The in the manner described above in the measurement volume 6 of the container 5 generated reflection measurement signals S _RX are, as already mentioned, from the device 1 by means of the measuring unit 3 received again and at the second signal output of the transmitting / receiving switch 23 decoupled. Accordingly, at the second signal output of the transmitting / receiving switch 23 one by means of the transmission signal S _TX and the reflection measurement signal S _RX formed total measuring signal S _TX + S _RX be tapped.

A direct evaluation of the second signal output of the transceiver 8th applied high-frequency total measurement signals S _TX + S _RX , in particular a direct measurement of the term t , is practically no longer possible or only with a high technical complexity, for example by the use of high-frequency electronic components possible. Due to this, the transmitting / receiving unit includes 2 Further, a sampling circuit 19 , which is the high-frequency transmitted total measuring signal S _TX + S _RX temporally in such a way that the high frequency f _HF and the pulse repetition frequency f _{PRF are transformed} into a lower frequency range of a few hundred kilohertz. For temporally stretching the total measurement signal S _TX + S _RX this is one with the second signal output of the transmitting / receiving switch 23 connected first signal input of the sampling circuit 19 fed. Simultaneously with the total measurement signal S _TX + S _RX is at a second signal input of the sampling circuit 19 a sampling signal S _{Sampl of} the sampling oscillator 12 created. A sampling frequency f _Sampl or clock rate at which the strobe signal S is clocked _Sampl is normally set to be slightly smaller than the pulse repetition frequency f _PRF of the transmission signal S _TX , By means of the sampling circuit 19 becomes the total measuring signal S _TX + S _RX to an intermediate frequency signal S _ZF imaged by a transformation factor K _T compared to the total measurement signal S _TX + S _RX time is stretched This sampling circuit 19 due to the frequency offset between the pulse repetition frequency f _PRF and the sampling frequency f _Sampl, the total measurement signal is sampled S _TX + S _RX in each period at different phase, resulting in a time-delayed intermediate frequency signal S _ZF with the transformation factor described above k _T arises. The sampling circuit 19 is, for example, as an RF frequency converter or RF mixer with a sampling pulse generator having the same phase position and frequency of the burst sequence as the transmit pulse generator 18 or a fast sampling switch. As a sampling switch, for example, RF diodes or fast transistors can be used.

The transformation factor k _T , with which the total measuring signal S _TX + S _RX in a lower frequency intermediate frequency signal S _ZF is converted, corresponds to a quotient of the pulse repetition frequency f _PRF of the transmission signal S _TX divided by a difference of the pulse repetition frequency f _{PRF of} the transmission signal S _TX and the sampling frequency f _{Sampl of} the sampling signal S _Sampl . $$k_T=\frac{f_{PRF}}{f_{Diff}}=\frac{f_{PRF}}{f_{PRF}-f_{Sampl}}\stackrel{\wedge}{=}\frac{f_{HF}}{f_{ZF}}$$

An intermediate frequency f _{IF of} the intermediate frequency _signal thus generated S _ZF lies with such devices 1 for determining the level I usually in a frequency range of 50 to 200 kHz; if necessary, the frequency range can be selected higher or lower. Empirically, the intermediate frequency f _{IF is set} to about 160 kHz in the measuring instruments of the applicant. The dependence of the intermediate frequency f _ZF on the ratio of the sampling frequency f _Sampl and the pulse repetition frequency f _PRF As shown in the second equation (equation 2), it can be deduced from the first equation (equation 1). $$f_{ZF}=f_{HF}\cdot \left(1-\frac{f_{Sampl}}{f_{PRF}}\right)$$

If necessary, the intermediate frequency signal S _ZF that compared to the total measuring signals S _TX + S _RX a transformation factor k _T is time-expanded, suitably by a filter / amplifier unit 22 as a filtered intermediate _{frequency signal} S _gefZF amplified and filtered before it in the control / evaluation unit 15 or further evaluation circuits is evaluated as echo curve or envelope.

This difference frequency f _sweep is determined for two reasons; Firstly, this loop is controlled by 24 the instantaneous control and triggering of the sampling clock oscillator 17 and possibly also the transmit clock oscillator 18 through the control / evaluation unit 15 secondly, it is calculated from the quotient of the known or measured pulse repetition frequency f _PRF and the difference frequency f _sweep a transformation factor k _T in the control / evaluation unit 15 determined. In the control / evaluation unit 15 can also already the term t the measurement signals, as well as the level I by echo signal evaluation of the filtered intermediate frequency signal S _ZF and by knowing the transformation factor k _T be determined.

In this embodiment, in the LF circuit part 20 the transmitting / receiving unit 17 by sequentially sampling the pulse repetition frequency f _PRF with the sampling frequency f _Sampl by means of a frequency converter 11 a difference frequency f _sweep between the sampling clock oscillator 13 and the transmit clock oscillator 12 determines this difference frequency f _sweep or their difference time t _sweep is in the control / evaluation unit 15 processed and measured. The difference time t _sweep corresponds to the reciprocal of the difference frequency f _sweep , The control / evaluation unit controls via a feedback branch 15 the controllable sampling clock oscillator 13 according to the determined difference frequency f _sweep or the difference time t _sweep back to. This structure is a loop 24 been created, the difference frequency f _sweep largely at the desired setpoint of the difference _frequency f _Sweep _ _setpoint , eg 21.73913 Hz. Driving the controllable sampling clock oscillator 13 and optionally the Sendetaktoszillators 12 done by a frequency control unit 10 , In voltage-controlled oscillators, eg VCO or oscillator with a parallel variable-frequency capacitance diode, a control voltage V _c as a corresponding controlled variable c_var outputs or the digitally controllable oscillators, eg NCO, a digital control value V _dig as a controlled variable c_var outputs. The frequency control unit 10 is by means of a so-called three-step control by an upward control signal R _Up and a down-control signal R _down from the control / evaluation unit 15 set so that one at the output of the frequency control unit 10 applied control variable c_var the controllable sampling oscillator 12 controls accordingly. The corresponding control of the sampling oscillator 12 takes place in such a way that the generation of the defined desired value of the difference _frequency f _{sweep_setpoint} between the pulse repetition _frequency f _PRF and the sampling frequency f _sample is effected. As a controlled variable c_var For example, either a standard voltage V _C or a digital control value V _dig be used. The frequency control unit 10 is designed, for example, as an RC element or a charge pump, which is the control voltage V _C representing the sampling frequency f _Sampl of the sampling oscillator 12 regulates, stabilizes. With up-level control signals R _Up For example, the charge voltage of the charge pump is increased and with down-control signals R _down decreased. The set controlled variable c_var or control voltage V _c is via a measuring line from the control / evaluation 15 Determined This determination is necessary because normally only the difference _frequency f _{sweep is} determined and for the determination of the gradient the actually generated control variable c_var or control voltage V _C is needed. In the control / evaluation unit 15 is for example a microcontroller 16 integrated, which controls the control and executes the control algorithm.

In 3 is an embodiment of the control characteristic con_char the difference time t _sweep with the safe control range e and critical control areas A . B . C . F , as well as the fault area D shown. On the ordinate of the coordinate system is the difference time t _sweep and on the abscissa of the coordinate system is the control voltage V _C as a controlled variable c_var displayed. As a controlled variable c_var is an analog control voltage in this example V _C have been displayed; regardless of this, every other variable is also valid c_var such as a digital control value V _dig or a mechanical controlled variable applicable. The control characteristic con_char the difference time t _sweep is in 3 shown completely, but it is also possible that only sections of individual properties, such as exceeding the limit and / or the gradient grad of the control characteristic con_char at a current control voltage V _act , be determined. The control characteristic con_char The difference time t _sweep or the properties determined in sections can be generated, for example, by applying to the sampling oscillator 12 out 2 the control voltage V _C or controlled variable c_var is changed continuously and corresponding values of the difference time t _sweep or difference frequency f _sweep from the rule / evaluation 15 be determined and stored. From the determination of these values can be based on the location of the current operating point operating room in the corresponding control areas A . B . C . D . e , F be closed back. Is the working point operating room in a critical control area A . B . C . F or a fault area D , the control algorithm is interrupted and a renewed adjustment in a safe control range e started. The initiation of control in a safe control area e takes place, for example, in that the control voltage V _C to a minimum value of the control voltage V _min is brought and thus steadily increased until a target value of the operating point OP_setpoint or a setpoint of the difference time t _sweep _ _setpoint by the control / evaluation unit 15 is determined. An operating point operating room can not be in the critical control areas A . B lie, because the control voltage V _C According to the invention only between a minimum value of the control voltage V _min and a maximum value of the control voltage V _max can be set. These limiting values of the control voltage V _C were generated because of this difference time t _sweep below a minimum limit of the difference time t _{sweep_min} , eg less than 1 ms (millisecond) lie, and here operating points operating room and gradients Degree can not be determined exactly.

The position of the setpoint of the operating point OP_setpoint is chosen so that adjusting the difference time t _sweep or difference frequency f _sweep by a corresponding control voltage V _C is easily possible. On the other hand, is the position of the setpoint of the difference time t _{Sweep_setpoint} or the setpoint of the difference _frequency f _{Sweep_setpoint selected} so that a slight change in the control voltage V _C a big change in the difference time t _sweep or difference frequency f _sweep caused by the working point operating room at one point of the control characteristic con_char lies where the gradient Degree is very large, so is an exact adjustment to the setpoint of the operating point OP_setpoint only difficult is possible. Due to the gradient Degree is determined with which accuracy the control voltage V _C must be changed so that a certain change in the difference time t _sweep or difference frequency f _sweep can be achieved. As a result, when a large gradient is detected, the control voltage becomes V _C with a higher accuracy by a more exact, slower control algorithm that changes only small changes in voltage. Is the gradient, on the other hand Degree small, becomes the control voltage V _C changed in larger change steps, since this is a change of the difference time t _sweep not so strong.

The control characteristic con_char or the control characteristic of the difference time t _sweep has at a certain Polstellen-control voltage V _{Pol on} a pole. At this pole of the control characteristic con_char are the sampling frequency f _sample the driven sampling oscillator 12 and the pulse repetition frequency f _PRF the pulse repetition oscillator 13 exactly the same. Due to the pole of the control characteristic con_char the difference time t _sweep is it possible to have the same difference time t _sweep by means of a first control voltage V _C1 in the safe control area e and by means of a second control voltage V _C2 in the critical control range F to create. The first control voltage V _C1 in the safe control area e is correspondingly smaller than the Polstellen-control voltage V _pol and the second control voltage V _C2 in the critical control range F is correspondingly larger than the pole position control voltage V _pol designed. Above a maximum limit value of the difference time t _{Sweep_max} , which is above the setpoint of the difference time t _{Sweep_setpoint} is, the control algorithm is interrupted and the control of the difference time t _sweep or the corresponding difference frequency f _sweep in the safe control area e started again. By forming this maximum limit of the difference time t _{Sweep_max} is a fast regulation of the difference time t _sweep to a setpoint of the difference time t _sweep _ _setpoint only possible. The regulation in the critical control range C around the pole, in which the difference time t _sweep high, inappropriate values and thereby causes long measurement and control times, is thus prevented. The contrary case that the control characteristic con_char or the control characteristic of the difference frequency f _sweep - in a sense the reciprocal of the difference time t _sweep - has a zero point with a reversal point, is due to the analogy to the above in 2 not explicitly executed.

In 4 is a state diagram of the signals of the control loop 24 out 2 demonstrated. The state diagram consists of five signal characteristics, all the same time unit or time scale t _s consequences. The highest, first signal characteristic of the supply voltage P shows the timing of the initialization of the microcontroller Int_μC or the start time of the power supply of the microcontroller 16 or the entire control / evaluation unit 15 at. The third and fourth signal characteristic are the control signals R _C for up-regulation R _UP and for down-regulation R _down on, the signals for controlling a frequency control unit 10 show. The fifth signal characteristic represents the control voltage V _C which is output, for example, from the frequency control unit 10 and controls a voltage-controlled sampling clock oscillator 12. The feedback of the control loop 24 or the control of the sampling clock oscillator 12 via a frequency control unit 10 is achieved by the signal characteristics three, four and five. The second signal characteristic of the difference signal S _sweep with the varying difference time t _sweep represents the generated control result of the control loop 24 represents.

By switching on the supply voltage P is the microcontroller 16 and the control / evaluation 15 initialized. It happens that due to past control process phases and the operating state or transient response of the electrical components in this state, a not previously definable current control voltage V _act at the sampling oscillator 12 is applied. For this reason, in the method according to the invention first a work point detection OP_ID performed in a first control _step, a first difference time t _sweep1 with the corresponding current control voltage V _act or fourth control voltage V _C4 determined. Subsequently, in a further control step, a second control voltage V _C2 set and a corresponding second difference time t _Sweep2 is determined. From the quotient of the difference of the first difference time t _Sweep1 and second difference time t _Sweep2 to the difference of the corresponding control voltages V _C4 . V _C2 becomes a gradient Degree or a slope determines the same is also true with generating a first control voltages V _C1 and a third control voltage V _C3 and determining the corresponding difference times t _Sweep1 . t _Sweep2 feasible. By determining the gradient Degree and by determining the crossing or-compliance with the limit value of the difference frequency _sweep t _ _setpoint can be determined very quickly whether the operating point operating room the regulation in a critical control area A . B . C . F or in a safe control area e lies. In 4 is the example 3 been picked up that the current control voltage V _act during the initialization of the microcontroller Init_μC or the initialization of the control / evaluation unit 15 in the critical control range F was localized and this critical control area F corresponding gradient Degree exist. In this case, the control algorithm regulates the current control voltage V _act to a minimum value of the control voltage V _min down, creating a re-lock phase in the safe control area e is started. At the start of this Einregelungsphase is a large deviation of the current difference time t _sweep to the setpoint of the difference time t _sweep _ _setpoint . Why the current difference frequency t _sweep by a plus-wide-modulated control C_PWM with pulse width modulated control signals R _PWM acting as both up-level control signals R _Up as well as down-control signals R _down are designed, changed. The upward control signals R _Up and down control signals R _Down control the frequency control unit 10 on which a corresponding control voltage V _C generated. One at the frequency control unit 10 applied upstream control signal R _Up causes an increase in the control voltage V _C at the output of the frequency control unit 10 , On the other hand causes a at the frequency control unit 10 applied down-regulation signal R _down a drop in the control voltage V _C , Is the deviation of the current difference time t _sweep to the setpoint of the difference time t _{Sweep_setpoint} only small, so is in a pulse train modulated control C_Toggle with pulse train modulated control signals R _toggle changed in the frequency control unit 10 only with short pulse trains of very short pulses is driven. Due to the short pulses of the pulse train modulated control signals R _toggle It is possible that only a slight change in the difference frequency t _sweep can be made. By this procedure, that with small deviations of the difference time t _sweep from the setpoint of the difference time t _{Sweep_setpoint} a pulse train modulated control signal R _toggle is used for fine adjustment, occurs only a low transient response of the scheme. Is the difference time t _sweep adjusted to the setpoint of the difference time t _sweep _ _setpoint , then the setpoint of the operating point OP_setpoint in the safe control area e set. By single short pulses of the down-regulation signal R _down and the up-control signal R _Up With an adjusted differential time t _{sweep can be set} to the setpoint value of the difference time t _{Sweep_setpoint} a tendency to be counteracted. For this purpose, for example, the period is interrupted depending on the tendency of the control deviation, or one or more short pulses are omitted.

LIST OF REFERENCE NUMBERS

1

contraption

2

transmitters

3

measuring unit

3a

antenna

3b

Surface waveguide

4

Surface, boundary layer

5

contents; medium

6

process space

7

container

8th

Power line

9

fieldbus

10

Frequency control unit

11

frequency converter

12

first oscillator, sampling clock oscillator

13

second oscillator, transmit clock oscillator

14

Communication / supply unit

15

Control / evaluation unit

16

microcontroller

17

Transmit / receive unit

18 19

Transmit pulse generator sampling circuit

20

LF-circuit portion

21

RF circuit part

22

Amplifier / filter unit

23

Transmit / receive switch

24

loop

S _HF

high-frequency measurement signal

S _TX

send signal

S _RX

reflection signal

S _ZF

Intermediate frequency signal

S _sweep

difference signal

S _PRF

Pulsreptiersignal

f _PRF

Pulsreptierfrequenz

S _sample

sampling

f _sample

sampling

f _sweep

difference frequency

t _sweep

time difference

t _Sweep1

first difference time

t _Sweep2

second difference time

f _{Sweep_setpoint}

Setpoint of the difference frequency

t _{Sweep_setpoint}

Setpoint of the difference time

t _{Sweep_max}

maximum limit value of the difference time

t _{sweep_min}

minimum limit value of the difference time

P

supply voltage

Init_μC

Initialization of the microcontroller

C_PWM

plus-width modulated control

C_Toggle

pulse sequence modulated control

operating room

working

OP_setpoint

Setpoint of the operating point

OP_ID

Working point detection

R _C

control signals

R _Up

Up-control signals

R _down

Down control signals

R _PWM

pulse width modulated control signal

R _toggle

pulse sequence modulated control signal

c_var

controlled variable

c_var1

first controlled variable

c_var2

second controlled variable

Degree

Gradient; of change; pitch

V _C

control voltage

V _C1

first control voltage

V _C2

second control voltage

V _C3

third control voltage

V _C4

fourth control voltage

V _max

Maximum value of the control voltage

V _min

Minimum value of the control voltage

V _act

current control voltage, actual value of the control voltage

V _set

Setpoint of the control voltage

V _pol

Pole control voltage

V _dig

digital control value

con_char

Control characteristic

e

safe control areas

A, B, C, F

critical control areas

D

Error scope

k _T

transforming factor

d

distance

H

height

l

level

t

running time

t _s

timescale

x

running track

Claims (11)

Method for determining the filling level (l) on the basis of the transit time (t) of a high-frequency measuring signal (S _HF ) which is transformed by means of a transformation process with a specific transformation factor (k _T ) into a lower-frequency intermediate-frequency signal (S _ZF ), - the transformation factor ( k _T ) from a difference frequency (f _sweep ) or a difference time (t _sweep ) of a difference signal (S _sweep ) between a pulse repetition signal (S _PRF ) with a pulse repetition frequency (f _PRF ) and a sampling signal (S _Sampl ) with a sampling frequency (f _Sampl ) is generated and determined, - whereby the pulse repetition frequency (f _PRF ) and / or the sampling frequency (f _Sampl ) are so changed by a control with a controlled variable (c_var) by a corresponding control algorithm that the difference frequency (f _sweep ) on a setpoint of the difference _frequency (f _{sweep_setpoint} ) or the difference time (t _sweep ) to a setpoint of the difference time (t _{sweep_se tpoint} ), wherein a gradient (grad) is determined between at least two values of the difference frequency (f _sweep ) or between at least two values of the difference time (t _sweep ) as a function of the controlled variable (c_var), wherein on the basis of the gradient (grad) and the difference frequency (f _sweep ) or the difference time (t _sweep ) with set control variable (c_var) an operating point (OP) of the control is determined, and accordingly the control algorithm is adjusted. Method according to Claim 1 , wherein the polarization or the sign of the difference frequency (f _sweep ) of the difference signal (S _sweep ) or the difference time (t _sweep ) by means of the gradient (grad) is determined. Method according to Claim 1 in which a control characteristic (con_char) of the difference signal (S _sweep ) is recorded by determining and storing the difference frequency (f _sweep ) or the difference time (t _sweep ) as a function of the changing controlled variable (c_var). Method according to Claim 1 or 3 wherein the control algorithm resets the operating point (OP) to a defined safe control range (E) by resetting the controlled variable (c_var) to a minimum value when the operating point (OP) is in a critical control range (A; B; C; F) the scheme is determined. Method according to Claim 3 or 4 , wherein the control algorithm adjusts the operating point (OP) to the setpoint of the operating point (OP_setpoint) by the control variable (c_var) is continuously changed when the operating point (OP) in a defined safe control range (E) of the control is determined Method according to Claim 3 . 4 or 5 in which a fault is detected by the control algorithm if the operating point (OP) is permanently determined in the fault area (D) above a maximum limit value of the difference frequency (f _sweep ) or below a minimum limit value of the difference time (t _sweep ). Method according to Claim 1 . 3 . 4 . 5 or 6 wherein at a defined limit value of the deviation of the difference _frequency (f _sweep ) from the setpoint of the difference _frequency (f _{sweep_setpoint} ) or the difference time (t _sweep ) from the setpoint of the difference time (t _{sweep_setpoint} ) in the control algorithm, an error counter is incremented, if this limit value is exceeded or decremented if this limit is met. Method according to Claim 7 , wherein a maximum count of the error counter is set, in which the error counter is deleted, the control algorithm is interrupted and the operating point (OP) in the defined safer control ranges (E) of the control characteristic (con_char) by returning the controlled variable (c_var) to a minimum value is offset. Method according to Claim 1 or 7 , wherein as a controlled variable (c_var) up to a first tolerance value a large deviation of the difference frequency (f _sweep ) from the setpoint of the difference _frequency (f _{sweep_setpoint} ) or the difference time (t _sweep ) from the setpoint of the difference time (t _{sweep_setpoint} ) a pulse width modulated control signal (R _PWM ) is effected by the control algorithm. Method according to Claim 1 . 7 or 9 , wherein as a controlled variable (c_var) up to a second tolerance value of a mean deviation of the difference frequency (f _sweep ) from the setpoint of the difference _frequency (f _{sweep_setpoint} ) or the difference time (t _sweep ) from the setpoint of the difference time (t _{sweep_setpoint} ) a pulse train modulated control signal (R _Toggle ) is effected by the control algorithm. Method according to Claim 1 . 7 . 9 or 10 , up to a third tolerance value of a tendency deviation of the difference _frequency (f _sweep ) from the setpoint of the difference _frequency (f _{sweep_setpoint} ) or the difference time (t _sweep ) from the setpoint of the difference time (t _{sweep_setpoint} ) not immediately with a corresponding control variable (c_var) contrary is controlled, but a tendency of the deviations is determined and a tendency control signal (S _drift ), which is added to the pulse train modulated control signal (R _toggle ) or to the pulse width modulated control signal (R _PWM ) is generated.

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