Fill level measuring device

Abstract

The invention relates to a radar-based fill level measuring device (1) for measuring a fill level (L) of a filling material (2) in a container (3), the functionality of a high-frequency unit (12) of the fill level measuring device (1) being able to be checked. The high-frequency unit (12) is designed in a redundant manner and thus comprises two activatable high-frequency sources (121, 121') for generating a high-frequency signal (S HF ) and two activatable receivers (123, 123') for sampling a received signal (E HF ) in order to generate a time-extended evaluation signal (A(t)). A correspondingly designed diagnostic unit (15) is thus able to switch between the active high-frequency sources (121, 121') and / or between the active receivers (123, 123'), wherein a defined property of the evaluation signal (A(t)), such as a signal amplitude, is ascertained before and after the switching. If the defined property of the evaluation signal (A(t)) changes at least by a defined value as a result of the switching, the high-frequency unit (12) is classified as not functioning properly.

Description

TECHNICAL FIELD

[0001] The present application relates to a fill level measuring device whose functionality can be checked. BACKGROUND

[0002] In automation technology, in particular for process automation, field devices are frequently used which serve to detect various measured variables. The measured variables to be determined can be, for example, the fill level of a medium in a process plant, a flow rate, a pressure, a temperature, a pH value, a redox potential, a conductivity or a dielectric value. In order to detect the corresponding measured values, the field devices each comprise a suitable sensor or are based on a suitable measurement principle. The Endress+Hauser Group produces and sells various types of field devices.

[0003] In order to measure the fill level of a filling material in a container, radar-based measurement methods have been established as they are reliable and low-maintenance. One key advantage of radar-based measurement methods is the capability to measure the fill level more or less continuously. In the context of the present patent application, the terms "radar" and "high frequency" refer to radar signals with a frequency between 0.03 GHz and 300 GHz. Typical frequency bands in which fill level measurements are performed are 2 GHz, 26 GHz, 79 GHz or 120 GHz. Two common measurement principles here are the pulsed time-of-flight principle (also referred to by the term "pulse radar") and the FMCW ("Frequency Modulated Continuous Wave") principle.

[0004] In the case of the pulsed time-of-flight method, a high-frequency signal is emitted in the direction of the filling material in a pulsed manner with a defined clock speed by means of a high-frequency source. On the basis thereof, the time-of-flight of the high-frequency pulse reflected at the surface of the filling material before arrival is measured by receiving a corresponding reception signal. In order to determine the time-of-flight or the distance from the filling material more easily on the basis of the high-frequency reception signal in terms of circuitry, an evaluation signal which is time-extended is generated again on the basis of the reception signal. In this case, the evaluation signal is generated by means of subsampling of the reception signal due to the high pulse frequency. The reception high-frequency pulse is sampled by using a corresponding generated scanning pulse, the subsampling taking place in the receiver, wherein the deviation of its sampling rate from the clock speed of the transmitted high-frequency pulse is in the range of the kilo-hertz. In order to ensure that the corresponding target deviation between the sampling rate and the clock speed is adhered to, the sampling rate is readjusted by a control loop depending on the measured actual deviation. In summary, the evaluation signal thus reproduces the signal amplitude of the reception signal or of the reflected high-frequency pulse in a time-extended manner. In this case, the evaluation signal mirrors the signal amplitude of the reception signal depending on the distance of the measurement device from the filling material.

[0005] In contrast to the pulsed time-of-flight method, the FMCW is based on the fact that a high-frequency source continuously transmits a high-frequency signal, but at a modulated frequency. In this case, the frequency of the high-frequency signal to be transmitted is within a fixed frequency band in a standardized center frequency range. By default, the frequency of the FMCW varies linearly over time and assumes a sawtooth or triangular shape. However, in principle, a sinusoidal variation can also be used. In this case, when implementing the FMCW method, the distance from the filling material or the filling level is determined on the basis of the instantaneous frequency difference between the currently received signal and the currently transmitted high-frequency signal, since the receiver in turn generates an evaluation signal of the time extension by mixing the corresponding electrical HF signals. In this case, the distance can be ascertained on the basis of the frequency of the evaluation signal, since the frequency of the evaluation signal inherently varies proportionally to the distance.

[0006] On the basis of the pulsed time-of-flight method and the FMCW method, it is possible to implement a filling level measuring device with relatively low circuit complexity and a high filling level resolution in the sub-millimeter range. The radar-based measuring principle is described in more detail, for example, in "Radar Level Detection, Peter Devine, 2000".

[0007] In addition to the free-space radar measurement via the antenna for transmitting or receiving the high-frequency signal, there is also a variant of the guided radar. In this case, a conductive measuring probe, for example a coaxial cable or a metal rod, which is lowered into the container in order to guide the high-frequency signal, is used instead of the antenna as a transmission unit. Similar to the free-space radar, the high-frequency signal in the measuring probe is reflected at the level of the filling material surface and guided back along the measuring probe towards the filling level measuring device. This variant of the radar-based filling level measurement is also referred to by the term "TDR" (Time Domain Reflectometry). The advantage of this variant is that, due to the guided signal radiation, less power is required to operate the filling level measuring device. Similar to the free-space radar device according to the pulsed time-of-flight or FMCW principle, an evaluation signal for ascertaining the filling level is also generated when implementing the TDR principle.

[0008] Regardless of the implemented measuring principle, for safety-relevant fill level measuring devices it is required that different functional units of the device can be monitored so that possible malfunctions of each unit can be detected sufficiently safely. For example, according to the IEC 61508 standard series, corresponding safety specifications are defined, for example, as "safety integrity level x (SIL x)". If a fill level measuring device cannot or can only partially comply with such safety specifications, it is considered to be unsafe and can only be operated, if at all, with a suitably short test cycle. However, such a test cycle is complex and therefore undesirable in ongoing production processes. However, for high-frequency units comprising a high-frequency source and a receiver, the ability to check the correction function is difficult, since faults in the analog range cannot usually be clearly identified as defects. This is related to, for example, creepage changes, which affect the measuring accuracy and are therefore safety-relevant. SUMMARY

[0009] The present application is therefore based on the object of providing a fill level measuring device which is able to check the function of its high-frequency unit.

[0010] The present application achieves this object by a fill level measuring device for measuring the fill level of a filling material in a container, which operates according to a radar-based measuring principle and comprises at least the following units:

[0011] - a transmission unit by means of which a high-frequency signal can be transmitted to the filling material and can be received as a reception signal after reflection at the filling material surface,

[0012] - a high-frequency unit having

[0013] o two actively switchable high-frequency sources, each of which is configured to generate a high-frequency signal,

[0014] o two actively switchable receivers by means of which the reception signal can be time-stretched in each case,

[0015] - a control unit designed to

[0016] o in each case, one of the high-frequency sources is actively switched so that the active high-frequency source generates a high-frequency signal, and

[0017] o in each case, one of the receivers is actively switched so that the active receiver converts the reception signal into a time-stretched evaluation signal,

[0018] - an evaluation unit designed to:

[0019] o determining the filling level on the basis of the signal transit times of the signal maxima, and

[0020] o determining the filling level on the basis of the signal transit times of the signal maxima, and

[0021] - a diagnostic unit which is designed to

[0022] o controlling the control unit such that the high-frequency source active in each case and / or the receiver active in each case is switched to inactive and vice versa,

[0023] o controlling the evaluation unit such that the defined property of the evaluation curve is ascertained before and after the switching in each case, and

[0024] o classifying the high-frequency unit as malfunctioning if the defined property of the evaluation signal changes at least by a defined value as a result of the switching.

[0025] The application is therefore based on the idea of designing all essential components of the high-frequency unit in a redundant manner, wherein the functionality of the high-frequency unit is checked by a switching of at least one of the redundant components, i.e. by switching off the component active before this point and activating the corresponding component inactive before this point. In this case, the check is carried out by checking the change in the evaluation signal caused by the switching in the evaluation signal. Accordingly, if the diagnostic unit classifies the high-frequency unit as malfunctioning, the diagnostic unit of the level measuring device is able to generate a corresponding error signal, for example, depending on the field of application. Alternatively or additionally, it is also conceivable for the diagnostic unit to generate a corresponding "OK signal" as long as the high-frequency unit is classified as functioning.

[0026] In the context of the application, the term "unit" refers in principle to any electronic circuit designed in a manner suitable for the intended purpose. It can therefore be an analog circuit for generating or processing a corresponding analog signal, depending on the requirements. However, it can also be a digital circuit, for example an FPGA, or a storage medium which interacts with a program. In this case, the program is designed to carry out the corresponding method steps or to apply the necessary calculation operations of the respective unit. In this context, the various electronic units of the measuring device can also potentially access a common physical memory or be physically operated by means of the same digital circuit in the sense of the application.

[0027] In the context of the application, several properties are possible in principle as the defined property of the evaluation signal which is checked during the switching; in this respect, the diagnostic unit can be designed to compare, for example, the edge steepness of one of the signal maxima, the signal amplitude and / or the corresponding signal transit time as the defined property of the evaluation signal.

[0028] If the fill level measuring device is based on the TDR principle and the transmission unit is correspondingly designed as an electrically conductive measuring probe which, in the installed state of the fill level measuring device, extends approximately perpendicularly to the container base, then, in addition, advantageously, one of the two high-frequency sources can be designed in an inverting manner and / or one of the two receivers can be designed in an inverting manner. Thus, when the high-frequency unit is functioning properly, the polarity of the evaluation signal changes if the component active in each case is switched. This reduces the emission of the fill level measuring device, as described, for example, in the publication WO 2005 / 062002 Al. In this case, the evaluation unit is preferably designed such that, if the polarity of the evaluation signal changes as a result of the switching, the evaluation unit inverts the polarity of the evaluation signal again and the evaluation unit ascertains the defined property on the basis of the evaluation signal which can be inverted back in terms of polarity.

[0029] In addition, the inverting design of one of the receivers or one of the high-frequency sources can be used to check the functioning of the high-frequency unit; for this purpose, the evaluation unit must be designed to ascertain the polarity of the evaluation signal. In addition, a diagnostic unit must be designed to control this evaluation unit such that, before and after the switching of the high-frequency source active in each case or of the receiver active in each case, its polarity is ascertained as the defined property of the evaluation signal. Thus, if the polarity of the evaluation signal does not change as a result of the switching, the diagnostic unit can classify the high-frequency unit as not functioning properly.

[0030] In principle, it is irrelevant in the sense of the present application whether the fill level measuring device is based on the FMCW or on the pulsed time-of-flight principle. In the case of the pulsed time-of-flight principle, the receivers can be designed, for example, as samplers. In order to implement the pulsed time-of-flight principle, the control unit must also be correspondingly designed in order to actuate the high-frequency source active in each case such that the high-frequency signal is generated in a pulsed shape in accordance with the pulsed time-of-flight method. In addition, in the case of the pulsed time-of-flight method, the evaluation unit controls the sampler active in each case such that the received signal is subsampled in accordance with the pulsed time-of-flight method such that the evaluation signal is time-discretized.

[0031] In the case of the FMCW and in the case of the pulsed time-of-flight method, the high-frequency source can be designed as a high-frequency oscillator, wherein, at least in the case of the FMCW, its frequency can be controlled by means of, for example, a "phase-locked loop (PLL)". In the case of the TDR, it is sufficient for the high-frequency source to be designed as a capacitor. In the case of the FMCW method, the receivers can be designed, for example, as mixers in order to mix the incoming received signal with the high-frequency signal to be emitted, thereby obtaining a time-extended evaluation signal in a manner corresponding to the pulsed time-of-flight method.

[0032] Similar to the fill level measuring device according to the application, the object of the application is also achieved by a corresponding measuring method for checking the functionality of a fill level measuring device. In this case, the method comprises the following method steps:

[0033] - generating a high-frequency signal by means of a high-frequency source which is active in each case by switching,

[0034] - transmitting the high-frequency signal to the filling material and receiving a corresponding reception signal after reflection at the surface of the filling material,

[0035] - receiving the reception signal by means of a receiver which is active in each case by switching, so that an evaluation signal which is time-extended is generated,

[0036] - ascertaining a defined property of the evaluation signal,

[0037] - switching the high-frequency source which is active in each case to inactive and vice versa, and / or switching the receiver which is active in each case to inactive and vice versa.

[0038] Subsequently, the preceding method steps are repeated, in addition to the switching of the active high-frequency source or the active receiver. The repetition of the method steps makes it possible to finally

[0039] - compare the defined property before and after the switching, and

[0040] - classify the high-frequency unit as functionally abnormal if the defined property of the evaluation signal has changed by a defined minimum value as a result of the switching.

[0041] In this case, the method can be automated, for example, the switching of the high-frequency source which is active in each case or the receiver which is active in each case, the comparison of the defined property before and after the switching and the classification of the high-frequency unit in terms of its functionality are carried out cyclically in a measuring operation. BRIEF DESCRIPTION OF DRAWINGS

[0042] The application is explained in more detail with reference to the following drawings, in which:

[0043] Figure 1 a fill level measuring device based on TDR according to the prior art is shown;

[0044] Figure 2 is a schematic diagram of an evaluation signal;

[0045] Figure 3 is a block diagram of a fill level measuring device according to the application; and

[0046] Figure 4 shows the possible influence of a defective high-frequency unit on the evaluation curve. DETAILED DESCRIPTION

[0047] To gain a basic understanding of this invention, Figure 1 A block diagram of a fill level measuring device 1' constructed according to the prior art is shown. This fill level measuring device 1' is used to measure the fill level L of fill material 2 located in a container 3. The fill level measuring device 1' shown is based on the pulse transit time principle, wherein, according to the TDR method, it includes a measuring probe as a transmission unit 13. To determine the fill level L, the measuring probe 13 extends accordingly from the top side inside the container to just above the container base. In this case, the mounting height h of the measuring probe 13 above the container base is known and stored in the evaluation unit 14 of the fill level measuring device 1'.

[0048] According to the pulse transit time method, the measuring probe 13 accordingly transmits a high-frequency signal S in a pulse manner in the direction of the filling material 2. HF Due to the abrupt change in dielectric value there, the high-frequency signal S... HF The signal is reflected at the horizontal level of the filling material surface 2 in the measuring probe 13, and accordingly becomes the received signal E after the corresponding signal transit time t. HF It is received in the fill level measuring device 1'. In this case, signal S HF E HF The signal transit time depends on the distance d = hL between the top of the container and the surface of the filling material.

[0049] In order to generate a high-frequency signal S HF The filling level measuring device 1' includes a first high-frequency source 121 as a component of the high-frequency unit 12. In this case, the first high-frequency source 121 can be designed, for example, as a capacitor according to the TDR method, which discharges accordingly to generate a pulse lasting from 100 ps to approximately 1 ns. In the case of free radiation radar according to the pulse transit time or FMCW method, the first high-frequency source 121 can be designed, for example, as a frequency-controlled high-frequency oscillation circuit or a crystal oscillator. To enable the first high-frequency source 121 to generate a high-frequency signal S at the desired cycle rate according to the TDR method... HF The first high-frequency source 121 is actuated by a control unit 11 outside the first high-frequency unit 12 in a corresponding clock manner. In this case, the first high-frequency source 121 guides the high-frequency signal S. HF Transmission is sent to the measurement probe 13 via transceiver switch 122. In this case, the design of transceiver switch 122 is not strictly defined. In the case of TDR, as... Figure 1 In the variant shown, the transceiver switch 122 can be designed simply as, for example, an electrical node. Especially in the case of free-radiation radar, the transceiver switch 122 can be implemented as, for example, a duplexer.

[0050] The reception signal E HF is guided via the transceiver switch 122 to the first receiver 123. In the first receiver 123, the reception signal E HF is sampled twice in accordance with the pulse transit time principle, so that an evaluation signal A(t) is generated which reproduces the reception signal E HF in time with a defined factor. In this case, the time extension factor depends on the sampling rate. In order to achieve sufficient time extension, the corresponding sampling rate must be selected such that it differs from the clock rate of the transmission signal pulse S HF only in the range of a few percent. Correspondingly, the sampling rate with which the first receiver 123 samples the reception signal E HF is again set at the first receiver 123 by the control unit 11 which also specifies the clock rate of the transmission signal pulse S HF . From a circuitry point of view, the time extension simplifies the determination of the filling level L based on the reception signal E HF . In contrast to the variant shown in Figure 2 , in the case of the free-space radar, in addition to the time extension, the reception signal E HF is rectified in the first receiver 123, so that the evaluation signal A(t) has only one polarity - positive or negative - with respect to a fixed reference potential.

[0051] In order to determine the filling level L, the first receiver 123 transmits the evaluation signal A(t) to the evaluation unit 14. If the first receiver 123 is a digital sampler, this already occurs in digitized form. In this case, reference Figure 2 is made to the diagram shown in Fig. 3, which illustrates in more detail the determination of the filling level L by means of the evaluation signal A(t) by the evaluation unit 14.

[0052] Figure 2 Fig. 4 illustrates the time amplitude development of the reception signal E HF or the time-extended evaluation signal A(t). In this case, the distance d between the container top and the filling material surface is proportional to the time axis of the evaluation signal A(t) and to the time axis of the reception signal E HF . In the ideal case, i.e. without the influence of any external disturbances, the reception signal E HF comprises three signal maxima M a . The first signal maximum M a in time is due to the internal reflection of the high-frequency signal S HF at the transceiver switch 122. The second signal maximum M HF in time in the reception signal E a is generated at the surface of the filling material 2, while the third signal maximum Ma is caused by the probe tip 131 of the measuring probe 13.

[0053] Using an arbitrary filtering method, the evaluation unit 14 is able to ascertain the signal maximum M a of the signal transit time t M . Based on the corresponding calibration data, the evaluation unit 14 calculates the corresponding distance d from the signal transit time t M and, due to the relationship L = h - d or a known installation height h, is able to determine the filling level L again based on the distance d

[0054] A central prerequisite for the evaluation unit 14 to be able to determine the filling level L correctly and accurately is the error-free functioning of the high-frequency unit 12, since depending on a functional impairment, this does not lead to a noticeable failure of the high-frequency unit 12. As Figure 4a indicated, a malfunctioning high-frequency unit 12 can lead to a creeping shift of the reception signal E HF or a shift of the evaluation signal A(t) for example with increasing use time. As a result, the signal transit time t A of the filling level maximum M M may not be noticed and thus the ascertained distance value d is distorted. Figure 4b Again, the result of a gradual degradation of the high-frequency amplifier of the high-frequency unit 12 is illustrated; by this failure mechanism, the amplitude of the evaluation signal S HF or of the underlying reception signal E HF can be damped, so that the evaluation unit 14 in doubt does not identify the signal maximum M A based on which the filling material surface is based, but instead erroneously uses a different signal maximum M A to ascertain the distance d or the filling level L. This malfunction of the high-frequency unit 12 cannot be directly recognized from the outside either. However, a sudden failure of one of these units is conceivable and can be identified depending on the case. Thus, the filling level measuring device 1' cannot be used in applications in which corresponding safety requirements such as "SIL" must be adhered to.

[0055] Therefore, a possible embodiment of the filling level measuring device 1 according to the application is described in more detail in Figure 3 , by means of which a possible malfunction of the high-frequency unit 12 can be diagnosed; in principle, Figure 3 the design and operating mode of the filling level measuring device 1 illustrated in Figure 1The variation shown. However, in addition, the high-frequency unit 12 of the fill level measuring device 1 according to the invention includes a second high-frequency source 121' and a second receiver 123'. In this case, it is advantageous if the second high-frequency source 121' is designed in the same manner as the first high-frequency source 121—for example, also as a capacitor. The same applies to the second receiver 123' as to the first receiver 123.

[0056] In the case of free radiation radar, that is, with Figure 3 The variant shown is the opposite; for high-frequency sources 121 and 121', if they generate high-frequency signals S at the same frequency... HF This is advantageous to ensure consistent signal behavior.

[0057] The control unit 11 can selectively activate one of the two high-frequency sources 121, 121' by means of a first switch 120. By means of a second switch 124, the control unit 11 can again selectively activate one of the two receivers 123, 123'. In this context, switches 120, 124 can be designed, for example, as transistors, whose gates / bases are actuated by the control unit 11.

[0058] Regarding high-frequency sources 121 and 121', the term "activation" refers to turning on the high-frequency source 121 or 121' to be activated, and connecting the high-frequency source 121 or 121' to the measurement probe 13 or transceiver switch 122. Regarding the term "switching," this means that the high-frequency source 121 or 121' to be switched to inactivity is disconnected from the measurement probe 13 and / or turned off. Regarding the two receivers 123 and 123', the term "activation" refers to the activation of the received signal E. HF Sampling is performed, and the corresponding evaluation signal A(t) is transmitted to evaluation unit 14. In conjunction with the term "switching," this means that after activating another receiver 123, 123', the receiver 123, 123' to be switched to inactive will no longer receive signal E. HF Sampling and / or no longer transmitting evaluation signal A(t) to evaluation unit 14.

[0059] exist Figure 3 In the variant of the fill level measuring device 1 according to the invention shown, the control unit 11 also provides, in principle, the clock rates of the two high-frequency sources 121, 121'. This also applies to the two receivers 123, 123', whose sampling rates are predefined by the control unit 11.

[0060] By means of the redundant design of the high-frequency unit 12 with two high-frequency sources 121, 121' and two receivers 123, 123', the control unit 14 can cyclically switch the first switch 120 or the second switch 124, for example, during a measurement operation or during a test operation of the fill level measuring device 1. In this case, the switching of the two switches 120, 124 can occur simultaneously or in an offset manner to each other and optionally cyclically. In this case, before each switching and after each switching, at least one evaluation curve A(t) is recorded, wherein the control unit 14 determines the previously defined properties, such as the polarity of one of the signal maxima M A , the signal amplitude and / or the corresponding signal transit time t M .

[0061] The switching, the recording of the corresponding evaluation curves A(t) and the corresponding determination of the defined properties before and after the switching are coordinated by means of a correspondingly designed diagnostic unit 15 external to the high-frequency unit 12. Furthermore, the diagnostic unit 15 compares the defined properties before the switching with the corresponding variables after the switching. If the properties change beyond a limit value, the diagnostic unit 15 classifies the high-frequency unit 12 as not functioning properly and optionally outputs a corresponding error signal. If the defined properties are the signal amplitudes of one of the signal maxima M A , a decrease in the amplitude beyond a limit value due to the switching can be interpreted as, for example, a creep degradation of the high-frequency amplifier of the high-frequency unit 12, as shown in Figure 4b .

[0062] In the embodiment of the fill level measuring device 1 according to the application shown in Figure 3 , the second high-frequency source 121' and the second receiver 123' are designed to be inverting. Since the fill level measuring device 1 is based on the TDR method, this provides the possibility of inverting the polarity of the received signal E HF or the evaluation signal A(t) when switching the active receiver 123, 123' without switching the active high-frequency source 121, 121', or vice versa. This not only reduces the emission of the fill level measuring device 1, but can also be used again to check the functionality of the high-frequency unit 12; if the diagnostic unit 15 does not detect a change in polarity, despite the active high-frequency source 121, 121' or the active receiver 123, 123', the high-frequency unit 12 will be classified as not functioning properly.

[0063] It goes without saying that, before and after the switching, the diagnostic unit 15 in principle checks not only one property of the evaluation signal A(t) but also a plurality of properties, wherein the diagnostic unit 15 has classified the high-frequency unit 12 as not functioning properly in this case if one of the defined properties of the evaluation signal A(t) has changed at least a defined value as a result of the switching.

[0064] Figure 3 The embodiment of the level measuring apparatus 1 according to the application shown in the middle is based on the pulse transit time principle and comprises, according to the TDR method, the measuring probe 13 as a transmission unit. In this connection, it should be noted that, according to the application, the redundant design of the high-frequency unit 12 with two high-frequency sources and two receivers and the corresponding checking of the function can in principle also be implemented in the case of a free- running radar or when the FMCW principle is implemented.

[0065] List of reference signs

[0066] 1, 1' filling level measuring apparatus

[0067] 2 filling material

[0068] 3 container

[0069] 11 control unit

[0070] 12 high-frequency unit

[0071] 13 transmission unit

[0072] 14 evaluation unit

[0073] 120 first switch

[0074] 121, 121' high-frequency source

[0075] 122 transceiver switch

[0076] 123, 123' receiver

[0077] 124 second switch

[0078] A(t) evaluation signal

[0079] d distance

[0080] E HF received signal

[0081] h installation height

[0082] L filling level

[0083] M a signal maximum

[0084] S HFhigh frequency signal

[0085] t signal transit time

Claims

1. Radar-based fill level measuring device for measuring a fill level (L) of a filling material (2) in a container (3), comprising: - an electrically conductive measuring probe, which in the installed state of the fill level measuring device (1) extends substantially perpendicularly to the container base, by means of which an high-frequency signal (S HF ) can be transmitted to the filling material (2) and, after reflection on the filling material surface, the high-frequency signal (S HF ) can be received as a reception signal (E HF ); - a high-frequency unit (12), which comprises o two activable high-frequency sources (121, 121'), each of which is designed to generate the high-frequency signal (S HF ) o two activatable receivers (123, 123'), by means of which the received signal (E HF ) can be time-extended in each case; o wherein one of the two high-frequency sources (121, 121') is designed in an inverter fashion, and / or wherein one of the two receivers (123, 123') is designed in an inverter fashion; - a control unit (11), which is designed to o in each case activating one of the high-frequency sources (121, 121'), such that the activated high-frequency source (121, 121') generates the high-frequency signal (S HF ) and the other high-frequency sources (121, 121') are deactivated; and o activating one of the receivers (123, 123'), so that the activated receiver (123, 123') converts the received signal (E HF ) into a time-extended evaluation signal (A(t)); - an evaluation unit (14), which is designed to Based on the evaluation signal (A(t)), at least one signal maximum value (M) is identified. A The properties defined by ) and signal transit time (t) M );as well as o determining the fill level (L) based on the signal transit time (t A ) of the signal maximum (M M ); and - a diagnostic unit (15), which is designed to o controlling the control unit (11) such that in each case the high-frequency source (121, 121') active and / or in each case the receiver (123, 123') active is deactivated, and vice versa, to invert the polarity of the evaluation signal (A(t)) or the received signal (E HF ) o control the evaluation unit (14) in such a way that in each case at least a defined property of the evaluation signal (A(t)) is ascertained before and after a switching-over; o classify the high-frequency unit (12) as not functioning properly, if the defined property of the evaluation signal (A(t)) has changed at least by a defined value as a result of the switching-over.

2. The fill level measuring apparatus according to claim 1, wherein The diagnostic unit (15) is designed to compare the edge steepness, the signal amplitude and / or the corresponding signal transit time (t M ) of the at least one signal maximum (M A ) as the defined property of the evaluation signal (A(t)).

3. The fill level measuring apparatus according to claim 1 or 2, wherein The evaluation unit (14) is designed to o invert the polarity of the evaluation signal (A(t)) if the polarity has changed as a result of the switching-over; and o ascertain the defined property on the basis of the polarity-inverted evaluation signal (A(t)).

4. The fill level measuring apparatus according to claim 1 or 2, wherein The evaluation unit (14) is designed to ascertain the polarity of the evaluation signal (A(t)), wherein the diagnostic unit (15) is designed to o control the evaluation unit (14) in such a way that in each case the polarity of the evaluation signal (A(t)) is ascertained as the defined property of the evaluation signal (A(t)) before and after a switching-over of the active high-frequency source (121, 121') or of the active receiver (123, 123'); and o classify the high-frequency unit (12) as not functioning properly, if the polarity of the evaluation signal (A(t)) has not changed as a result of the switching-over, or if the polarity of the evaluation signal (A(t)) has changed as a result of the switching-over.

5. The fill level measuring apparatus according to claim 1 or 2, wherein The receivers (123, 123') are designed as samplers, and wherein the control unit (11) is designed to - actuating the high-frequency source (121, 121') active in each case such that the high-frequency signal (S HF ) is generated in a pulsed manner; and - actuating the samplers (123, 123') active in each case, such that the received signal (E HF ) is subsampled according to the pulse transit time method, thereby time-discretizing the evaluation signal (A(t)).

6. The fill level measuring apparatus according to claim 1 or 2, wherein The diagnostic unit (15) is designed to generate an error signal, if it classifies the high-frequency unit (12) as not functioning properly.

7. Method for checking the functionality of a fill level measuring device (1) according to any one of claims 1 to 6, comprising the following method steps: - generating a high-frequency signal (S HF ) by means of a high-frequency source (121, 121') which is switched active in each case; - sending said high frequency signal (S HF ) to the filling material (2) and receiving a corresponding reception signal (E HF ) after reflection on the filling material surface; - receiving the reception signal (E HF ) by means of a receiver (123, 123') which is switched active in each case, such that a time-extended evaluation signal (A(t)) is generated; - determining at least one signal maximum (M A ) of the evaluation signal (A(t)); - determining at least one defined property of the at least one signal maximum (M A ,M A ) - switching the active high-frequency source (121, 121') in each case into the inactive state and vice versa, and / or switching the active receiver (123, 123') in each case into the inactive state and vice versa, and subsequently repeating the preceding method steps; - comparing the defined property before and after the switching-over; and - classifying the high-frequency unit (12) as not functioning properly, if the defined property of the evaluation signal (A(t)) has changed at least by a defined value as a result of the switching-over.

8. Method according to claim 7, wherein the defined property is the polarity of the evaluation signal (A(t)). - switching the high frequency source (121, 121') active in each case, or switching the receiver (123, 123') active in each case, - comparing the defined property before and after the switching, and - the classification of the high frequency unit (12) with respect to its functionality is performed cyclically.

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