The invention relates to a fill-level measuring device suitable for high-temperature applications.
In automation technology, especially for process automation, field devices are often used, which serve to detect various measured variables. The measured variable to be determined may, for example, be a fill level, a flow, a pressure, the temperature, the pH value, the redox potential, a conductivity, or the dielectric value of a medium in a process plant. In order to detect the corresponding measured values, the field devices each comprise suitable sensors or are based on suitable measuring principles. A variety of such types of field devices is produced and marketed by the Endress+Hauser group of companies.
For measuring the fill level of filling materials in containers, radar-based measuring methods have become established since they are robust and require minimum maintenance. Within the scope of the invention, the term “container” is also understood to mean containers that are not closed, such as basins, lakes, or flowing bodies of water. A key advantage of radar-based measuring methods lies in the ability to measure the fill level quasi-continuously. In the context of this patent application, the term “radar” refers to radar signals having frequencies between 0.03 GHz and 300 GHz. Typical frequency bands at which fill-level measurement or distance measurement is generally performed are 2 GHz, 26 GHz, 79 GHz, or 120 GHz. The two common measuring principles here are the pulse time-of-flight principle (also known under the term “pulse radar”) and the FMCW principle (“frequency-modulated continuous wave”). A fill-level measuring device which operates according to the pulse time-of-flight method is described, for example, in published patent application DE 10 2012 104 858 A1. For a typical construction of FMCW-based fill-level measuring devices, reference is made by way of example to published patent application DE 10 2013 108 490 A1.
In the case of both measuring principles, the fill level or the distance to the filling material is measured cyclically at a defined measurement rate. The measuring principles of FMCW and pulse radar are described in greater detail in “Radar Level Detection, Peter Devine, 2000,” for example.
Fill-level measuring devices are generally used in process environments in which the outside temperature or room temperature prevails. Specifically in the case of hygienically sensitive process plants, such as in the food industry, temporary cleaning cycles are, however, often performed in corresponding process containers at temperatures of more than 130° C., which is close to the junction temperature of semiconductor-based circuits of about 150° C. For this reason, the corresponding semiconductor circuit and thus the fill-level measuring device do not work properly above this temperature. Due to the resulting low temperature reserve, the fill-level measuring device can therefore not be used at least during and after the cleaning cycles.
The invention is therefore based on the object of providing a fill-level measuring device that can also be used at an increased ambient temperature.
The invention achieves this object by means of a radar-based fill-level measuring device for measuring a fill level of a filling material located in a container. For this purpose, the fill-level measuring device comprises at least:
According to the invention, the signal-generating unit and the evaluation unit are designed to control the measurement rate as a function of the measured temperature in such a way that the measurement rate is reduced, especially, linearly or stepwise, as the temperature increases, at least above a defined limit temperature of, for example, 100° C. Consequently, in spite of their proximity to cleaning processes, primarily components that heat up significantly in the signal-generating unit and the evaluation unit, such as any amplifiers, only heat up to a limited extent. As a result, the components of the fill-level measuring device reaching the junction temperature can be flexibly prevented. By the control according to the invention, the fill-level measuring device can thus be used at a correspondingly low measurement rate even during any cleaning cycles if appropriate.
With reference to the fill-level measuring device, the term “unit” within the scope of the invention is understood in principle to mean any electronic circuit that is suitably designed for the respective intended purpose. Depending on the requirement, it can therefore be an analog circuit for generating or processing corresponding analog signals. However, it can also be a digital circuit, such as a microcontroller or a storage medium in interaction with a program. In this case, the program is designed to perform the corresponding method steps or to apply the necessary calculation operations of the respective unit. In this context, various electronic units of the fill-level measuring device in the sense of the invention can potentially also access a common physical memory or be operated by means of the same physical digital circuit.
The fill-level measuring device according to the invention can be developed by designing the signal-generating unit and the evaluation unit to completely stop the fill-level measurement if the measured temperature exceeds a predefined maximum value, especially, the junction temperature of 150° C. This is advantageous especially if the signal-generating unit or the evaluation unit is designed as a monolithic component of an ASIC. If the signal-generating unit comprises a transmission amplifier for amplifying the high-frequency signal, it is also possible to arrange the temperature sensor directly on the transmission amplifier since such an amplifier heats up by far the most in practice and thus represents the hottest location, at least in the case of an ASIC-based design. In addition, the transmission amplifier is normally arranged close to the antenna in relation to most of the other components of the fill-level measuring device and thus very close to any cleaning cycles.
Within the scope of the invention, the fill-level measuring device can be designed both according to the pulse time-of-flight method and according to the FMCW method: In the case of the pulse time-of-flight method, the signal-generating unit is to be designed such that the high-frequency electrical signal is generated according to the pulse time-of-flight method. The evaluation unit is correspondingly designed to determine the fill level according to the pulse time-of-flight method on the basis of the sampled receive signal.
When implementing the FMCW method, the signal-generating unit is to be correspondingly designed to generate the high-frequency electrical signal according to the FMCW method, or the evaluation unit is to be designed such that the fill level is determined according to the FMCW method by mixing the high-frequency signal and the receive signal.
In correspondence with the fill-level measuring device according to the invention, the object on which the invention is based is also achieved by a method for operating a measuring device according to one of the previously described embodiment variants. Accordingly, the method comprises at least the following method steps:
In this case, the measurement rate is controlled as a function of the measured temperature in such a way that, at least above a defined limit temperature, the measurement rate is reduced as the temperature increases.
The invention is explained in more detail with reference to the following figures. The following is shown:
For a basic understanding of the distance measurement according to the invention,
As a rule, the fill-level measuring device 1 is connected via a bus system, such as “Ethernet,” “PROFIBUS,” “HART,” or “Wireless HART,” to a higher-level unit 4, such as a process control system or a decentralized database. On the one hand, information about the operating status of the fill-level measuring device 1 can thus be communicated. On the other hand, information about the fill level L can also be transmitted via the bus system in order to control any inflows or outflows that may be present at the container 3.
Since the fill-level measuring device 1 shown in
The radar signal SHF is reflected at the surface of the filling material 3 and, after a corresponding signal time-of-flight, is correspondingly received as an electrical receive signal eHF by the transmitting/receiving antenna 12. The signal time-of-flight of the radar signal SHF, EHF depends on the distance d=h−L of the fill-level measuring device 1 from the filling material surface.
In contrast to the embodiment variant shown, it is also possible for two separate antennas to be used for separate transmission and reception of the radar signal SHF, EHF, instead of a single transmitting/receiving antenna 12. Another alternative consists of using an electrically conductive probe, such as a waveguide or a coaxial cable, which extends toward the container bottom. This embodiment variant is known as TDR (“time-domain reflectometry”).
The basic circuit design of a fill-level measuring device 1 operating according to the FMCW method is illustrated in
As shown in
With the ramp-shaped frequency change according to the FMCW principle, the frequency of the high-frequency signal sHF increases in a periodically repeating manner within a predefined frequency band Δf at a constant rate of change. The periodicity of the individual frequency ramps may be within a range of a few 100 ms. The duration of the individual ramp can be within the range between 100 μs and 100 ms. The position of the frequency band Δf is to be set taking into account regulatory requirements, for which reason the frequency bands about frequencies of 6 GHz, 26 GHz, 79 GHz, or 120 GHz are preferably implemented as frequency band Δf. The bandwidth lies especially between 0.5 GHz and 10 GHz, depending on the position of the frequency band Δf.
In practice, the high-frequency signal sHF is not continuously generated in the case of FMCW. Rather, the ramp-shaped change is interrupted for a defined pause time after a defined number of successive frequency ramps. In the case of FMCW, the corresponding measurement rate rm at which the FMCW-based fill-level measuring device 1 cyclically redetermines the fill level L results from this number of successive frequency ramps, or from their respective ramp duration, and the subsequent pause time. In practice, the cycle duration in this case is between 0.3 Hz and 30 Hz.
For transmission, the high-frequency electrical signal sHF in the signal-generating unit 11 is supplied to the antenna 12 via a signal divider 116, a transmission amplifier 113, and a transmitting/receiving switch 114. The incoming radar signal EHF, which is reflected by the filling material surface, is converted back into a purely electrical receive signal eHF by the transmitting/receiving antenna 12. Subsequently, after any reception amplification (not shown in
In order to determine the frequency of the evaluation signal IF, an analog/digital converter of a computing unit 134 digitizes the evaluation signal IF in the evaluation unit 13. The computing unit 134 can thus subject the digitized evaluation signal to a (fast) Fourier transformation, or FFT for short. The frequency of the global maximum of the corresponding FFT spectrum ideally corresponds to the distance d from the filling material surface.
A circuit diagram of a fill-level measuring device 1, which operates according to the pulse time-of-flight method, is shown in
The frequency of the microwave pulses SHF, EHF is established by the oscillation frequency of the high-frequency oscillator 112. In the simplest case, the high-frequency oscillator 112 can be designed as an oscillating crystal. A VCO (“voltage-controlled oscillator”) can also be used. In this case, the high-frequency oscillator 112 is actuated by the pulse generator 111 by means of a corresponding DC voltage signal. The pulse generator 111 thereby defines the pulse duration of the individual microwave pulses SHF and the clock rate fc at which the microwave pulses SHF are emitted. As standard, a semiconductor-based digital resonant circuit is used as the high-frequency oscillator 112. The clock rate fc at which the individual microwave pulses sHF are excited is between 100 KHz and 1 MHz in practice. The high-frequency pulses sHF thereby generated by the high-frequency oscillator 112 are supplied to the antenna 12 analogously to the FMCW method via a transmission amplifier 113 and a transmitting/receiving switch 114 so that they are correspondingly emitted as microwave pulses SHF.
Since the reflected microwave pulses EHF are also received via the antenna 121, the transmitting/receiving switch 114 supplies the corresponding receive signal EHF to a mixer 131 in the evaluation unit of the fill-level measuring device 1. In contrast to the shown embodiment variant, an electrically conductive probe, such as a waveguide or a coaxial cable, which extends toward the container bottom can also be used instead of the antenna 12. In contrast to the circuit shown in
By means of the mixer 131, the undersampling of the receive signal eHF characteristic of the pulse time-of-flight method is carried out. For this purpose, the receive signal eHF is mixed with electrical sampling pulses s′HF by the mixer 131. In this case, the sampling rate f′c at which the sampling pulses s′HF are generated differs by a defined relative deviation Φ of far less than 0.1 per thousand from the clock rate fc of the generated high-frequency pulses sHF.
The sampling pulses sHF are generated in the evaluation unit 13 analogous to the signal-generating unit 13 by a second pulse generator 133 which actuates a second high-frequency oscillator 134. Thus, correspondingly to the high-frequency pulses sHF, the frequency of the sampling pulses s′HF is defined by the second high-frequency oscillator 134. In this case, the frequency of both high-frequency oscillators 112, 134 is set identically in practice. In this case, the second pulse generator 134 in turn controls the sampling rate fc at which the sampling pulses s′HF are generated.
Mixing the receive signal eHF with the electrical sampling pulses s′HF by means of the mixer 131 generates an evaluation signal IF which is typical for the pulse time-of-flight method and represents the receive signal eHF in a time-expanded manner. The time expansion factor is proportional to the deviation Φ between the clock rate fc and the sampling rate fc. Accordingly, in the case of the pulse time-of-flight method, the measurement rate rm is defined as the clock rate fc divided by the deviation Φ according to
The advantage of the time expansion is that the evaluation signal IF can be evaluated considerably more easily from a technical point of view due to the time expansion in comparison to the pure receive signal eHF: The reason for this is that the receive signal eHF has a correspondingly short time scale tin the nanosecond range due to the high speed of propagation of the microwave pulses SHF, EHF at the speed of light. The time expansion results in the evaluation signal ZF having a time scale in the millisecond range.
By means of the time-expanded evaluation signal IF, the computing unit 132, after corresponding analog/digital conversion, subsequently again determines the fill level L by determining the signal maximum corresponding to the distance d in the evaluation Signal IF.
Both the FMCW-based embodiment variant shown in
According to the invention, the two embodiment variants of the fill-level measuring device 1, which are described in
According to the graph therein, the fill-level measuring device 1 measures the fill level L below a defined limit temperature Tg at the temperature sensor 115 of, for example, 100° C. at a constant or undiminished measurement rate rm at which it also measures under normal conditions. However, above the limit temperature Tg, the fill-level measuring device 1 reduces the measurement rate rm linearly as the temperature increases. In contrast to a linear reduction, a stepwise reduction is also conceivable in contrast to the shown illustration. This counteracts primarily the development of heat by the transmission amplifier 113. Thus, with a corresponding reduction in the measurement rate rm per ° C., despite a high ambient temperature (for example caused by a cleaning step in the container 2), the temperature in the fill-level measuring device 1 does not exceed a critical maximum temperature Tmax of, for example, 150° C. If the temperature sensor 115 nevertheless detects that the critical maximum temperature Tmax is being exceeded, the fill-level measuring device 1 can in this case be automatically shut off, for example, given a corresponding design. This ensures, on the one hand, that the fill-level measuring device 1 does not measure any faulty fill-level values L and, on the other hand, does not suffer irreparable damage.
It goes without saying that the control of the measurement rate rm according to the invention can also be used for fill-level measuring devices 1 that do not operate according to the pulse time-of-flight or FMCW method. Likewise, the method according to the invention can also be generally used in radar-based distance measurement.
1 Fill-level measuring device
2 Object/filling material
3 Container
4 Higher-level unit
11 Signal-generating unit
12 Antenna arrangement
13 Evaluation unit
111 Ramp-generating unit or pulse generator
112 High-frequency oscillator
113 Amplifier
114 Transmitting/receiving switch
115 Temperature sensor
116 Signal divider
131 Mixer
132 Microcontroller
133 Pulse generator
134 Second high-frequency oscillator
d Distance
EHF, eHF Received radar signal or receive signal
fc Clock rate
f′c Sampling rate
h Installation height or measuring range
IF Evaluation signal
L Fill level
rm Measurement rate
SHF, SHF Radar signal or high-frequency signal
S′HF Sampling signal
Φ Relative deviation between the clock rate and the sampling rate
Number | Date | Country | Kind |
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10 2019 133 245.1 | Dec 2019 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/082849 | 11/20/2020 | WO |