WO2016041726A1

WO2016041726A1 - Device and method for monitoring a process variable of a medium

Info

Publication number: WO2016041726A1
Application number: PCT/EP2015/069058
Authority: WO
Inventors: Maik WEISHAAR; Dietmar FRÜHAUF; Armin Wernet; Kaj Uppenkamp; Gerd BECHTEL
Original assignee: Endress+Hauser Gmbh+Co. Kg
Priority date: 2014-09-19
Filing date: 2015-08-19
Publication date: 2016-03-24
Also published as: DE102014113545A1

Abstract

The invention relates to a device for monitoring at least one physical or chemical process variable of a medium (2) in a container (3), comprising at least one measurement probe (1) operated in the capacitive measuring mode and an electronic unit (7), the electronic unit (7) being designed to apply an adjustable excitation signal to the measuring probe (1), wherein a measuring circuit (8) is provided inside the electronic unit (7), which is designed to transform the response signal received from the measurement probe (1) into a response signal of a predetermined frequency independently of the frequency of the excitation signal, wherein an evaluation unit (16) is provided, which is designed to determine the process variable from the transformed response signal received from the measurement probe (1).

Description

Device and method for monitoring a process variable of a

medium

The invention relates to a method and a device for monitoring a process variable of a medium in a container with a capacitive measuring probe. The process variable may be given for example by a level, the electrical conductivity and / or the permittivity of the medium, or it is possible to monitor whether the sample has formed on the measuring probe. Corresponding field devices are marketed by the applicant in many different embodiments, for example under the name Liquicap or Solicap.

A capacitive measuring probe generally comprises a rod-shaped measuring probe and, if appropriate, a guard electrode which coaxially surrounds the measuring probe and which lies at the same electrical potential. The guard electrode serves to improve the measurement when forming a deposit on the probe. This procedure has become known from the document DE 32 12 434 C2. The capacitive measuring method is known per se from the prior art. The measuring probe is acted on by a starting signal in the form of an alternating current, and the filling level is then determined from the response signal received by the measuring probe. This depends on the capacitance of the capacitor formed by a measuring probe and the wall of the container or a second electrode. Depending on the conductivity of the medium, either the medium itself or a probe insulation forms the dielectric of the capacitor. The capacitance, in turn, is often determined from an apparent current reading, in which the amount of the probe is charged by the probe

AC flowing apparent current is measured. However, since this has an active and reactive component, is often an admittance measurement, in which in addition to the apparent current of the phase angle between the apparent current and at the

Probe voltage applied I is measured, more suitable. The additional determination of the phase angle also allows statements about possible formation of a formation, such as in the

DE102004008125A1 described.

For capacitive probes, the problem arises that the frequency of the applied AC voltage is lower the longer the probe is because of resonance effects. On the other hand, the influence of build-up decreases at higher measurement frequencies. Approach is equally important for short and long probes. Since usually the corresponding electronic units are designed only to load the measuring probes with a constant frequency, it is important to find the best compromise for the frequency. Therefore, a frequency is usually selected for the excitation signal, which is equally suitable for all lengths of the probes. However, this is below the optimum frequency for shorter probes. Another problem occurs with media whose conductivity values are in one

Transition region between a permittivity dependent and a

Permittivitätsunabhängigen range lie. The level is in this

Transition area can not be reliably determined, so that the corresponding media of the capacitive level measurement are usually not accessible.

To overcome these two problems, was in the document

DE10201 1003158A1 a measuring device and method based on the capacitive measuring principle proposed, in which the probe is acted upon at least temporarily by means of a frequency sweep with an excitation signal having a plurality of lying within a predetermined frequency band, consecutive discrete frequencies, and wherein at Hand of the

Frequency scan determines an optimal for each application measurement frequency and is determined from the belonging to this measurement frequency response signal level. Depending on the length of the measuring probe and / or the nature of the medium, in particular the conductivity value, a different measuring frequency is accordingly determined in which the filling level can be determined most accurately. To implement the procedure described in this document, among other things, therefore, a signal generator is needed, which for applying the

Measuring probe with a frequency sweep is suitable. Such a dynamic signal generator is described for example in the hitherto unpublished document DE102013107120.1. This signal generator generates the frequency sweep by means of a clock, which has a constant sampling frequency for

Which is greater than the maximum discrete signal frequency

predetermined frequency range. Moreover, in a memory unit at each of the discrete signal frequencies, the amplitude values of the corresponding periodic signals can be stored as a function of the sampling frequency. The stored amplitude values are then successively read out of the memory unit by means of a control and / or arithmetic unit with the sampling frequency of the clock generator. This then generates the periodic signals. Both approaches are also designed so that they are only a small

Ensure power consumption of the respective components of an electronic unit. These stringent power requirements are with regard to common interfaces like a 4-20mA interface or a NAMUR

Interface necessary. The power consumption plays a particular role when the corresponding field device is to be operated in an explosive atmosphere.

In addition to a dynamic signal generation and the determination of the optimum measurement frequency for the detection of the level must be ensured by means of a suitable measuring circuit that the probe can be acted upon with a variety of frequencies, and that the response signal of the

Measuring probe can be evaluated at different frequencies. A problem here is that the capacitive load of a probe can vary with frequency.

The invention is therefore based on the object to provide a cost-effective and universally applicable measuring circuit for a capacitive probe. The object is achieved by a device for monitoring at least one physical or chemical process variable of a medium in a container with at least one operated in the capacitive measurement mode

Measuring probe and an electronic unit, the electronic unit to do so

is designed, the probe with an adjustable signal to stimulate

apply, wherein within the electronic unit, a measuring circuit

is provided, which is configured to transform the response signal received from the probe independent of the frequency of the exciting signal into a response signal of a predetermined frequency, wherein an evaluation unit is provided, which is adapted to the process variable from that of the

Probe to be obtained transformed response signal. Such a configured device offers various advantages: Regardless of the

Frequency of the start signal and without loss of information can for determining the respective at least one process variable in each case a response signal of a

be evaluated at certain constant predetermined frequency. The frequency of the start signal may be in a range of 1 kHz to at least 10 MHz. Due to the fact that the response signal is transformed to a certain predefinable frequency, the evaluation unit does not need to evaluate a

Response signal to be designed with a wide frequency spectrum. This is particularly advantageous with respect to the respective microcontroller. In particular, high-frequency signals can generally not be processed with simple and / or low-power microcontrollers. However, this is possible by the procedure according to the invention. Because, in spite of a dynamic start signal, an evaluation can be carried out at a fixed frequency, the solution according to the invention is furthermore inexpensive and at the same time has a low power consumption.

In a first embodiment, the measuring circuit is configured to

To transform the start signal to the same frequency as the response signal, and the evaluation unit is configured to determine from the transformed start signal and the transformed response signal, the at least one process variable. This makes it possible, the at least one process variable by means of a

Admittanzmessung, in which in addition to the apparent current of the phase angle between the apparent current and the voltage applied to the probe voltage is to be determined. This is particularly advantageous in terms of buildup.

In a further possible embodiment, the excitation signal for acting on the measuring probe is a constant signal of adjustable frequency. In this case, the invention allows the frequency of the excitation signal at the beginning optimally to the

To adjust the composition of the medium and the length of the probe and avoids that only the frequency, which allows the best compromise for all lengths of the probe must be used. Thus, also advantageous media with a conductivity value from the transition region of

Capacitive level measurement accessible.

Alternatively, in another embodiment, the excitation signal for acting on the probe is given by a frequency sweep with successive signals discrete frequencies, which within a predetermined

Frequency range are. By transforming the response signal into a signal with a predefinable frequency, the evaluation can also be considerably simplified for the multiplicity of response signals generated by means of the frequency sweep. It is advantageous if the starting signal is a rectangular signal, triangular signal or a sinusoidal signal.

In a particularly preferred embodiment, within the electronics unit for generating the start signal, a signal generator and a broadband

Driver circuit arranged. The signal generator can be, for example, a dynamic signal generator as in the introduction of the description

described. The driver circuit in turn serves to drive the measuring probe with the start signal. The driver circuit should be broadband and designed to drive large capacitive loads. This has the following reason: While the capacity of the probe remains the same at a given level, the reactance changes with the frequency of the start signal and hangs, if the

Measuring probe is subjected to a frequency sweep, on the bandwidth of the frequency interval. This leads to a greater load on the driver circuit, which is used to drive capacitive loads over more than an order of magnitude must be suitable. In particular, the driver circuit should be designed to drive loads between 400pF and 4000pF.

Further criteria for the selection of a suitable driver circuit are the highest possible amplitude stability, low power consumption and power consumption that is as constant as possible regardless of the respective capacitive load. In addition, the lowest possible costs are desirable. All these criteria lead to the selection of the above possibilities for a suitable driver circuit.

Therefore, it is advantageous if the driver circuit has a complementary

Push-pull stage, a voltage follower with current-feedback

Operational amplifier, or voltage follower with voltage feedback operational amplifier is.

In addition, it is advantageous if an adjustable resistor network with differently adjustable shunt resistors is provided. A shunt resistor serves to generate a voltage signal proportional to the current flowing at the measuring probe and is connected in series therewith. Now the start signal can be selected from a wide frequency interval, or a

Frequency sweep must be used, then the shunt resistor

be adjusted accordingly, which can be achieved by the adjustable resistor network. This possibility of range switching can always achieve the best possible accuracy or resolution for the respective measurement.

In a particularly preferred embodiment, a mixer is arranged within the measuring circuit, which mixer is designed to generate from the response signal of the probe a transformed response signal of constant frequency, wherein a first signal for the mixer received by the probe

Response signal is, wherein a second signal for the mixer is the excitation signal, which is applied to the probe, and wherein a reference signal for the mixer is a signal with a constant predeterminable frequency difference from the excitation signal. There are two signals from the actual measuring branch to the mixer forwarded, received by the probe response signal and the Anregebzw. Drive signal. These can be recorded directly with reference to the mass. Mixers are found in the prior art mainly in the high frequency range again; however, the mixer is used here for comparatively low frequencies. The use of a mixer is distinguished by its insensitivity to interference and its low power consumption. In particular, since high-frequency signals can not be processed with the usual lower-cost microcontrollers, a mixer is in particular due to the possibility of such frequencies to lower

Transforming frequencies is advantageous.

It is advantageous if the mixer is an analog down mixer,

in particular, a discretely constructed JFET mixer. Such a discretely constructed circuit arrangement makes it possible by multiplying the

Response signal and the excitation or drive signal with an additionally generated signal to produce different mixing frequencies. In this case, a constant frequency difference is selected for the second generated signal to the actual response signal, wherein at the output of the mixer, the difference frequency of the two

Input frequencies is realized.

In addition, it is advantageous if the electronic unit is designed such that the excitation signal and the response signal by the same switching branch for

Evaluation unit flow. For this purpose, for example, an analog switch can be used in front of the mixer. Advantageously, this results in a lower sensitivity to interference, such as temperature and / or voltage fluctuations.

In a preferred embodiment is between the measuring circuit and the

Evaluation unit integrated a static filter. It is advantageous if the filter is a static low-pass filter, in particular a static low-pass filter 6.

Order, whose frequency is matched to the transformed response signal. This measure results from the fact that many different frequencies are present in the output signal of the mixer after the downward mixing. The filter then filters out the signal at the desired frequency and amplifies it. In that the response signal to a certain predetermined frequency is transformed, a static filter can be used. The filter ensures a particularly störunempfindliche circuitry.

In a preferred embodiment, the at least one process variable is a continuous or predetermined level of a medium in a container.

The object according to the invention is also achieved by a method for monitoring at least one physical or chemical process variable of a medium in a container with at least one measuring probe operated in the capacitive measuring mode and an electronic unit, wherein the measuring probe is acted on by an adjustable starting signal, wherein the received from the measuring probe Response signal is transformed independently of the frequency of the start signal into a response signal of a predetermined frequency, and wherein the process variable is determined from the transformed response signal obtained from the measuring probe.

In summary, the present invention enables a capacitive

Measuring probe with a start signal with a variety of different

To drive and evaluate frequencies. The invention is also characterized by a simple inexpensive and space-saving design, large

Noise immunity and high accuracy and is suitable for both static and dynamic loading of the probe. Another advantage to be highlighted is the low power consumption, which

in particular makes it possible to operate the device in an explosive atmosphere.

The invention will be described in more detail with reference to the following Figures 1 to 7. It shows:

Fig. 1: a schematic representation of a measuring probe in a container according to the prior art.

2 shows a block diagram of an electronic unit according to the invention FIG. 1 shows a typical construction for a measuring probe 1, by means of which a predetermined fill level can be monitored in the capacitive measuring method. The measuring probe 1 is arranged on a container 2 and protrudes partially into it. The container in turn is at least partially filled with a medium 3. The measuring probe 1 is composed in the present example of a measuring electrode 5 and a guard electrode 6, which serves to avoid the formation of approach. The measuring probe is connected outside the container to an electronic unit 7, which is used for signal acquisition, evaluation and / or supply

responsible for. In particular, the electronic unit of the determined by the

Probe received response signal or the response signals the level of the medium 3 in the container. 2

A block diagram of an electronic unit 7 according to the invention is the subject of FIG. 2. By means of a signal generator 9, a periodic signal is generated. This can either be a constant signal of adjustable frequency or else a frequency sweep. By means of a measuring circuit 8, the

Probe 1 operated, and that the response signal of the probe 1 evaluated at different frequencies. For this purpose, an admittance measurement is made in the example shown here.

To act on the probe 1 is a suitable broadband

Driver circuit 10 is provided. This amplifies the start signal and impresses it to a series circuit consisting of the measuring probe 1 and a shunt resistor 12a, 12b, which can be selected via an analog switch 11 for range switching, in a resistor network 12. About the respective shunt resistor

12a, 12b, the apparent current is generated for the admittance measurement.

About a further analog switch 13 are for applying the

Probe 1 used stimulus or driver signal U _t and and the received from the probe 1 response signal U _a to a mixer 14 passed. The mixer 14 is a discretely constructed circuit arrangement which makes it possible to generate different mixing frequencies by multiplying the measurement signal by a second additionally generated signal U _z . In particular, for this purpose, a downward mixing is realized, in which the reference signal used second additionally generated signal U _z in the form of a rectangular signal has a constant frequency difference to the actual response signal U _a .

Accordingly, the difference frequency of the two capture frequencies is achieved at the output of the mixer 14. In this way _, a transformed response signal of constant frequency can be generated from the response signal of the measuring probe U _a .

Finally, the mixer 14 is followed by a static filter 15, because the output signal of the mixer 14 by the simple discrete mixture contains many different frequencies. The response signal transformed in this way is finally forwarded to the microcontroller 16 and processed there. By downconverting to a low constant frequency, it is possible to work with a simple microcontroller 16 with an integrated analog-to-digital converter. As a consequence, the measuring circuit is characterized by a low power consumption and a low cost.

Reference symbol measuring probe

container

medium

Wasting the container

measuring electrode

Guard electrode

electronics unit

measuring circuit

signal generator

driver stage

Analog switch for range switching Resistance network

a, 12b shunt resistors

Analog switches

mixer

filter

microcontroller

Claims

claims

1 . Device for monitoring at least one physical or

chemical process variable of a medium (2) in a container (3) with at least one measuring probe (1) operated in the capacitive measuring mode and an electronic unit (7),

wherein the electronic unit (7) is configured to apply an adjustable starting signal to the measuring probe (1),

wherein within the electronic unit (7) a measuring circuit (8) is provided, which is designed to transform the response signal received from the measuring probe (1) into a response signal of a predetermined frequency independently of the frequency of the exciting signal,

wherein within the electronic unit an evaluation unit (16) is provided, which is designed to determine the process variable from the transformed response signal received by the measuring probe (1).

2. Apparatus according to claim 1,

wherein the measuring circuit is configured to transform the excitation signal to the same frequency as the response signal, and wherein the evaluation unit is configured to determine from the transformed excitation signal and the transformed response signal the at least one process variable.

3. Apparatus according to claim 1,

wherein the exciting signal for acting on the measuring probe (1) is a constant signal of adjustable frequency.

4. Apparatus according to claim 1,

wherein the excitation signal for applying to the measuring probe (1) is given by a frequency sweep with successive signals of discrete frequencies, which are within a predetermined frequency range.

5. Device according to at least one of the preceding claims, wherein the start signal is a square wave, triangular or a

Sinusoidal signal is.

6. Device according to at least one of the preceding claims,

wherein within the electronic unit (7) for generating the start signal, a signal generator (9) and a broadband driver circuit (10)

are arranged.

7. Device according to at least claim 5,

wherein the driver circuit (10) is a complementary push-pull stage, a voltage follower with a current-feedback operational amplifier, or a voltage follower with a voltage-feedback operational amplifier.

8. Device according to at least one of the preceding claims,

wherein an adjustable resistor network (12) with differently adjustable shunt resistors (12a, 12b) is provided.

9. Device according to at least one of the preceding claims,

wherein within the measuring circuit (8) a mixer (14) is arranged, which mixer (14) is adapted to the response signal of the

Measuring probe (1) to generate a transformed response signal of constant frequency,

wherein a first signal for the mixer (14) is the response signal received from the probe (1), a second signal for the mixer being the excitation signal applied to the probe (1) and a reference signal for the mixer Signal with constant

specifiable frequency difference to the start signal.

10. Device according to at least one of the preceding claims,

the mixer (14) being an analog down mixer, in particular a discretely constructed JFET mixer.

1 1 .Vorrichtung according to at least one of the preceding claims, wherein the electronic unit (7) is designed such that the starting signal and the response signal flow through the same switching branch to the evaluation unit (16).

12. Device according to at least one of the preceding claims,

wherein between the measuring circuit (8) and the evaluation unit (16) a static filter (15) is integrated.

13. Device according to at least one of the preceding claims,

wherein the filter (15) is a static low-pass filter, in particular a sixth-order static low-pass filter whose frequency is transformed to the one

Response signal is adjusted.

14. Device according to at least one of the preceding claims,

wherein the at least one process variable is a continuous or

predetermined level of a medium (2) in a container (3).

15. Method for monitoring at least one physical or chemical process variable of a medium (2) in a container (3) with at least one measuring probe (1) operated in the capacitive measuring mode and one

Electronic unit (7),

wherein the measuring probe (1) is subjected to an adjustable starting signal,

wherein the response signal obtained from the measuring probe (1) is transformed into a response signal of a predetermined frequency independent of the frequency of the exciting signal, and

wherein the process variable is determined from the transformed response signal obtained by the measuring probe (1).