DE102017128440A1 - Impedance level sensor - Google Patents

Abstract

Impedance level sensor (100) with
a measuring probe (102) which can be influenced by a medium surrounding the measuring probe (102) in a capacity,
a measuring resonant circuit in which the measuring probe 102 is arranged as a capacitance-determining element,
- An electronic unit (101) with a signal generator () for exciting the measuring resonant circuit and a signal detector (104) for determining a response signal of the measuring resonant circuit,
- A signal processing unit () for generating a measurement signal, which is connected to the electronic unit (101) and wherein the impedance threshold sensor (100) is designed as a two-wire field device and an energy buffer (304) for at least partially supplying the impedance level sensor (100) with energy ,

Description

The present invention relates to an impedance threshold sensor according to the preamble of patent claim 1 and a method for operating such a sensor.

Impedanzgrenzstandsensoren are basically known from the prior art, for example. For the measurement of limits or levels. Typical applications for the detection of a predefined level are process vessels, such as process tanks, storage tanks, silos or pipelines in the process industry. Impedance threshold sensors are often referred to as so-called limit switches, i. to determine whether a filling medium exceeds or falls below a certain filling level, the so-called limit level, in different liquids, as well as granulated and powdered bulk materials.

There are also other types of limit switches or level sensors known, which are selected depending on the application, process conditions and properties of the filling medium. In addition to impedance threshold sensors, sensors based on the TDR (Time Domain Reflectometry) principle or vibration level sensors or capacitive sensors are used. For example, a switching command of the limit switch can start or stop filling devices or emptying devices in order to correspondingly avoid overflowing or idling of the respective process container.

In the present application, instead of the term impedance threshold sensor, the terms impedance sensor and limit switch are equally used for the sake of simplicity.

A known impedance sensor 100 is in 1 shown.

1 shows a simplified sectional view with circuit blocks of an impedance sensor 100 according to the prior art. Essentially, the impedance sensor is made 100 , also referred to as point level sensor or limit level detector, according to the prior art from an electronic unit 101 and a measuring probe 102 , The measuring probe 102 is formed in the present embodiment as a series resonant circuit between a measuring electrode 106 and a reference electrode 108 a measuring capacity is formed 110 out, which with a discrete inductance 109 is connected to the designed as a series resonant circuit resonant circuit.

The measuring electrode 106 is rotationally symmetrical to a longitudinal axis L of the impedance sensor 100 trained and over an insulation 107 from a process room 90 separated. Advantageously, the discrete inductance 109 chosen so that there is a resonant frequency fres of the resonant circuit for a wide variety of media or coverage conditions (empty, full and dirty) between 100 MHz and 200 MHz sets.

An amount of frequency varying complex valued impedance | Z | This measuring resonant circuit is advantageously analyzed between 100 MHz and 200 MHz, ie the measuring resonant circuit is determined by means of a frequency generator 103 with a frequency sweep with frequencies between 100 MHz and 200 MHz excited and a response signal (frequency response) of the resonant circuit with a frequency detector 104 detected. If there is a medium in the area of the probe 102 , The impedance behavior of the measuring resonant circuit changes, ie in particular shifts its resonant frequency fres at which a minimum of the impedance is formed.

A frequency sweep is understood to mean the sequential excitation with a plurality of successive frequencies within a frequency range, wherein the frequency range should ideally contain all possible resonant frequencies of the resonant circuit.

The change in the impedance of the measuring resonant circuit is for an evaluation in an evaluation and control unit 105 used. Specifically, the frequency response becomes a frequency change .delta.f and a change in the amplitude of a minimum of the impedance .DELTA.Z , also referred to as amplitude change, evaluated and generated from a switching command.

In 2 are exemplary of the medium Ketchup the frequency responses of the impedance sensor 100 listed in the prior art.

A first turn 200 shows the resonance behavior of a clean probe 102 , Shown is the amount of impedance Z over the frequency f ,

The behavior of a probe soiled with ketchup buildup 102 is in a second turn 201 and that of a completely ketchup-covered probe 102 in a curve 202 shown.

Switching commands (empty, full) are provided by the evaluation and control unit 105 realized, according to the prior art, only the minima of the resonance curves are used for the evaluation. These will change with respect to a frequency change .delta.f and amplitude change ΔZ evaluated. Is the minimum of the resonance curve in a first range I , so gives the evaluation and control unit 105 the switching command "empty". However, if the minimum is in a second range II the switching command "full" is output. The two defined switching ranges I . II can factory-fixed in the impedance sensor 100 programmed or adjusted by a customer comparison and changed. Ideally, the ranges should be defined so that as many different media as possible the defaults are sufficient, as customer matching is time consuming and therefore undesirable.

The point level sensors 100 According to the prior art signal the switching state by means of one or more switching outputs, for example in the form of a transistor output, and are supplied via a separate line with energy. The point level sensors 100 According to the prior art are offered only with one or two switching outputs. These switching output cables and the two supply cables are connected to a connector plug of the sensor and must therefore be wired with additional effort. This cabling is perceived as a disadvantage.

It is the object of the present invention to provide an impedance threshold sensor which allows a reduced cabling effort.

This object is achieved by an impedance threshold sensor having the features of claim 1.

Advantageous developments are the subject of dependent claims.

An impedance threshold sensor according to the invention with a measuring probe which can be influenced by a medium surrounding the measuring probe in a capacitance, a measuring resonant circuit in which the measuring probe is arranged as capacitance determining element, an electronic unit with a signal generator for exciting the measuring resonant circuit and a signal detector for determining a response signal of Measuring resonant circuit and a signal processing unit for generating a measurement signal, which is connected to the electronic unit characterized by the fact that the impedance threshold sensor is designed as a two-wire field device and has an energy buffer for at least partially supplying the impedance level sensor with energy.

A two-wire field device according to the present invention is understood to mean a field device which is connected via two lines to a higher-order unit, with both a power supply and a measured value transmission taking place via these two lines.

The energy and / or signal transmission between the two-wire field device and the higher-level units is carried out according to the known 4 mA to 20 mA standard, in which a 4 mA to 20 mA current loop, i. a two-wire line is formed between the field device and the higher-level unit. In addition to the analogue transmission of signals, there is the possibility that the measuring devices, according to various other protocols, in particular digital protocols, transmit or receive further information to or from the higher-level unit. By way of example, the HART protocol or the Profibus PA protocol may be mentioned here.

The power supply of these field devices is also provided via the 4 mA to 20 mA current signal, so that no additional supply line is required in addition to the two-wire line. In order to minimize the wiring and installation costs and the safety measures, for example when used in explosion-proof areas, it is also not desirable to provide additional power supply lines.

With the impedance threshold sensor according to the invention, it is thus possible to significantly reduce the required cabling and to allow a reliable measurement despite the limited availability of energy that is provided via the two-wire line.

The energy store can advantageously be designed as a capacitor or as an accumulator. In this way, a high number of charging cycles, high availability and high energy density of the energy storage can be achieved.

In a further development, the impedance sensor can have an energy management device which is connected to the intermediate energy store and / or a controller, for example, in an evaluation and control unit and / or measuring electronics. In this way, an efficient energy management can be achieved, for example, the controller and / or the measuring electronics for the duration in which energy must be cached, disabled or put into a power saving mode.

• - Überprüfen einer momentan zur Verfügung stehenden Energiemenge,
• - wenn eine für eine erfolgreiche Messung notwendige Mindestenergiemenge zur Verfügung steht,
  1. i) Aktivieren eines Controllers sowie einer Messelektronik,
  2. ii) Durchführen einer Impedanzmessung,
  3. iii) Ausgeben eines Messwertes und nach Abschluss der Messung oder wenn die Mindestenergiemenge nicht erreicht ist
  4. iv) Puffern der mittels der Zweidrahtleitung übertragenen Energie.

An inventive method for operating an impedance threshold sensor according to the present application comprises the following steps:

• - Checking a currently available amount of energy,
• - if the minimum energy required for a successful measurement is available,
  1. i) activating a controller and a measuring electronics,
  2. ii) performing an impedance measurement,
  3. iii) Outputting a measurement and after completion of the measurement or when the minimum energy level is not reached
  4. iv) buffering the energy transmitted by the two-wire line.

In order to be able to carry out a new measurement more quickly, the controller and / or the measuring electronics can also be deactivated or put into an energy-saving mode.

• 1 ein vereinfachtes Schnittbild eines Impedanzsensor gemäß dem Stand der Technik (schon behandelt),
• 2 das Impedanzverhalten des Impedanzsensors gemäß 1 (schon behandelt),
• 3 ein vereinfachtes Blockschaltbild eines Impedanzgrenzstandsensors gemäß der vorliegenden Anmeldung und
• 4 einen Verfahrensablauf zum Betreiben des Sensors gemäß 3.

The present invention will be explained in detail below with reference to embodiments and with reference to the accompanying figures. Show it:

• 1 a simplified sectional view of an impedance sensor according to the prior art (already discussed),
• 2 the impedance behavior of the impedance sensor according to 1 (already treated),
• 3 a simplified block diagram of an impedance threshold sensor according to the present application and
• 4 a procedure for operating the sensor according to 3 ,

3 shows a simplified block diagram of an impedance threshold sensor 100 according to the present application.

To the impedance sensor 100 As a two-wire field device to run this has a power supply 301 - 304 which forms a two-wire interface and essentially a current regulator 301 , a voltage regulator 302 , an energy management device 303 as well as an energy buffer 304 having.

Since both a power supply and an output of the switching state via two lines, the power supply has the power controller 301 on, which can output the possible switching states "uncovered" and "covered" as a loop current of, for example, 8 mA or 16 mA. In addition, the current regulator 301 also designed to output a device error, for example in the form of a third current value, which deviates from the two switching states.

In the simplest case, such a current regulator 301 designed as a series transistor, which impresses depending on the detected state of the sensor, for example, the current value 8 mA in the state "uncovered" and 16 mA in the state "covered" in the supply loop. For the signaling of the fault condition, it is customary to let a current value less than 3.6 mA or greater than 21 mA flow.

About the additional voltage regulation 302 Ensures that the energy stored in the energy store 304 for the following sensors and the evaluation and control unit 105 is available is not fed back into the two-wire line and thus distorts the switching state. Furthermore, one at the energy buffer 304 voltage is limited to a maximum value, so that it is not operated outside the permissible values for operation. The energy cache 304 has the task to buffer the power, which is available through the two-wire supply for the subsequent measurement, ie between store.

It can happen the situation that a power consumption of the sensor 306 , in the present case in particular the frequency generator 103 , the frequency detector 104 and the probe 102 includes, the evaluation and control unit 105 for the duration of a measurement exceeds the instantaneous power available across the two supply lines. In this case, the available energy in the energy storage for a long time 304 buffered, so long that sufficient energy is available in this memory for a measurement. The current state of charge of the energy buffer 304 is being used by the energy management unit 303 checked. To shorten the time to reach this state, for example, the sensors 306 during this time disabled and the controller of the evaluation and control unit 105 are in a power saving mode.

These differences in power consumption are appropriately controlled by an energy management and organized accordingly.

A possible procedure for operating the arrangement according to 3 is in 4 shown.

Before starting a measurement will be in step 401 the current state of the energy buffer 304 detected. Based on the stored energy will be in step 402 decided whether a measurement can be performed. If enough energy is stored, it will be in step 403 performed an impedance measurement and its result in step 404 evaluated. Depending on a location of the minimum of the impedance curve is in step 405 issued a corresponding switching state.

Will in step 402 found that there is still too little energy to carry out a measurement in the energy buffer 304 is stored, or after performing a measurement becomes step 401 returned.

LIST OF REFERENCE NUMBERS

90

process space

100

impedance sensor

101

electronics unit

102

probe

103

frequency generator

104

frequency detector

105

Evaluation and control unit

106

measuring electrode

107

insulation

108

Reference electrode / Housing

109

inductance

110

measuring capacitance

200

first turn

201

second bend

202

third turn

301

current regulator

302

voltage regulators

303

Energy management unit

304

Storage of energy

401-405

steps

I

first area

II

second area

.delta.f

frequency change

Az

amplitude change

fres

resonant frequency

Claims (6)

Impedanzgrenzstandsensor (100) with - a measuring probe (102) which is influenced by a measuring probe (102) surrounding medium in a capacity, - a measuring resonant circuit in which the measuring probe (102) is arranged as a capacity determining element, - an electronic unit (101 ) with a signal generator (103) for exciting the measuring resonant circuit and a signal detector (104) for determining a response signal of the measuring resonant circuit, - a signal processing unit (105) for generating a measuring signal which is connected to the electronic unit (101) and characterized in that Impedanzgrenzstandsensor (100) is designed as a two-wire field device and an energy buffer (304) for at least partially supplying the impedance level sensor (100) with energy. Impedance threshold sensor (100) according to Claim 1 , characterized in that the energy buffer (304) is designed as a capacitor or as an accumulator. Impedanzgrenzstandsensor (100) according to one of the preceding claims, characterized by an energy management device (303) which is connected to the energy buffer (304), a controller of an evaluation and control unit (105) and a measuring electronics (101). Impedance threshold sensor (100) according to one of the preceding claims, characterized by the complete power supply of the sensor through the two-wire line. Method for operating an impedance threshold sensor (100) according to one of the preceding claims, comprising the following steps: Checking a currently available amount of energy, - if the minimum energy required for a successful measurement is available, i) performing an impedance measurement, (ii) outputting a reading, and after completion of the measurement or when the minimum amount of energy has not been reached, iii) buffering the energy transmitted by the two-wire line. Method according to Claim 5 , characterized in that a controller of an evaluation and control unit 105 and / or an electronics unit 101 before performing the measurement is activated and after completion of the measurement or when the minimum amount of energy is not reached, the controller and / or the measuring electronics (101) deactivated or in a power-saving mode is set.

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