CN102538905A - Capacitive radio-frequency admittance level transmitter and operating method thereof - Google Patents
Capacitive radio-frequency admittance level transmitter and operating method thereof Download PDFInfo
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- CN102538905A CN102538905A CN2010105786900A CN201010578690A CN102538905A CN 102538905 A CN102538905 A CN 102538905A CN 2010105786900 A CN2010105786900 A CN 2010105786900A CN 201010578690 A CN201010578690 A CN 201010578690A CN 102538905 A CN102538905 A CN 102538905A
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Abstract
The invention relates to a capacitive radio-frequency admittance level transmitter and an operating method thereof. A high-frequency sinusoidal excitation signal output by a high-frequency driver is loaded into a capacitance sensor; through analyzing the sizes of an AC (alternating current) component and a DC (direct current) component of a response signal of the capacitance sensor, the impedance and condensance of the capacitance sensor are calculated through the operation of an MCU (micro-programmed control unit); and the condensance of an attachment is obtained through backstepping according to a conclusion that in a radio-frequency admittance theory, a real part and an imaginary part which are subjected to equivalent admittance by the attachment are numerically equal, so that the real capacitance is equal to a value obtained by subtracting the attachment capacitance from the measured capacitance, thereby eliminating the measurement errors of the attachment caused by that a measured medium is adhered to an electrode, solving the difficult problem that the measurement errors of viscous measured materials or solid powdered measured materials are large, improving the measurement precision, and guaranteeing the measurement stability.
Description
Technical field
The present invention relates to a kind of detection technique, particularly a kind of condenser type radio frequency admittance level transmitter.
Background technology
The thing position is meant the interface location between liquid in the hold-up vessel or the equipment of industrial product, pulverulent solids, the gas, also can be owing to density does not wait the interface location that forms between mutual exclusive two kinds of liquid.Be divided into liquid level, material level, boundary's level sensor or transmitter according to concrete purposes.The thing position is one of major parameter related in the control procedure.The parameter of material consumption or output metering is not only in the thing position, also is the important parameter that guarantees production continuously and device security.Particularly in the modern chemical industry industry, production scale is big, and speed is high, and high temperature, high pressure, severe corrosive or inflammable and explosive material are often arranged, and is most important especially with automatic control for the supervision of thing position.Therefore, account for critical role in the process control of thing position detection technique modern chemical industry industry.Level transmitter in the power station, industrial circle such as petrochemical industry, oil field is widely used.Through measurement to the thing position; Obtain the continuous variable or the switching value signal of thing position; To obtain the quantity of material in the container; So that regulate the inflow and outflow flow of material, thereby realize processing and the optimized control of process, and useful information is provided storage management at line material to material.
Capacitance type sensor is a kind of sensor that the variation of tested non electrical quantity is converted into electric capacitance change.Though capacitance type sensor is subject to the influence of distributed capacitance, and antijamming capability and signal long-distance transmissions ability are not high yet.But capacitance type sensor has simple in structure, plurality of advantages such as resolution height.And can under mal-conditions such as hyperthermia radiation and judder, work.The capacitance object position transmitter is applicable to that industrial enterprise measures and control production run in process of production, and is main as type conduction and the liquids position of non-conductive medium or the remote continuous coverage and the indication of powder shape solid material level.As the classic method of measurement object position, measurement by capacitance is less demanding to site environment, and simple, and is easy to maintenance, also can be applicable to many industrial occasions.The capacitance object position transmitter is product moulding the most at present, and function is the most perfect, uses the most a kind of level meter.
The defective that exists in the practical application: for the bridging situation on the potential electrode, promptly when material level is descended by higher level, have some materials attached on the potential electrode, especially tested material is the situation of thickness or pressed powder.Because the material of surveying is a conducting medium; This moment, the earth point of potential electrode surface of insulating layer covered whole material and bridging district; Make and measure electric capacity two parts that electric capacity has comprised corresponding electric capacity in true thing position and the formation of bridging layer, the increasing that causes measuring electric capacity causes the dummy position.
The size of bridging error is influenced by the electric conductivity of measured medium also, and electric conductivity is strong more, and its influence is big more; The bridging error that produces is also just big more, and this is that material electric capacity is that conductive material is medium formation with the sensing electrode core with the sensing electrode insulation course because when measuring conductive medium; And the bridging layer has electric conductivity; Electric conductivity is strong more, and it is more little to be equivalent to bridging layer resistance, and bridging layer current potential just approaches the material current potential more; Thereby the bridging electric capacity that is produced is big more, and nature is also just big more to the influence of measurement result.
Summary of the invention
The present invention be directed to existing capacitance object position transmitter is big, the problem of unstable of measuring error of thickness or pressed powder at tested material; A kind of condenser type radio frequency admittance level transmitter and method of work have been proposed; Detect electric capacity and resistance simultaneously and can eliminate the measuring error of the bridging that produces owing to the measured medium adhesive electrodes, arrive accurate measurement result.
Technical scheme of the present invention is: a kind of condenser type radio frequency admittance level transmitter; Comprise sample circuit, HART communication interface/display interface, MCU control module, man-machine interface; Sample circuit keeps several modules to form by high-frequency drive, capacitance detecting, bridging separation, sampling successively; Sample circuit detects measurand, sends into the MCU control module after the capacitance of measuring is handled; The MCU control module receives the instruction that man-machine interface comes, and send HART communication interface/display interface after the signal Processing that reception comes to sample circuit simultaneously, and standard signal is seen off and video data.
Said high frequency drive circuit produces a high frequency sinusoidal excitation signal; Sinusoidal signal obtains the square wave signal shaping and filtering through BPF.; Obtain meeting the pumping signal of design at last through driving circuit, pumping signal is carried on the capacitive transducer of capacitance detecting.
A kind of method of work of condenser type radio frequency admittance level transmitter; Comprise condenser type radio frequency admittance level transmitter; The high frequency sinusoidal excitation signal of said high-frequency drive output is loaded into capacitive transducer; The AC compounent through analyzing its response signal and the size of DC component are tried to achieve the impedance and the capacitive reactance of capacitive transducer through the computing of MCU, and obtain the bridging capacitive reactance by the real part of the bridging equivalent admittance in the radio frequency admittance theory and counter the pushing away of conclusion of imaginary part numerically equal; Actual capacitance=measurement electric capacity-bridging electric capacity then; Through HART communication interface circuit, MCU control circuit unit are set, the simulating signal of sensor sample compares computing through the MCU control circuit unit again; Carry out exporting 4～20mA simulating signal by D/A conversion unit after the data processing according to sampled value, realize the level gauging of capacitance object position transmitter; Simultaneously, HART communication interface electric circuit inspection demodulates the digital signal of stack to the external control signal that is superimposed upon on 4～20mA direct current signal, sends CPU to and handles; Process result is the alternating signal of HART semaphore request and is superimposed upon on the direct current signal through HART communication interface circuit modulation conversion.
Beneficial effect of the present invention is: condenser type radio frequency admittance level transmitter of the present invention and method of work; Given radio frequency admittance admittance position measuring technique in conjunction with utilization; Having solved tested material is the big difficult problem of measuring error of thickness or pressed powder; Improve the precision of measuring, guaranteed the stable of measurement.
Description of drawings
Fig. 1 is the bridging equivalent circuit diagram;
Fig. 2 is a condenser type radio frequency admittance level transmitter structural representation of the present invention;
Fig. 3 is condenser type radio frequency admittance level transmitter medium-high frequency driving circuit figure of the present invention;
Fig. 4 is HART communication interface figure in the condenser type radio frequency admittance level transmitter of the present invention.
Embodiment
Radio frequency admittance admittance position measuring technique is a kind ofly to grow up from the capacitance object position measuring technique; Anti-bridging, more reliably, more accurately, level gauging technology that applicability is wider; The implication of " admittance " is the inverse of electricity middle impedance in " radio frequency admittance "; It is comprehensively formed by resistive composition, capacitive composition, perceptual composition, and " radio frequency " is high frequency, so the radio frequency admittance technology is appreciated that to using the high frequency measurement admittance.Stable measuring-signal source of high frequency pure oscillator output, with the admittance between accurate measurement capacitive transducer and the material pot metallic walls, under the output of instrument two-wire system 4～20mA quantities, the output of instrument increases with the rising of thing position.
In above-mentioned article, can know the analysis of capacitance object position instrument measurement principle; Cause the instrument measurement reasons of error to be because due to the additional capacitor that the bridging when measuring the electric conductivity material of thickness causes; And the electric conductivity of material is strong more, and the measuring error that causes also can be big more.
The size of resistance is confirmed by conductivity and resistance size; The resistance value of conducting medium and its cross-sectional area are inversely proportional to; And the cross-sectional area of material that we survey is far longer than the cross-sectional area of bridging layer, so the size of sensor impedance part mainly is that bridging produces.In the practical application conductive material is handled as a pole plate of electric capacity, thing position electric capacity promptly is that conductive material and center sensing electrode are the electric capacity that medium forms with the insulation course.The bridging layer then can have very big resistance, and the layer of bridging simultaneously can be that medium forms an electric capacity with the insulation course with the center sensing electrode also.Can carry out the differential equivalence to the impedance capacitive reactance that sensor electrode produces; Regard the uniform transmission line network that constitutes by infinite a plurality of infinitesimal capacity cells and infinitesimal resistive element as; Therefore; Bridging layer and corresponding sensing electrode can be abstract for a pair of be the uniform transmission line of medium with the insulation course, its equivalent electrical circuit is as shown in Figure 1.
Can draw real part and the conclusion of imaginary part numerically equal of the equivalent admittance
of bridging by the uniform transmission line theory.When sensor rod when having bridging extremely; Actual measured results is to measure true thing position and the common complex impedance that forms of bridging layer; The electrical resistance property of bridging layer shows as this real part of impedance; The capacitive properties of bridging layer and thing position electric capacity sum show as the imaginary part of this impedance, and the characteristic based on the capacitor value numerically equal of the electric capacity that capacitive properties showed of the resistance value that electrical resistance property showed of bridging layer and bridging layer deducts the resistance value of measuring by total capacitor value of measuring; Promptly obtain the pairing capacitor value in actual thing position, and then try to achieve actual thing position.
Condenser type radio frequency admittance level transmitter as shown in Figure 2 comprises sample circuit 1, HART communication interface/display interface 2, MCU control module 3, man-machine interface 4; Sample circuit keeps 16 several modules to form by high-frequency drive 11, capacitance detecting 14, bridging separation 15, sampling successively; 1 pair of measurand 12 of sample circuit detects, and sends into MCU control module 3 after the capacitance of measuring is handled; MCU control module 3 receives the instruction of man-machine interface 4; Send HART communication interface/display interface 2 after simultaneously sample circuit 1 being received next signal Processing; Standard signal is seen off and video data, and HART communication interface 2 also comprises the modulation-demodulation circuit that is made up of special-purpose HART modem chip.
The present invention is loaded into capacitive transducer by one road high frequency sinusoidal excitation signal; The AC compounent through analyzing its response signal and the size of DC component; Try to achieve the impedance and the capacitive reactance of capacitive transducer through the computing of MCU; And obtain the bridging capacitive reactance by the real part of the bridging equivalent admittance in the radio frequency admittance theory
and counter the pushing away of conclusion of imaginary part numerically equal; Actual capacitance=measurement electric capacity-bridging electric capacity then, thus the error that bridging brings eliminated.Again through HART communication interface circuit, MCU control circuit unit are set; The simulating signal of sensor sample is through the MCU control circuit unit; Compare computing; Carry out exporting 4～20mA simulating signal by the D/A converting unit after the data processing according to sampled value, realize the level gauging of capacitance object position transmitter; Simultaneously, HART communication interface electric circuit inspection demodulates the digital signal of stack to the external control signal that is superimposed upon on 4～20mA direct current signal, sends CPU to and handles; Process result is the alternating signal of HART semaphore request and is superimposed upon on the direct current signal through HART communication interface circuit modulation conversion, thus the digital communication of realization and external information.Among the present invention, main variables and control information are transmitted by 4～20mA, and under the situation of needs, other procedure parameters, device configuration, calibration and configuration info are visited with the mode of digital communication through the HART agreement.
High frequency drive circuit is responsible for producing a high frequency sinusoidal excitation signal, and it obtains square-wave signal by the MCU frequency division, and sinusoidal signal obtains the square wave signal shaping and filtering through BPF., obtains meeting the pumping signal of design at last through driving circuit, like Fig. 3.
Capacitive transducer is formed potential-divider network with a known capacitor C 0 and resistance R 0 respectively; Sinusoidal excitation signal is loaded into the input of this network; Output after the potential-divider network through resistance is that the DC channel of sinusoidal excitation signal is corresponding; What this voltage obtained after through LPF is the relation between known resistance R0 and the bridging resistance, thus can be in the hope of bridging resistance, and be the alternating current path response of pumping signal through the potential-divider network that capacitor C 0 and sensor are formed; The amplitude that exchanges records through AD637 true rms measurement chip; Because we know amplitude, frequency, capacitor C 0, the resistance R 0 of pumping signal, it is little that we can obtain capacitance-resistance Chinese People's Anti-Japanese Military and Political College of sensor fully, can eliminate the influence of bridging thus through the calculating of MCU.
Inner integrated 12 bit A/D converters of MCU control module, have super low-power consumption and can directly drive LCD, can save many peripheral chips, simplify hardware circuit design greatly up to the FLASH storer of 60KB and the RAM of 2KB.
The HART communication interface is with MCU and the HART MODEM chip development capacitance object position transmitter based on the HART communication, and is simple and convenient, dwindled space and cost.The main functional modules of HART MODEM is made up of steering logic, carrier wave detection, modulation and demodulation four parts.The function of it is integrated input filtering and output Shaping can be simplified the reliability of interface circuit design realization system.
As shown in Figure 4 when occurring the frequency signal of 1200 (± 10) Hz or 2200 (± 20) Hz on the IRXA pin of HART chip A5191; Then the OCD pin of A5191 is a high level; MCU puts high level with P1.3 after P1.6 detects high level; This moment, the INRTS pin of HART MODEM detected high level, opens detuner detected digital signal is carried out demodulation and send valid data to MCU by ORXD handling.
If OCD is low level, explain that bus is idle, the P1.3 of MCU setting at this moment also is a low level, then the modulator of A5191 is opened, and is sent by the OTXA pin after the data-modulated shaping that will receive from ITXD.Data by OTXA sends are transformer coupled to 4～20mA direct current signal by operational amplifier amplification and process.
The HART agreement adopts the Bell202 frequency shift keying signal of standard to communicate with 1200 baud rates.Its Physical layer adopt two kinds of different frequencies (1200Hz and 2200Hz, represent respectively 2 systems 1 and 0) sinusoidal signal, be superimposed upon on 4～20mA signal and communicate, owing to the average of sinusoidal signal is 0, do not disturb direct current signal.It can make simulating signal and digital signal can carry out two-way communication simultaneously and do not interfere with each other.
What the present invention adopted is semiduplex communication, adopts the three layer model structure of simplifying, i.e. ground floor Physical layer, second layer data link layer and layer 7 application layer.Physical layer specifies the transmission method of signal, transmission medium; Data link layer has been stipulated the form of HART frame, and the link communication function realizes setting up, safeguards, terminates; Application layer regulation HART command set.Order is divided three classes, i.e. generic command, normal commands and specific command.Generic command provides all and field apparatus function associated, and these orders comprise: read manufacturing firm and device type; Read initializaing variable and unit; Read the number percent of electric current output and range; Read 4 predefined dynamic variables; Read and write 8 character items, 16 character descriptors, date; Read and write 32 character informations; Read range, unit and damping time constant; Read transducer series number and boundary; The Assembly part numeral that read-write is the most last; Write polling address.Commonly used command provides the function in the common multiple field apparatus, but is not whole.Comprise and read a group selection of 4 dynamic variables altogether; Write variable damping time; Write range; (zero setting, put maximum range) is measured in calibration; Fixing output current is set; Carry out self check; Carry out host reset; Adjustment is initial to become zero; Write initializaing variable unit; Adjustment digital-analog convertor zero point and gain; Write tansfer function; Write transducer series number; Read-write dynamic variable assignment.Specific command only provides the function to special field apparatus, and it is peculiar, not compatible to be that the present invention controls oneself.
Because HART agreement predetermined data transfer rate is 1200bit/s, 10 milliseconds around an of character of every transmission, so adopt the mode transceive data of interrupting in the software, all the other times can make CPU get into low power consumpting state or handle other affairs.The reception of the USART module of MSP430, transmission buffer memory all can produce independently and interrupt, and program is accomplished the various processing to the HART information frame in this interrupts.In the receiving course of frame, unusual whenever detecting, corresponding state information can be set automatically, program interrupts focusing on mistake, the delay that occurs in the transmission course at timer, and calls corresponding processing procedure according to the difference of system works pattern.The process of transmitting of information frame similarly; When system works during in group schema; The command information whether steady arm constantly has main frame to upload on the detection line at a certain time interval if do not have, can send the form of the relevant information of equipment operation with the HART information frame automatically; If just there is main frame to communicate by letter, then preferentially carries out Host Command, or postpone or cancel the transmission of burst data according to the Host Command type.
Claims (3)
1. condenser type radio frequency admittance level transmitter; It is characterized in that; Comprise sample circuit, HART communication interface/display interface, MCU control module, man-machine interface; Sample circuit keeps several modules to form by high-frequency drive, capacitance detecting, bridging separation, sampling successively, and sample circuit detects measurand, sends into the MCU control module after the capacitance of measuring is handled; The MCU control module receives the instruction that man-machine interface comes, and send HART communication interface/display interface after the signal Processing that reception comes to sample circuit simultaneously, and standard signal is seen off and video data.
2. according to the said condenser type radio frequency admittance of claim 1 level transmitter; It is characterized in that; Said high frequency drive circuit produces a high frequency sinusoidal excitation signal; Sinusoidal signal obtains the square wave signal shaping and filtering through BPF., obtains meeting the pumping signal of design at last through driving circuit, and pumping signal is carried on the capacitive transducer of capacitance detecting.
3. the method for work of a condenser type radio frequency admittance level transmitter; Comprise condenser type radio frequency admittance level transmitter; It is characterized in that; The high frequency sinusoidal excitation signal of said high-frequency drive output is loaded into capacitive transducer; The AC compounent through analyzing its response signal and the size of DC component; Try to achieve the impedance and the capacitive reactance of capacitive transducer through the computing of MCU; And obtaining the bridging capacitive reactance by the real part of the bridging equivalent admittance in the radio frequency admittance theory
and counter the pushing away of conclusion of imaginary part numerically equal, actual capacitance=measurement electric capacity-bridging electric capacity then is again through being provided with HART communication interface circuit, MCU control circuit unit; The simulating signal of sensor sample is through the MCU control circuit unit; Compare computing, carry out exporting 4～20mA simulating signal by D/A conversion unit after the data processing, realize the level gauging of capacitance object position transmitter according to sampled value; Simultaneously, HART communication interface electric circuit inspection demodulates the digital signal of stack to the external control signal that is superimposed upon on 4～20mA direct current signal, sends CPU to and handles; Process result is the alternating signal of HART semaphore request and is superimposed upon on the direct current signal through HART communication interface circuit modulation conversion.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN105784060A (en) * | 2014-12-26 | 2016-07-20 | 中核建中核燃料元件有限公司 | Material level detection device and method for uranium dioxide powder |
| CN108896132A (en) * | 2018-05-08 | 2018-11-27 | 鲁东大学 | Level gauging unit and material level gauge in a kind of RF admittance level meter |
| CN108896131A (en) * | 2018-05-08 | 2018-11-27 | 鲁东大学 | Level gauging unit and material level gauge in RF admittance level meter based on temperature-compensating |
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| CN101210838A (en) * | 2006-12-27 | 2008-07-02 | 德菲电气（北京）有限公司 | Four-wire radio frequency admittance level gauge and material level meter and its method |
| CN101393461A (en) * | 2007-09-21 | 2009-03-25 | 上海平迪凯特仪器仪表有限公司 | Radio frequency admittance admittance level controller |
| CN201974212U (en) * | 2010-12-08 | 2011-09-14 | 上海自动化仪表股份有限公司 | Capacitive radio frequency admittance level transmitter |
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN101210838A (en) * | 2006-12-27 | 2008-07-02 | 德菲电气（北京）有限公司 | Four-wire radio frequency admittance level gauge and material level meter and its method |
| CN101393461A (en) * | 2007-09-21 | 2009-03-25 | 上海平迪凯特仪器仪表有限公司 | Radio frequency admittance admittance level controller |
| CN201974212U (en) * | 2010-12-08 | 2011-09-14 | 上海自动化仪表股份有限公司 | Capacitive radio frequency admittance level transmitter |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN105784060A (en) * | 2014-12-26 | 2016-07-20 | 中核建中核燃料元件有限公司 | Material level detection device and method for uranium dioxide powder |
| CN108896132A (en) * | 2018-05-08 | 2018-11-27 | 鲁东大学 | Level gauging unit and material level gauge in a kind of RF admittance level meter |
| CN108896131A (en) * | 2018-05-08 | 2018-11-27 | 鲁东大学 | Level gauging unit and material level gauge in RF admittance level meter based on temperature-compensating |
| CN108896131B (en) * | 2018-05-08 | 2019-11-26 | 鲁东大学 | Level gauging unit and material level gauge in RF admittance level meter based on temperature-compensating |
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Application publication date: 20120704 |