CN201974212U

CN201974212U - Capacitive radio frequency admittance level transmitter

Info

Publication number: CN201974212U
Application number: CN2010206477632U
Authority: CN
Inventors: 王鑫; 杨振荣
Original assignee: Shanghai Automation Instrumentation Co Ltd
Current assignee: Shanghai Automation Instrumentation Co Ltd
Priority date: 2010-12-08
Filing date: 2010-12-08
Publication date: 2011-09-14
Anticipated expiration: 2020-12-08

Abstract

The utility model relates to a capacitive radio frequency admittance level transmitter and a working method; a high frequency sine excitation signal which is output by a high frequency driver is loaded to a capacitive sensor; the impedance and capacitive reactance of the capacitive sensor are obtained by analyzing the size of alternating component and direct-current component of a response signal of the capacitive sensor and the operation of an MCU (microprogrammed control unit); a hanging capacitive reactance is obtained by the reverse derivation of a conclusion that the real part and the imaginary part of handing equivalent admittance in a radio frequency admittance theory are equivalent in numerical values, then the actual capacitance is equal to a result of the hanging capacitance being substracted from a measuring capacitance; the level transmitter eliminates the measuring error of hanging, which is generated as a measured medium is adhered to an electrode, solves the difficult problem of big measuring error of thick or solid powder as a measured material, increases the measuring precision and ensures the stability of the measurement.

Description

Condenser type radio frequency admittance level transmitter

Technical field

The utility model 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 modernized industry, production scale is big, the speed height, and high temperature, high pressure, severe corrosive or inflammable and explosive material are often arranged, most important especially for the supervision and the automatic control of thing position.Therefore, thing position detection technique accounts for critical role in the industrial process control of modernization.Level transmitter in the power station, industrial circle such as petrochemical industry, oil field is widely used.By 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 provide useful information 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 to 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, mainly as class 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 measuring the thing 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, a kind of level meter that is most widely used.

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 that measuring electric capacity has comprised the electric capacity of true thing position correspondence and electric capacity two parts that the bridging layer forms, 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, electric conductivity is strong more, its influence is big more, the bridging error that produces is also just big more, this is because when measuring conductive medium, material electric capacity is that conductive material and sensing electrode core are that medium forms with the sensing electrode insulation course, and the bridging layer has electric conductivity, electric conductivity is strong more, it is more little to be equivalent to bridging layer resistance, 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 utility model is to be big, the problem of unstable of measuring error of thickness or pressed powder at tested material at existing capacitance object position transmitter, a kind of condenser type radio frequency admittance level transmitter has 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.

The technical solution of the utility model 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 sent and video data.

Described high frequency drive circuit produces a high frequency sinusoidal excitation signal, sinusoidal signal obtains the square wave signal shaping and filtering by bandpass filter, obtain meeting the pumping signal of design at last by driving circuit, pumping signal is carried on the capacitive transducer of capacitance detecting.

The beneficial effects of the utility model are: the utility model condenser type radio frequency admittance level transmitter, 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, has improved the precision of measuring, and has guaranteed the stable of measurement.

Description of drawings

Fig. 1 is the bridging equivalent circuit diagram;

Fig. 2 is the utility model condenser type radio frequency admittance level transmitter structural representation;

Fig. 3 is the utility model condenser type radio frequency admittance level transmitter medium-high frequency driving circuit figure;

Fig. 4 is HART communication interface figure in the utility model condenser type radio frequency admittance level transmitter.

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 reliable, more accurate, 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, uses the high frequency measurement admittance so the radio frequency admittance technology can be understood as.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 to the analysis of capacitance object position instrument measurement principle as can be known, 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 determined 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 the equivalent admittance of bridging by the uniform transmission line theory

Real part and the conclusion of imaginary part numerically equal.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, 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, deduct 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 comprises sample circuit 1, HART communication interface/display interface 2, MCU control module 3, man-machine interface 4 as shown in Figure 2, 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 sent and video data, and HART communication interface 2 also comprises the modulation-demodulation circuit that is made of special-purpose HART modem chip.

The utility model is loaded into capacitive transducer by one road high frequency sinusoidal excitation signal, the AC compounent by analyzing its response signal and the size of DC component, try to achieve the impedance and the capacitive reactance of capacitive transducer by the computing of MCU, and by the bridging equivalent admittance in the radio frequency admittance theory

Real part and counter the pushing away of conclusion of imaginary part numerically equal obtain the bridging capacitive reactance, actual capacitance=measurement electric capacity-bridging electric capacity then, thus eliminated the error that bridging brings.Again by 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; The result who handles 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.In the utility model, 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 in the mode of digital communication by 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 by bandpass filter, obtains meeting the pumping signal of design at last by driving circuit, as 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 by resistance is that the DC channel of sinusoidal excitation signal is corresponding, this voltage obtains after by low-pass filtering is 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 by the potential-divider network that capacitor C 0 and sensor are formed, the amplitude that exchanges records by AD637 true rms measurement chip, because we know the amplitude of pumping signal, frequency, capacitor C 0, resistance R 0, 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 by 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.

HART communication interface usefulness MCU and HART MODEM chip development are based on the capacitance object position transmitter of HART communication, and be 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 that interface circuit design realizes 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, the bus free time be described, 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.Transformer coupled to 4～20mA direct current signal by the data that OTXA sends by operational amplifier amplification and process.

Claims

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, 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 sent and video data.

2. according to the described condenser type radio frequency admittance of claim 1 level transmitter, it is characterized in that, described high frequency drive circuit produces a high frequency sinusoidal excitation signal, sinusoidal signal obtains the square wave signal shaping and filtering by bandpass filter, obtain meeting the pumping signal of design at last by driving circuit, pumping signal is carried on the capacitive transducer of capacitance detecting.