Sensor and transmitter in instruments, meters and plays an important role in industrial automation field. Unlike sensor, transmitter in addition to convert non-electricity into measurable quantity, general also has certain effect. This paper simply introduces the features of all kinds of transmitter, for users to choose.

A, integrated temperature transmitter

Integrated temperature transmitter by general temperature measuring probe (thermocouple or heat resistance sensors) and two wire system solid electronic units. In the form of solid module temperature measuring probe directly installed on the terminal box, thus forming the integration of the transducer. Integrated temperature transmitter with thermal resistance and thermocouple generally fall into two types.

Heat resistance temperature transmitter is the R/V conversion unit, the benchmark unit, linear circuit, reverse connect protection, current limiting protection, V/I conversion unit, etc. Measuring temperature resistance signal amplification, and then by linear circuit of temperature compensate nonlinear relationship with the resistance, after the V/I conversion circuit outputs a measured a linear relationship with temperature and constant current signal of 4 ~ 20 ma.

Thermocouple temperature transmitter by the reference source, cold junction compensation, amplifier unit, linearization, V/I conversion, broken accidentally processing, reverse connect protection, current limiting circuit protection units. It is the cold junction compensation of thermocouple thermoelectric potential enlarged, cap again by linear circuits eliminate the nonlinear error of thermoelectric potential and temperature, the final amplification into 4 ~ 20 ma current output signal. In order to prevent the thermocouple measurement by thermocouple wire break and make the temperature control failure caused by accident, transmitter will also include a power protection circuit. When the thermocouple wire break or pick up solution, transmitter will output the maximum 28 (ma) in order to make the instrument to cut off the power supply.

Integrated temperature transmitter has simple structure, saves the wire, big output signal, strong anti-interference ability, good linear, simple display instrument, solid module earthquake moisture-proof, reverse connection protection and current limiting protection, reliable work, etc. The output of the integrated temperature transmitter for unification of 4 ~ 20 ma signal; Can be used with microcomputer system or other conventional instrument match. Can also be user requirements in explosion-proof type or fire type measuring instrument.

Second, the pressure transmitter

Pressure transmitter also calls the difference transmitter, mainly by the load cell sensor, module circuit and display meter, watchcase and process fittings, etc. It can will receive such as gas, liquid pressure signal into standard current voltage signal, in order to supply instructions alarming device, recorder, controller and other secondary instrument measurement, instructions and process adjustment.

Pressure transmitter measuring principle diagram is shown in figure 3. Its measurement principle is: the process pressure and reference, respectively applied to the ends of the integrated silicon pressure sensor, the differential pressure make the silicon wafer deformation (displacement is very small, only microns), so that the silicon wafers made of semiconductor technology, full-motion wheatstone bridge in external driven by current source output voltage signal is proportional to the pressure of the mV level. Due to the strong excellent silicon material, so the output signal of the linearity and variation are very high. Work, pressure transmitter to physical quantity being measured into the mV level of voltage signal, and sent to a high magnification and can cancel each other out temperature drift of a differential amplifier. After amplification of the signal by voltage current transform into the corresponding current signal, again through the nonlinear correction, and finally with the input pressure into the standard linear relation between the current voltage signal.

Pressure transmitter according to the pressure range can be divided into general pressure transmitter (0.001 MPa ~ 20 mp3) and differential pressure transmitter (0 ~ 30 kpa) two kinds.

Third, liquid level transmitter

1, the ball float liquid level transmitter

Ball float liquid level transmitter ducts, the signal is measured by the magnetic floating ball, unit, the electronic unit, junction box and install.

Generally the proportion of magnetic float is less than 0.5, can float on the surface and move up and down along the measuring tube. Intraductal equipped with measuring element, it can will be under outer magnetic liquid level measurement signal is proportional to the resistance of the liquid level change signal, and converting electronic unit 4 ~ 20 ma, or other standard signal output. The transmitter module circuit, has the advantages of acid, moisture, shock, anti-corrosion, internal circuit has permanently within the current feedback circuit and protect circuit, can make the output current is not more than 28 biggest ma, and thus can be reliably protect the power supply and the secondary instrument is damaged.

2, Jane type liquid level transmitter

Buoy type liquid level transmitter is change the magnetic float to buoy, it is designed according to the principle of Archimedes. Buoy type liquid level transmitter is to use small metal film strain sensor technology to measure the liquid level, the position or density. It can be done in the scene button at work routine set *.

3rd, static pressure and liquid level transmitter

The transmitter using hydrostatic pressure measuring principle of work. It usually chooses silicon pressure measured by pressure sensor will be converted to electrical signals, then through amplifying circuit amplification compensation and compensation circuit, and finally to 4 ~ 20 ma or 0 ~ 10 ma current output.

Fourth, capacitive level transmitter

Capacitive level transmitter is suitable for the industrial enterprises in the process of production to measure and control the production process, mainly used as a kind of conductive liquid level and the conductive medium distance continuous measurement of powder or granular solid material level and instructions.

Capacitive liquid level transmitter is composed of capacitor sensor and electronic module circuit, it with two wire system 4 ~ 20 ma constant current output type as the base, through the transformation, can use the way of three lines or four lines output, the output signal form for 1 ~ 5 v, 0 ~ 5 v, 0 ~ 10 ma, and other standard signal. Insulated electrode capacitance sensor and a measuring of medium cylindrical metal container. When material level rise, due to the dielectric constant of conductive material significantly less than the dielectric constant of the air, so the capacitance changes with the height of the material. Transmitter module circuit by the reference source, pulse width modulation, transformation, constant gain, feedback and current limit and other units. On the principle of pulse width tuning, measuring is the advantage of low frequency, radio frequency interference on the surrounding yuan, good stability, good linear, no temperature drift, etc.

Five, the ultrasonic transducer

Ultrasonic transducer is divided into general ultrasonic transducer (no header) and the integration of the ultrasonic transducer, the integration of the ultrasonic transducer is more commonly used.

Integrated ultrasonic change new device by the header (such as LCD displays) and probe in two parts, this direct 4 ~ 20 ma output signal of the transmitter is the miniaturization of sensitive element (probe) and the electronic circuit assembly together, so that a smaller, lighter weight, the price is cheaper. Ultrasonic transducer can be used in the liquid level. Level measurement and channelization, open channel flow measurement, and can be used to measure the distance.

Six, antimony electrode transmitter acidity

Antimony electrode transmitter acidity PH detection, automatic cleaning, electrical signal is converted to the integration of industrial online analytical instrument, it is made up of antimony electrode and reference electrode PH measurement system. Antimony in acid solution was measured, due to the electrode surface will generate antimony trioxide oxidation layer, so the metal antimony and antimony trioxide can form between the potential difference. The size of the potential difference depends on three antimony oxide concentration, the concentration of measured hydrogen ions in acid solution and modest. If the antimony and antimony trioxide and aqueous solution of moderate as 1, the electrode potential is available nernst formula calculated.

Solid module of antimony electrode acidity transmitter circuit is composed of two parts. In order to field effect on the safe side, the power part adopts ac 24 v for secondary instrument power supply. The power supply in addition to provide clean motor drive power supply, also should through the current dc voltage is converted to the corresponding conversion unit, used for analog circuit. The second part is measuring circuit in the transmitter, it makes the reference signals from sensors and PH PH signal after amplification to slope adjustment and positioning circuit, in order to reduce signal resistance and can be adjusted. Will be of the signal and the temperature of the PH value of the enlarged signal superposition and then sent into transform circuit, the output and the PH value corresponding to 4 ~ 20 ma current signal with constant current to the secondary instrument to complete the display and control of PH value.

Seven, acid, alkali, salt concentration transmitter

Acid, alkali, salt concentration transmitter by measuring the conductivity value to determine the concentration of solution. It can continuous online testing industry in the process of acid, alkali, salt concentrations in the aqueous solution. The transducer is mainly applied to the confecting of boiler feed water treatment, chemical industry and environmental protection and other industrial production process.

Acid, alkali, salt concentration transmitter working principle is: in a certain range, the concentration of acid and alkali solution and its conductivity is proportional to the size of the. Therefore, as long as the measure of solution conductivity size change is that the concentration of acid and alkali. When into the special test solution conductance pool, if you ignore the electrode polarization and the capacitance, can be equivalent to a pure resistance. In persistence pressure alternating current to flow through, its output current and a linear relationship with the conductivity, and conductivity with acid, alkali concentration in the solution is proportional to the relationship. So as long as the current measure solution, can calculate the concentration of acid, alkali, salt.

Acid, alkali, salt concentration transducer is mainly composed of conductance pool, electronic module, the display of header and shell. Electronic module circuit, by the excitation power supply, conductance, transconductance amplifier, phase-sensitive rectifier, modem pool, temperature compensation, overload protection and current conversion and other units.

Eight, conductivity transmitter

It is by measuring the conductivity of the solution values to indirect measurement of ion concentration process instrumentation (integration) in the transmitter, can online aqueous solution in the process of continuous detection of industrial electric conductivity. Because the electrolyte solution and metal conductor as good conductors of electricity, so there will be resistance when current flows through the electrolyte solution, and in accordance with ohm's law. But the liquid resistance temperature characteristic, in contrast to the metal conductor, with negative temperature characteristic. To distinguish it from the metal conductor, conductive ability of electrolyte solution using conductance (the reciprocal of resistance) or electrical conductivity (the reciprocal of resistivity). When two electrodes of the insulation conductance pool, each other if placed between the test solution, and access to constant voltage alternating current, the current loop can be built up. If the size of the voltage and electrode size is fixed, the loop current as a function of conductivity is certain. Thus, measuring the flow of current in the solution under test, will be able to measure the conductivity of the solution under test. Conductance transducer structure and the circuit with acid, alkali, salt concentration transmitter is the same.

Nine, intelligent transmitter

Intelligent transmitter is made up of sensors, and microprocessor (microcomputer) phase structure. It made full use of the computing and storage capacity of the microprocessor, to process the data of sensors, including the measurement signal conditioning (such as filtering, amplifying, A/D conversion, etc.), data display, automatic calibration and compensation, etc.

The microprocessor is the core of the intelligent transmitter. It is not only to calculate the measurement data, storage, and data processing, also can be adjusted through the feedback loop of sensor, in order to make the best acquisition data. Because the microprocessor has a variety of software and hardware function, thus it can complete the traditional transducer difficult tasks to complete. So intelligent transmitter to reduce the manufacturing difficulty of sensors, and the great master to improve the performance of the sensor. In addition, intelligent transmitter has the following features:

1. With automatic compensation ability, can through the software of sensor's nonlinearity, temperature drift, drift and automatic compensation.

Since diagnosis, after electrify to self-inspection of sensors, to check each part of the sensor is normal, and make a judgment. Convenient accurate data processing, automatic processing data according to internal applications, such as statistical processing, which can reduce abnormal values.

2. Have the function of two-way communication. Microprocessor can not only receive and process the sensor data, but also the information feedback to the sensor, so as to adjust and control the measurement process. Sensor for information storage and memory, it can store the characteristics of the data, configuration information and compensation characteristics, etc.

3. Interface with the digital quantity output function, can the output digital signal easily and computer and field bus connections.