Transmitter

1 The main concepts

Sensors that are used in industrial fields and can output standard signals are called transmitters.

This term is sometimes used in common with sensors.

In "Principles of Automatic Control", the transmitter is a converter that converts the output signal of the sensor into a signal that can be recognized by the controller. Sometimes it is common to use sensors because the output signal of most modern sensors is already a signal that can be received by a universal controller. This signal can be directly recognized by the controller without conversion from the transmitter. Therefore, the traditional sense of “transmitter” should be: “convert the output signal of the sensor into an instrument that can receive the standard signal received by the controller or measuring instrument”. In the automatic control: Source -> Sensor -> Transmitter -> Operator Controller -> Actuator -> Control Output.

There are many types of transmitters. In general, a signal is sent from the transmitter to the secondary instrument to display the measurement data of the secondary instrument.

A device that converts a physical measurement signal or an ordinary electrical signal into a standard electrical signal output or can be output as a communication protocol. Generally divided into: temperature / humidity transmitter, pressure transmitter, differential pressure transmitter, liquid level transmitter, current transmitter, power transmitter, flow transmitter, weight transmitter and so on.

2 protection function

1, input overload protection;

2, output over-current limit protection;

3, output current long-term short circuit protection;

4, two-wire port transient inductive and surge current TVS suppression protection;

5, the working power overvoltage limit protection ≤ 35V;

6, reverse working power protection.

Principle 3

Transmitter is the Chinese name, English is: TRANSMITTER
As the name implies, the transmitter contains the meaning of "change" and "send."
The so-called "change" refers to the conversion of various physical quantities from sensors into an electrical signal. For example, thermocouples are used to convert temperature to potential; current transformers are used to convert large currents to small currents. Since electrical signals are the easiest to handle, modern transmitters convert all kinds of physical signals into electrical signals. Therefore, the transmitters we talk about are usually turned into "electrical."
The so-called "send" refers to the various electrical signals that have become, in order to facilitate other instruments or control devices to receive and transmit, once again through the electronic circuit, the electrical signals from the sensor, unified (such as 4-20MA). The method is implemented by multiple operational amplifiers. This "change" + "send" constitutes the most commonly used modern transmitter.
For example: SST3-AD is a current transducer that converts the output current of a current transformer into a standard 4-20MA current transducer; for example: SST4-LD, the weight signal from a weight sensor can be converted into a standard 4 -20MA weight transmitter.

4 advantages and disadvantages

After the marketization of the means of production, intensified fierce competition, which is difficult to distinguish between true and false, and because the transmitter is a marginal discipline, many engineering designers are relatively unfamiliar with this, and some manufacturers' product industry level and civil commercial level indicators are confused (industrial grade The price is 2-3 times that of civilian commercial grade).

Take the commonly used 0.5-level precision current-voltage transmitter as an example, start from the following methods to identify the true and false advantages and disadvantages:

1, the reference must be stable, 4mA is the corresponding input zero reference, reference instability, talk about the accuracy of linearity, cold start 3 minutes 4mA zero drift within 40% less than 0.5%; (ie 3.98- 4.02mA), the pressure drop on the load 250Ω is 0.995-1.005V, the foreign IC chip often uses the expensive energy gap reference, the temperature drift coefficient changes by 10ppm per degree;

2. The total consumption current of the internal circuit is less than 4mA, equals to 4.000mA after the setting is added, and the active rectifier filter amplifies the constant current circuit, and the current consumption also changes due to the change of the primary input. The IC chip in foreign countries adopts constant current power supply;

3. When the working voltage is 24.000V, when the full-scale range is 20.000mA, the full-scale reading of 20.000mA will not change due to the change of the load 0-700Ω; the change does not exceed 20.000mA0.5%;

4, when full scale 20.000mA, load 250Ω,

Full-scale reading of 20.000mA will not change due to a change in the operating voltage of 15.000V-30.000V; variation does not exceed 20.000mA0.5%;

5. When the primary side is overloaded, the output current does not exceed 25.000mA+10%. Otherwise, the 24V power supply and A/D input clamp circuit used in the PLC/DCS for the transmitter are damaged due to excessive power consumption. The transmitter-to-output in the transmitter is also damaged due to excessive power consumption, and the A/D input clamp circuit is even more vulnerable.

6. When the working voltage is 24V reversed, the transmitter must not be damaged and must have polarity protection;

7, when the two lines due to induction and induced surge voltage exceeds 24V to clamp, do not damage the transmitter; generally in parallel between the two lines 1-2 TVS transient protection diode 1.5KE can inhibit every 20 The pulse of positive and negative pulse of 20 millisecond pulse width of one time interval of one second, transient withstand impact power 1.5KW-3KW;

8. The linearity of the product labeling 0.5% is an absolute error or a relative error. It can be clearly identified by the following method: the following indicators are true linearity of 0.5%;

When the primary input is zero, the output is 4mA plus or minus 0.5% (3.98-4.02mA).

The pressure drop on the load 250Ω is 0.995-1.005V;

When the input of the primary side is 10%, the output voltage is 5.6mA plus or minus 0.5% (5.572-5.628mA). The load voltage at 250 ohms is 1.393-1.407V.

When the primary input is 25%, the output voltage is 8mA plus or minus 0.5% (7.96-8.04mA), and the load voltage at 250Ω is 1.990-2.010V.

When the input voltage is 50% on the primary side, the output voltage is 12mA plus or minus 0.5% (11.94-12.06mA). The pressure drop on the load 250Ω is 2.985-3.015V;

When the input of the primary side is 75%, the output voltage is 16mA plus or minus 0.5% (15.92-16.08mA). The load pressure at 250Ω is 3.980-4.020V.

When the primary input is 100%, the output is 20mA plus or minus 0.5% (19.90-20.10mA). The load is 250mA with a voltage drop of 4.975-5.025V.

9. The current limit must be limited when the primary input is overloaded:

When the primary input overload is greater than 125%, the output overcurrent limit 25mA+10% (25.00-27.50mA) load 250Ω pressure drop is 6.250-6.875V;

10, whether the sensing surge voltage exceeds 24V when there is no clamping recognition: In the two-wire output port and an AC 50V pointer meter, with AC 50V then two lines to instantly touch the two-wire output port to see if there is no clamp Bit, how many volts can be clamped at a glance

11, with or without polarity protection identification: with an analog multimeter multimeter by 10K positive and negative measurement of the two-wire output port, there is always an infinite resistance Ω, there is polarity protection;

12, there is no pole output current long-term short-circuit protection: the original input 100% or when the overload is greater than 125% -200%, the load 250Ω short circuit, measuring the short-circuit protection limit is 25mA+10%;

13. Industrial-grade and civil-commercial-grade discrimination: Industrial-grade operating temperature range is -25 degrees to +70 degrees, temperature drift coefficient is 100ppm per degree, that is, temperature changes by 1 degree per degree, and accuracy changes to one ten-thousandth; Civilian commercial grade operating temperature range is 0 degrees (or -10 degrees) to +70 degrees (or +50 degrees), temperature drift coefficient is 250ppm per degree, that is, temperature changes every degree 1 degree, accuracy changes to 2/10000 Point five; temperature and voltage transmitter temperature drift coefficient can be tested using the incubator or high and low temperature box to test more cumbersome.

The above 13 methods can also be used to distinguish between the true and false advantages and disadvantages of other transmitters.

5 common faults

1. Install the transmitter so that the pressure sensitive part is perpendicular to the direction of gravity.

If the installation conditions are limited, adjust the transmitter zero to the standard value after the installation is fixed.

2. The residual pressure is not released, so the sensor's zero position will not come back. The best way to eliminate this reason is to remove the sensor and see if the zero position is normal. If the seal is replaced normally, try again.

3, pressure transmitter output does not change, and then pressurize the transmitter output suddenly changes, pressure transmitter zero can not go back. The cause of this phenomenon is most likely caused by the pressure sensor seal.

4. Whether it meets the power supply requirements; there is no wiring error between the power supply and the transmitter and load device. No voltage or reversed polarity on the transmitter terminals can cause the transmitter to output no voltage signal.

5, pressure sensor and transmitter housing generally need to be grounded, signal cable must not be mixed with the power cable laying, sensors and transmitters should avoid strong electromagnetic interference around. Sensors and transmitters in use

The periodic inspection shall be carried out in accordance with the regulations of the industry.

6. When selecting pressure sensors and transmitters, the user should fully understand the working conditions of the pressure measurement system and make reasonable choices according to their needs so that the system can work in the best condition and reduce the project cost.

7. Transfer to the two sides of the measuring diaphragm through the spacer and the filling liquid in the element. Measure the diaphragm and the electrodes on both sides of the insulating sheet to form a capacitor.

8, pressure transmitter requires inspection once a week, once a month inspection, mainly to remove the dust inside the instrument, a serious inspection of electrical components, the current value of the output should be always proofread, the pressure transmitter is weak inside, certain To be separated from the outside world by strong electricity.

6 types of features

There are many types of transmitters. The transmitters used in industrial control instruments mainly include temperature transmitters.

Pressure transmitters, flow transmitters, current transmitters, voltage transmitters, etc.

Transmitters play an important role in the field of instruments, meters and industrial automation. Different from the sensor, the transmitter generally has a certain amplification effect, in addition to converting the non-electricity into measurable electricity.

Pressure Transmitters:

Pressure transmitter, also called differential transmitter, is mainly composed of load cell sensor, module circuit, display meter, case and process connector. It can convert the received gas, liquid and other pressure signals into standard current and voltage signals to provide secondary instruments such as alarm indicators, recorders, and regulators for measurement, indication and process adjustment.

Pressure transmitter measurement principle is: process pressure and reference pressure acting on the two sides of the integrated silicon pressure sensitive element, the differential pressure of the silicon wafer deformation (displacement is very small, only μm level), so that the semiconductor technology using silicon The resulting full-dynamic Wheatstone bridge outputs an mV-level voltage signal proportional to pressure driven by an external current source. Since the silicon material has excellent strength, the linearity and variation index of the output signal are both high. At work, the pressure transmitter converts the measured physical quantity into a mV-level voltage signal and sends it to a differential amplifier that has a very high amplification and cancels temperature drift. The amplified signal is converted into a corresponding current signal by voltage-current conversion, and then subjected to a non-linear correction. Finally, a standard current-voltage signal having a linear relationship with the input pressure is generated.

Pressure transmitters can be divided into two general pressure transmitters (0.001MPa to 20MP3) and differential pressure transmitters (0 to 30kPa) according to the pressure measurement range.

Integrated temperature transmitter:

The integrated temperature transmitter generally consists of a temperature probe (thermocouple or RTD sensor) and a two-wire solid electronic unit. The solid-state module is used to mount the temperature probe directly in the junction box to form an integrated transmitter. Integrated temperature transmitters are generally classified into two types: thermal resistance and thermocouple type.

Thermal resistance temperature transmitter is composed of reference unit, R/V conversion unit, linear circuit, reverse connection protection, current limiting protection, V/I conversion unit and so on. After the temperature-resistor resistance signal is converted and amplified, the linear circuit compensates the nonlinear relationship between temperature and resistance. After the V/I conversion circuit, a constant-current signal of 4-20 mA which is in linear relationship with the measured temperature is output.

The thermocouple temperature transmitter is generally composed of a reference source, a cold junction compensation, an amplifying unit, a linearization process, a V/I conversion, a burnout processing, a reverse connection protection, and a current limit protection circuit unit. It is the thermal potential generated by the thermocouple through the cold-end compensation after amplification, and then cap the linear circuit to eliminate the thermal potential and temperature of the nonlinear error, and finally amplified into 4 ~ 20mA current output signal. In order to prevent accidents caused by temperature control failure due to galvanic disconnection in thermocouple measurement, a power failure protection circuit is also provided in the transmitter. When the thermocouple breaks or fails to connect properly, the transmitter will output the maximum value (28mA) to make the meter cut off the power.

The integrated temperature transmitter has the advantages of simple structure, saving lead wire, large output signal, strong anti-interference ability, good linearity, simple display instrument, solid module moisture resistance, reverse connection protection, current limiting protection, and reliable work.

The output of the integrated temperature transmitter is a unified 4-20mA signal; it can be used in conjunction with a microcomputer system or other conventional instruments. Can also be made of explosion-proof or fire-proof measuring instruments.

Level Transmitter:

1、Floating ball level transmitter

The float level transmitter is composed of a magnetic float, a measurement catheter, a signal unit, an electronic unit, a junction box, and a mounting member.

The magnetic float generally has a specific gravity of less than 0.5 and can float above the liquid surface and move up and down along the measuring tube. The catheter is equipped with a measuring element, which can convert the measured liquid level signal into a resistance signal proportional to the change of liquid level under the action of external magnetic flux, and converts the electronic unit into 4-20 mA or other standard signal output. The transmitter is a module circuit, which has the advantages of acid resistance, moisture resistance, shockproof, anti-corrosion, etc. The circuit contains a constant current feedback circuit and internal protection circuit, so that the maximum output current does not exceed 28mA, which can reliably protect the power supply and make the two The secondary meter is not damaged.

2, floating type liquid level transmitter

Float type liquid level transmitter is to change the magnetic float to buoy, which is designed according to the Archimedes buoyancy principle. Float type liquid level transmitters use tiny metal film strain sensing technology to measure liquid level, boundary level, or density. When it is working, it can be set by the on-site button

3, static pressure or liquid level transmitter

The transmitter operates using the principle of hydrostatic pressure measurement. It generally selects the silicon pressure pressure sensor to convert the measured pressure into an electric signal, and then compensates by the amplifying circuit and the compensating circuit, and finally outputs the current in a 4-20 mA or 0-10 mA current mode.

Capacitive level transmitter:

Capacitive position transmitter is suitable for industrial enterprises to measure and control the production process during the production process. It is mainly used as a continuous measurement and indication of long-distance measurement of liquid level or powdered solid material level in conductive and non-conductive media.

The capacitive level transmitter consists of a capacitive sensor and an electronic module circuit. It uses a two-wire constant current output of 4 to 20mA as the base type. After conversion, it can be output in three-wire or four-wire mode. The output signal is formed as 1 to 1 5V, 0 ~ 5V, 0 ~ 10mA and other standard signals. The capacitive sensor consists of an insulated electrode and a cylindrical metal vessel equipped with a measuring medium. When the material level rises, the dielectric constant of the non-conductive material is significantly smaller than the dielectric constant of air, so the capacitance changes with the height of the material. Transmitter module circuit consists of reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting unit. The advantage of using the pulse width modulation principle to measure is that it has a low frequency, interference with ambient radio frequency, good stability, good linearity, and no significant temperature drift.

Ultrasonic transmitter:

Ultrasonic transmitters are divided into general ultrasonic transmitters (without headers) and integrated ultrasonic transmitters. Integral ultrasonic transmitters are more commonly used.

The integrated ultrasonic transformer consists of a watch head (such as an LCD display) and a probe. This transmitter, which directly outputs a 4-20 mA signal, assembles a miniaturized sensor (a probe) and an electronic circuit. Make it smaller, lighter, and cheaper. Ultrasonic transmitters can be used for level. Level measurement and flow measurement of open channels, open channels, etc. can be used to measure distances.

锑 Electrode pH Transmitter:

锑 Electrode pH Transmitter is an industrial online analytical instrument that integrates PH detection, automatic cleaning, and electrical signal conversion. It is a pH measurement system consisting of a helium electrode and a reference electrode. In the measured acidic solution, a germanium trioxide oxide layer is formed on the surface of the germanium electrode, so that a potential difference is formed between the germanium surface and the germanium oxide. The magnitude of this potential difference depends on the concentration of trioxotide, which corresponds to the appropriate level of hydrogen ions in the acidic solution being tested. If the appropriate values ​​of germanium, antimony trioxide, and aqueous solution are all taken as 1, the electrode potential can be calculated using the Nernst formula.

The solid module circuit in the pH transmitter consists of two major components. For the sake of safety in the field, the power supply uses AC 24V to power the secondary instrument. In addition to supplying power to the cleaning motor, this power supply should also be converted into a corresponding DC voltage by the current conversion unit for use by the transmission circuit. The second part is the measurement transmitter circuit, which sends the reference signal and PH acidity signal from the sensor to the slope adjustment and positioning adjustment circuit so that the internal resistance of the signal can be reduced and adjusted. The amplified PH signal and temperature are compensated

After the signal is superposed, the differential conversion circuit is added. Finally, the 4-20 mA constant current signal corresponding to the PH value is output to the secondary meter to complete the display and control the PH value.

Acid, alkali, salt concentration transmitters:

The acid, alkali, and salt concentration transmitters determine the concentration by measuring the conductivity of the solution. It can continuously monitor the concentration of acid, alkali and salt in aqueous solution in the industrial process. This transmitter is mainly used in boiler water treatment, chemical solution preparation and environmental protection and other industrial production processes.

The working principle of the acid, alkali, and salt concentration transmitters is: within a certain range, the concentration of the acid-base solution is proportional to the size of its electrical conductivity. Therefore, as long as the measurement of the conductivity of the solution changes, the concentration of acid and alkali can be known. When the measured solution flows into a dedicated conductivity cell, if the electrode polarization and distributed capacitance are ignored, it can be equivalent to a pure resistance. When there is constant voltage alternating current flow,

The output current is linear with the conductivity, and the conductivity is proportional to the concentration of acid and alkali in the solution. Therefore, as long as the solution current is measured, the concentration of acid, alkali and salt can be calculated.

The acid, alkali, and salt concentration transmitters consist of a conductivity cell, an electronic module, a display meter, and a housing. The electronic module circuit is composed of excitation power supply, conductivity cell, conductance amplifier, phase sensitive rectifier, demodulator, temperature compensation, overload protection and current conversion.

Conductivity transmitters:

It is a process meter (integrated transmitter) that measures the ion concentration indirectly by measuring the conductance value of a solution, and can continuously detect the conductivity of an aqueous solution in an industrial process on-line.

Because the electrolyte solution is the same good electrical conductor as the metal conductor, there must be an electrical resistance when the current flows through the electrolyte solution, and it is in accordance with Ohm's law. However, the resistance temperature characteristics of liquids are contrary to those of metal conductors and have negative temperature characteristics. In order to distinguish from the metal conductor, the conductivity of the electrolyte solution is represented by conductance (reciprocal of resistance) or conductivity (reciprocal of resistivity). When two electrodes that are insulated from each other form a conductivity cell, a current loop is formed if the solution to be measured is placed in between, and a constant voltage alternating current is passed through. If the voltage size and the electrode size are fixed, the loop current and the conductivity have a certain function. In this way, measuring the current flowing in the solution to be measured, the conductivity of the solution to be measured can be measured.

The conductance transmitter has the same structure and circuit as the acid, alkali, and salt concentration transmitters.

Smart Transmitters:

The smart transmitter is a sensor and microprocessor (microcomputer) phase structure.

It makes full use of the microprocessor's computing and storage capabilities, and can process sensor data, including conditioning of measurement signals (such as filtering, amplification, A/D conversion, etc.), data display, automatic correction, and automatic compensation.

The microprocessor is the heart of the smart transmitter. It can not only calculate, store and process the measurement data, but also adjust the sensor through the feedback loop to make the collected data the best. Because the microprocessor has a variety of software and hardware functions, it can accomplish tasks that traditional transmitters are difficult to accomplish. Therefore, the intelligent transmitter reduces the manufacturing difficulty of the sensor and improves the sensor performance largely. In addition, the intelligent transmitter also has the following features:

1. With automatic compensation ability, it can automatically compensate the nonlinearity, temperature drift, time drift, etc. of the sensor through software;

2, self-diagnosis, after the power can be self-test sensor to check the sensor is normal, and make a judgment;

3, data processing is convenient and accurate, can automatically process data according to internal procedures, such as statistical processing, removal of abnormal values;

4, with two-way communication. The microprocessor not only receives and processes sensor data, but also feeds information back to the sensor to adjust and control the measurement process.

5, can carry on the information storage and the memory, can store the characteristic data, the configuration information and the compensation characteristic and so on of the sensor;

6. With digital interface output function, the output digital signal can be conveniently connected with computer or field bus. [1]

Two-wire transmitter:

The two-wire system refers to the use of only two conductors to connect the transmitter on the spot with the control room. These two lines are both the power line and the signal line.

Two-wire system and three-wire system (one positive power line, two signal lines, one common GND) and four-wire system (two positive and negative power lines, two signal lines, one common GND), Low measurement accuracy.

A thermal resistance is a primary component that converts a change in temperature into a change in resistance. It is usually necessary to pass the resistance signal through a wire to a computer control device or other primary meter. Industrial thermal resistance is installed at the production site and there is a certain distance between it and the control room. Therefore, the wire of the thermal resistance will have a greater impact on the measurement results.

Line classification:

Two-wire system: The method of connecting a wire to the two ends of the RTD to induce the resistance signal is called two-wire system: This method of wiring is very simple, but since the connecting wire must have a wire resistance r, r size and the material and length of the wire With regard to factors, this lead method is only suitable for applications where measurement accuracy is low;

Three-wire system: The method of connecting a lead wire at one end of the root of a thermal resistor and connecting the two lead wires at the other end is called a three-wire system. This method is usually used in conjunction with a bridge to eliminate the influence of the lead resistance. It is an industrial system. The most commonly used in process control;

Four-wire system: The method of connecting two wires at both ends of the thermal resistor is called four-wire system, in which two wires provide a constant current I for the thermal resistance, converts R into a voltage signal U, and then passes the other two wires. Lead U to the secondary instrument. It can be seen that this lead wire method can completely eliminate the influence of the resistance of the wire, and is mainly used for high-precision temperature detection.

Thermal resistance adopts three-wire connection method. The use of three-wire system is to eliminate the measurement error caused by connecting wire resistance. This is because the circuit for measuring the thermal resistance is generally an unbalanced bridge. Thermistor acts as a bridge arm resistor of the bridge, and its connecting wire (from the thermal resistance to the central control room) also becomes a part of the bridge arm resistance. This part of the resistance is unknown.

And with the ambient temperature changes, causing measurement errors. Using a three-wire system, one wire is connected to the power supply terminal of the bridge, and the other two are respectively connected to the bridge arm on which the thermal resistor is located and the adjacent bridge arm. This eliminates the measurement error caused by the wire line resistance.

Two-wire system advantages:

1. It is not easily affected by parasitic thermocouple and resistance drop and temperature drift along the wire resistance. It can use very cheap and finer wires; it can save a lot of cables and installation cost;

2. When the output resistance of the current source is large enough, the voltage in the wire loop is sensed by the magnetic field coupling and will not have a significant impact. Because the current caused by the interference source is extremely small, the twisted wire can be used to reduce the interference; three-wire system Shielded cables must be used for four-wire systems. The shielding of shielded cables must be properly grounded.

3, capacitive interference will cause the receiver resistance related error, for 4 ~ 20mA two-wire loop, the receiver resistance is usually 250Ω (sample Uout = 1 ~ 5V) This resistance is small enough to produce significant errors, therefore, can The allowable length of wire is longer and longer than the voltage telemetry system;

4. Each single reading device or recording device can switch between different channels with different lengths of wire. It does not cause the difference in precision due to the unequal length of the wire, and achieves distributed acquisition. The advantage of distributed acquisition is that: Acquisition, centralized control....

5, 4mA is used for zero level, making it easy to judge open circuit and short circuit or sensor damage (0mA state).

6, in the two-wire output port is very easy to add one or two lightning surge devices, is conducive to safety lightning explosion.

Both the three-wire and four-wire transmitters will not be replaced by the two-wire transmitter.

Different kinds of transmitters

Different kinds of transmitters (24 photos)

From the foreign industry dynamics and the supply and demand of the transmitter chip can be a little known, the current transmitter should be installed in the power line of the field device when using, and the monitoring system with the single-chip microcomputer as the core is located far away from the equipment site. In the surveillance room, the two are generally tens or hundreds of meters apart or even farther apart. The environment of the equipment site is rather harsh. The strong electric signal will generate various electromagnetic interferences. The lightning induction will generate strong surge pulses. In this case, a thorny problem encountered in the SCM application system is how to remotely operate in a harsh environment. Reliably transmit tiny signals.

The output of the two-wire current transducer is 4 to 20mA, and it is converted into an analog voltage signal of 1 to 5V or 2 to 10V through a 250Ω precision resistor. There are many ways to convert it to a digital signal, if the system is in a poor environment The industrial site is used for a long time, so the safety and reliability of the hardware system work must be considered. The input module of the system uses a voltage-frequency conversion device LM231 to convert the analog voltage signal into a frequency signal, and the opto-coupled device TL117 isolates the analog and digital quantities.

At the same time, the analog signal processing circuit and the digital signal processing circuit respectively use two independent sets of power supplies, and the analog ground and the digital ground are separated from each other, so that the safety of the system operation can be improved. The use of voltage-frequency conversion device LM231 also has some anti-high-frequency interference.

In many applications controlled by the SCM, the transmitter must be used to convert the signal that cannot be directly measured by the SCM into an electrical analog signal that can be processed by the SCM, such as a current transmitter, a pressure transmitter, a temperature transmitter, and a flow rate change. Sender etc.

Most of the early transmitters were voltage output type, which means that the measurement signal was converted to 0-5V voltage output. This is the direct output of the op amp and the signal power is less than 0.05W. The digital signal is converted by the analog/digital conversion circuit for microcontroller to read and control. . However, when the signal needs to be transmitted over a long distance or when the power grid has large interference in the environment, the use of the voltage output type sensor is greatly limited, and the disadvantages such as poor anti-interference ability, line loss and accuracy are exposed. Wire-based current output transmitters have been widely used for their high anti-interference ability.

The voltage output type transmitter has extremely poor anti-interference ability and the destruction of the line loss, not to mention how high the accuracy, and sometimes the AC voltage is superimposed on the output DC voltage, which causes the SCM to make erroneous judgments and control errors. Will damage the equipment, output 0-5V absolutely can not be remote transmission, remote line voltage drop, accuracy is greatly reduced, many ADC, PLC, DCS input signal ports are made two-wire current output transmitter 4- 20mA, proved the inevitable trend of the voltage output transmitter was eliminated.

7 development trends

Pressure transmitter technology development:

Pressure and differential pressure transmitters are the most important ones in process variable transmitters. They have a wide range of applications. They can be used not only for pressure and differential pressure measurements, but also for other parameters such as flow, level, and specific gravity. For a 5000t/d cement production line, pressure transmitters must be installed at key parts of the process, such as the kiln head, the kiln tail, the top and bottom of the preheaters at all levels, the various air ducts and the cooling chambers, etc. To monitor the normal operation of the process. According to statistics, a total of 80 pressure transmitters are required for a new dry process cement production line.

The pressure transmitter is divided according to the measurement mechanism, the most common being capacitive, inductive, diffusion silicon, vibrating wire and so on. In addition to the accuracy of the DR differential pressure transmitter, which is used to measure the negative pressure of the kiln head, the accuracy is 0.5%. The rest is mostly 0.25%. It is characterized by the use of analog instruments with 4mA to 20mA transmission. The local display header is The pointer type has a range ratio of about 6:1 and a stability of 6 months. This is the first generation of analog transmitters. The second generation of products is smart transmitters. The so-called smart concept is that sensors and transmitters are microprocessor-driven and have the ability to communicate and self-diagnose. In addition to high precision (0.1% to 0.075%), high ratio (up to 100:1) and high stability (1 to 5 years), the smart pressure transmitter generally has a HART protocol or a production company. The agreement, the later product also has the FF or PROFIBUS-PA agreement that accords with the field bus international standard, it changes the on-the-spot display table to the digital type, but also may carry on the communicator or the control system long-distance configuration, realizes the long-distance establishment or Remote modify transmitter configuration data.

Into the 21st century, the third-generation transmitter - digital intelligent transmitter has gradually entered the field of vision. The third-generation transmitter adopts advanced detection technology to eliminate the effects of humidity, dust, and other harsh environments on the transmitter. The accuracy is higher, and its accuracy is reported to be better than 0.05%. The sub-range of the range is even smaller, the range ratio is expanded to 200?1, the stability is more than 5 years, and the communication protocol is more complete. The new transmitter also passed the safety argument and can guarantee safe shutdown when the process condition exceeds the critical value. Since the second-generation transmitter can meet the cement plant monitoring requirements, and the third-generation transmitter price is still relatively high, the author believes that the cement plant should not be promoted temporarily.

Technical development of smart temperature transmitters:

Temperature is also an important class of process variable transmitters. It is one of the basic elements for measuring flow, density, and other process variables. A 5000t/d cement production line must have temperature transmitters installed in key parts of the process and important equipment. According to statistics, a new dry process cement line requires a total of 80 to 100 pressure transmitters.

Based on the great progress in the development of temperature measurement, it also promotes the accuracy, reliability and repeatability of temperature transmitters in the field of process control industry, and at the same time, it also contributes to the continuous improvement of process control quality. contribution.

The so-called smart temperature transmitter refers to a temperature transmitter that combines the temperature sensor technology with additional electronic components. It can realize remote setting or remote modification of configuration data. Looking at the current temperature transmitter market and the application of the cement industry, there are mainly three types of different intelligent temperature transmitter products. From the application and cost perspective, each type of smart temperature transmitter has its advantages and disadvantages.

1. Weatherproof Temperature Transmitters These types of temperature transmitters are usually installed in high-demand and demanding applications such as kiln exhaust, precalciners, and five-stage preheaters. This type of temperature transmitter is housed in a hermetically sealed housing, which is usually made of stainless steel. It is characterized by high accuracy, high reliability, good safety and weather protection. Its main disadvantage is its relatively low price. high. It usually has an on-site header that can be monitored, adjusted and configured on site.

2, DIN rail mounting, instrument panel mounted temperature transmitter This type of temperature transmitter can use DIN rail mounting, usually installed in the control room. It is cheap, simple to install and maintain, and can be configured to match different types of temperature sensors. The measurement accuracy is lower due to the longer connection wires between the remotely mounted sensors. This type of temperature transmitter can be recommended for cement mills.

3, integrated temperature transmitter This type of temperature transmitter can be directly installed on the DIN connector of the temperature sensor, its advantages are low installation costs, small size, compatible with various types of temperature sensors. Since this temperature transmitter is directly mounted on the temperature sensor's connector, the electrical connection and sensor wiring are very simple. Cement plant kiln system in addition to the requirements of the site, the use of integrated temperature transmitter, grinding system can also be used.

The communication protocol used by the smart temperature transmitter is the same as that of the pressure transmitter. It is in the dominant position of the HART protocol, and it also has a product that conforms to the field bus international standard FF or PROFIBUS-PA protocol.

Technological evolution of process variable transmitters:

The main trends in current process variable transmitters are wireless applications, smaller form factors, and multivariable process transmitter applications. As some cement large groups have adopted wireless technology for local auxiliary processes, wireless transmitters will also receive attention from China's cement industry. It is worthwhile to add the wireless function to the process variable transmitter. Considering that some cement plants occupy a large amount of physical space, some process measurement locations are difficult to install, and some process measurement point environments are very harsh and use traditional hard links. The cost of wire-mounting devices is too high. If the connection between the process variable transmitter and the control circuit is eliminated, switching to wireless communication can save energy and reduce wiring and maintenance costs. Therefore, the wireless function is added to the process variable transmitter. Value for money.

In the new dry cement production line, the process variable transmitter is very important as the eye of the monitoring. Its accuracy, reliability, stability, and safety are improved, and the process control quality and the final product quality are continuously improved. Contributed.

8 Selection Note

In all kinds of meters, the transmitter is the most widely used and most popular. The transmitter is roughly divided into a pressure transmitter and a differential pressure transmitter. Transmitters are often used to measure pressure, differential pressure, vacuum, level, flow, and density. Transmitter has two-wire system and four-wire system, especially two-wire system transmitter; smart and non-intelligent, more intelligent transmitters; pneumatic and electric points, electric transmitters In addition, according to the application of intrinsically safe and explosion-proof type; according to the application of operating conditions, the main types of transmitter are as follows:

Low (micro) pressure / low differential pressure transmitter;

Medium pressure / medium pressure transmitter;

High pressure / high differential pressure transmitter;

Absolute pressure / vacuum / negative pressure differential pressure transmitter;

High temperature / pressure, differential pressure transmitter;

Corrosion / pressure, differential pressure transmitter;

Easy crystallization/pressure, differential pressure transmitter.

The selection of the transmitter is usually based on installation conditions, environmental conditions, instrument performance, economics, and application media. The actual application is divided into direct measurement and indirect measurement; its use is process measurement, process control and device interlocking. Common transmitters include common pressure transmitters, differential pressure transmitters, single flange transmitters, dual flange transmitters, and plug-in flange transmitters.

Pressure transmitters and differential pressure transmitters measure pressure and the difference between two pressures in a single word, but there are many parameters that they indirectly measure. For example, pressure transmitters, in addition to measuring pressure, can also measure the level in the device. When measuring the liquid level in an atmospheric pressure vessel, a pressure change is required. When measuring the pressure vessel level, two pressure changes can be used, that is, one lower limit measurement, one upper limit measurement, and their output signals can be subtracted to measure the liquid level. Generally, differential pressure transmitters are used. It can also be used to measure the density of a medium when the level and pressure in the container are constant. The measurement range of the pressure transmitter can be very wide, from absolute pressure 0 to 100 MPa (general).

Selection principle:

The main basis for the selection of pressure/differential pressure transmitters is based on the nature of the medium to be measured, which is based on the principle of saving money, facilitating installation and maintenance. If the measured medium is high-viscosity, easily crystallized and strongly corroded, an isolated transmitter must be used.

When selecting the type, consider the media's corrosion of the film box metal. Be sure to choose the material of the film box. Otherwise, the outer film piece will be corroded badly after use, and the flange will be corroded to cause equipment and personal accidents. , so material selection is very important. Transmitter's film box material is common stainless steel, 304 stainless steel, 316L stainless steel, decant film box material.

When selecting the type, the temperature of the measured medium should be taken into consideration. If the temperature is generally 200 °C ~ 400 °C, use a high temperature type, otherwise the silicone oil will produce vaporized expansion, which will make measurement inaccurate.

When selecting the type, the working pressure level of the equipment must be taken into account. The pressure level of the transmitter must be consistent with the application. From the economic point of view, the material of the outer film box and the inserting part is more appropriate, but the connecting flange can use carbon steel and chrome plating, which will save a lot of money.

隔离型压力变送器选用最好是选用螺纹连接形式的，这样既节约资金安装又方便。

对于普通压力和差压变送器选型，也要考虑被测介质的腐蚀性问题，但使用的介质温度可以不考虑，因为普通型压变是引压到表内，长期工作温度为常温，但普通型使用的维护量要比隔离型大。首先是保温问题，在北方冬季零下，导压管会结冰，变送器无法工作甚至损坏，这就需要增加伴热和保温箱等。

从经济角度上来讲，选用变送器时，只要不是易结晶介质都可以采用普通型变送器，而且对于低压易结晶介质也可以加吹扫介质来间接测量（只要工艺允许用吹扫液或气），应用普通型变送器就是要求维护人员多进行定时检查，包括各种导压管是否泄漏、吹扫介质是否正常、保温是否良好等，只要维护好，大量使用普通型变送器一次性投资会节约很多。

从选用变送器测量范围上来说，一般变送器都具有一定的量程可调范围，最好将使用的量程范围设在它量程的1/4~3/4段，这样精度会有保证，对于微差压变送器来说更是重要。实践中有些应用场合（液位测量）需要对变送器的测量范围迁移，根据现场安装位置计算出测量范围和迁移量，迁移有正迁移和负迁移之分。

差压变送器根据以下几点选型：

1、测量范围、需要的精度及测量功能；

2、测量仪表面对的环境，如石油化工的工业环境，有可燃（有毒）和爆炸危险气氛的存在，有较高的环境温度等；

3、被测介质的物理化学性质和状态，如强酸、强碱、粘稠、易凝固结晶和气化等工况；

4、操作条件的变化，如介质温度、压力、浓度的变化。 Sometimes it is also necessary to take into account the changes in gas phase and liquid phase concentrations and densities from start-up until the parameters reach normal production;

5、被测对象容器的结构、形状、尺寸、容器内的设备附件及各种进出口料管口都要考虑，如塔、溶液槽、反应器、锅炉汽包、立罐、球罐等；

6、其他要求，如环保及卫生等要求；

7、工程仪表选型要有统一的考虑，要求尽可能地减少品种规格，减少备品备件，以利管理；

8、工艺专利商的具体要求。

9、实际的工艺情况：

1）考虑被测对象是属于哪一类设备。如槽、罐类，槽的容积较小，测量范围不会太大，罐的容积较大，测量范围可能较大；

2）要看介质的物化性质及洁净程度，首选常规的差压式变送器及浮筒式液位变送器，还要对接触介质部分的材质进行选择；

3）对有些悬浮物、泡沫等介质可用单法兰式差压变送器。有些易析出、易结晶的用插入式双法兰式差压变送器；

4）对高黏度的介质的液位及高压设备的液位，由于设备无法开孔，可选用放射液位计来测量

5）除了测量方法上和技术上问题外，还有仪表投资问题。

综上所述，变送器的选型，从技术上要可行，经济上要合理，管理上要方便。

9选购原则

1.安装时应使变送器的压力敏感件轴向垂直于重力方向，如果安装条件限制，则应安装固定后调整变送器零位到标准值。2.残存的压力释放不出，因此传感器零位又下不来。排除此原因的最佳方法是将传感器卸下，直接察看零位是否正常，如果正常更换密封圈再试。

3.加压变送器输出不变化，再加压变送器输出突然变化，泄压变送器零位回不去。 产生此现象的原因极有可能是压力传感器密封圈引起的

4.是否符合供电要求；电源与变送器及负载设备之间有无接线错误。 No voltage or reversed polarity on the transmitter terminals can cause the transmitter to output no voltage signal.

5.压力传感器及变送器的外壳一般需接地，信号电缆线不得与动力电缆混合铺设，传感器及变送器周围应避免有强电磁干扰。 Sensors and transmitters should be cycled according to industry regulations during use.

6.用户在选择压力传感器及变送器时，应充分了解压力测量系统的工况，根据需要合理选择，使系统工作在最佳状态，并可降低工程造价。

7.通过隔离片和元件内的填充液传送到测量膜片两侧。 Measure the diaphragm and the electrodes on both sides of the insulating sheet to form a capacitor.

8.压力变送器要求每周检查一次，每个月检验一次，主要是清除仪器内的灰尘，对电器元件认真检查，对输出的电流值要经常校对，压力变送器内部是弱电，一定要同外界强电隔开。