Transmitters play an important role in the fields of instruments, meters and industrial automation. Different from sensors, transmitters can not only convert non-electric quantities into measurable quantities, but also have a certain amplification effect.
Pressure transmitter:
Pressure transmitter, also known as differential pressure transmitter, is mainly composed of weighing sensor, module circuit, display meter head, shell and process connector. It can convert the received pressure signals such as gas and liquid into standard current and voltage signals, and supply them to secondary instruments such as indicator alarm, recorder and regulator for measurement, indication and process adjustment.
The measuring principle of the pressure transmitter is that the process pressure and the reference pressure act on the two ends of the integrated silicon pressure sensitive element respectively, and the pressure difference causes the silicon wafer to deform (the displacement is very small, only microns), so that the fully dynamic Wheatstone bridge made by the semiconductor process on the silicon wafer can output the mV voltage signal proportional to the pressure driven by the external current source. Because of the excellent strength of silicon material, the linearity and change index of output signal are very high. When working, the pressure transmitter converts the measured physical quantity into a millivolt voltage signal, which is sent to the differential amplifier with high magnification to offset the temperature drift. The amplified signal is converted into corresponding current signal by voltage-current conversion, and then nonlinear correction is carried out, and finally a standard current-voltage signal with a linear corresponding relationship with the input pressure is generated.
Pressure transmitter can be divided into general pressure transmitter (0.001MPa ~ 20m3) and micro differential pressure transmitter (0 ~ 30kpa) according to the pressure measurement range.
Integrated temperature transmitter:
An integrated temperature transmitter usually consists of a temperature probe (thermocouple or thermal resistance sensor) and a two-wire solid-state electronic unit. The temperature measuring probe is directly installed in the junction box in the form of solid module, thus forming an integrated transmitter. Integrated temperature transmitters are generally divided into thermal resistors and thermocouples.
Thermal resistance temperature transmitter consists of reference unit, R/V conversion unit, linear circuit, reverse connection protection, current limiting protection and V/I conversion unit. After the thermal resistance signal of temperature measurement is converted and amplified, the nonlinear relationship between temperature and resistance is compensated by linear circuit, and a constant current signal of 4 ~ 20 mA with linear relationship with the measured temperature is output by V/I conversion circuit.
Thermocouple temperature transmitter is generally composed of reference source, cold end compensation, amplification unit, linearization processing, V/I conversion, decoupling processing, reverse connection protection, current limiting protection and other circuit units. The thermoelectric potential generated by thermocouple is amplified by cold end compensation, and then the nonlinear error between thermoelectric potential and temperature is eliminated by linear circuit, and finally amplified and converted into 4 ~ 20 mA current output signal. In order to prevent accidents caused by temperature control failure caused by thermocouple disconnection, the transmitter is also equipped with power-off protection circuit. When the thermocouple is damaged or poorly connected, the transmitter will output the maximum value (28mA) to cut off the power supply of the instrument.
The invention has the advantages of simple structure, wire saving, large output signal, strong anti-interference ability, good linearity, simple display instrument, solid module seismic and moisture-proof, reverse connection protection and current limiting protection, reliable operation and the like.
The output of the integrated temperature transmitter is a unified 4 ~ 20mA signal; It can be used in conjunction with microcomputer systems or other conventional instruments. Can also be made into explosion-proof or fire-proof measuring instruments according to user requirements.
Liquid level transmitter:
1, float liquid level transmitter
Floating ball type liquid level transmitter consists of magnetic floating ball, measuring conduit, signal unit, electronic unit, junction box and installation parts.
The specific gravity of magnetic floating ball is generally less than 0.5, and it can float above the liquid level and move up and down along the measuring conduit. A measuring element is installed in the catheter, which can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under the action of an external magnetic field, and convert the electronic unit into a 4 ~ 20mA or other standard signal for output. The transmitter is a modular circuit with the advantages of acid resistance, moisture resistance, impact resistance and corrosion resistance. The circuit includes constant current feedback circuit and internal protection circuit, which can make the maximum output current not exceed 28mA, thus reliably protecting the power supply and preventing the secondary instrument from being damaged.
2. Floating simple liquid level transmitter
Float-type liquid level transmitter is designed according to Archimedes buoyancy principle, which changes the magnetic float into a float. Float-type liquid level transmitter uses micro-metal film strain sensing technology to measure liquid level, boundary or density. When working, you can press the button on the spot to carry out routine setting operation.
3. Static pressure or liquid level transmitter
The transmitter works based on the measurement principle of hydrostatic pressure. Generally, a silicon pressure sensor is selected to convert the measured pressure into an electrical signal, which is amplified by an amplifier circuit and compensated by a compensation circuit, and finally output in a current mode of 4 ~ 20ma or 0 ~ 10ma.
Capacitive liquid level transmitter:
Capacitive liquid level transmitter is suitable for measuring and controlling the production process of industrial enterprises, and is mainly used for continuously measuring and indicating the liquid level of conductive and non-conductive media or powdery solid at a long distance.
Capacitive liquid level transmitter consists of capacitive sensor and electronic module circuit. Based on 4 ~ 20ma double-wire constant current output. After conversion, it can be output in three or four wires, and the output signals form standard signals such as 1 ~ 5v, 0 ~ 5v, 0 ~ 10ma. Capacitance sensor consists of insulated electrode and cylindrical metal container filled with measuring medium. When the material level rises, because the dielectric constant of non-conductive materials is obviously lower than that of air, the capacitance changes with the material height. The module circuit of transmitter consists of reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting. The advantages of using the principle of pulse width modulation are low frequency, radio frequency interference to surrounding components, good stability, good linearity and no obvious temperature drift.
Ultrasonic transmitter:
Ultrasonic transmitter is divided into general ultrasonic transmitter (without meter head) and integrated ultrasonic transmitter, which is commonly used.
The integrated ultrasonic sensor consists of a meter head (such as LCD display) and a probe. The transmitter that directly outputs 4 ~ 20mA signals is a miniaturized sensitive element (probe) and electronic circuit, which makes it smaller, lighter and cheaper. Ultrasonic transmitter can be used for liquid level measurement. Measurement of material level and flow measurement of canals and open channels, and can be used to measure distance.
Antimony electrode acidity transmitter;
Antimony electrode acidity transmitter is an industrial on-line analytical instrument integrating PH detection, automatic cleaning and electrical signal conversion. It is a PH measuring system composed of antimony electrode and reference electrode. In the acid solution to be tested, an antimony trioxide oxide layer will be formed on the surface of antimony electrode, which will form a potential difference between the metal antimony surface and antimony trioxide. The potential difference depends on the concentration of antimony trioxide, which corresponds to the moderation of hydrogen ions in the acid solution to be measured. If a proper amount of antimony, antimony trioxide and aqueous solution are all regarded as 1, the electrode potential can be calculated by Nernst formula.
The solid module circuit in the antimony electrode acidity transmitter consists of two parts. For the safety of field operation, the power supply part uses AC 24V to supply power to the secondary instrument. This power supply not only provides driving power for the cleaning motor, but also converts it into corresponding DC voltage for the transmission circuit through the current conversion unit. The second part is the measuring and transmitting circuit, which amplifies the reference signal and PH and acidity signals from the sensor and sends them to the slope adjustment and positioning adjustment circuit to reduce and adjust the internal resistance of the signal. The amplified PH signal and temperature are compensated.
The signals are superimposed and then sent to the conversion circuit. Finally, the 4 ~ 20mA constant current signal corresponding to the PH value is output to the secondary instrument to display and control the PH value.
Acid, alkali and salt concentration transmitter;
The concentration transmitter of acid, alkali and salt determines the concentration by measuring the conductivity of the solution. It can continuously detect the concentration of acid, alkali and salt in aqueous solution in industrial process on line. The transmitter is mainly used in industrial production processes such as boiler feed water treatment, chemical solution preparation and environmental protection.
The working principle of acid-base salt concentration transmitter is: within a certain range, the concentration of acid-base solution is directly proportional to its conductivity. Therefore, the acid-base concentration can only be known by measuring the conductivity of the solution. When the measured solution flows into a special conductivity cell, it can be equivalent to a pure resistance if the electrode polarization and distributed capacitance are ignored. When constant voltage alternating current flows, its output current has a linear relationship with conductivity, and the conductivity is directly proportional to the concentration of acid and alkali in the solution. So as long as the solution current is measured, the concentrations of acid, alkali and salt can be calculated.
The transmitter of acid-base salt concentration is mainly composed of conductivity cell, electronic module, display instrument and shell. The electronic module circuit consists of excitation power supply, conductance cell, conductance amplifier, phase-sensitive rectifier, demodulator, temperature compensation, overload protection and current conversion.
Conductivity transmitter:
It is a process instrument (integrated transmitter) that indirectly measures the ion concentration by measuring the conductivity value of the solution, and can continuously detect the conductivity of the aqueous solution in the industrial process online.
Because electrolyte solution is a good conductor of electricity like metal conductor, there must be resistance when current flows through electrolyte solution, which conforms to ohm's law. However, the temperature resistance of liquid is opposite to that of metal conductor, and it has negative temperature characteristics. In order to distinguish it from metal conductor, the conductivity of electrolyte solution is expressed by conductance (reciprocal of resistance) or conductivity (reciprocal of resistivity). When two mutually insulated electrodes form a conductivity cell, if the solution to be measured is placed in the middle and constant voltage alternating current is applied, a current loop is formed. If the voltage and electrode size are fixed, there is a certain functional relationship between loop current and conductivity. In this way, by measuring the current flowing in the solution to be measured, the conductivity of the solution to be measured can be measured.
The structure and circuit of conductance transmitter are the same as those of acid, alkali and salt concentration transmitter.
Intelligent transmitter:
Intelligent transmitter is composed of sensor and microprocessor. It makes full use of the computing and storage capacity of the microprocessor, and can process the data of the sensor, including the conditioning of the measured signal (such as filtering, amplification, A/D conversion, etc.). ), data display, automatic correction and automatic compensation, etc.
Microprocessor is the core of intelligent transmitter. It can not only calculate, store and process the measured data, but also adjust the sensor through the feedback loop to optimize the collected data. Because the microprocessor has various software and hardware functions, it can accomplish tasks that are difficult for traditional transmitters. Therefore, the intelligent transmitter reduces the manufacturing difficulty of the sensor and greatly improves its performance. In addition, the intelligent transmitter also has the following characteristics:
1, which has the ability of automatic compensation, and can automatically compensate the nonlinearity, temperature drift and time drift of the sensor through software;
2, self-diagnosis, self-check the sensor after power-on, check whether each part of the sensor is normal, and make a judgment;
3, data processing is convenient and accurate, and data can be automatically processed according to internal procedures, such as statistical processing and eliminating abnormal values;
4, with two-way communication function. Microprocessor can not only receive and process the sensor data, but also feed back the information to the sensor, thus adjusting and controlling the measurement process.
5. It can store and memorize information, and can store sensor characteristic data, configuration information and compensation characteristics.
6. It has the function of digital output interface, and can conveniently connect the output digital signal with a computer or a field bus.
Two-wire transmitter:
Two-wire system means that only two wires are used to connect the field transmitter and the instruments in the control room. These two wires are both power lines and signal lines.
Compared with three-wire system (one positive power line and two signal lines, one of which is ***GND) and four-wire system (two positive and negative power lines and two signal lines, one of which is ***GND), the two-wire system has lower measurement accuracy.
Thermal resistance is the main component that converts temperature change into resistance change. Usually, it is necessary to transmit resistance signals to computer control devices or other primary instruments through wires. The industrial thermal resistor is installed in the production site, which is at a certain distance from the control room, so the lead of the thermal resistor will have a great influence on the measurement results.
Classification of line system:
Two-wire system: the method of connecting a wire at both ends of the thermal resistance to lead out the resistance signal is called two-wire system: this lead method is very simple, but because there must be lead resistance R in the connecting wire, the size of R is related to the material and length of the wire, this lead method is only suitable for occasions with low measurement accuracy;
Three-wire system: one end of the root of the thermal resistor is connected with a lead, and the other end is connected with two leads, which is called three-wire system. This method is usually used in combination with electric bridge, which can better eliminate the influence of lead resistance and is the most commonly used method in industrial process control.
Four-wire system: The way of connecting two wires at the root of the thermal resistor is called four-wire system, in which two wires provide constant current I for the thermal resistor, convert R into voltage signal U, and then lead U to the secondary instrument through the other two wires. It can be seen that this lead mode can completely eliminate the influence of lead resistance and is mainly used for high-precision temperature detection.
The thermal resistance is connected by three wires. The three-wire system is adopted to eliminate the measurement error caused by the resistance of the connecting wire. This is because the circuit for measuring thermal resistance is generally an unbalanced bridge. Thermal resistance is an arm resistance of the bridge, and its connecting line (from the thermal resistance to the central control room) also becomes a part of the arm resistance, which is unknown.
And changes with the ambient temperature, resulting in measurement error. Using three-wire system, one wire is connected to the power supply end of the bridge, and the other two wires are connected to the bridge arm where the thermal resistance is located and its adjacent bridge arm respectively, thus eliminating the measurement error caused by the resistance of the wire.
Advantages of two-wire system:
1, which is not affected by parasitic thermocouple and resistance voltage drop and temperature drift along the conductor, and can use very cheap and thin conductor; Can save a lot of cables and installation costs;
2. When the output resistance of the current source is large enough, the voltage in the wire loop induced by magnetic field coupling will not have a significant impact, because the current generated by the interference source is very small, and the interference can be generally reduced by using twisted pair; Three-wire system and four-wire system must use shielded wire, and the shielding layer of shielded wire should be properly grounded.
3. Capacitive interference will lead to errors related to receiver resistance. For 4 ~ 20mA two-wire circuit, the receiver resistance is usually 250Ω (sampling UOUT = 1 ~ 5V), which is too small to cause obvious error. Therefore, the allowable wire length is longer and farther than that of the voltage telemetry system.
4. Each single reading device or recording device can switch between different channels with different line lengths, and the difference in accuracy will not be caused by different line lengths, thus realizing decentralized acquisition. The advantages of decentralized acquisition are decentralized acquisition and centralized control. ....
5. The zero level is 4mA, which is very convenient to judge the open circuit and short circuit or the sensor is damaged (0mA state).
6. It is very easy to add one or two lightning protection and surge protection devices to the two-wire output port, which is beneficial to lightning protection and explosion protection.
Three-wire and four-wire transmitters do not have the above advantages and will be replaced by two-wire transmitters, which can be seen from the trend of foreign industries and the supply and demand of transmitter chips. The current transformer should be installed on the power line of the field equipment when it is used, and the monitoring system based on single chip microcomputer is located in the monitoring room far away from the equipment site, and the distance between them is generally tens to hundreds of meters or even more. The field environment of the equipment is harsh, and strong electric signals will produce all kinds of electromagnetic interference, and lightning induction will produce strong surge pulses. In this case, a thorny problem encountered by single chip microcomputer application system is how to transmit small signals reliably over a long distance in harsh environment.
The output of the two-wire current transformer is 4 ~ 20 mA, which is converted into an analog voltage signal of 1 ~ 5 V or 2- 10V through a precision resistor of 250 Ω. There are many ways to convert it into a digital signal. If the system is used in harsh industrial sites for a long time, the security and reliability of the hardware system should be considered. The input module of the system adopts voltage-frequency conversion device LM23 1 to convert analog voltage signal into frequency signal, and photoelectric coupling device TL 1 17 to isolate analog and digital signals.
At the same time, the analog signal processing circuit and the digital signal processing circuit use two independent power supplies, and the analog ground and the digital ground are separated from each other, which can improve the security of the system. The voltage-frequency converter LM23 1 also has certain anti-high frequency interference effect.
In many applications controlled by single chip microcomputer, transmitters are used to convert signals that cannot be directly measured by single chip microcomputer into electrical analog signals that can be processed by single chip microcomputer, such as current transducer, pressure transmitter, temperature transmitter and flow transmitter.
Early transmitters were mostly voltage output type, that is, the measured signal was converted into 0-5V voltage output, which was the direct output of operational amplifier, and the signal power was
The anti-interference ability of voltage output transmitter is poor, and the line loss is destroyed, not to mention the high accuracy. Sometimes, the output DC voltage will be superimposed with AC component, which will make the single chip microcomputer misjudge, make the control wrong, and even damage the equipment in serious cases. The output of 0-5V can never be transmitted over a long distance. After long-distance transmission, the line voltage drop is large and the accuracy is greatly reduced. Many input signal ports of ADC, PLC and DCS are made into 4-20mA two-wire current output transmitters.