There are many types of transmitters. The transmitters used on industrial control instruments mainly include temperature transmitters, pressure transmitters, flow transmitters, current transmitters, voltage transmitters, etc. wait.
Transmitters play an important role in the fields of instruments, instruments and industrial automation. Unlike sensors, transmitters generally have a certain amplification effect in addition to converting non-electrical quantities into measurable electrical quantities.
Pressure transmitter:
Pressure transmitter is also called a differential transmitter. It mainly consists of a load cell sensor, a module circuit, a display head, a watch case and a process connection. etc. composition. It can convert the received pressure signals such as gas and liquid into standard current and voltage signals to supply secondary instruments such as indicating alarms, recorders, and regulators for measurement, indication, and process regulation.
The measurement principle of the pressure transmitter is: the process pressure and the reference pressure act on both ends of the integrated silicon pressure sensitive element respectively, and the differential pressure causes the silicon wafer to deform (the displacement is very small, only μm level), so as to The fully dynamic Wheatstone bridge made of semiconductor technology on the silicon chip is driven by an external current source to output a mV level voltage signal proportional to the pressure. Due to the excellent strength of silicon material, the linearity and variation index of the output signal are very high. When working, the pressure transmitter converts the measured physical quantity into a mV-level voltage signal and sends it to a differential amplifier with a high amplification factor that can offset temperature drift. The amplified signal is converted into a corresponding current signal through voltage and current conversion, and then undergoes nonlinear correction, and finally a standard current and voltage signal that is linearly corresponding to the input pressure is generated.
Pressure transmitters can be divided into two types according to the pressure measurement range: general pressure transmitters (0.001MPa~20MP3) and micro-differential pressure transmitters (0~30kPa).
Integrated temperature transmitter:
Integrated temperature transmitter generally consists of a temperature measurement probe (thermocouple or thermal resistance sensor) and a two-wire solid-state electronic unit. The temperature measuring probe is installed directly in the junction box in the form of a solid module to form an integrated transmitter. Integrated temperature transmitters are generally divided into two types: thermal resistance and thermocouple types.
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, etc. After the temperature measurement thermal resistance signal is converted and amplified, the nonlinear relationship between temperature and resistance is compensated by a linear circuit. After passing through the V/I conversion circuit, a 4-20mA constant current signal that is linearly related to the measured temperature is output.
Thermocouple temperature transmitters are generally composed of circuit units such as reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, burnout processing, reverse connection protection, current limiting protection and other circuit units. It amplifies the thermoelectric potential generated by the thermocouple through cold end compensation, then uses a linear circuit to eliminate the nonlinear error between the thermoelectric potential and temperature, and finally amplifies and converts it into a 4-20mA current output signal. In order to prevent accidents caused by temperature control failure due to broken wires in the thermocouple measurement, the transmitter is also equipped with a power-off protection circuit. When the thermocouple wire is broken or the connection is poor, the transmitter will output the maximum value (28mA) to cause the instrument to cut off the power supply.
The integrated temperature transmitter has the characteristics of simple structure, saving leads, large output signal, strong anti-interference ability, good linearity, simple display instrument, solid module shock-proof and moisture-proof, reverse connection protection and current limiting protection. Reliable work and other advantages.
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. It can also be made into explosion-proof or fire-proof measuring instruments according to user requirements.
Liquid level transmitter:
1. Float-type liquid level transmitter
The float-type liquid level transmitter consists of a magnetic float, It consists of measuring conduit, signal unit, electronic unit, junction box and installation parts.
Generally, the specific gravity of a magnetic float is less than 0.5, and it can float above the liquid surface and move up and down along the measuring tube. The conduit is equipped with a measuring element, which can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under the influence of external magnetism, and convert the electronic unit into 4~20mA or other standard signal output.
The transmitter is a modular circuit with the advantages of acid resistance, moisture resistance, shock resistance, corrosion resistance, etc. The circuit contains a constant current feedback circuit and an internal protection circuit, which can prevent the maximum output current from exceeding 28mA, thus reliably protecting the power supply and enabling the secondary The secondary instrument is not damaged.
2. Float-type liquid level transmitter
The float-type liquid level transmitter changes the magnetic float ball into a float. It is designed based on Archimedes' buoyancy principle. of. The float level transmitter uses tiny metal film strain sensing technology to measure the level, boundary or density of the liquid. It can perform regular setting operations through on-site buttons during operation
3. Static pressure or liquid level transmitter
This transmitter uses the measurement principle of hydrostatic pressure Work. It generally uses a silicon pressure pressure sensor to convert the measured pressure into an electrical signal, which is then amplified by an amplifier circuit and compensated by a compensation circuit, and finally output in the form of 4-20mA or 0-10mA current.
Capacitive level transmitter:
Capacitive level transmitter is suitable for industrial enterprises to measure and control the production process during the production process. It is mainly used for conductive and Long-distance continuous measurement and indication of liquid level or powdery solid material level in non-conductive media.
The capacitive liquid level transmitter is composed of a capacitive sensor and an electronic module circuit. It uses a two-wire 4~20mA constant current output as the basic type. After conversion, it can be output in a three-wire or four-wire mode. , the output signal is formed into standard signals such as 1~5V, 0~5V, 0~10mA, etc. Capacitive sensors consist of insulated electrodes and a cylindrical metal container containing the measuring medium. When the material level rises, because the dielectric constant of the non-conductive material is significantly smaller than the dielectric constant of air, the capacitance changes with the change of the height of the material. The module circuit of the transmitter is composed of reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting units. The advantages of using the pulse width modulation principle for measurement are low frequency, radio frequency interference to surrounding elements, good stability, good linearity, and no obvious temperature drift.
Ultrasonic transmitter:
Ultrasonic transmitter is divided into two categories: general ultrasonic transmitter (no meter) and integrated ultrasonic transmitter. Integrated ultrasonic transmitter The device is more commonly used.
The integrated ultrasonic transducer consists of a meter (such as an LCD display) and a probe. This transmitter that directly outputs a 4-20mA signal is a combination of a miniaturized sensitive component (probe) and a probe. Electronic circuits are assembled together to make them smaller, lighter, and cheaper. Ultrasonic transmitters can be used for liquid levels. Measurement of material level and flow measurement in open channels and open channels, and can be used to measure distances.
Antimony electrode acidity transmitter:
The antimony electrode acidity transmitter is an industrial online analysis instrument that integrates pH detection, automatic cleaning, and electrical signal conversion. It is made of antimony A pH measurement system composed of electrodes and reference electrodes. In the acidic solution being measured, an oxide layer of antimony trioxide will be formed on the surface of the antimony electrode, resulting in a potential difference between the metal antimony surface and the antimony trioxide. The magnitude of this potential difference depends on the concentration of antimony oxide, which corresponds to the concentration of hydrogen ions in the acidic solution being measured. If the proportions of antimony, antimony trioxide and aqueous solution are all regarded as 1, the electrode potential can be calculated using the Nernst formula.
The solid module circuit in the antimony electrode acidity transmitter consists of two parts. For the safety of on-site operation, the power supply part uses AC 24V to power the secondary instrument. In addition to providing driving power for the cleaning motor, this power supply should also be converted into the corresponding DC voltage through the current conversion unit for use by the transmission circuit. The second part is the measurement transmitter circuit, which amplifies the reference signal and PH acidity signal from the sensor and then sends them to the slope adjustment and positioning adjustment circuit to reduce the internal resistance of the signal and make it adjustable. The amplified PH signal and the temperature compensated
signal are superimposed and then differentially entered into the conversion circuit. Finally, a 4~20mA constant current signal corresponding to the PH value is output to the secondary instrument to complete the display. And control the pH value.
Acid, alkali, and salt concentration transmitters:
Acid, alkali, and salt concentration transmitters determine the concentration by measuring the conductance value of the solution.
It can continuously detect the concentration of acids, alkalis and salts in aqueous solutions online during industrial processes. This kind of transmitter is mainly used in industrial production processes such as boiler feed water treatment, chemical solution preparation, and environmental protection.
The working principle of the acid, alkali and salt concentration transmitter is: within a certain range, the concentration of the acid-base solution is proportional to its conductivity. Therefore, as long as the conductivity of the solution is measured, the concentration of acid and alkali can be known. When the measured solution flows into a special conductivity cell, if the electrode polarization and distributed capacitance are ignored, it can be equivalent to a pure resistance. When a constant voltage alternating current flows through, the output current is linearly related to the conductivity, and the conductivity is proportional to the acid and alkali concentration in the solution. Therefore, as long as the current of the solution is measured, the concentration of acid, alkali, and salt can be calculated.
Acid, alkali and salt concentration transmitters are mainly composed of conductivity cells, electronic modules, display heads and shells. The electronic module circuit is composed of excitation power supply, conductivity cell, conductivity amplifier, phase sensitive rectifier, demodulator, temperature compensation, overload protection and current conversion units.
Conductivity transmitter:
It is a process instrument (integrated transmitter) that indirectly measures the ion concentration by measuring the conductivity value of the solution. It can continuously detect the industrial process online. Conductivity of aqueous solutions.
Since the electrolyte solution is a good conductor of electricity like a metal conductor, there must be resistance when current flows through the electrolyte solution, and it conforms to Ohm's law. However, the resistance temperature characteristics of liquids are opposite to those of metal conductors and have negative temperature characteristics. To distinguish it from metallic conductors, the ability of an electrolyte solution to conduct electricity is expressed in terms of 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 between them and a constant voltage alternating current is passed, a current loop is formed. If the voltage and electrode size are fixed, there will be a certain functional relationship between the 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 the conductivity transmitter are the same as those of acid, alkali and salt concentration transmitters.
Smart transmitter:
Smart transmitter is composed of a sensor and a microprocessor (microcomputer). It makes full use of the computing and storage capabilities of the microprocessor to process sensor data, including conditioning of measurement signals (such as filtering, amplification, A/D conversion, etc.), data display, automatic correction and automatic compensation, etc.
The microprocessor is the core of the intelligent transmitter. It can not only calculate, store and process measurement data, but also adjust the sensor through a feedback loop to optimize the collected data. Because the microprocessor has various software and hardware functions, it can complete tasks that are difficult to complete with traditional transmitters. Therefore, the smart transmitter reduces the difficulty of manufacturing the sensor and improves the performance of the sensor to a large extent. In addition, the intelligent transmitter also has the following characteristics:
1. It has automatic compensation capability, and can automatically compensate for the nonlinearity, temperature drift, time drift, etc. of the sensor through software;
2. It can be self-diagnosed. After powering on, the sensor can be self-tested to check whether each part of the sensor is normal and make a judgment;
3. The data processing is convenient and accurate, and the data can be automatically processed according to the internal program. , such as statistical processing, removal of abnormal values, etc.;
4. It has two-way communication function. The microprocessor can not only receive and process sensor data, but also feed information back to the sensor to adjust and control the measurement process;
5. It can store and remember information, and can store characteristic data of the sensor , configuration information and compensation characteristics, etc.;
6. It has a digital interface output function, which can easily connect the output digital signal to a computer or field bus.
Two-wire transmitter:
Two-wire system means that only two wires are used to communicate between the on-site transmitter and the control room instrument. These two wires are both power wires and It's a signal line.
Two-wire system and three-wire system (one positive power wire, two signal wires, one of which is GND) and four-wire system (two positive and negative power wires, two signal wires, Compared with one of them (***GND), the measurement accuracy is lower.
Thermal resistor is a primary component that converts temperature changes into resistance value changes. It usually needs to transmit the resistance signal to a computer control device or other primary instruments through leads. Industrial thermal resistors are installed at the production site, and there is a certain distance between them and the control room. Therefore, the leads of the thermal resistors will have a greater impact on the measurement results.
Classification of wire systems:
Two-wire system: The method of connecting a wire at both ends of the thermal resistor to draw out the resistance signal is called two-wire system: This wiring method is very simple. However, since the connecting wire must have a lead resistance r, and 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: in thermal resistance The method in which one end of the root is connected to a lead and the other end is connected to two leads is called a three-wire system. This method is usually used in conjunction with a 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 method of connecting two wires at each end of the root of the thermal resistor is called a four-wire system. Two of the leads provide a constant current I for the thermal resistor and convert R into a voltage. The signal U is then led to the secondary instrument through the other two leads. It can be seen that this lead method can completely eliminate the resistance influence of the lead and is mainly used for high-precision temperature detection.
The thermal resistor adopts three-wire connection method. The three-wire system is used to eliminate measurement errors caused by the resistance of the connecting wires. This is because the circuit used to measure thermal resistance is generally an unbalanced bridge. The thermal resistor is an arm resistance of the bridge, and its connecting wire (from the thermal resistor to the central control room) also becomes part of the bridge arm resistance. The resistance of this part is unknown
and changes with the ambient temperature, causing Measurement error. Using a three-wire system, connect one wire to the power end of the bridge, and the other two wires to the bridge arm where the thermal resistor is located and the adjacent bridge arm, thus eliminating the measurement error caused by the resistance of the wire line.
Advantages of the two-wire system:
1. It is not easily affected by parasitic thermocouples and resistance voltage drops and temperature drift along the wires. Very cheap and thinner wires can be used; it can save a lot of money. Cables and installation costs;
2. When the output resistance of the current source is large enough, the voltage in the wire loop induced through magnetic field coupling will not have a significant impact because the current caused by the interference source is extremely small. Generally, interference can be reduced by using twisted pairs; shielded wires must be used in three-wire and four-wire systems, and the shielding layer of the shielded wire must be properly grounded.
3. Capacitive interference will cause errors related to the receiver resistance. For a 4~20mA two-wire loop, the receiver resistance is usually 250Ω (sampling Uout=1~5V). This resistance is too small to be sufficient. Significant errors occur, so the allowable wire length is longer and further than the voltage telemetry system;
4. Each single reading device or recording device can be replaced between different channels with different wire lengths. connection, there is no difference in accuracy due to different wire lengths, and decentralized collection is achieved. The benefits of decentralized collection are: decentralized collection, centralized control....
5. Use 4mA for zero power flat, which makes it very convenient to judge open circuit, short circuit or sensor damage (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 safety, lightning protection and explosion protection.
Three-wire and four-wire transmitters do not have the above advantages and will soon be replaced by two-wire transmitters. This can be understood from foreign industry trends and the supply and demand of transmitter chips. Current The transmitter must be installed on the power line of the field equipment when in use, while the monitoring system with the microcontroller as the core is located in the monitoring room far away from the equipment site. The two are usually tens to hundreds of meters or even further apart. The environment at the equipment site is relatively harsh. Strong electrical signals will produce various electromagnetic interferences, and lightning induction will produce strong surge pulses. In this case, a thorny problem encountered in the microcontroller application system is how to transmit signals over long distances in harsh environments. Reliably transmits small signals.
The output of the two-wire current transmitter is 4~20mA, which is converted into an analog voltage signal of 1~5V or 2-10V through a 250Ω precision resistor. There are many ways to convert it into a digital signal. If The system is used for a long time in industrial sites with harsh environments, so the safety and reliability of the hardware system need to be considered. The input module of the system uses the voltage-to-frequency conversion device LM231 to convert the analog voltage signal into a frequency signal, and the photoelectric coupling device TL117 is used to isolate the analog and digital quantities.
At the same time, the analog signal processing circuit and the digital signal processing circuit use two independent sets of power supplies, and the analog ground and digital ground are separated from each other, which can improve the safety of the system. The use of voltage-to-frequency conversion device LM231 also has a certain anti-high-frequency interference effect.
In many applications controlled by microcontrollers, transmitters are used to convert signals that cannot be directly measured by the microcontroller into electrical analog signals that can be processed by the microcontroller, such as current transmitters, pressure transmitters, Temperature transmitter, flow transmitter, etc.
Most of the early transmitters were voltage output types, that is, the measurement signal was converted into a 0-5V voltage output. This was the direct output of the op amp, and the signal power was <0.05W, which was converted through an analog/digital conversion circuit. The digital signals are read and controlled by the microcontroller. However, in situations where signals need to be transmitted over long distances or where power grid interference is large in the use environment, the use of voltage output sensors is greatly restricted, exposing shortcomings such as poor anti-interference ability, line loss destroying accuracy, etc., and the two Wire current output transmitters are widely used because of their extremely high anti-interference ability.
The anti-interference ability of the voltage output type transmitter is extremely poor. The damage caused by the line loss cannot be said to be high in accuracy. Sometimes the output DC voltage is superimposed with AC components, causing the microcontroller to misjudge. If there is a control error, the equipment will be damaged in serious cases. The output of 0-5V must not be transmitted remotely. After the remote transmission, the line voltage drop will be large and the accuracy will be greatly reduced. The input signal ports of many ADCs, PLCs, and DCSs are made into two-wire current systems. The 4-20mA output transmitter proves the inevitable trend that voltage output transmitters will be eliminated.