Transmitter selection considerations - Solutions - Huaqiang Electronic Network

Spot Remarks 2017+

SMD aluminum electrolytic capacitor

Photocoupler

First, the integrated temperature transmitter integrated temperature transmitter is generally composed of a temperature probe (thermocouple or thermal resistance sensor) and a two-wire solid electronic unit. The temperature probe is mounted directly in the junction box in the form of a solid module to form an integrated transmitter. Integrated temperature transmitters are generally classified into two types: thermal resistance and thermocouple type.
The thermal resistance temperature transmitter is composed of a reference unit, an R/V conversion unit, a linear circuit, a reverse connection protection, a current limiting protection, a V/I conversion unit, and the like. After the temperature-measuring thermistor signal is converted and amplified, the nonlinear relationship between the temperature and the resistance is compensated by the linear circuit, and a constant current signal of 4-20 mA which is linear with the measured temperature is output after the V/I conversion circuit.
The thermocouple temperature transmitter is generally composed of circuit elements such as reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, burn-off processing, reverse connection protection, and current limiting protection. It is to compensate the thermoelectric potential generated by the thermocouple through the cold end compensation, and then the non-linear error of the thermoelectric potential and the temperature is eliminated by the linear circuit, and finally amplified and converted into a 4-20 mA current output signal. In order to prevent accidents caused by temperature control failure due to galvanic breakage in the thermocouple measurement, the transmitter also has a power-off protection circuit. When the thermocouple breaks or the connection is poor, the transmitter will output the maximum value (28mA) to turn off the power.
The integrated temperature transmitter has the advantages of simple structure, saving lead wires, large output signal, strong anti-interference ability, good linearity, simple display instrument, anti-seismic and moisture-proof of solid modules, reverse connection protection and current limiting protection, and reliable operation.
The output of the integrated temperature transmitter is a uniform 4 to 20 mA signal; it can be used with a microcomputer system or other conventional instruments. It can also be made into explosion-proof or fire-proof measuring instruments.
Second, the pressure transmitter pressure transmitter is also called differential transmitter, mainly composed of load cell sensor, module circuit, display head, case and process connection. It can convert the received gas, liquid and other pressure signals into standard current and voltage signals to supply secondary instruments such as alarms, recorders and regulators for measurement, indication and process adjustment.
The measurement schematic of the pressure transmitter is shown in Figure 3. The measuring principle is that the process pressure and the reference pressure respectively act on the two ends of the integrated silicon pressure sensitive element, and the differential pressure causes the silicon wafer to be deformed (the displacement is small, only μm level), so that the silicon wafer is made of semiconductor technology. The fully dynamic Wheatstone bridge outputs an mV-level voltage signal proportional to the pressure driven by an external current source. Due to the excellent strength of the silicon material, the linearity and variation index of the output signal are high. In operation, 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 cancels the temperature drift. The amplified signal is converted into a corresponding current signal by voltage and current conversion, and then subjected to nonlinear correction, and finally a standard current voltage signal linearly corresponding to the input pressure is generated.
Pressure transmitters can be divided into general pressure transmitters (0.001MPa ~ 20MP3) and differential pressure transmitters (0 ~ 30kPa) according to the pressure range.
Third, the liquid level transmitter
1. Float type liquid level transmitter The float type liquid level transmitter consists of magnetic float ball, measuring tube, signal unit, electronic unit, junction box and mounting parts.
Generally, the magnetic float has a specific gravity of less than 0.5 and can float above the liquid surface and move up and down along the measuring catheter. The measuring tube 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 action of external magnetic field, and convert 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, shock resistance, corrosion resistance, etc. The circuit contains a constant current feedback circuit and an internal protection circuit, so that the maximum output current does not exceed 28 mA, thus reliably protecting the power supply and enabling The secondary meter is not damaged.
2, floating simple liquid level transmitter float type liquid level transmitter is to change the magnetic float to the float, it is designed according to Archimedes buoyancy principle. The float type liquid level transmitter uses a small metal film strain sensing technology to measure the liquid level, boundary or density of the liquid. It can be used to perform regular setting operations by pressing the button at work.
3. Static pressure or liquid level transmitter The transmitter works by the measurement principle of hydrostatic pressure. It generally uses a silicon pressure load cell sensor to convert the measured pressure into an electrical signal, and then is amplified by an amplifier circuit and compensated by a circuit, and finally outputted at a current of 4 to 20 mA or 0 to 10 mA.
4. Capacitive level transmitter Capacitive level transmitter is suitable for industrial enterprises to measure and control the production process in the production process. It is mainly used as liquid level or powdery solid material level for conductive and non-conductive media. Long distance continuous measurement and indication.
Capacitive liquid level transmitter consists of a capacitive sensor and an electronic module circuit. It is based on a two-wire 4-20mA constant current output. After conversion, it can be output in three-wire or four-wire mode. The output signal is formed as 1~ Standard signals such as 5V, 0~5V, 0~10mA. The capacitive sensor consists of an insulated electrode and a cylindrical metal container containing the measuring medium. When the material level rises, since the dielectric constant of the non-conductive material is significantly smaller than the dielectric constant of the air, the capacitance changes with the height of the material. The module circuit of the transmitter consists of reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting. The advantage of using pulse width modulation principle is that the frequency is low, the radio frequency interference to surrounding elements, good stability, good linearity, and no obvious temperature drift.
Five, ultrasonic transmitter Ultrasonic transmitter is divided into two types of general ultrasonic transmitter (no head) and integrated ultrasonic transmitter, integrated ultrasonic transmitter is more commonly used.
The integrated ultrasonic transformer is composed of a meter head (such as an LCD display) and a probe. The transmitter that directly outputs a 4-20 mA signal assembles the miniaturized sensor (probe) and electronic circuit. Makes the volume smaller, lighter and cheaper. Ultrasonic transmitters can be used for liquid levels. Measurement of level and flow measurement such as open channel, open channel, etc., and can be used to measure distance.
Sixth, 锑 electrode acidity transmitter 锑 electrode acidity transmitter is an industrial online analytical instrument that integrates PH detection, automatic cleaning and electrical signal conversion. It is a PH value measurement system composed of 锑 electrode and reference electrode. In the acid solution to be tested, a tantalum trioxide oxide layer is formed on the surface of the tantalum electrode, so that a potential difference is formed between the metal tantalum surface and the tantalum trioxide. The magnitude of this potential difference depends on the concentration of the three cerium oxides, which corresponds to the moderate amount of hydrogen ions in the acidic solution being tested. If the appropriateness of bismuth, antimony trioxide and aqueous solution is regarded as 1, the electrode potential can be calculated by the Nernst formula.
The solid-state module circuit in the 锑 electrode acidity transmitter consists of two major components. For the safety of the field function, the power supply part uses AC 24V to supply power to the secondary instrument. In addition to providing the driving power for the cleaning motor, this power supply should be converted into a corresponding DC voltage by the current conversion unit for use in the transmission circuit. The second part is the measuring transducer circuit, which amplifies the reference signal and pH acidity signal from the sensor and sends it to the slope adjustment and positioning adjustment circuit to reduce and adjust the internal resistance of the signal. The amplified PH signal and the temperature compensated signal are superimposed and then sent to the conversion circuit, and finally a 4-20 mA constant current signal corresponding to the PH value is output to the secondary meter to complete display and control the PH value.
7. Acid, alkali, and salt concentration transmitters Acid, alkali, and salt concentration transmitters determine the concentration by measuring the conductivity value of the solution. It can continuously detect the concentration of acid, alkali and salt in aqueous solution in the industrial process. This type of transmitter is mainly used in boiler feed water treatment, chemical solution preparation and environmental protection industrial production processes.
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 the conductivity. Therefore, as long as the magnitude of the conductivity of the solution is measured, the level of the acid-base concentration can be known. When the measured solution flows into the dedicated 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, the output current is linear with the conductivity, and the conductivity is proportional to the acid and alkali concentrations 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 are mainly composed of a conductivity cell, an electronic module, a display head and a casing. The electronic module circuit consists of an excitation power supply, a conductivity cell, a conductivity amplifier, a phase sensitive rectifier, a demodulator, temperature compensation, overload protection, and current conversion.
8. Conductivity Transmitter It is a flow meter (integrated transmitter) that indirectly measures the ion concentration by measuring the conductance value of the solution, and can continuously detect the conductivity of the aqueous solution in the industrial process on line.
Since the electrolyte solution is the same good electrical conductor as the metal conductor, the electric current must flow through the electrolyte solution and must comply with Ohm's law. However, the resistance temperature characteristics of the liquid are opposite to those of the metal conductor and have a negative temperature characteristic. In order to distinguish it from a metal conductor, the conductivity of the electrolyte solution is expressed by conductance (reciprocal of resistance) or electrical conductivity (reciprocal of resistivity). When two mutually insulated electrodes constitute a conductivity cell, if a solution to be tested is placed in between and a constant voltage alternating current is passed, a current loop is formed. If the voltage magnitude and electrode size are fixed, there is a certain functional relationship between the loop current and the conductivity. Thus, by measuring the current flowing through the solution to be tested, the conductivity of the solution to be tested can be measured.
The structure and circuit of the conductivity transmitter are the same as those of the acid, alkali and salt concentration transmitters.
Nine, smart transmitter Intelligent transmitter is made up of sensor and microprocessor (microcomputer) phase structure. It makes full use of the computing and storage capabilities of the microprocessor, and can process the sensor data, including conditioning of the measurement signal (such as filtering, amplification, A / D conversion, etc.), data display, automatic correction and automatic compensation.
The microprocessor is the heart of a smart 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 a variety of software and hardware functions, it can accomplish tasks that are difficult for traditional transmitters to accomplish. Therefore, the smart transmitter reduces the manufacturing difficulty of the sensor and improves the performance of the sensor over a large range. In addition, the smart transmitter has the following features:
With automatic compensation, the sensor can automatically compensate for nonlinearity, temperature drift, time drift, etc. of the sensor.
Self-diagnosis, the sensor can be self-tested after power-on to check whether the sensor parts are normal and make a judgment.
The data processing is convenient and accurate, and the data can be automatically processed according to internal procedures, such as performing statistical processing and removing abnormal values.
Has two-way communication capabilities. The microprocessor not only receives and processes sensor data, but also feeds back information to the sensor to adjust and control the measurement process.
Information storage and memory can be stored, and sensor characteristic data, configuration information and compensation characteristics can be stored.
It has a digital interface output function, which can easily connect the output digital signal to a computer or field bus.