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Characteristics and application of Fieldbus transmitter


transmitter is the basis of automatic control system. It is not only a sensor to obtain information, but also the source of information industry. At present, the transmitter has developed from analog and intelligent to the body weight, and the weight has also decreased significantly by 10 (3) 0% of the on-site general line

intelligent transmitter (refers to the transmitter using Hart and de protocol) is a transmitter with mixed digital signal and analog signal, and its connection with DCS (distributed control system) still adopts two-wire system. When the intelligent transmitter communicates with DCS, the DCS must also be equipped with a smart card (digital detection card), otherwise, even the products of the same manufacturer cannot communicate. Generally, digital communication cannot be carried out between DCs and intelligent transmitters from different manufacturers, and most of them are still 4~20ma analog transmission. Therefore, this kind of intelligent transmitter has poor interoperability and little intelligent effect. With the development of computer, communication and microprocessor technology, new changes have taken place in the control system, and a fieldbus control system (FCS) has been produced to replace the traditional DCS (DCs that does not absorb Fieldbus Technology). In order to meet the requirements of fieldbus control system, fieldbus transmitter has developed rapidly

I. features

compared with the intelligent transmitter widely used at present, the fieldbus transmitter has the following characteristics

(1) all digital

fieldbus, as a digital communication network, extends from the control room to the production site, making the point-to-point analog signal transmission used in the past into the serial digital transmission of multiple points and one line. In Fieldbus transmitter, both sensor circuit and converter circuit are digital. Unlike intelligent transmitters, there are both digital and analog circuits. Therefore, the all digital nature of the fieldbus transmitter makes the structure of the transmitter simpler, and its resolution, measurement speed and stability are higher than those of the intelligent transmitter

(2) high accuracy

in intelligent transmitters, some of their sensors are not directly converted into digital, but act on the diaphragm to produce displacement, change capacitance or resistance, first output voltage, and then convert it into digital signal through analog/digital (a/d) conversion, and then send it to the microprocessor. Of course, there are also some intelligent transmitters. The displacement of the differential pressure diaphragm is directly converted into a frequency signal, and there is no a/d converter. However, in order to retain 4~20ma analog signals, digital to analog (d/a) converters still exist. Therefore, in the intelligent transmitter, the signal transmission accuracy is reduced in the repeated a/d and d/a conversion. However, there is no a/d and d/a conversion circuit in the fieldbus transmitter, so the conversion error of a/d and d/a will not be generated, so the signal transmission accuracy is improved, so the accuracy of Fieldbus transmitter is higher than that of intelligent transmitter

(3) strong anti-interference ability

in analog and intelligent transmitters, the signal exists in a continuous form of 4~20ma, and the change of signal value can be infinitely small. Therefore, noise and signal distortion cannot be avoided in analog signal transmission. In other words, the transmitter has poor stability when transmitting analog signals, and the zero point is easy to drift. In the field bus transmitter, there are only two effective values of the signal: 0 or 1. The level is high, and it is difficult for general noise to distort it. Only when there is a large interference, can the signal be distorted. Therefore, the anti-interference ability of Fieldbus transmitter is stronger than that of intelligent transmitter (when it transmits 4~20ma analog signal)

(4) embedded control function

fieldbus transmitters not only maintain the excellent measurement performance of intelligent transmitters, but also add calculation and control functions. PID control, logic operation, arithmetic operation, accumulation and other modules are embedded in each fieldbus transmitter. Users can call these functional modules arbitrarily through configuration software to realize the field control of parameters. Since the fieldbus transmitter is installed near the production equipment through the computer data collection and processing system, the distance of signal transmission is greatly shortened, which not only improves the control quality of the loop and reduces the instability of the loop, but also cancels the control cabinet, further saving the space of the control room

(5) high speed communication

whether using HART communication protocol or de communication protocol, the communication rate of intelligent transmitter is not high, so it can not effectively realize real-time control. The fieldbus transmitter needs higher communication rate to realize closed-loop real-time control like the 4~20ma system. Since high-speed requires greater power consumption, this requirement conflicts with intrinsic safety. Therefore, it is necessary to select an appropriate communication rate to make the communication negative column spacing of the system: 540mm; The load should be as small as possible

fieldbus transmitter adopts communication scheduling system to control variable sampling, algorithm execution and communication system optimization. For example, FF (field Foundation) divides data into periodic and non periodic parts: periodic data must be processed within the cycle required by the system; Non periodic data can be transmitted when the communication channel is idle. Such communication scheduling makes the fieldbus transmitter have the same communication rate as the analog transmitter, and achieve the same high-speed closed-loop control performance as the analog control system

(6) multivariable measurement

the so-called multivariable measurement refers to that a transmitter can measure multiple process variables at the same time. In the past analog communication mode, measuring a variable requires a pair of wires, so each transmitter can only measure one process variable. After adopting the fieldbus communication mode, because each fieldbus transmitter is equipped with multiple sensing elements, it can measure multiple process variables at the same time and transmit them through the fieldbus. For example, in the field bus flow transmitter, in addition to measuring fluid flow, it can also measure process pressure, temperature and other variables; For another example, in the fieldbus liquid level transmitter, in addition to measuring the liquid level of the container, it can also measure various variables such as medium density, pressure and temperature in the container

(7) the comprehensive cost of the system is low

under the 4~20ma communication mode, the transmitter can only measure one physical variable. If you want to convert this measured variable into necessary data for control, you also need other auxiliary instruments. For example, when measuring steam flow with temperature and pressure compensation, three transmitters are required to measure temperature, pressure and differential pressure (flow) respectively. In the fieldbus control system, only one fieldbus transmitter is needed. This not only greatly simplifies the system structure, but also reduces the comprehensive cost of the system and the cost of system installation and debugging

(8) real interoperability

fieldbus communication mode is advancing towards international standardization, which ensures the realization of interoperability. Interoperability refers to the ability of devices from different manufacturers to communicate with each other and complete functions in a multi manufacturer environment. Interoperability enables the equipment of different manufacturers to use each other, and the composition of its control system is free. All fieldbus devices that conform to the international fieldbus communication standard can exchange information with each other no matter which manufacturer produces them. In this way, users do not have to choose equipment around a certain instrument company, and the freedom of control system composition is greatly increased. Users can form a control system that meets their requirements with the optimal performance/price ratio

II. Products and applications

although many detection instruments are not traditionally called hydraulic cylinder 1 cylinder transmitter, they still have the function of transmitter in terms of their functions, such as oxygen analyzer, electromagnetic flowmeter, vortex flowmeter, ultrasonic flowmeter, etc. At present, the world's major instrument manufacturers have launched fieldbus transmitters one after another. There are more than 60 kinds of transmitters that have passed FF certification, such as ld302 and tt302 of SMAR company; Rosemount 3051, 3244mv, oxymitter4000, 4081c/t, 8742; Ffx-t series and ffx-p series of Fuji company; EJA, yta-302, yewflow, exaxtph of Yokogawa company; Ffoption and stt35f Fieldbus of Honeywell; Iaspt10, rtt25, iap10, iap20 of foxbro; Cerabars of endress+hauser; 600ten of ABB, etc

fieldbus transmitters have been used in the control systems of domestic enterprises, such as the boiler control system of Zhongyuan Petrochemical Complex, the control system of acetonitrile recovery unit of Anqing Petrochemical Acrylic Fiber Plant, the boiler control system of Binzhou chemical self provided power plant, the sulfur and sulfur recovery/rich liquid regeneration control system of Anqing refinery, etc. The structure of these control systems is simplified, there is no huge control cabinet and i/o cabinet, and multiple transmitters share a cable, which greatly reduces the installation and commissioning costs and obtains greater economic benefits

III. conclusion

to sum up, fieldbus transmitter is no longer a transmitter in the traditional sense, but a field device integrating transmission, control and communication functions. The emergence of Fieldbus transmitter will bring new revolutionary changes to the field of instrumentation and automation. (end)

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