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Features:
The all-digital fieldbus extends from the control room to the production site as a digital communication network, enabling the use of point-to-point analog signal transmissions to become multipoint, one-line, serial digital transmissions. In the pressure transmitter, both the sensor circuit and the converter circuit are digital. Unlike smart transmitters, there are both digital and analog circuits. Therefore, the all-digital nature of fieldbus transmitters makes the structure of the transmitter simpler, and its resolution, measurement speed, and stability are all higher than those of smart transmitters.
High precision in the pressure calibrator, some of its transmitter is not directly converted to digital, but the pressure difference is applied to the diaphragm, causing it to move, change the capacitance or resistance, the first output voltage, and then through After analog/digital (A/D) conversion becomes a digital signal, it is sent to the microprocessor. Of course, there are also some smart transmitters. The displacement of the differential pressure diaphragm is directly converted into a frequency signal and there is no A/D converter. However, digital/analog (D/A) converters also exist to retain 4~20mA analog signals. Therefore, in smart transmitters, the signal transmission accuracy is reduced in repeated A/D and D/A conversions. Fieldbus type transmitter does not have A/D and D/A conversion circuits, so it will not produce A/D and D/A conversion errors. Therefore, the accuracy of signal transmission has been improved, so the fieldbus type changes. The transmitter is more accurate than the smart pressure transmitter.
Strong anti-interference ability In the analog and intelligent pressure transmitters, the signal is in the form of continuous 4~20mA, and the change of signal value can be infinitely small. Therefore, noise and signal distortion cannot be avoided in the transmission of analog signals. of. In other words, the pressure transmitter is less stable when transmitting analog signals, and the zero point tends to drift. In fieldbus type transmitters, the effective value of the signal is only two: 0 or 1, the level is high, the general noise is difficult to distort it, only when a large amount of interference occurs, the signal may be distorted . Therefore, the fieldbus type transmitter has stronger anti-interference ability than the intelligent transmitter (when it transmits 4~20mA analog signal).
Built-in control functions Fieldbus type pressure transmitters, in addition to maintaining the excellent measurement performance of the intelligent transmitters, also add computing and control functions. PID control, logic operation, arithmetic operation, accumulation and other modules are embedded in each fieldbus type transmitter. Users can make arbitrary calls to these function modules through the configuration software to achieve field control of parameters. Because the fieldbus transmitter is installed near the production equipment, the signal transmission distance is greatly shortened. This not only improves the control quality of the loop, reduces the instability of the loop, but also cancels the control cabinet, further saving control. Room space.
High-speed communication intelligent transmitters adopt either HART communication protocol or DE communication protocol, and their communication speed is not high, so real-time control cannot be realized effectively. Fieldbus-type pressure transmitters require higher communication speeds to implement closed-loop real-time control as well as 4-20mA systems. This requirement contradicts intrinsic safety due to the need for higher power consumption at high speeds. Therefore, it is necessary to select an appropriate communication rate so that the communication load of the system can be used as a field bus type transmitter to adopt a communication scheduling system to control variable sampling, algorithm execution, and optimization of the communication system. For example, FF (Field Foundation, Fieldbus Foundation) divides data into two parts: periodic and non-periodic: periodic data must be processed within the period required by the system, while non-periodic data can be idle in the communication channel. Transmission. This communication schedule makes the fieldbus pressure transmitter have the same communication rate as the analog transmitter and achieves the same high-speed closed-loop control performance as the analog control system.
Multivariable Measurements Multivariate measurements are the measurements of a single transmitter that can measure multiple process variables simultaneously. In past analog communication methods, measuring a variable required a pair of wires, so each transmitter can only measure one process variable. With fieldbus communication, because each fieldbus transmitter is equipped with multiple sensing elements, it can simultaneously measure multiple process variables and transmit it via the fieldbus. For example, in the fieldbus type flow pressure transmitter, in addition to measuring fluid flow, various variables such as process pressure and temperature may also be measured; for example, in a field bus type liquid level transmitter, in addition to measuring fluid in a container. In addition, various variables such as medium density, pressure and temperature in the vessel can also be measured.
Low overall system cost In the 4-20 mA communication mode, the transmitter can only measure one physical variable. If this measurement variable is to be converted into data necessary for control, additional instrumentation is required. 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 type transmitter is needed. This not only greatly simplifies the system structure, but also reduces the overall cost of the system and reduces the system installation and commissioning costs.
The true interoperability fieldbus communication method is advancing toward international standardization, and standardization ensures the realization of interoperability. Interoperability refers to the ability of devices from different vendors to communicate with each other and to perform functions in a multi-vendor environment. Interoperability allows equipment from different manufacturers to use each other, and the composition of its control system is free. Any fieldbus device that conforms to the fieldbus international communication standard can exchange information with each other regardless of which manufacturer produces it. In this way, the user does not have to select equipment around a certain instrument company, the degree of freedom of the control system structure is greatly increased, and the user can construct a control system that meets his own requirements with an optimal performance/price ratio.
Fieldbus-type transmitters are now being used in control systems of domestic companies, such as the boiler control system of Zhongyuan Petrochemical Plant, the control system of an acetonitrile recovery plant of Anqing Petrochemical Factory, and the boiler control system of Binzhou Chemical's own power plant. , Anqing refinery sulfur recovery / rich liquid regeneration control system. These control systems are simple in structure, there are no large control cabinets, I/O cabinets, and multiple transmitters share a single cable, which greatly reduces the installation and commissioning costs and achieves greater economic benefits.
The fieldbus transmitter is no longer a transmitter in the traditional sense. It is a field device that integrates transmission, control and communication functions. The emergence of fieldbus-type transmitters will certainly bring new and revolutionary changes to the field of instrumentation and automation.
The Characteristics and Application of Fieldbus Pressure Transmitter
Fieldbus pressure transmitters are more and more widely used. With the reform and development of control technology, fieldbus pressure transmitters have gradually evolved into the development trend of pressure transmitters.