Features and Applications of Fieldbus Transmitters

Introduction

Transmitter is the foundation of automatic control system. It is not only a sensor for obtaining information , but also the source of information industry. At present, transmitter has developed from analog type, intelligent type to fieldbus type. Intelligent transmitter (referring to transmitter using HART and DE protocols) is a transmitter that mixes digital signals and analog signals. Its connection with DCS (distributed control system) still adopts two-wire system. When intelligent transmitter communicates with DCS, it must also be equipped with smart card (digital detection card) in DCS, otherwise, even products of the same manufacturer cannot communicate. Generally, digital communication cannot be carried out between DCS and intelligent transmitters of different manufacturers. Most of them are still 4~20mA analog transmission. Therefore, the interoperability of this intelligent transmitter is poor and the intelligent function is not great. With the development of computer, communication and microprocessor technology, new changes have taken place in control system, resulting in fieldbus control system (FCS for short) that replaces traditional DCS (referring to DCS that does not absorb fieldbus technology). In order to meet the requirements of fieldbus control system, fieldbus transmitter has developed rapidly. 1. Features Compared with the intelligent transmitters that are now widely used, fieldbus transmitters have the following features. (1) Full digitality As a digital communication network, fieldbus extends from the control room to the production site, transforming the point-to-point analog signal transmission used in the past into a multi-point serial digital transmission. In fieldbus transmitters, both the sensor circuit and the converter circuit are digital. Unlike intelligent transmitters, which have both digital and analog circuits. Therefore, the full digitality of fieldbus transmitters makes the transmitter structure simpler, and its resolution, measurement speed, and stability are higher than those of intelligent transmitters. (2) High accuracy In intelligent transmitters, some of their sensors are not directly converted into digital, but the pressure difference acts on the diaphragm to cause displacement, change capacitance or resistance, output voltage first, and then convert it into a digital signal through analog/digital (A/D) conversion and send it to the microprocessor. Of course, there are also some intelligent transmitters that directly convert the displacement of the pressure difference diaphragm into a frequency signal without an A/D converter. However, in order to retain the 4~20mA analog signal, the digital/analog (D/A) converter still exists. Therefore, in the intelligent transmitter, the signal transmission accuracy is reduced in the repeated A/D and D/A conversion. In the fieldbus transmitter, there is no A/D and D/A conversion circuit, so there will be no A/D and D/A conversion error, so the signal transmission accuracy is improved, so the fieldbus transmitter is more accurate than the 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 are unavoidable 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 fieldbus transmitter, the signal has only two effective values: 0 or 1, and the level is high. It is difficult for general noise to distort it. Only when there is a lot of interference, the signal may be distorted. Therefore, the anti-interference ability of fieldbus transmitters is stronger than that of intelligent transmitters (when it transmits 4~20mA analog signals). (4) Embedded control function In addition to maintaining the excellent measurement performance of intelligent transmitters, fieldbus transmitters also add calculation and control functions. Each fieldbus transmitter is embedded with PID control, logic operation, arithmetic operation, accumulation and other modules. Users can call these function modules arbitrarily through the configuration software to achieve on-site control of parameters. Since fieldbus transmitters are installed near production equipment, the signal transmission distance is greatly shortened, which not only improves the control quality of the loop and reduces the instability of the loop, but also eliminates the control cabinet, further saving space in the control room. (5) High-speed communication Whether the intelligent transmitter uses the HART communication protocol or the DE communication protocol, its communication rate is not high, so it cannot effectively achieve real-time control. Fieldbus transmitters need a higher communication rate to achieve 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 minimize the communication load of the system. Fieldbus transmitters use a communication scheduling system to control the sampling of variables, the execution of algorithms, and the optimization of the communication system. For example, FF (Field Foundation) divides data into two parts: periodic and non-periodic: periodic data must be processed within the cycle required by the system; while non-periodic data can be transmitted when the communication channel is idle. Such communication scheduling enables fieldbus transmitters to have the same communication rate as analog transmitters and achieve the same high-speed closed-loop control performance as analog control systems. (6) Multivariable measurement Multivariable measurement means that a transmitter can measure multiple process variables at the same time. In the past analog communication method, a pair of wires was required to measure one variable, so each transmitter could only measure one process variable. After adopting the fieldbus communication method, since 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 a fieldbus flow transmitter, in addition to measuring fluid flow, multiple variables such as process pressure and temperature can also be measured; in another example, in a fieldbus level transmitter, in addition to measuring the liquid level of a container, multiple variables such as medium density, pressure in the container and temperature can also be measured. (7) Low overall system cost In the 4~20mA communication mode, the transmitter can only measure one physical variable. If this measured variable is to be converted into necessary control data, other auxiliary instruments are required. For example, when measuring steam flow with temperature and pressure compensation, three transmitters are required to measure temperature, pressure and pressure difference (flow) respectively. In a fieldbus control system, only one fieldbus transmitter is required. 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. (8) True interoperability Fieldbus communication methods are moving towards international standardization, and standardization ensures the realization of interoperability. Interoperability refers to the ability of devices from different manufacturers to communicate with each other and to complete functions in a multi-manufacturer environment. Interoperability allows the devices of different manufacturers to be used with each other, and the composition of the control system is free. All fieldbus devices that meet the fieldbus international communication standards can exchange information with each other regardless of which manufacturer they are produced by. In this way, users do not have to choose equipment based on a certain instrument company, and the freedom of control system composition is greatly increased. Users can construct a control system that meets their requirements with the best performance/price ratio. 2. Products and Applications











































Although many detection instruments are not called transmitters in custom, they still have the functions of transmitters, such as oxygen analyzer, electromagnetic flowmeter, vortex flowmeter, ultrasonic flowmeter, etc. At present, major instrument manufacturers in the world have successively launched fieldbus transmitters, and there are more than 60 transmitters that have passed FF certification, such as SMAR's LD302 and TT302; Rosemount's 3051, 3244MV, OXYMITTER4000, 4081C/T, 8742; FUJI's FFX-T series and FFX-P series; Yokogawa's EJA, YTA-302, YEWFLOW, EXAXTPH; Honeywell's FFOPTION, STT35F Fieldbus; Foxbro's IASPT10, RTT25, IAP10, IAP20; Endress+Hauser's CERABARS; ABB's 600TEN, etc.

Fieldbus transmitters have now begun to be used in the control systems of domestic enterprises, such as the boiler control system of Zhongyuan Petrochemical General Plant, the control system of the acetonitrile recovery device of Anqing Petrochemical Acrylic Fiber Plant, the boiler control system of Binzhou Chemical Self-provided Power Plant, and the sulfur recovery/rich liquid regeneration control system of Anqing Refinery. These control systems have a streamlined structure, without large control cabinets and I/O cabinets. Multiple transmitters share one cable, which greatly reduces the installation and commissioning costs and achieves greater economic benefits.

3. Conclusion

In summary, fieldbus transmitters are no longer transmitters in the traditional sense. They are field devices that integrate transmission, control and communication functions. The emergence of fieldbus transmitters will surely bring new revolutionary changes to the instrumentation and automation fields.