Abstract: This paper introduces the advantages of HART, methods for establishing long-term continuous communication between the supervisory system and local instruments, and proposes a plan to expand the measurement range and flow rate of energy metering instruments, thereby solving a series of problems in traditional measurement methods. Keywords: HART protocol, all-digital communication, energy metering, wide-range measurement. Yang Jiangguo, Li Wenxia, ​​Wu Shujing (Anyang Iron and Steel Group Co., Ltd) These methods significantly expand the function of the instruments, solving a series of difficult problems in traditional measuring methods. KEY WORDS HART Agreement full numeric energy sources measuring broad range measuring 0 Introduction The HART protocol has been in use for 21 years since it became an open protocol in 1983. It has become a de facto standard in the field of process automation. The price of HART instruments has also dropped significantly, and in China, it is now very close to the price of analog instruments. Supporting hardware and software for communication between HART instruments and host computers is also abundant. In today's booming fieldbus technology market, value, cost, and risk are the three main factors to consider when choosing fieldbus communication technology. The HART protocol is currently the de facto international standard for intelligent instruments that has entered the practical application stage. Its advantages include a large information capacity; each standard HART device has 35-40 data items. HART intelligent field devices have continuous self-testing, instrument monitoring capabilities, and continuously provide the collected real process data. 1. HART Protocol HART uses Frequency Shift Keying (FSK) conforming to the BEL1202 standard to superimpose digital signals as AC signals onto a 4–20mA DC signal. The bits corresponding to "0" and "1" are encoded as 2200Hz and 1200Hz sine waves, respectively, with a modulation amplitude of approximately 0.5mA. During transmission, information bits are converted to the corresponding frequency; during reception, the frequency is converted back to the corresponding state of the information bits. The frequency signal is sinusoidal and perfectly symmetrical, without adding a DC component. Furthermore, the frequency signal is filtered out by a low-pass filter at the analog device input in the current loop. Thus, digital communication does not interfere with the analog 4–20mA signal. A single HART communication chip is responsible for signal modulation and demodulation (as shown in Figure 1). HART communication applications fall into two categories: The first category involves the coexistence of digital communication and 4–20mA signals. This category is further divided into: communicating with transmitters in the analog loop using a handheld terminal (as shown in Figure 2) and connecting transmitters in the existing analog loop to a PC or DCS using an interface. The second category is all-digital communication. This category is further divided into "point-to-point" connections (as shown in Figure 3(a)) and "multi-station structure" connections (as shown in Figure 3(b)). The multi-station structure is particularly suitable for data acquisition where speed requirements are not high, such as energy metering, regional temperature and pressure compensation, and pipeline management, because it saves on cables. Commonly used equipment includes multiplexers (which have multiple HART communication ports, each of which can connect to field devices in a point-to-point manner; in some cases, the ports can also connect to field devices in a multi-point manner), RS-485 serial ports, and gateways that convert HART signals to Ethernet, Modbus, and Profibus. Multiplexers that convert HART signals to RS-485 can also be used. If a master multiplexer and several slave multiplexers are connected in series, up to 7905 HART instruments can be connected to a PC. Data from the HART network is sent to another network, such as Modbus, PROFIBUS, or a dedicated protocol based on RS485, via a multiplexer, and then to the master station. In this sense, the multiplexer is actually a gateway unit. A HART network requires some impedance between the devices and the power supply. The impedance located on the host input module is typically 250Ω. It serves two purposes: preventing the DC power supply from short-circuiting the AC signal; and acting as a load for the communication signal. When the FSK current passes through this impedance, it generates a 0.125V AC voltage drop. All devices on the network pick up this AC voltage, allowing reception even when there is signal attenuation on the line. In other words, transmission is through current, and reception is through voltage. In multidrop mode, several transmitters are connected in parallel; typically, the 4–20mA function is disabled, and the output is fixed at 4mA, independent of the input. 2. Advantages and Applications of the HART Protocol 2.1 Advantages Each HART instrument's digital signal can provide an average of 3540 pieces of information. To fully utilize this information, the only condition is to establish a continuous, long-term communication connection with the HART instrument. If communication with the HART instrument is only occasional and brief, using a handheld device, the potential of the HART protocol cannot be realized. Once a long-term communication connection is established with the HART instrument, the following advantages will be available: 1) Facilitates instrument and control loop debugging. HART instruments provide information about the instrument itself, which debugging personnel can easily verify on the screen and readjust parameters (range, upper and lower limits, etc.) without having to go to the site to verify them one by one. 2) Assists operators in making correct judgments during system operation, reducing unexpected system shutdowns. Because the HART instrument provides instrument status information in every communication, it is possible to accurately determine whether the cause is instrument-related or process-related. The HART instrument can also verify its output analog signals. 3) Improves the efficiency of instrument maintenance by enabling network-based maintenance management of field instruments. More detailed fault information obtained from the HART instrument allows for a preliminary determination of the instrument's fault type. Therefore, the correct tools and spare parts can be brought to the site, allowing maintenance personnel to go to the site with a clear purpose. Additionally, the date when the instrument needs maintenance can be predicted based on the statistical information within the HART instrument. 4) Utilizing multiple process variable measurements in the HART instrument, or adopting a multi-connection method, can reduce the number of instruments. Generally, a HART instrument can provide measurements for up to four variables. Utilizing these signals can reduce the number of instruments and also reduce the number of instrument mounting holes on the main equipment. In a multi-connection state, if 15 instruments are connected to a network segment, its information update time is approximately 10-15 seconds, which is generally sufficient for energy metering. The data acquisition time of the entire energy metering network can be shortened by dividing the network into multiple substations. In a 4-20mA-based system, the control loop signal undergoes several analog-to-digital (A/D) and digital-to-analog (D/A) conversions. Each conversion adds an error, called "quantization error," and the sum of these conversion errors often exceeds 1%. Using a digital network eliminates the need for these conversions, greatly improving system accuracy. HART, PROFIBUS, and Foundation Fieldbus transmit variables digitally using engineering units. If the fieldbus value is 200 kPa, it will be transmitted as 200, not x% or ymA. This eliminates range conversion, reducing equipment configuration and certification work. 2.2 Applications and Effects in Energy Metering Instruments 1) Intelligent transmitters can transmit various physical quantities such as pressure, flow (differential pressure), level, temperature, weight, and position (potentiometer type). They can typically be connected in a multi-station configuration to save on wiring. Each HART bus can connect up to 15 transmitters, and the host computer can be a PC. In this case, the transmitters operate in a fully digital communication state, exchanging measurement data with the host computer using digital signals. It is commonly used for data acquisition and energy management. Figure 4 shows a schematic diagram of a 5-point temperature and pressure compensated gas flow meter consisting of 5 pressure transmitters, 5 differential pressure transmitters, and 5 temperature transmitters. These 5 points are achieved using only one cable (C can be up to 16 kin) and one host computer, thus reducing system costs. This system has two key features: wide-range measurement and metering balance correction. The dynamic range of flow measurement using conventional differential pressure methods is only about 1:5. This is primarily because orifice plates and other throttling devices deviate from the square relationship at low flow rates; secondly, the error at small signals is significantly amplified due to the square root relationship, necessitating small signal cutoff. Both of these problems are easily solved in this system. The intelligent transmitter has both high measurement accuracy and user-defined curve functionality, so small flow curve correction will greatly improve wide-range accuracy. Because traditional measurement methods have low accuracy, and the operating conditions at several measurement points are often different, the total pipe flow rate often does not equal the sum of the branch pipe flow rates. This metering imbalance often causes headaches for metering management departments. In this system, on the one hand, the measurement accuracy at each point is improved, and on the other hand, a fair correction formula can be used for balance correction at any time, thereby improving the metering level. 2) The arithmetic module provides calculation functions for various purposes. It can be used to perform common calculations in process industries, such as pressure and temperature compensation for flow, static pressure tank measurement, and controlled flow setpoints in proportional control. The module's process variables are derived from two inputs: a primary input and a low-end input. The low-end input can be used in automatic range changing schemes such as using two transmitters for differential pressure flow measurement to achieve a higher range ratio (as shown in Figure 5). This solves a series of problems in traditional measurement methods, such as wide-range measurement requiring dual throttling devices, as well as switching and judgment control. The structure in Figure 5 can also be used for the measurement of bidirectional flowing media (the pressure should not exceed the transmitter's withstand pressure). 3 Conclusion Due to the compatibility of HART instruments with the original 4-20mA standard instruments, the development and application of HART instruments have progressed rapidly, especially in equipment retrofitting. Although HART communication is considered a transitional standard, its rapid development is remarkable; it has moved beyond the field of smart instruments and is developing towards system-level connection and exchange. In 2000, HART 6.0 was released. It is an enhanced version of the original HART protocol, which has overcome the shortcomings of the original communication technology and improved the support for existing and future smart instrument products. The data utilization rate has been greatly improved. Internationally, its sales are still on the rise, while domestically it can be said to be just the beginning. Several well-known international companies are still vigorously developing HART instruments, focusing on laying a solid foundation for instruments, forming professional advantages, and then building an asset management system (AMS) on this basis; at the same time, the products can be incorporated into the DCS control system to realize integrated management and control. Any technology has a stage for development, and insightful people can find their own development space in this field. 4 References [1] Yang Xianhui, ed. Fieldbus Technology and Its Application. 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