Analysis of the overall design of an optimized electromagnetic flowmeter

 As a precise flow measurement instrument, electromagnetic flowmeter works through the interaction of a whole set of systems. Not only hardware factors should be considered, but also the working effect of the software part should be evaluated and analyzed. Based on the actual needs of users and the analysis and research of electromagnetic flowmeter excitation technology at home and abroad, this paper proposes and implements the functions of using AC excitation as the excitation mode of electromagnetic flowmeter and HART communication interface. In particular, a more ideal solution is proposed for the anti-interference performance. By analyzing various anti-interference measures of software and hardware and user needs, using the high-performance and universal computing unit combining advanced microprocessors (ARM) and digital signal processors (DSP), and the method of software anti-interference and hardware anti-interference working together, various interferences under the traditional power frequency AC excitation mode are better overcome. In terms of humanization, intelligence, ease of operation and convenience of maintenance, infrared remote control, WEB server interface and other functions are integrated to achieve the optimal design of electromagnetic flowmeter.
 
　　
1. Electromagnetic flowmeter solution selection
　　Considering that high-end electromagnetic flowmeters not only meet the general functional requirements of flow settlement, cumulative flow, empty pipe detection, etc., but also have HART communication, WEB server and other functions, as well as the need to implement digital filtering algorithms, this system uses ARM+DSP in the core structure of the electromagnetic flowmeter converter to form a high-performance computing platform. And due to the limitations of communication wiring between dual processor boards and the requirements of versatility, programmable logic devices are selected to realize serial-to-parallel communication between ARM and DSP. The
　　ARM+DSP structure can meet the core processing capabilities of high-end electromagnetic flowmeters in terms of multiple parallel tasks and complex algorithms; in terms of communication, traditional general-purpose logic devices are generally selected, which consumes a lot of power, is troublesome to wire, has poor anti-interference, and can only realize simple combinational logic or sequential logic functions. Complex programmable logic devices (CPLDs) can realize complex sequential logic and complex pulse assertion functions. The device itself also has online debugging functions and strong anti-interference characteristics, so complex programmable logic devices are the first choice for communication on the two core processors of ARM and DSP.
　　
2.  Hardware structure design of electromagnetic flowmeter
　　Through the selection of electromagnetic flowmeter solutions and the development trend of intelligentization, integration and networking of electromagnetic flowmeter products at home and abroad, the hardware system structure is shown in the figure.
　　Hardware system
　　block diagram The hardware subsystem of the electromagnetic flowmeter is mainly composed of dual processor module, programmable logic device module, signal input module, excitation output module, signal output module, HART interface module, WEB server interface module, display module, debugging interface module, power supply module, etc. The functions of each module are described as follows:
　　Dual processor core module: The minimum system is composed of ARM processor, DSP processor, Flash, SRAM, FRAM and related circuits, and the hardware platform of the software system.
　　Programmable logic device module: Use complex programmable logic device (CPLD) to realize serial-to-parallel communication between ARM and DSP, and generate 4-20mA current output pulse and flow accumulation pulse.
　　Signal input module: Amplify and convert the weak electrode signal, reference signal and feedback excitation signal output by the sensor into digital quantity, so as to send it to the digital signal processor (DSP) for software filtering.
　　Excitation output module: The excitation signal is generated by outputting low amplitude through the D/A converter, and is output to the sensor through power amplification. The excitation signal is generated by DSP control.
　　Signal output module: 4-20mA current and flow accumulation pulse output controlled by the program, and the analog output circuit is simplified.
　　HART interface module: ARM's UART1 interface is used as the interface with the HART output board to realize the interface with the control system.
　　WEB server interface module: It is composed of a 10M Ethernet control chip to realize the user's remote online modification of instrument parameters and other functions in the software.
　　Display module: It is composed of a 51 single-chip microcomputer, an infrared remote control receiver, an OLED display and buttons, which is convenient for users to operate and observe flow information. Users can set system parameters through buttons or infrared remote controllers, and the OLED display shows instantaneous flow and cumulative flow. At the same time, in order to meet the requirements of different languages, this system can also display an English interface.
　　Debug interface module: It is composed of a JTAG interface and an ISP interface.
　　Power supply module: Provides multiple levels of stable and reliable power supply for the entire system.
　
　3. Functional design of electromagnetic flowmeter software
　　The software system mainly includes the following modules: signal sampling module, system monitoring module, system algorithm implementation module, human interaction module, inter-chip communication program module and digital filtering module.
　　3.1 Signal acquisition module The
　　signal acquisition module completes the acquisition function of electrode signal, reference signal, excitation voltage signal and excitation current signal. In system implementation, it receives data collected by A/D converter through the interrupt of digital signal processor (DSP). This system uses high-precision A/D converter complex programmable logic device. This A/D chip can collect six channels of information. It is directly connected to DSP chip. The interface between them is DSP multi-channel buffered serial interface (McBSP). To realize real-time data acquisition, A/D converter must be initialized first. In the system, DSP interrupt program 1 is used to send control word to A/D, and DSP interrupt program 2 is used to collect the required six channels of data.
　　3.2 System monitoring module
　　The system monitoring module mainly includes system self-diagnosis module, system function detection module, system error alarm module and system log module.
　　The system self-diagnosis module completes the diagnosis of the system working status, determines whether the system is working normally, whether the system is in an abnormal state, and gives corresponding error alarms according to different phenomena. The
　　system function detection module completes the system's functional detection. The detection functions include system display, system instantaneous flow simulation output, system current output, system pulse output, flow direction of the measured fluid, data storage device FRAM, empty pipe detection, insulation detection, manual or automatic adjustment of various parameters, system alarm setting, and cumulative flow reset. The
　　system error alarm module completes the system due to abnormal events that cause the system to work abnormally, and the flow exceeds the specified range. The
　　system log module completes the recording of the system's working conditions, such as the number of system power failures, DSP reset records, error number records and error content, alarm number and alarm content. The excitation frequency of the system, the upper and lower limits of the system's work, and the cumulative flow history records completed by the system. This is a function that increases the friendly interaction between the system and the user.
　　3.3 System Algorithm Implementation Module
　　The system algorithm implementation module completes the algorithm of the system, including the calculation of instantaneous flow, the calculation of cumulative flow, the adjustment of excitation coefficient, the calculation of current output, the calculation of pulse output, the system working zero adjustment algorithm, the system electrical zero adjustment algorithm, the system calibration coefficient adjustment algorithm, the system positive and negative full scale adjustment algorithm, the instantaneous flow representation conversion algorithm, etc.
　　3.4 Human-computer interaction module
　　The human-computer interaction module includes the display module, the key detection module and the external trigger processing module.
　　The display module completes the display of the measured fluid flow direction, the display of multiple representations of instantaneous flow, the display of multiple representations of cumulative flow, the display of excitation coefficient amplification gain, the display of electrode values, the display of reference values, the display of system menus, the display of system error alarms and more than 20 other display information.
　　The key detection module completes the value represented by the key being pressed and notifies the system to perform the corresponding operation. The
　　external trigger processing module completes the response task of the external trigger button. The external trigger buttons designed in this system are mainly external chip reset, external zero adjustment, and external cumulative flow reset.
　　3.5 Inter-chip data communication module
　　The data communication module completes the ARM microprocessor downloading the DSP program to the internal RAM of the DSP chip, as well as the data exchange between the ARM microprocessor and the DSP digital signal processor. This communication module is based on the host interface component inside the DSP chip, and its programming design realizes real-time data transmission between the ARM microprocessor and the DSP.
　　3.6 Digital filtering module
　　The digital filtering module completes the digital filtering of the data collected by the A/D. This module is very important. It is proposed on the basis that the analog filter cannot eliminate various interference signals. The quality of its filtering effect directly affects the zero point stability and accuracy requirements of the system. In order to eliminate the system zero point instability caused by industrial frequency sine wave excitation, we studied and analyzed a large amount of field data and concluded that within a certain frequency band, the influence of the industrial field environment on the system zero point is almost non-existent. Therefore, we proposed to use this frequency as the excitation frequency of the sine wave. After removing other interference signals through the designed digital filter, all data in the system are based on this frequency to achieve the zero point stability and high precision requirements of the entire system.
　　
4. Conclusion
Laboratory and field user-level tests show that the system has good anti-interference ability and stability, and has good application effects on solid-liquid two-phase difficult-to-measure fluids such as mortar and slurry, and has obvious advantages in major technical indicators such as flow output sway.