Design of multi-loop monitoring unit based on ADE7758+MC9S08AW32 solution

Introduction
　　In the field of power distribution system, intelligence and networking are the mainstream development direction. However, in actual use, if each power distribution circuit is equipped with an intelligent network monitoring instrument, the user's hardware investment cost is very high. In view of this, integration will be a development direction, that is, the electrical parameter measurement of multiple power distribution circuits will be realized by one intelligent instrument. Therefore, the emergence of a multi-circuit intelligent monitoring unit with advanced design, high reliability and high measurement accuracy can greatly reduce the user's hardware investment cost and use cost while ensuring the realization of user measurement requirements.
　　This article introduces a design method of an AMC series multi-circuit intelligent monitoring unit (see Figure 1), which can realize the electrical parameter measurement of up to 3 three-phase circuits (or 9 single-phase circuits); combined with RS485 bus technology and host computer software, it can realize remote transmission of data and status information, meeting the development needs of low-voltage power distribution intelligence, networking and integration.

Figure 1 Device diagram

1 Technical features of AMC multi-circuit monitoring unit

　　AMC multi-circuit monitoring unit is mainly used for monitoring the electrical parameters of multiple distribution circuits. It centrally measures and displays the bus voltage in the circuit, the current, power, electric energy of multiple distribution circuits and the switch status of each circuit, and has communication output, realizing the centralized measurement and monitoring of the distribution line circuit with simpler monitoring requirements. One AMC multi-circuit monitoring unit can complete the monitoring function of the above-mentioned multiple circuits, greatly facilitating the wiring, installation and commissioning of the system; saving the user's investment and reducing the system cost. See Table 1 for specific models and product functions.

                              Table 1 Product models and functions                    　　

	model	Function	application
Simplex	AMC16-1I9	9-way single-phase I, ULN, RS485/Modbus	9 single-phase circuit electrical parameter monitoring, switch monitoring, and communication
	AMC16-1E9	9-way single-phase I, ULN, kW, kWh, RS485/Modbus
	AMC16-1E9/K	9 single-phase I, ULN, kW, kWh, RS485/Modbus, 18 DI, 1 DO
Three-phase	AMC16-3I3	3-way three-phase I, U, RS485/Modbus	3 three-phase circuit electrical parameter monitoring, switch monitoring, and communication
	AMC16-3E3	3-way three-phase I, U, kW, kWh, RS485/Modbus
	AMC16-3E3/K	3 three-phase I, U, kW, kWh, RS485/Modbus, 18 DI, 1 DO

2 System Structure
　　The overall system consists of a central processing unit, power supply, AC sampling operation, human-machine interface, switch quantity control, communication interface module, etc. The hardware structure of the device is shown in Figure 2.

Figure 2 Device hardware structure diagram

2.1 Power chip dedicated to the central processing unit
　　The central processing unit uses Freescale's high-performance processor MC9S08AW32. MC9S08AW32 is a highly energy-saving processor based on the S08 core of Freescale. It is the first microcontroller approved for the automotive market. It can be used in highly integrated high-performance devices such as home appliances, automobiles, and industrial control. It has the best EMC performance in the industry.
　　The CPU bus frequency can reach up to 20MHz, and the maximum operating speed can reach 40MHz. Rich on-chip resources: 32K Flash memory, internal clock generator, timer with 8 programmable channels, 10-bit, 16-channel ADC, dual SCI ports, rich I/O ports, SPI, I2C and other interfaces, which greatly facilitates the expansion of hardware. And supports BDM on-chip debugging.
2.2 Power supply
　　The power supply module used is a universal +5V switching power supply module. The circuit principle is shown in Figure 3. The input voltage of this power module is AC85V～265V or DC100V～350V, the input frequency is 45Hz～60Hz, the output voltage is stable, the failure rate is small, the output ripple is <1%, and the conversion efficiency is ≥75%. It has overvoltage and overcurrent protection. The module has been used in actual field and has high stability, reliability and anti-interference ability. [page]

Figure 3 Power supply circuit principle

2.3 AC sampling and operation
　　The AC sampling operation unit includes AC sampling and a dedicated power chip.
　　After the bus voltage of the system passes through the voltage transformer, sampling circuit, and filtering circuit, the voltage signal enters the voltage channel of the dedicated power chip. After
　　the currents of multiple loads pass through the current transformer, sampling circuit, and filtering circuit, the current signals enter the input channel of the high-speed signal switching switch. The on and off of the high-speed signal switching switch controls the current signals of each load to enter the current channel of the power chip.
　　The dedicated power chip uses the high-precision three-phase power measurement chip ADE7758 from ADI of the United States. The chip has high measurement accuracy and powerful functions. It has a serial port, two pulse outputs, integrated digital integration, reference voltage source, temperature sensitive components, etc., and can be used for the measurement of active power, complex power, apparent power, effective value, and the signal processing circuit required to correct system errors (gain, phase and offset, etc.) in a digital way. This chip is suitable for measuring active power, complex power, and apparent power in various three-phase circuits (whether three-wire or four-wire).
2.4 Human-machine interface
　　The human-machine interface uses LED digital display. The system uses 2 rows of 4-digit LED digital tubes plus 1 row of 6-digit digital tubes to display the electrical parameters of each circuit. The meaning of the displayed data is indicated by the red LED light-emitting diode. The default display mode is to display the electrical parameters of each circuit in a cycle, and the user can also set it according to actual needs. The display range of the electrical parameters is 0 to 9999, and the menu and parameters are displayed in the programming state, see Figure 4 Instrument interface. The digital tube display adopts dynamic scanning mode, and its drive circuit uses a 74HC595 plus a triode.

Figure 4 Instrument interface

2.5 Switching quantity control module
　　The switching quantity control module consists of switching quantity input and alarm output. The circuit principle is shown in Figure 5. The switching quantity input is connected to the CPU via a photoelectric coupler. The alarm output is connected to the output relay via the GPIO port via a photoelectric coupler. There are 18 switching quantity inputs in total, which monitor the opening and closing states of the three three-phase circuits respectively. There is 1 alarm output, and its alarm condition can be set arbitrarily. As long as a set condition is met, an alarm signal will be output. [page]

Figure 5 Switching module circuit principle

2.6 Communication interface module
　　The communication interface module adopts the universal RS-485 and Modbus RTU communication protocols. The circuit principle is shown in Figure 6. It can realize functions such as telemetry, remote control, and remote communication.

Figure 6 Communication module circuit principle

3 Implementation functions and principles
　　The main purpose of this design is to use a single power chip to measure the current, voltage, power, electric energy and other parameters of multiple loop loads. Considering the requirements of cost and performance, this design adopts a power chip plus multiple electronic switches to measure and monitor various electrical parameters of three three-phase loops. The
　　implementation method of this solution is to connect the bus voltage of the loop to the voltage channel of the power chip ADE7758, and the current of multiple loads is switched in sequence by the electronic switch under the control of the CPU, so that ADE7758 can measure and calculate the electrical parameters of each load in sequence, and the measured data is processed by the CPU in various ways. The host structure of the monitoring
　　unit is divided into three major sections: power supply, main board and display board. Among them, the power board is mainly the layout of components of switching power supply, communication and switching quantity, the main board is mainly the layout of components such as sampling operation circuit, CPU and peripheral circuit, and the display board is mainly the layout of display circuit components. The overall structure adopts modular design, and various additional functions can be added or subtracted according to customer requirements.
4 Software Design Methods
　　The system software design includes the following four parts: main program, measurement control module, display module and communication module.
　　The main program completes power-on or reset initialization, reset watchdog, task scheduling and other functions. The program design process is shown in Figure 7.

Figure 7 Main program flow chart  

　　Program initialization includes the initial state of the CPU's I/O port, the configuration of SPI, I2C, various timers, clocks, RAM initialization, and the guidance of various configuration information.
　　Task scheduling is mainly divided into 7 tasks. 1 to 3---data acquisition of loops 1 to 3, 4---power information display, 5---communication tasks, 6---fault judgment and output, 7---power accumulation processing.
　　Tasks are triggered by external interrupts.
　　Event flags mainly include: programming settings, device calibration, fault reset and other information configuration.
　　Interrupts mainly include: external interrupt 1---data acquisition, timer interrupt 1---LED dynamic display, timer interrupt 2---switch monitoring, fault judgment, soft clock RTI---display event flag, communication interrupt---data reception and transmission.
　　Communication reception and transmission processing, display and measurement and control of electrical parameters are all implemented in interrupt mode, and the priority order is: serial communication interrupt (highest) → display interrupt → measurement control interrupt (lowest).

　　The system communication adopts the standard MODBUS-RTU protocol, which is convenient for the design of host computer management software and can be networked with other network instruments to achieve complete monitoring of the power supply and distribution system.