Functional design and demonstration of smart meter system based on ARM processor

　　For many years, electricity management and charging have been based on the traditional operation mode of using electricity first, reading the meter later, and paying the bill. The calculation of electricity value cannot achieve higher accuracy and has large deviations. In order to adapt to the needs of society and ensure that users use electricity safely, reasonably and conveniently, it is necessary to redesign the traditional electricity meter and electricity consumption to meet the needs of social development.

　　1. Development prospects of smart meters:

　　Phase 1 2000-2007

　　Gradually replace traditional electromechanical meters with electronic meters; promote one-way communication networks in the United States and Europe;

　　Second phase 2008-2012

　　Electromechanical meters are being phased out worldwide; two-way communication networks based on the Automatic Metering Infrastructure (AMI, an automatic two-way communication architecture between smart meters with IP addresses and power companies) are being promoted on a large scale in Europe, the United States and China; smart meters serve as simple home gateways and can be used for automatic meter reading (AMR) and load management of various utilities.

　　Phase 3 2013-onwards

　　Smart meters become home gateways, realizing various residential control functions such as security, alarm, etc.; and realizing intelligent power generation (solar and wind power generation) and distribution in homes and units.

　　From the above three stages, we can see that smart meters have not developed in the direction of small size and single function. On the contrary, in the future, smart meters will be given more functions, making them an indispensable part of the family.

　　With the introduction of policies on peak-valley time-of-use electricity prices and peak-avoidance electricity prices, the market demand for multi-rate meters will further increase, especially for large industrial users, the demand for smart multi-rate meters will grow rapidly. With the advancement of the information age and the development of technology, smart meters, as the nerve endings of smart grids, will play a greater role in the information society in the near future and have broader application prospects.

　　2. The proposal of ARM smart meter:

　　It is precisely because of the above background that smart meters came into being. The so-called smart meters are meters that use computer technology and communication technology to form smart chips as the core, with functions such as high-precision calculation of electricity, communication with the host computer, and display of electricity-related data.

　　The energy meter is the only tool for power companies to collect electricity bills. The stability of its performance, the accuracy of its measurement, and its reliability are related to the settlement between the power department and the user. Among them, the electricity value measurement technology of the smart meter is particularly important. Most of the designed smart meter systems are generally controlled by 8-bit or 16-bit single-chip microcomputers. Their control functions are relatively simple, and it is difficult to achieve networking and wireless transmission. The scalability of future smart meter systems is also relatively limited. Based on such limitations, we propose a smart meter system based on an ARM processor.

　　At present, the smart meter we need to design mainly realizes the following functions:

　　1. Realize high-precision calculation and scalability of smart meters;

　　2. Realize two-way data transmission, including wireless meter reading. The meter sends the customer's electricity usage to the power department, and the power department sends the customer's account balance back to the meter and displays it on the meter;

　　3. Realize LED display of data, display of electricity consumption of last month and this month, and balance display, etc.;

　　Based on the above functional design of smart meters, higher requirements are put forward for the software and hardware of meters, which are obviously difficult to achieve with traditional meters. ARM technology has the characteristics of high performance, low cost and low energy consumption, and is suitable for a variety of fields, such as embedded control, consumer/educational multimedia and mobile applications. So far, the application of ARM microprocessors and technologies has penetrated into almost every field, and there are precedents for their application in smart instruments and meters, so we choose ARM technology for smart meters.

　　Smart meters add multiple functions to digital meters to make them more adaptable to residents' needs and read users' power consumption more intelligently, conveniently and quickly, making it possible to distribute and transform power to individual users in the future.

　　3. Design of various functions of smart meter:

　　3.1. Implementation of remote meter reading:

　　In terms of system integrity, the power system has basically achieved network management from power generation, distribution, transmission and regional substations. However, the user terminal is the only one that is not connected to the network, which causes the incompleteness of the system and directly or indirectly affects the potential of the system. Therefore, we have designed the technology of remote meter reading of smart meters.

　　The collector collects the meter signals from the multi-channel branch line connection in a regular order and processes and stores the data. The collectors are connected by a bus and finally connected to the computer. Its typical feature is that each household meter is connected to the collector through a branch line, and then the collector sends it to the server subsystem through wireless transmission. This avoids installing a wireless radio frequency module for each smart meter to save costs. The system is generally divided into four hierarchical structures: front-end acquisition subsystem, server subsystem, communication subsystem, and management center subsystem.

　　The initial idea is to install a collector in each building, or install several collectors in a community to realize the front-end collection subsystem of the entire meter reading system. The collector connects to each smart meter through the CAN bus; then the data is sent to the server installed on the street or within a certain range through wireless radio frequency; finally, the server sends the data to the management center through the communication subsystem. The management center will also send the customer's information back to the meter according to the IP address of each meter. The path is just the reverse of the data collection mentioned above. In this way, we build a smart meter based on the automatic metering infrastructure, namely the AMI architecture. The so-called AMI architecture is an automatic two-way flow architecture between smart meters with IP addresses and power companies.

　　3.2, sampling filtering:

　　The key part of the entire hardware design is signal acquisition, which converts 220V AC power into a corresponding 0 to 3V AC signal. This is related to the accuracy of the entire system. If the acquisition is distorted, it will not only lead to a decrease in accuracy, but also may cause serious errors.

　　This technology uses a dedicated transformer to directly transform the voltage. After obtaining the required AC signal, it passes through a filtering circuit and is directly output to the MCU's A/D interface for sampling.

　　The current passes through a special transformer and converts the voltage into a 0.5V voltage Ui; then it passes through the first-order low-pass filter R1 and C1. The low-pass filter is mainly used to filter out high-frequency signals. The capacitor can pass high-frequency signals but block low-frequency signals. By grounding the capacitor C1, the high-frequency signal can be filtered out. Since the signal obtained after filtering is relatively weak, an Ur is added through the resistor R2. Ur is a 0-3V DC signal, and finally a 0-3V AC signal U0 will be obtained.

　　3.3、Smart meter hardware design method:

　　Smart meters are mainly composed of electronic components. Their working principle is: first, through real-time sampling of the user's power supply voltage and current, the sampled voltage and current signals are transformed using integrated circuits, and a 0-3V analog signal is obtained through filtering. Then, the digital signal is converted through four processes of sampling, holding, quantization and encoding by an A/D converter. The digital signal processing unit calculates the power per second through the formula and accumulates it. Finally, the obtained power value is processed and controlled by the control unit, and the power value is stored or output.

　　It uses a 16/32-bit ARM7TDMI CPU that supports real-time simulation and embedded tracing, and has embedded high-speed Flash memory. The memory interface and unique acceleration structure enable the 32-bit code to run at the maximum clock frequency.

　　For applications with strict control over code size, 16-bit Thumb mode can be used to reduce code size by more than 30% with little performance loss. The A/D converter needs to sample three-phase industrial frequency AC power without distortion. Multiple serial interfaces can be easily expanded.

　　The peripheral interface can be connected to an external handheld device for staff debugging and maintenance, etc. Data storage, output and display all require different data structures, so the structure of the power data needs to be converted into the corresponding storage structure, output structure and display structure, which are all performed in the MCU's data conversion module.

　　3.4、Software design method of smart meter:

　　The software of the smart meter is mainly the calculation part of the power value. We use software programming to calculate the power value by formula calculation, which is mainly executed in the digital signal processing unit module. The demonstration and design of the application of ARM technology in smart meters is calculated by U0 and I0 obtained from the A/D converter, but integration is too complicated in the binary calculation of the computer. Considering the cost problem, the accumulation method is used instead of the integral operation. The accumulation formula is. The demonstration and design of the application of ARM technology in smart meters, if N is 100000, the cycle accumulation 100000 times is the power value calculated by the smart meter in one second.

　　Summarize:

　　This design mainly discusses the application of ARM in smart meters. The main difficulty of ARM in smart meters is the cost issue, so cost saving has been considered many times in the design, including using as few wireless modules as possible in wireless transmission and considering the realization of multi-user smart meters. The application of ARM in smart meters is mainly to consider the future development trend of smart meters, to achieve more scalability and compatibility of smart meters, and to add more practical functions to ARM-based smart meters in the future.