Design of multi-user intelligent electric meter based on ZigBee

0 Introduction

With the advent of the "post-PC" era in 2000, the development of electric meters has entered the "intelligent" era from the "mechatronics" era, which is marked by the transformation of electric meters from simple passive meter reading to automatic remote transmission of electric energy data and the diversification of ancillary functions (such as multiple rates, automatic power off, etc.). The emerging wireless sensor network technology is the latest direction for the development of electric meters; fundamentally speaking, the automatic meter reading system is a monitoring system with less monitoring content and weak real-time requirements, while the wireless sensor network technology has the characteristics of wireless transmission, free transmission frequency band, small amount of transmission data, slow transmission rate, large number of nodes, flexible networking mode, and low construction cost, which are exactly the goals pursued by the automatic meter reading system. As the hardware foundation of the automatic meter reading system, the combination of electric meters and wireless sensor networks meets the requirements of technological development.

ZigBee wireless communication technology, as the most widely used representative of wireless sensor network technology, has been initially applied in automatic meter reading products. However, it is mostly used in the form of meter reader uploading communication. With the development of microelectronics technology and embedded technology, the functions of meter readers can be fully compatible with smart electricity meters. Therefore, the meter reader in the automatic meter reading system and its lower-level attached meters can be completely replaced by a multi-user smart meter that uses ZigBee communication. The smart meter designed in this article adopts this technical solution.

1 Design

The design in this article is aimed at community users. A smart meter is installed in each unit of each residential building in the community. In the standard design, one meter can realize the automatic measurement of electricity and automatic power off and other auxiliary functions for up to 16 households in one unit (the number of user points in one unit can be increased by increasing the number of data selectors). The system structure block diagram is shown in Figure 1. From the perspective of system functional structure, the meter can be divided into three parts: control unit, measurement and execution unit, and communication unit. The specific function analysis is as follows:

1.1 Control Unit

The control unit of the meter is composed of a controller module based on MSP430F133. The controller module receives the pulse signal transmitted by the corresponding electric energy metering module through the corresponding operation of the data selector according to the pre-set control program. According to the metering method designed in the article, after internal calculation and processing, the power data in the cycle is obtained, which is accumulated and stored in real time and uploaded regularly via ZigBee communication. At the same time, the actuator module/communication module is controlled to realize the execution of real-time/regular arrears power off/real-time copy command. Moreover, the controller unit, as the brain of the smart meter, is also responsible for the operation and maintenance of the daily work of the entire meter system.

1.2 Measurement and execution unit

The measuring unit of the electric meter consists of 16 electric meter measurement modules and a 16-to-1 data selector. Each metering module is composed of the ADE7755 electric energy metering chip and its associated circuits, which realizes the automatic measurement of electric energy of a family user, meeting the current requirement of "one household, one meter". The ADE7755 can realize the automatic measurement of electricity consumption, and transmit the current electricity consumption to the controller through the data selector in the form of a pulse signal, while the 16-to-1 data selector can transmit the pulse signals of each metering module to the controller one by one through the set program. For the characteristics of small power consumption and flat power curve of household residential users, the average power of a certain time period in a cycle time period is used to replace the average power of the entire time range. In this paper, 32s is selected as the cycle period. Specifically, in the cycle period of 32s, a data measurement channel is connected one by one every 2s, and the controller records the accumulated pulse number of the corresponding electric energy metering module within 2s.

This metering method not only ensures measurement accuracy, but also greatly saves the port resources of the controller chip, thereby effectively reducing system costs. The data selector uses the controller to realize the successive connection of 16 data upload channels at different positions of its 4 control pins. The execution unit of the meter consists of a 4-wire to 16-wire data output controller and 16 SSRs. Once the microcontroller receives a power-off signal caused by arrears or other reasons from the upper concentrator, it immediately controls the corresponding user's solid-state relay to cut off its power supply by setting the 6 pins G0, G1, A, B, C, and D of the 4-wire to 16-wire data output controller. [page]

1.3 Communication Unit

The communication method between the meter and the upper concentrator adopts ZigBee communication. In terms of process, the CC2420 chip is used to send the current power consumption of 16 households to the data concentrator in the form of passive query or scheduled upload through the ZigBee local area network. Or the query command of the upper data concentrator and the automatic power-off command for arrears are downloaded to the controller unit. In terms of data transmission structure, a certain number of multi-user smart meters and a concentrator constitute a central structure ZigBee local area network.

2 Hardware Design

2.1 Controller unit schematic circuit construction

The control unit uses the MSP430F133 microcontroller as the core control chip. The MSP430 series microcontroller is a 16-bit ultra-low power mixed signal processor (Mixed Signal Processor) launched by Texas Instruments (TI) in 1996. It is called a mixed signal processor mainly because it integrates many analog circuits, digital circuits and microprocessors on one chip to provide a "single-chip" solution in response to actual application needs.

The MSP430F133 microcontroller controls the 4-bit selection pin of the data selector within a 32-second cycle, and connects one channel every 2 seconds to accumulate the accumulated pulse signal from the corresponding electric energy metering module to the corresponding accumulator unit. Every 10 minutes, the accumulated metering pulses are converted into corresponding power consumption data and the current time constant is added to store them in the corresponding memory unit. At the end of each hour, the stored data is uploaded regularly according to the pre-set program, or the copy command is executed at any time to immediately convert the accumulated pulses into power consumption data and the current time constant is added, and sent to the upper concentrator together with the accumulated storage data from the end of the previous hour to the present moment. The sending method is to send it to the upper end of the data concentrator in the community through the ZigBee communication module. At the same time, the power-off command caused by arrears or other reasons transmitted from the upper end can be sent to the execution module at any time to realize the automatic power-off function. The specific hardware construction of the controller is as follows:

The system is powered by a +3.3V DC power supply after rectification by LM7805, and a +4.5V battery pack is used as a backup power supply. The system uses a 4×4 matrix keyboard for parameter setting. The FM24C16 is expanded to increase the system data storage capacity. The user's power consumption display can be accurate to the latest power consumption within 10m. The dynamic scanning display circuit composed of 6 common cathode LED digital tubes uses a hexadecimal number to represent 16 users in the first digit, and the subsequent 5 digits display the current power consumption of the corresponding user, with an accuracy of 2 decimal places. The power consumption data can also be directly cyclically displayed in sequence after being processed by the single-chip microcomputer. The calculation and processing methods of different rates at different times can also realize the multi-rate function of billing. The specific hardware principle connection circuit diagram is shown in Figure 2.

2.2 Measurement and execution unit schematic circuit construction

The digital energy metering chip in the measurement unit adopts ADE7755, and the 16-to-1 data selector adopts 74LS150. The 4-wire to 16-wire data output controller in the execution unit adopts 74HC154, and the SSR adopts MOC3061+IGBT.

ADE7755 is a pulse output energy metering integrated chip designed and produced by the famous American company ADI. It can maintain extremely high accuracy and long-term stability under harsh environmental conditions. It integrates the core circuit of energy metering operation including phase correction, multiplier, digital-to-frequency converter, and signal processing circuit, and can provide the power to the MCU in the form of pulse output proportional to the instantaneous power. The single-chip microcomputer only needs to automatically record the number of pulses transmitted within a certain time interval through the counter, and then multiply it with the power/frequency conversion parameter to obtain the power consumption within this time period. The single-chip microcomputer can easily realize complex power billing by using different rate/power parameters at different times. [page]

In ADE7755, SCF=0, S1=S0=1. CF output selects the highest frequency output mode, the meter pulse/power output constant is 204800imp/kWhr, the maximum current value of household electricity is 20A, then PMAX=2.2kW, CFMAX=125Hz, the system design requires CFMIN=0.5Hz, then PMIN=8.8W. That is, the minimum power that the multi-user smart meter in this article can measure is 8.8W.

MSP430F133 realizes the sequential connection of 16 channels from E0 to E15 by setting the four pins A, B, C, and D of 74LS150. ADE7755 transmits the current power consumption to MSP430F133 in the form of pulse frequency to realize automatic meter reading; once the upper concentrator sends a power-off command, the microcontroller controls the corresponding SSR solid-state relay through the setting of the 4-wire-16-wire data output controller 74HC154 to realize power off. The principle is to drive the IGBT to turn off through the optocoupler MOC3061. The specific hardware principle connection circuit diagram is shown in Figure 3.

2.3 Communication unit schematic circuit construction

CC2420 requires only a few peripheral components. Its peripheral circuits include crystal oscillator clock circuit, RF input/output matching circuit and microcontroller interface circuit. Its hardware principle connection circuit diagram is shown in Figure 4.

CC2420 uses 1.8V internal working voltage, so the power consumption is very low, suitable for battery-powered devices; the external digital I/O interface uses 3.3V voltage, so for devices with only 3.3V power supply, no additional voltage conversion circuit is required to work normally. CC2420 RF signal transmission and reception adopts differential transmission, and its optimal differential load is 115+j180. The impedance matching circuit should be adjusted according to this value. If a single-ended antenna is used, a balanced/unbalanced conversion circuit is required to achieve the best transceiver effect. CC2420 is very convenient to connect with the processor. It uses four pins, SDF, FIFO, FIFOP, and CCA, to indicate the state of transceiver data; the processor exchanges data and sends commands with CC242O through the SPI interface; CC2420 sets the working mode of the chip through the 4-wire SPI bus (SI, SO, SCLK, CSn) and realizes read/write cache data, read/write status register, etc. The transmit/receive buffer can be set by controlling the state of the FIFO and FIFOP pin interfaces. [page]

3 Software Design

The software design of the meter system includes the main program design and the communication program design. Here we only talk about the design of the main program, and its flow chart is shown in Figure 5.

The system process is first initialized, and the process after success is divided into two parallel parts. The first is the reading of each household's electricity consumption: the round-read counter in the microcontroller is set to 01, and the MSP430F133 sets the four control terminals ABCD of 74LS150 to 0000. At this time, the channel corresponding to pin EA0 is connected to the input terminal of MSP430F133, and the microcontroller counts the number of pulses accumulated within 32 seconds. After 2 seconds, the round-read counter is added by 1 to 02, ABCD is set to 1000, and the channel corresponding to EA1 is connected, and the corresponding user's counted accumulated pulses are read. In this way, the corresponding user's copy is sent in a cycle. When the number of round-read counters = 16, a total of 16 households are read, and the round-read counter is set to 01 for the next round of reading. Generally speaking, the whole meter system is a cycle of 32s, and the user's electricity consumption is copied cyclically. Every 10m, the accumulated pulses counted by each user's accumulator unit are converted into electricity data (electricity consumption = number of pulses × 16 × meter pulse/power output constant, because the statistical pulse number is 1/16 of the real value) and the attached time constant is stored in the corresponding memory unit, and the electricity consumption/electricity fee in the latest 10m is displayed in real time. The electricity consumption data (electricity consumption and corresponding time constant) is transmitted regularly at the end of every hour. The second is the upload of meter data and the execution of power off: start CC2420 to connect to the ZigBee LAN, wait for the command of the upper concentrator after success, and restart CC2420 to reconnect to the LAN once the network is disconnected; the commands downloaded by the upper concentrator are divided into data upload and power off due to arrears. If it is a power off command, MSP430F133 controls the SSR of the corresponding user to disconnect the power connection, and automatically switches to standby state after success. If unsuccessful, repeat the power off operation until the operation is successful. If it is an upload command, MSP430F133 will immediately convert the accumulated pulses in the accumulator into power data and store them in the corresponding memory unit with a time constant, and then upload the stored data in the RAM from the end of the previous hour to the current moment to the upper concentrator through CC2420 in ZigBee communication mode. After success, it will automatically enter the standby state.

4 Conclusion

The multi-user smart meter using ZigBee communication technology is the hardware foundation and constituent element of the remote automatic meter reading system. A certain number of meters and the concentrator using the same CC2420-based ZigBee communication technology constitute a central structure of the ZigBee local communication network. Each meter exchanges data with the central node concentrator, and the concentrator uploads the collected meter reading data to the remote control center to realize remote automatic meter reading. In the laboratory environment, the MSPRF-430F2618-PK professional development system (some modules have been replaced) was used for verification tests, which proved that the multi-user smart meter operates normally and communicates smoothly with the concentrator. However, there are also defects such as insufficient communication distance (less than 50m) and being easily interfered by the external electromagnetic environment. It is possible to consider using the latest CC2591 (with enhanced antenna power, its theoretical communication distance can reach 1000m) to replace CC2420 to improve communication efficiency.

In general, the multi-user smart meter in this paper is compatible with the meter reader function, simplifies the meter reading system structure under the premise of "one meter per household", and saves system costs. Compared with the current smart meters using wired communication technology (such as power carrier/bus communication, etc.), the multi-user smart meter based on ZigBee realizes wireless meter reading within a short distance, which not only avoids the difficulty of wired meter reading wiring construction, but also has the function of wireless sensor network node in the remote meter reading system, that is, its entry and exit in the meter reading communication network only need remote control without any hardware modification, thus greatly reducing the labor intensity of daily maintenance work after installation and operation. Compared with GSM technology, which is also wireless meter reading, ZigBee technology is based on the free 2.4GHz channel, and there is no need to pay expensive communication channel rental fees. In short, the combination of low-cost ZigBee communication technology and electric meters not only meets the current market's functional requirements for smart meters, but also meets the requirements of the future development of "smart homes". Automatic meter reading is a functional unit of the "smart home" three-meter centralized reading system. ZigBee communication technology, as the most promising communication method in "smart home", is not only the most economically efficient when combined with the former, but also represents the future development direction of electricity meters.