Design of concentrator for wireless meter reading system in smart community based on nRF2401

With the continuous development of electronic technology, computer technology and communication technology, the automation process of various industries is gradually accelerating, so that higher requirements are put forward for the real-time, reliability and information volume of data collection in automatic meter reading. The current meter reading systems mainly include: wired meter reading system, handheld meter reading system and wireless meter reading system. Among them, the wired meter reading system increases the cost and difficulty of integrated wiring, reduces the application flexibility of the system, and limits the promotion and application of the wired meter reading system. The handheld meter reading system requires the meter reading department to use a handheld meter reader to read data, thus reducing the degree of automation. The wireless meter reading system uses wireless transceiver equipment to transmit data, does not require special wiring, has a simple system structure, saves manpower and material resources, and has greater advantages than the wired meter reading system and the handheld meter reading system. This paper proposes a wireless meter reading system based on wireless network technology, focusing on the design of a wireless communication concentrator based on nRF2401 , through which the collected data can be transmitted quickly and reliably.

Wireless meter reading system solution

The wireless meter reading system of smart community mainly consists of three-meter data collection terminal, concentrator and computer of property center. The three-meter data collection terminal and concentrator (lower layer), concentrator and computer of property center (upper layer) all adopt star structure. The structure of wireless meter reading system of smart community is shown in Figure 1.

The upper layer is connected to the nRF2401 wireless communication module on the concentrators in each building in the community through the nRF2401 wireless communication module, forming a one-to-many connection form, realizing the real-time online connection between the concentrator and the computer of the community property center. The lower layer communication includes the collection, storage, and forwarding of the three-meter data by the concentrator through wireless means, as well as forwarding the instructions issued by the host computer and controlling the three meters. The concentrator is the communication bridge of the entire wireless meter reading system. Its working condition determines the reliability and stability of the system. Its functions include: querying the data of the three meters of the terminal at the agreed time every month, and accumulating and saving the collected data; when receiving the collection command issued by the computer of the property center, immediately packing the data and transmitting it; it can also actively send data to the computer of the property center, report emergency situations, etc.

Concentrator Hardware Design

In the wireless meter reading system, the concentrator is in the middle of the information transmission path. The concentrator in the system uses wireless data transmission and is the core of the entire system. The wireless data concentrator mainly consists of a wireless data transmission module nRF2401, an external storage unit, a local communication interface, a microprocessor (MCU) and a power module. The data concentrator hardware structure block diagram is shown in Figure 2, and the data concentrator circuit schematic diagram is shown in Figure 3.

Considering the characteristics of wireless data concentrators such as low power consumption, cost-effectiveness, stable performance, simple interface circuit and high degree of automation, it is crucial to choose the right circuit chip.

Wireless data transmission module nRF2401

nRF2401 is a single-chip 2.4GHz wireless transceiver chip from Nordic, Norway. It integrates RF, 8051 MCU, 9-channel 12-bit ADC, peripheral components, inductors and filters into a single chip, and can provide functions such as ShockBurst, DuoCeiver, on-chip CRC and address calculation encoding. Like Bluetooth, nRF2401 wireless transceiver chip works in the 2.4GHz free frequency band. nRF2401 supports multi-point communication, with a maximum transmission rate of more than 1Mbit/s, and has a higher transmission speed than Bluetooth. Unlike Bluetooth, it is user-transparent and products of the same type can communicate freely. More importantly, nRF2401 is cheaper than Bluetooth products.

Power Module

In the concentrator circuit, the power supply is the energy center and plays a very important role in the concentrator. Since the chip used in this concentrator system needs to be powered by +3V or +5V DC, the AC-DC module is first used in the power supply part to convert 220V AC into +5V DC. In this system, the LM1117, which is specially used to provide conversion voltage for communication control chips, is selected as the conversion chip. Because there are many high-frequency interference sources in the voltage, these high-frequency components can easily enter the communication system through the power supply. In addition, the system's own transmission frequency will also be fed back to the communication system through the power supply induction to cause interference. Therefore, a 220mH inductor is added to the power supply circuit, and a filter circuit composed of multiple capacitors with different capacitance values ​​is incorporated to suppress various high-frequency signals. The circuit schematic diagram of the power supply is shown in Figure 4.

Concentrator Circuit Design Considerations

The circuit design of the RF part is the key and difficulty of the concentrator design, and it is also the key to the success of the concentrator design. Anti-interference design is directly related to the RF performance and the operation of the entire concentrator. When wiring the RF part, reasonable layout and wiring and the use of multi-layer boards are both necessary for wiring and effective means to reduce electromagnetic interference and improve anti-interference capabilities.

The following points should be noted when wiring: First, the area of ​​the RF circuit that is not used for wiring must be filled with copper and connected to the ground to provide RF shielding to achieve effective anti-interference; second, the bottom of the nRF2401 chip should be grounded; in order to reduce delays, reduce crosstalk, and ensure the transmission of high-frequency signals, use multiple ground vias to connect the bottom of the nRF2401 chip to the ground layer; third, to reduce crosstalk as much as possible and reduce the impact of distributed parameters, the devices should be closely distributed around the nRF2401 and use a smaller package.

Concentrator software design

The main task of the concentrator is to complete the communication with the community property center and the three-meter data collection terminal. The communication of the concentrator includes two parts: the computer of the community property center communicates with the concentrator through the nRF2401 wireless communication module; the concentrator communicates with the three-meter data collection terminal through the nRF2401 wireless communication module.

In the wireless data transmission between the concentrator and the community property center and the three-meter data collection terminal, the data must be processed in a specified format to effectively reduce the bit error rate during data transmission. The data packet format used in this system consists of data sequence number, destination address, source address, sent data length, data body and check code.

When the receiving end receives a data packet, it sends a confirmation signal to the sending end. If the verification is correct, the judgment process is started and the data packet is processed. The data sequence number is added before the data packet to detect packet loss and leakage in time.