Implementation of Micro Nodes in Wireless Sensor Networks

Wireless sensor networks (WSNs) are composed of randomly distributed micro nodes that integrate sensors, data processing units and communication modules, forming a network through self-organization. Sensor networks have many advantages brought by distributed processing, such as high-precision monitoring, high fault tolerance, large coverage area, and remote monitoring. They have become one of the important hotspots in international network research in recent years.

introduction

Wireless sensor networks (WSNs) are composed of randomly distributed micro nodes that integrate sensors, data processing units and communication modules, forming a network through self-organization. Sensor networks have many advantages brought by distributed processing, such as high-precision monitoring, high fault tolerance, large coverage area, and remote monitoring. They have become one of the important hotspots in international network research in recent years.

Wireless sensor network micro nodes are disposable, requiring low node cost and as long working time as possible. There should be no dedicated router nodes in the wireless sensor network, and each node is both a terminal node and a router node. The nodes are connected by a mobile self-organizing network and communicate using a multi-hop routing mechanism. Therefore, on a single node, on the one hand, the hardware must be low-energy and use wireless transmission; on the other hand, the software must support multi-hop routing protocols. The IEEE802.15.4/ZigBee protocol fully considers the needs of wireless sensor network applications and is currently widely favored by the industry.

Wireless communication protocol. Based on these basic ideas, this paper designs a wireless sensor network micro node with high-end 8-bit AVR microcontroller ATmega128L as the core, combined with peripheral sensors and 2.4GHz wireless transceiver module CC2420, and it has been applied in practice.

Micronode structure

The wireless sensor network micro node consists of four parts: data acquisition unit, data processing unit, data transmission unit and power management unit, as shown in Figure 1. The data acquisition unit is responsible for the collection and data conversion of information in the monitoring area. In this design, the data acquisition unit includes temperature, humidity, light intensity, acceleration and atmospheric pressure sensors; the data processing unit is responsible for controlling the processing operation, routing protocol, synchronous positioning, power consumption management, task management, etc. of the entire node; the data transmission unit is responsible for wireless communication with other nodes, exchanging control messages and sending and receiving collected data; the power management unit selects the sensors used, and the node power supply consists of two 1.5V alkaline batteries. In the future, micro button batteries will be used to further reduce the size. In order to facilitate debugging and scalability, the data acquisition unit is separated and made into two scalable motherboards that can be connected to each other.

Figure 1 Structure diagram of micro-node in wireless sensor network

Micro node module design

Data processing unit

The data processing unit in this design uses Atmel's ATmega128L microcontroller, which is an 8-bit microcontroller based on RISC structure produced using low-power COMS technology. It is the most powerful single-chip microcomputer in the AVR series. The AVR core connects 32 working registers and a rich instruction set together. All working registers are directly connected to the ALU, which enables the operation of accessing two independent registers while executing a single instruction in one clock cycle, and has a good cost-effectiveness. This structure improves code efficiency and improves performance by about 10 times compared to ordinary CISC single-chip microcomputers.

ATmega128L has rich resources and extremely low power consumption. It has 128KB of on-chip program Flash, 4KB of data SRAM, and can be expanded to 64KB of E2PROM. In addition, it has 8 10-bit ADC channels, 2 8-bit and 2 16-bit hardware timers/counters, and can work in a variety of different modes; 8 PWM channels, programmable watchdog timer and on-chip oscillator, on-chip analog comparator; UART, SPI, I2C bus interface; JTAG interface. In addition to the normal operating mode, it also has six different levels of low-power operating modes, each with different power consumption.

To collect environmental parameter signals, a CPU with a high sampling rate and large data volume is required. If the traditional 51 series is used as the CPU, the speed of the peripheral A/D device and the CPU speed will have a bottleneck that restricts each other; if more complex data processing and storage are added, it is necessary to expand the external ROM and RAM; so many peripheral devices limit the stability and speed improvement of the system, and also greatly increase the power consumption of the system. After comprehensive comparison, the data processing unit uses ATmega128L for development. The interface circuit of the data processing unit is shown in Figure 2.

Figure 2 Data processing unit interface circuit

Data transmission unit design

The data transmission unit module circuit is composed of the low-power, short-range wireless communication module CC2420 produced by Chipcon. CC2420 is a highly integrated industrial RF transceiver device that complies with ZigBee technology. Its MAC layer and PHY layer protocols comply with the 802.15.4 specification and operate in the 2.4GHz frequency band. The chip requires very few external components to ensure the effectiveness and reliability of short-range communication. The data transmission unit module supports a data transmission rate of up to 250kbps, which can achieve multi-point to multi-point rapid networking. The system is small in size, low in cost, and low in power consumption, which is suitable for long-term battery power supply. It has the characteristics of hardware encryption, security and reliability, flexible networking, and strong anti-destruction. The data transmission unit interface circuit is shown in Figure 3. The connection between CC2420 and the processor is very simple, using the four pins SFD, FIFO, FIFOP and CCA to indicate the state of data transmission and reception; the processor exchanges data with CC2420 through the SPI interface (MISO, MOSI, SCK) and sends commands.

Figure 3 Data transmission unit interface circuit

Data acquisition unit

The entire node is powered by batteries, requiring the sensors in the data acquisition unit to be small, low power, and simple in peripheral circuits. It is best to use digital sensors that do not require signal conditioning circuits. The sensors selected in this design are all digital sensors:

(1) Temperature sensor MLX90601: analog linear output, PWM output, SPI programmable interface; accuracy ±0.2℃.

(2) Pressure sensor MS5534AP: It integrates a piezoresistive pressure sensor and an ADC interface IC. The sensor provides a 16-bit pressure parameter output with a pressure range of 300-1100 mbar. In addition, the module also contains 6 readable parameters to facilitate software calibration and high accuracy. It can automatically disconnect the power supply, and the 3-wire interface can meet various communications with the microprocessor.

(3) Humidity sensor SHT11: Using CMOSens technology, it not only combines temperature and humidity sensors together, but also integrates signal amplifiers, analog/digital converters, calibration data storage, standard I2C bus and other circuits into one chip; full-scale calibration, two-line digital output; humidity measurement range is -40_+123.8℃; temperature measurement accuracy is ±0.4℃.

(4) Light intensity sensor TSL2550D: Contains two photodetectors, one for sensing visible light and infrared light, and the other for sensing only infrared light. The two photodetectors generate two signals. The sensor simulates the principle of the human eye and determines the intensity of the surrounding light based on the strength of the two signals. It can directly convert the light intensity into a digital quantity. The compression expansion A/D converter of this device has a resolution of 12 bits. Due to the use of integral conversion technology, there will be no jitter when measuring the light of AC lamps, which improves the measurement stability.

(5) Two-dimensional digital accelerometer ADXL202AE: Using advanced MEMS technology, a polysilicon coded micromechanical sensor is etched in the same silicon wafer, and a set of precise signal processing circuits are integrated. The signal processing circuit converts the analog signal generated by the surface micromechanical sensor into a duty cycle modulated (DCM) digital signal and outputs it. This duty cycle modulated signal can be directly sent to the microcontroller, which is very convenient to use. The interface circuit of the temperature measurement data acquisition unit is shown in Figure 4.

Figure 4 Temperature measurement data acquisition unit interface circuit

Power Management Unit Design

Electric energy is the most precious resource in sensor networks, and it determines the life of sensor networks. Once the power of a node is exhausted, it is declared to have expired and exit the network, and the remaining nodes will re-form the network. Therefore, the power management of nodes is very important. In this design, the multiplexer chip ADG715BRU is used to select the used sensors under the control of the I2C bus, and the unused sensors are not powered, so as to achieve the purpose of turning off the power supply in time when there is no data collection task and saving energy.

Conclusion

Based on the summary of existing research results, the author elaborated on the wireless sensor network micro node based on ATmega128L combined with peripheral sensors and 2.4GHz wireless transceiver module CC2420. The node was well applied in the experiment and could collect high-precision temperature, humidity, light, acceleration and atmospheric pressure data, and smoothly transmit it back to the host through the network, and achieved the low power consumption required by the sensor network. It provides a basis for the design of future communication structure and specific protocol.

References:

1.ATmega128(L)datasheet.AtmelCorporation, 2001.

2.ChipconAS.CC2420PreliminaryDatasheet(1.2).2O04.