Design of intelligent control system for power supply of wireless sensor network

0 Introduction
Wireless Sensor Network (WSN) is a network composed of sensor nodes, which can monitor, sense and collect various information of the objects of interest to the observers in the node deployment area in real time (such as physical phenomena such as light intensity, temperature, humidity, noise and harmful gas concentration), and send these information wirelessly after processing, and finally send them to the observers through wireless networks. Wireless sensor networks have broad application prospects in military reconnaissance, environmental monitoring, medical care, smart home, industrial production control and business.
Most wireless sensor networks are powered by batteries, and the working environment is usually harsh. Moreover, the number is large, and it is very difficult to replace the battery, so low power consumption is one of the most important design criteria for wireless sensor networks. When some modules of the network node are not working or in a dormant state, their power supply circuit can be disconnected to save power. When there is an instruction to wake them up, their power supply circuit is connected to ensure the normal operation of the system. This can effectively save energy, extend the battery power supply time and service life, and also ensure the working quality of the entire network system and extend its service life.

1 System power module hardware design
The system power module mainly designs power detection, power switch intelligent control and power conversion. The circuit system hardware block diagram is shown in Figure 1.

The main functions of each part of this power supply system are as follows: the power supply is composed of two 12 V lead-acid batteries connected in series, providing the system with a 24 V DC voltage; the battery detection module mainly obtains the battery power consumption information by sampling the output voltage of the power supply, and charges the battery in real time to ensure the normal and stable operation of the entire system; the power supply switch control module mainly determines whether to supply power to the load through switch control according to the working status of the system load, thereby reducing standby power consumption and extending the battery life; the power conversion module mainly uses a modular DC-DC converter as a building unit to realize the conversion of the power supply voltage to provide a suitable voltage for the load.
1.1 Battery detection module circuit design
The battery detection module determines whether the battery is sufficient by sampling the output voltage of the power supply. Here, the power supply output voltage is divided into five states: 25 V or more for full battery state, 24.2-25 V for sufficient battery state, 23.5-24.2 V for normal battery operation state, 22.8-23.5 V for insufficient battery but still working state, and 22.8 V or less for battery failure. The state reading is used to determine whether the battery can normally power the entire system and decide whether to charge the battery to ensure the normal operation of the entire system. The circuit structure of this module is shown in Figure 2.

The module circuit is mainly composed of a low-dropout linear regulator (LMlll7), sampling resistors (R1, R2, R3, R4, R5), a voltage comparator (LMl39), an inverter (74HC04) and an encoder (74HCl48). The low-dropout linear regulator LMlll7 provides a 3.3 V reference voltage, which is compared with the sampling voltage obtained by the sampling resistor and input into the voltage comparator LMl39. The high and low levels output by the voltage comparator are then input into the encoder 74HCl48 through the inverter 74HC04 for encoding. The binary code output by the encoder reflects the battery power information.
The battery voltage sampling resistor network samples the output voltage of the battery through a combination of sampling resistors. The battery voltage status values ​​corresponding to each resistor endpoint are: R2 corresponds to 22.8 V, R3 corresponds to 23.5 V, R4 corresponds to 24.2 V, and R5 corresponds to 25 V. When the voltage value output by the battery is equal to the state value set by each resistor, the sampled voltage at the resistor end is 3.3 V; when the voltage value output by the battery is greater than the state value set by each resistor, the sampled voltage at the resistor end is greater than 3.3 V; when the voltage value output by the battery is less than the state value set by each resistor, the sampled voltage at the resistor end is less than 3.3 V. The values ​​of the sampling resistors R1, R2, R3, R4, and R5 can be obtained by the following equations:

The solution is:

The encoder 74HCl48 is valid at low level, so the inverter 74HC04 is connected after the comparator LMl39. The relationship between the binary code (truth table) of the encoder input/output and the battery voltage is shown in Table 1.

1.2 Circuit design of switch control module for each power supply
The switch control of each branch of the power supply system is mainly realized by a switch circuit composed of field effect transistors. The circuit structure to realize this function is shown in Figure 3.

The field effect tubes selected are the enhanced P-channel field effect tube IRF9640 and the enhanced N-channel field effect tube VN2222L. The field effect tube IRF9640 is used as a switch tube to control the on and off of the circuit, and the field effect tube VN2222L is used as a switch control tube to control the on and off of the field effect tube IRF9640. When a high level is input to the control signal input end, the switch circuit is turned on; when a low level is input to the control signal input end, the switch circuit is turned off. After testing, it was measured that when the control signal voltage gradually increases from 0 V to 1.8 V, the switch is turned on; when the voltage gradually decreases from a high level (such as 3.3 V) to 1.8 V, the switch is turned off. The performance of this switch circuit is shown in Figure 4.

1.3 Power conversion module circuit design
The power conversion circuit chip mainly uses the DC/DC module power supply of Jinshengyang Company and is equipped with a 78 series three-terminal voltage regulator.
The characteristics of the DC/DC module power supply product are: wide voltage input (2:1~4:1), efficiency up to 85%, good high and low temperature characteristics, can meet the technical requirements of industrial-grade products, working temperature: -40~+85℃, isolation voltage 1500V DC, dual output, metal shielding package, international standard pin method, MTBF>1000000h.
In order to ensure that it can still maintain the best working state under full load conditions, an external capacitor is required. In order to further reduce the input/output ripple, the output capacitor Cout capacitance value can be appropriately increased or a capacitor with a small series equivalent impedance value can be selected, but the capacitance value cannot be too large. The input/output external capacitors of the DC/DC module power supply VRA2412D are 100μF capacitors, and the input and output external capacitors of WRA2405CS are 22μF and 100μF capacitors respectively; the input and output external capacitors of the 78 series three-terminal voltage-regulated power supply are 0.33μF and 0.1μF capacitors respectively according to the typical circuit application.

2 Conclusion This paper
expounds the basic concepts of wireless sensor networks, analyzes the importance of power modules to the safe and reliable operation of the entire wireless sensor network system, introduces in detail the composition of the wireless sensor network power system and the circuit structure and function implementation process of each module, and proves through experimental debugging that its performance meets the predetermined performance requirements.