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 this information wirelessly after processing, and finally send it to the observers through the wireless network. Wireless sensor networks have broad application prospects in military reconnaissance, environmental monitoring, medical care, smart home, industrial production control and business.

Most of the 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. In this way, electric energy can be effectively saved, the battery power supply time and service life can be extended, and the working quality of the entire network system can be guaranteed at the same time, and the service life can be extended.

1 System power module hardware design

The power module of this system mainly designs power detection, intelligent control of power switch and power conversion. The hardware block diagram of the circuit system 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 based on the working status of the system load, thereby reducing standby power consumption and extending the battery life; the power conversion module mainly uses a module 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 the module is shown in Figure 2.



The module circuit is mainly composed of a low-dropout linear regulator (LM1117), sampling resistors (R1, R2, R3, R4, R5), a voltage comparator (LM139), an inverter (74HC04) and an encoder (74HC148). The low-dropout linear regulator LM1117 provides a 3.3 V reference voltage, which is compared with the sampled voltage obtained by the sampling resistor and input to the voltage comparator LM139. The high and low levels output by the voltage comparator are then input to the encoder 74HC148 for encoding through the inverter 74HC04. 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 the combination of sampling resistors. The battery voltage state 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 sampling resistors R1, R2, R3, R4, and R5 can be obtained from the following equations:

Solved:

R1=3.3 MΩ; R2=16.2 kΩ; R3=16.6 kΩ; R4=16.9 kΩ; R5=510 kΩ.

The encoder 74HCl48 is effective at low level, so an inverter 74HC04 is connected behind 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 the 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 a field effect tube. 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 the control signal input terminal is input at a high level, the switch circuit is turned on; when the control signal input terminal is input at a low level, the switch circuit is turned off.

After testing, it was found 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 features of DC/DC module power supply products 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, external capacitors are 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 selected as 100μF capacitors, and the input and output external capacitors of the WRA2405CS are selected as 22μF and 100μF capacitors respectively; the 78 series three-terminal voltage regulator selects 0.33μF and 0.1μF capacitors for input and output external capacitors according to typical circuit applications.

2 Conclusion

The basic concept of wireless sensor network is explained, the importance of power module to the safe and reliable operation of the whole system of wireless sensor network is analyzed, the composition of wireless sensor network power system and the circuit structure and function realization process of each module are introduced in detail, and experimental debugging proves that its performance meets the predetermined performance requirements.