Design of Solar Power System for Wireless Sensor Network Nodes (3)

Keywords : wireless sensor network, sensor node, solar energy, DC-DC

For wireless sensor network nodes, power supply is one of the key parts of the system. Herein, a power supply system is proposed to collect solar energy in the environment to power sensor nodes. The system adopts efficient and safe charging control technology, unique battery voltage monitoring circuit, and low-power DC-DC conversion circuit. Through experimental verification, the sensor node based on this solar power supply has good dynamic power consumption adjustment performance and significantly increased life cycle. The system can be applied to various outdoor monitoring nodes, such as environmental monitoring, precision agriculture, forest fire prevention, etc.

0 Introduction

Wireless sensor networks have broad application prospects in environmental monitoring, smart home, transportation, precision agriculture and other fields, and are increasingly valued by people. As an important component of wireless sensor networks, sensor nodes are usually scattered in a certain area to collaboratively monitor, perceive and collect information about various environments and monitored objects in real time. The deployment environment of sensor nodes and the requirements of actual applications determine that the node power supply cannot be connected to the normal power system in most cases. For example, if the MICAz node of Crossbow uses a 3,000 mAh battery set at a 1% duty cycle, the battery needs to be replaced every 17.35 weeks. In addition, since the nodes are often deployed in harsh and complex environments, the cost of replacing batteries is further increased. How to stably and effectively provide power supply for sensor nodes has become a key issue in sensor node design. The current research ideas for this problem are mainly how to collect energy from the environment where the node is located and store it effectively, so that the node has energy replenishment capabilities and effectively extends the node's life cycle. The environment has a variety of abundant energies, such as solar energy, wind energy, thermal energy, mechanical vibration energy, acoustic energy, electromagnetic energy, etc. At present, some companies have researched and developed systems that use environmental energy for wireless sensor network functions. For example, the solar energy collection module CBC-EVAL-08 has been successfully applied to TI's ultra-low power wireless sensor network node eZ430-RF2500-SHE to provide energy for it. The startup company Perpetuum launched the PMG7 micro vibration generator, which can generate up to 5 mW/3.3 V output power from a 100 mg vibration. However, current energy collection has some limitations. For example, the solar energy collection module CBC-EVAL-08 can only work when there is sunlight due to the small amount of energy collected by photovoltaic thin-film batteries and the lack of backup energy; the use of vibration energy restricts the layout environment of the node. Even in an intermittent vibration environment, the system cannot work stably and continuously.

By comparing and analyzing various energies in the environment, it is concluded that using solar energy to power outdoor sensor nodes is a good choice. This paper proposes a node power system design based on solar energy, which can automatically manage the charging process and perform effective energy storage. By monitoring the battery voltage and executing energy-saving solutions, the purpose of extending the node life cycle is achieved. In addition, since the voltages required by various devices on the node are inconsistent, efficient DC-DC conversion is also an indispensable link.

1 Power System Design

The power supply unit is the energy supply part of the sensor node, which determines the life of the sensor network, so the power supply design of the node is very important. The power supply unit is mainly composed of batteries, power management modules and peripheral circuits. The first thing to consider in power supply design is low power consumption. Since the power consumption of the load is proportional to the square of the voltage, it is best to use a lower operating voltage to ensure reliable operation of the system. Node components such as sensors, MCUs, and wireless RF modules all have low operating voltage options, such as +3.3 V. Taking the above factors into consideration, a power supply system as shown in Figure 1 is proposed.

In this system, the energy generated by the solar panel is stored in the lithium battery through the charging control unit; the power supply management unit selects the appropriate energy supply scheme by real-time monitoring of the battery voltage. As the battery terminal voltage gradually decreases when it discharges, it will affect ADC sampling, etc. In addition, the operating voltages of various devices are not consistent. In order to ensure the reliable operation of the system, a stable power supply voltage is required. Since the power supply unit itself should consume as little battery energy as possible, the conversion efficiency of the power supply must be improved. Therefore, a highly efficient DC-DC conversion unit is designed to provide a stable voltage for the load on the node.

1.1 Charging control unit

The charging control unit connects the solar panel and the lithium battery, and its function is mainly to effectively store the collected energy in the lithium battery. In this design, the solar panel uses an 80mm×45mm panel. The output voltage of this panel is 5.5 V, the current is 150 mA, and the conversion efficiency is 16% when the maximum output power is output. The lithium battery has no memory effect, and a lithium battery with a capacity of 2 000 mAh and an operating voltage of 3.7 V is selected. The control part of this unit uses the intelligent charging control chip LTC4070 for lithium-ion batteries launched by Linear Technology Corporation. With its 450 nA operating current, the device charges and protects the battery with very low current, intermittent or continuous charging that could not be used before. The function of this device is very suitable for continuous and intermittent, low-power charging power supply applications. The LTC4070 has pin-selectable 4.0V, 4.1 V or 4.2V settings, and its 1% accurate battery floating voltage allows users to optimize the balance between battery capacity and life. Independent low battery power and high battery power monitoring status outputs indicate that the battery is discharged or fully charged. With an external PFET in series with the load, the low battery status output implements a latch-off function that automatically disconnects the system load from the battery to protect the battery from deep discharge. The schematic diagram of the charging control unit is shown in Figure 2.

When the solar panel is not charging the lithium battery, in order to reduce the energy consumption of LTC4070, transistor Q1 is added. When the base voltage of Q1 drops, LTC4070 is isolated from the lithium battery. In normal charging mode, most of the current flows to the lithium battery through Q1. When VCC reaches the floating voltage set by ADJ, LTC4070 shunts the current of the bc junction in Q1 to continuously reduce the battery charging current until 0, and Q1 enters the saturation state. If the thermistor T increases and the floating voltage decreases, LTC4070 will shunt more current, and Q1 is forced to enter the reverse bias state until the battery voltage drops. The ADJ pin is used to set the floating voltage, which is 4.0 V when connected to ground, 4.2 V when connected to VCC, and 4.1 V when floating. When the lithium battery voltage is lower than 3.2 V, LBO pulls up D1 to light up. When the lithium battery is fully charged, HBO pulls up and D2 lights up.

1.2 Power Management Unit

The power management unit has two functions: one is to set the discharge threshold to prevent the lithium battery from being deeply discharged; the other is to obtain the current battery voltage to determine the power consumption mode adopted by the node.

The lithium battery discharge threshold setting circuit composed of MAX680 and MAX8211 is shown in Figure 3.

In this circuit, when the lithium battery voltage drops to the threshold voltage determined by R1 and R5, MAX8211 will cut off the supply voltage of MAX680, and finally turn off IRF541 to disconnect the power supply battery and the load circuit. The on-current of the IRF541 power switch is less than 0.5 mA, and the off-leakage current is only less than 8μA. The relationship between the startup threshold Vu and the cut-off threshold V1 of this circuit and the external resistors R5, R6 and R7 can be given by the following formula:

In order to perform effective power management, it is necessary to understand the storage of battery energy and adjust the working state and communication strategy according to the task requirements and its own energy state. The design uses the LM4041 voltage reference chip, and the microprocessor samples its terminal voltage and calculates the actual voltage value of the battery for program processing. Its schematic diagram is shown in Figure 4.

U4 is LM4041-1.2, which is a micro-power precision voltage regulator. Resistor Rs is responsible for providing the voltage regulator current IL and the load current IQ. The value of Rs should ensure that the current IQ flowing through the voltage regulator does not exceed IQmin and IQmax. The calculation formula of Rs is as follows:

Where: When VS is 4.2 V, VR is 1.2 V, IL+IQ is about 120 A, and the calculated Rs value is about 27 kΩ. In the implementation process, ADC0 is used to measure the stable voltage VQ, and the battery supply voltage is selected as the reference voltage Vref of ADC. When PC0 is set to "0", Q3 is turned on, and the reading of ADC0 is ADC_Data. The relationship between ADC_Data and the reference voltage Vref is shown in formula (4):