"Preface
Nowadays, the wide application of Aerial Robot Technology in many fields such as civil and national defense has been paid more and more attention, and has attracted the attention of experts and scholars all over the world. The small rotor robot takes the model helicopter as the carrier and is equipped with sensor unit, control unit and servo mechanism to realize autonomous flight. In order to improve the safety of aircraft, it is necessary to design a set of equipment monitoring system to monitor the attitude information of aircraft, the status of airborne equipment and power supply in real time.
The power supply used in the platform is a battery pack composed of two lithium-ion batteries in series. Using the charge and discharge characteristics of lithium-ion batteries, a charge and discharge management system with mega16l as the core is designed. Lithium battery has the advantages of small volume, high energy density, no memory effect, high cycle life, high voltage battery and low self discharge rate. Unlike nickel cadmium battery and nickel hydrogen battery, the safety during charging and discharging must be considered to prevent characteristic deterioration. Therefore, during the operation of the system, in order to protect the safety of lithium battery, a set of undervoltage protection circuit is designed to prevent the deterioration of battery characteristics and durability characteristics due to overuse.
1. Overall framework of power management system
UAV power management system is an important part of aircraft autonomous flight. Its general framework is shown in Figure 1. In this system, the 2212 / 34 generator produced by Axi company is used to convert kinetic energy into 220V AC, and then output 11.6v DC voltage after rectification and voltage stabilization, which can charge two lithium batteries. The controller of the power management system is MEG a161 The controller controls the relay switch to charge and discharge the battery by detecting the voltage of two lithium batteries.
Figure 1 power management system framework
After collecting the information in the power supply system, the controller transmits the data to the ground in real time through wireless transmission equipment. The ground monitoring platform can also send some commands to mega16l to control the battery charge and discharge by controlling the relay switch, so as to achieve the purpose of monitoring and controlling the aircraft.
The on-board power module is composed of two lithium batteries produced by intelman Battery Co., Ltd. when the battery pack is fully charged, the voltage is 8?? 4V. The charge of the battery is closely related to the reliability of the whole power supply system. The more the remaining battery, the higher the reliability of the system. Therefore, the remaining battery can be obtained in real time during flight, which will greatly improve the reliability of the aircraft.
2 implementation of power monitoring system
Sufficient power supply is indispensable for the helicopter to successfully complete its flight mission.
It can be seen from the characteristics of lithium battery that in the case of excessive discharge, the electrolyte will degrade the battery characteristics and reduce the charging times due to decomposition. Therefore, in order to protect the safety of the battery, the power supply system must pass through the undervoltage protection module and voltage stabilizing module before supplying power to the control system. In order to predict the remaining power in the power supply system, the method of detecting the power supply system voltage is adopted here. After measuring the power supply voltage of the system, the remaining power in the power supply system can be estimated by looking up the database established by the discharge curve.
The power supply voltage required by the single chip microcomputer is 2.7 ~ 5.5V, so the external reference voltage can be designed for MEG A16L as 2.5V. The reference voltage stabilizing circuit is shown in Figure 2. Therefore, to detect the battery voltage, the system needs to divide the battery resistance, and the maximum divided voltage value cannot exceed 2.5V. The real-time voltage in the power supply system can be obtained after multiplying the voltage value measured by the controller by the reduced multiple of the voltage partial voltage. The purpose of effectively using battery capacity and prolonging service life can be achieved by constantly monitoring the power consumption of lithium battery and preventing battery overuse.
Figure 2 reference voltage circuit
2.1 hardware design
2.1.1 circuit of Brushless DC motor
Brushless DC motor is composed of motor body and driver. It is a typical mechatronics product. The brushless DC motor has the same working principle and application characteristics as the general DC motor, but its composition is different. In addition to the motor itself, the former also has a commutation circuit. The brushless DC motor itself is an electromechanical energy conversion part. In addition to the motor armature and permanent magnet excitation, it also has sensors. Some AC-DC circuits of the generator are shown in Figure 3.
Fig. 3 AC-DC circuit of brushless motor
2.1.2 charging circuit
The charging characteristics of lithium-ion batteries are different from those of nickel cadmium and nickel hydrogen batteries. When charging lithium-ion batteries, the battery voltage rises slowly and the charging current decreases gradually. When the voltage reaches about 4.2V, the voltage basically remains unchanged and the charging current continues to decrease. Therefore, for the modified charger, the charging mode of constant current first and then constant voltage can be used for charging. The specific charging circuit is shown in Figure 4. The circuit uses lm2575adj to form a chopper switching regulator, and the maximum charging current is 1a
Fig. 4 partial circuit of high efficiency switch type constant current / constant voltage charger
The working principle of the circuit is as follows: when the battery is connected to the charger, the circuit outputs a constant current to charge the battery. The constant current control part of the charger consists of half of the dual operational amplifier lm358, gain setting resistors R3 and R4, current sampling resistor R5 and 1.23V feedback reference voltage source. Just after the battery is connected, the operational amplifier lm358 outputs a low level, the output voltage of the switching regulator lm2575-adj is high, and the battery begins to charge. When the charging current rises to 1a, the voltage drop at both ends of the sampling resistor R5 (50m Ω) reaches 50mV. After the voltage is amplified by an operational amplifier with a gain of 25, it outputs a voltage of 1.23V, which is added to the feedback terminal of LM2575 to stabilize the feedback circuit.
When the battery voltage reaches 8.4v, lm3420 starts to control the feedback pin of lm2575adj. Lm3420 turns the charger into the constant voltage charging process, and the voltage at both ends of the battery is stable at 8 ℃?? 4V. R6, R7 and C3 form a compensation network to ensure the stable operation of the charger under constant current / constant voltage. If the input supply voltage is interrupted, Diode D2 and PNP in op amp lm358 The input stage is reverse biased to isolate the battery from the charging circuit and ensure that the battery will not discharge through the charging circuit. When the charging is transferred to the constant voltage charging state, the diode D3 is reverse biased, so there is no perfusion current in the operational amplifier.
2.1.3 power undervoltage protection
The power supply undervoltage protection is easy to know from the battery discharge characteristics of lithium battery. When the battery is at 3.5V, the battery is about to run out. The battery should be charged in time, otherwise the battery voltage will drop sharply until the battery is damaged. Therefore, a set of undervoltage protection circuit is designed, as shown in Figure 5, which is obtained by resistance voltage division and TL431 The designed reference voltage comparison sends the comparison results to the LM324 amplifier circuit, and then triggers the switching system composed of triodes to control the on resistance of the load circuit. The test shows that when the system voltage reaches the critical dangerous voltage of 7V, the output current of the system is only 4mA, so as to prevent the excessive discharge of lithium battery in the system.
Figure 5 undervoltage protection circuit
Due to the high energy density of lithium-ion batteries, it is difficult to ensure the safety of batteries. In the overcharge state, the energy will be surplus after the battery temperature rises, so the electrolyte decomposes and produces gas, which is in danger of spontaneous combustion or rupture due to the rise of internal pressure; On the contrary, in the over discharge state, the decomposition of electrolyte leads to the deterioration of battery characteristics and durability, so as to reduce the number of rechargeable times. The charging circuit and the management system can effectively prevent overcharge and overuse of lithium battery, so as to ensure the safety of battery and improve the service life of lithium battery.
2.2 software design
The software design of the power management system mainly MEG A16L detects the voltage state of the battery through its 8-channel 10 bit ADC port, and takes corresponding measures according to different situations. Once the battery is lower than 7.0v, The single chip microcomputer switches the battery to the charging state and ensures that at least one group of batteries supply power to the load, and the priority of battery 1 is higher than that of battery 2. The main program flow chart is shown in Figure 6. The program is in an infinite cycle. The single chip microcomputer constantly monitors the voltage state of the two groups of batteries and remembers the current charging state. Once the discharged battery reaches below 7V, The single chip microcomputer drives the relay switch to switch the charging circuit to the battery and switch another group of batteries to the power supply of the load circuit.
Figure 6 Main program flow chart
During the operation of the program, timer 1 generates an interrupt every 1 second, receives the command information sent by the monitoring platform through the serial port, and sends the real-time voltage status of two groups of aircraft power supplies, relay status and other information to the ground station through the radio frequency module, so that the ground can know the power supply of the aircraft in real time.
2.3 host computer design
2.3.1 radio frequency module
The upper computer hardware of the power management system is mainly composed of radio frequency module, level conversion circuit and PC. the general block diagram is shown in Figure 1. Because the data to be received by the RF module is TTL Level, and then change it into RS232 level through Max 232 level conversion to transmit it to the computer, so as to realize the communication between the aircraft and the ground.
The reason why the system can realize long-distance monitoring of aircraft mainly depends on the characteristics of long-distance and high accuracy of radio frequency module. Its main features are as follows: (1) long distance characteristics: indoor / urban distance up to 450 meters; Outdoor visual range: up to 11km with 2.1dB dipole antenna and up to 32km with high gain antenna; Receiver sensitivity is - 110dbm. (2) advanced network and security: 7 frequency hopping channels, each channel can obtain 65K address, recovery and acknowledgement mechanism to ensure reliable packet transmission; It supports peer-to-peer network structure (without master / slave dependency), point-to-point, point to many and multipoint access network topology.
Therefore, XT end OEM radio frequency module provides the farthest distance in low-cost wireless data communication solutions. The module is easy to use, low power consumption, reliable data transmission for important data packets between devices, compact volume and saving valuable circuit board space. Fig. 7 shows a system block diagram of wireless connection between hosts composed of xtend OEM wireless RF module.
Fig. 7 system block diagram of wireless connection between hosts
2.3.2 ground monitoring platform
The monitoring platform is an important part of the whole equipment monitoring system. Duplex communication is required between the monitoring platform and the control program. On the one hand, the controller on the aircraft platform transmits the real-time information of the aircraft to the ground by data transmission. On the other hand, the ground station sends instructions to the aircraft to complete the required tasks.
The ground software is based on the VC + + 6.0 platform of MICROO ft and developed with the help of its MFC class library
The ground software is developed based on VC + + 6.0 platform of MICROO ft with the help of MFC class library. The specific software development process adopts object-oriented design method and C + + language. Each functional module corresponds to a class. In this way, the final software implementation structure is clear and reasonable, easy to maintain and upgrade. The program uses MFC technology combined with m sComm control and is written in C +. The program functions include: manual setting of serial port parameters, serial receiving data and sending instructions, displaying received data information and saving received data.
3 Analysis of experimental results
After obtaining the information of power supply voltage, relay status, charge and discharge, the controller transmits these information to the ground and saves them on the PC. Figure 8 shows the data collected by the aircraft during flight.
Fig. 8 charge and discharge data of battery 1.
As can be seen from the figure, firstly, battery 1 supplies power to the system as a load. After a period of use, it decreases from 7.5V to 7.0v. At this time, the single chip microcomputer drives the relay after detecting that the battery is low, and switches the battery to the charging circuit. After 10 minutes of charging, because the voltage of battery 2 is also less than 7V, the single chip microcomputer switches the power supply of the system to battery 1 again, and so on until the task is completed. Therefore, the system can convert kinetic energy into kinetic energy, effectively manage the power cycle of the system, improve the running time of the system, and improve the practicability and reliability of the whole system.
4 Conclusion
In this paper, a UAV power management system is designed. The system has the functions of automatic control of charge and discharge management and real-time monitoring of battery voltage. The system has been debugged and tested to verify its feasibility, but in order to ensure aircraft safety, more tests need to be done to ensure the safety and stability of UAV autonomous flight. In addition, high and low frequency filtering and battery power prediction are also important directions, which need in-depth research. Nowadays, lithium batteries are used more and more widely, and their price is relatively moderate. If we master advanced and scientific use methods and let lithium batteries play their due maximum utility, we will save a lot of resources and wealth.
Reprinted from - viku electronic market network“
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