农田信息采集用多旋翼无人机姿态稳定控制系统设计与试验

农田信息快速采集是精准农业的基础。为快速、高效、准确、节能获取农田信息,该文搭建了多旋翼无人机平台,设计了以STM32F407为主控制器的多旋翼飞行控制系统。采用了比例积分微分(proportion,integration,differentiation,PID)双闭环控制策略,外环为角度反馈,内环为角速度反馈。通过工程凑试法得到合适的PID控制参数。运用专家控制策略改进上述控制方法,使控制参数适应无人机姿态变化。对所设计的无人机控制系统进行抗干扰和阶跃响应试验。系统在受到30?横滚与俯仰角干扰后,其对应恢复平衡时间均在3.4 s内,航向角30?干扰后恢复时间在4 s内。系统横滚与俯仰角阶跃响应调节时间在1~2 s内,航向角在3.4 s内。试验结果表明:双闭环PID控制策略实现多旋翼无人机姿态稳定控制,专家控制策略增强无人机的抗干扰能力。在室外农田环境中,无人机能根据指令在1~2 s内快速调整姿态。当姿态受风影响发生倾斜时,陀螺仪测量角速度大于3(?)/s,采用的控制策略能迅速调整电机转速,保持无人机姿态稳定平衡。试验证明该控制系统稳定可控且具有较强抗干扰性,满足多旋翼无人机低空采集农田信息的要求。

Design and test of attitude stabilization control system of multi-rotor unmanned aerial vehicle applied in farmland information acquisition

Farmland information acquisition is the basis of precision agriculture.Multi-rotor unmanned aerial vehicle (UAV) can obtain farmland information quickly,efficiently and accurately.But as an under actuated system,the flight stability of multi-rotor UAV is susceptible to natural wind,electromagnetic interference and the near surface factors in farmland environment.The flight stability control method is important in multi-rotor UAV flight and affects qualified information acquisition.In this paper,multi-rotor UAV was adopted as research object and its body coordinate system and navigation coordinate system were established.Based on its motion analysis,changing 4 motors speed can realize multi-rotor UAV attitude regulation.The multi-rotor UAV platform consisted of brushless motors,electric speed controllers and composite-fiber fabric propellers.The flight control system was designed based on STM32F407 as master controller,MPU6050 as motion sensor integrated with 3-axis accelerometer and 3-axis gyroscope,AK8975 as 3-axis magnetometer.The attitude of multi-rotor UAV was measured and computed by the sensors above.The timers in the controller were used for capturing the input signals from remote control and generating PWM output signals for motors control.Multiple tasks including attitude measurement remote control input process,attitude stabilization management and motors control output were scheduled by task scheduling method in the control system.The mathematical model of multi-rotor UAV attitude control was established.Through matrix calculation,the multi-rotor UAV attitude angle and throttle control inputs were mapped to each motors speed control.Then the control principle applied in this study was explained.The double closed-loop proportional integral differential (PID) control strategy with angular velocity as the inner feedback loop and angle as the outer feedback loop was proposed.A multi-rotor UAV experimental platform was built to gain the proper PID control parameters of inner and outer control loops through engineer debugging method.The double closed loop PID control method was further improved by expert control strategy.The expert control rules included throttle input ratio definition,angle and angular velocity integration limitation,angle differential control parameter variation,input and output control limitation.By introducing the expert rules,the control parameters were diverse to be adapted to the multi-rotor UAV attitude change.Anti-interference tests and step response tests were taken to testify the designed control system on the experimental platform.In the traditional PID control,when interference angle increased,the rise time and the adjustment time of the system increased.But under the effect of variable derivative,when the interference angle became large,the rise time did not increase.The rise time of the system was less than 0.27 s.When the system was subjected to 30° interference angle,the adjustment times for roll angle,pitch angle and yaw angle restored to balance were less than 3.4 and 4 s respectively.In the step response tests,rise time,maximum overshoot,adjustment time and oscillation frequency of the system were recorded.According to the statistics,the maximum adjustment times for roll angle,pitch angle and yaw angle were less than 2.2 s and 3.4 s respectively.It proved that the double closed-loop PID expert control strategy adopted in this paper made the multi-rotor UAV have quick response,small fluctuation and stable control performance.Rice breeding base was chosen as outdoor farmland circumstance to test the multi-rotor UAV attitude control performance.Because the multi-rotor UAV flight cannot avoid wind interference in outdoor,higher inner loop proportion value,outer loop proportion and differential value in roll and pitch control made the multi-rotor UAV more agile,responsive to control input and stronger resistance to wind disturbance.In the outdoor farmland flight tests,the collected information were sent to computer through wireless data transmission and were stored and analyzed in computer.From the experiment results,it could be concluded that the multi-rotor UAV can regulate its motors speed in a short while and adjust its attitude according to the remote control input in 1-2 s.When the multi-rotor UAV flight control was disturbed by wind,the angular velocities measured by gyroscope were larger than 3 (°)/s,the control strategy made the multi-rotor UAV automatically adjust its attitude to keep stable flight and effectively resist to the wind disturbance.It proved that the control strategy designed in this research is suitable for multi-rotor UAV applied in farmland information acquisition.