Navigation guidance and control is the most complex system in the UAV system, and its functions can be divided into many methods. Today, we mainly introduce and summarize the basic principles and common methods of the UAV navigation guidance and control system.
Navigation: that is, the UAV obtains its current (under a certain reference frame) position, speed and other information. If necessary, it also needs to obtain the current (relative to a certain reference frame) attitude, attitude angular velocity and other information. For example, using pure inertial navigation can obtain the position, velocity and acceleration of the UAV in a certain inertial system, as well as the attitude angle and angular velocity relative to the inertial system; GPS navigation system can provide the speed, position and heading angle of UAV in WGS84 coordinate system; With the help of indoor positioning systems such as vicon and UWB, the speed and position of the UAV relative to a certain coordinate system in the room can be obtained. Therefore, the main task of briefly summarizing navigation is to “know where you are and your posture”.
Guidance: that is, the UAV finds (or inputs) the position, speed and other information of the target, and obtains the position or speed command required to reach the target according to its own position, speed, internal performance and constraints of the external environment. For example, when flying according to the planned waypoint, calculate the command of UAV flying directly or along a certain route to the waypoint; When optical guidance based on computer vision target tracking is adopted, the overload or attitude angular velocity command required for tracking the target is calculated according to the position of the target in the field of view (and the off-axis angle that may exist in the camera); When there are obstacles or no fly zones that need to be avoided in the pre installed (or Slam obtained) map, the feasible avoidance route or speed command is calculated according to the flight performance of the UAV. Therefore, the main task of briefly summarizing guidance is to “know where the target is and how to reach the target”.
Control: according to the current speed, attitude and other information, the UAV changes the attitude, speed and other parameters through the action of the actuator, so as to realize stable flight or track guidance instructions. For example, when the fixed wing UAV needs to climb altitude, it calculates the required pitch angle and pitch angle speed command, and the throttle command required to prevent the airspeed from greatly reducing; When flying along the route, but there is crosswind, calculate the required yaw angle command to offset the Crosswind Effect by using sideslip; Or when one rotor of the multi rotor UAV fails, calculate how to assign commands to the remaining rotors to achieve stable flight as much as possible. Therefore, the main task of the control is to “change the flight attitude and track the guidance command”.
While the quartz flexible accelerometer can measure the change of gravity acceleration in the gravity field. When its sensitive axis is perpendicular to the horizontal plane, the accelerometer indicates the unit gravity acceleration; If the sensitive axis inclines, its output is the product of the sine function of the gravitational acceleration and the inclination angle. This principle can be used for tilt measurement and leveling. This is the role of quartz accelerometer in navigation and guidance.
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