Summary of basic knowledge points of UAV

A. Basic Concepts

• Autopilot: The control system of the aircraft, wherein the hardware contains the sensor part IMU and the control part of the MCU, the software part contains the control algorithm.

• tilt:pitch, rotate around the y axis of the coordinate system and imagine the usual plane

• Course:Yaw, rotating around the z axis of the coordinate system, imagine the usual plane

• roll, rotate around the x-axis of the coordinate system, imagine the usual plane

• Gyroscope: sensing angular velocity, with high dynamic characteristics, but it is an indirect measurement of the angle of the device, which measures the angle of the derivative, angular velocity, the angular velocity to the time integral to obtain the angle. Generally not affected by external interference, but there will be problems with the integration error.

• accelerometers : sensing acceleration, including acceleration of gravity, gravitational acceleration, and geographic coordinates are fixed, so the angular relationship between the accelerometer and the ground can be obtained through this relationship, but if the rotation around the z axis is not changed, it is impossible to perceive horizontal rotation. The mobile phone in life automatically rotates the screen to use this.

• Magnetic Meter : HMC5883L is a method of using the Earth's magnetic field to orient the Arctic, sensing the direction of the planet's magnetic field similar to the Compass's function.

• MPU6050: Six-axis sensor, integrated 3-axis gyroscope and 3-axis accelerometer, as well as a reserved IIC interface can be connected to the magnetometer, while containing DMP can be a hardware solution of four yuan, and output is the output of the IIC ad value.

• Inertial navigation module: IMU, including gyroscope, accelerometer and magnetometer.

• MEMS: Micro-electromechanical systems (micro-electro-mechanical system), microelectronic micro-machined, with sensitive components such as capacitance, piezoelectric, piezoresistive, thermoelectric coupling, resonance, tunnel current to feel the conversion of electrical signals devices and systems.

• barometric pressure gauge: The MS5611 barometer mainly senses the relative and absolute height of the object through the change of barometric pressure, which can be used to set high, especially in the case of indoor GPS not working.

• attitude calculation: refers to the gyroscope, accelerometer, magnetic compass and other data fused together to obtain the aerial attitude of the aircraft.

• Deep Solution calculation: also known as long-term fusion, can be more accurate to solve the attitude of the figure.

• Fast solution: also called rapid fusion, more rough to get posture.

• four USD: in unmanned aerial vehicles the four-dollar number is a mathematical representation of the gesture, and is the most used, since the four-dollar number is more suitable for combination rotation. In the attitude calculation, the general use of four yuan to save the gesture, that is, to save the ad value obtained from the MPU6050.

• Euler angle: Another mathematical representation of attitude, which is mainly used in the attitude calculation to convert four yuan into Euler angle, then used for attitude calculation, the input of attitude solution can only be Euler angle.

• Rigid body: refers to an object in which the shape and size are constant and the relative position of the interior points is constant after the movement and the action of the Force.

• Geographic coordinate system: the coordinate system in which the Earth is located, the coordinate system is fixed, True north, east, and above are represented by X, Y, Z axis respectively.

• Headless mode: the four-axis direction is flown from the middle of the two motors

• with head mode: the direction of the four axes is flying from the direction of one of the motors

two. Basic Structure 1) Power part

1. Rack: DJI's F450

2. Motor: brush motor and brushless motor, brush motor refers to the motor has a commutation brush, and brushless motor refers to the motor does not have a commutation brush. Brush Stator is a permanent magnet, brushless stator is the coil, the intuitive view is brushless motor has three lines and with ESC use. Senior axis is generally used brushless, small four-axis with a brush of the hollow Cup motor.

3. Blade : Carbon fiber propeller, wood paddle, injection propeller, the same motor and battery, large kv value with small propeller, small kv value with large propeller

4. ESC: Electronic governor, also known as the Motor Drive module, is mainly used to control the motor start and stop and speed, by changing the direction of the internal current of the fixed coil, to ensure that the force between it and the permanent magnet is mutually exclusive, continuous rotation to continue.

5. Battery: model of lithium battery needs a strong discharge capacity, the general use of C to do units, which is the difference with ordinary lithium batteries.

2) Control Section

1. Flight control: mainly Pixhawk, which adopts the STM32 controller chip, wherein the hardware contains the sensor part IMU and the control part of the MCU, the software part contains the control algorithm.

2. Means of communication: NRF24L01, Bluetooth 4.0 or WiFi

3. Remote control: American hand and Japanese hand, Mei National champion refers to the throttle on the left side of the rocker, Japanese hand is the opposite. Note: The left hand is rational, the right hand is the feeling, the sword is dangerous thing, must use the reason to manipulate.

three. Algorithm Analysis 1) Attitude Solver algorithm

Attitude solution refers to the gyroscope, accelerometer, magnetic compass and other data fused together to obtain the aerial attitude of the aircraft, also known as Attitude Fusion. Attitude solving involves the reading and filtering of sensor data, the four-dollar number and rotation, the frame of attitude calculation and the long-term fusion. The aircraft obtains pitch, yaw and roll angles from the three-axis angular velocity of the gyroscope by four-element method, which is a fast solution , then combines the three-axis magnetometer and the triaxial accelerometer to obtain the drift compensation and depth calculation .

We use the coordinate system R to represent the earth's coordinate system (geographic coordinate system), the coordinate system is fixed, there are X, Y, z three axes, while the coordinate system r to represent the four-axis aircraft coordinate system. The so-called attitude solution is to measure the coordinate system r with the coordinate system R angle position relationship, through the sensor data acquisition and analysis measurement, finally get this relationship. The mathematical representation used in this method is Euler's angle, four-dollar number, matrix, and axis angle.

The attitude calculation can be realized by software calculation and hardware solution . The input parameters of the attitude control algorithm must be Euler angles. The ad value refers to the gyroscope and acceleration values of the MPU6050, the gyroscope values of 3 dimensions and the acceleration values of 3 dimensions, each of which is 16-bit precision. The ad value must first be converted to four and then converted to Euler angle by a four-dollar number. This four-dollar number may be a soft solution, the master chip (STM2) read to the ad value, the software from the ad value is calculated, it may be through the MPU6050 in the DMP hard solution, the Master chip (STM32) directly read to the four-dollar number.

Four. a little thought.

1) The drone's endurance problem

The small four-axis is up to more than 10 minutes, and the consumer-grade seniors axis is only 30 minutes.

2) Hysteresis of takeoff and descent of UAV

When the drone uses the remote control to take off the time will appear carelessly the throttle increases will suddenly rush up, the descent may also loose the throttle too much and suddenly drops. So we want to be able to rise when the plane, not because the throttle suddenly increased suddenly and sharply, nor because the throttle accidentally pulled down too quickly caused the aircraft to fall quickly to the ground.

Workaround:

1. Data curve

The value of the remote control to the aircraft, the aircraft is not a sudden output, the data curve is then output, which can play a certain role in buffering. Turning the data into a downward-concave parabola, the throttle joystick value range of the remote control has a one by one mapping relationship with the throttle volume of the aircraft. Because the parabolic line is concave, the maximum value is the same as the minimum value, and the process of the change is not linear, so it can solve the problem of the uplift to some extent.

2.　　 Negative feedback control

The current acceleration value of the UAV minus the local acceleration of gravity until a difference in acceleration, and then the current throttle amount minus the difference can play a role in a buffer, but this method requires us to know the local gravitational acceleration, which can be viewed through the serial assistant, but it is more troublesome, We can learn this optimal value through machine learning to achieve the effect of buffering. (Why is the throttle amount minus the difference?) Why is 150* this difference? ）

Drone Basics Summary