DIY flight controller from scratch
Pictures: First hardware version using Arduino Uno board and second hardware version using Arduino Nano board
---------------------------Table of contents----------------------------
1.2. Progress state
3.1. Angle feedback
3.2. Speed stabilization
4.1. Angles and angular speeds feedback
5.1. Components list
6.1 Using Arduino IDE
6.2 Using PlatformIO
6.3. Using Docker
7.1 Source Code overview
7.2 The state machine
7.3 Radio reception
7.4 Motors speed control
8.1 My BenchTest
8.2 Exemple of First Person View configuration(FPV)
Before starting this project, you must know that spinning propellers are very dangerous for your eyes and people around. When using the benchtest indoor, you must wear large protective glasses, and set up a power limit. For outside tests, choose a very large area, with nobody around.
The aim of this project is to develop a very simple quadrirotor flight controller from scratch, using an Arduino and inertial sensors. There is two benefits:
In this project, the two main flight modes are addressed:
I strongly advice to start by implementing the accrobatic mode, since a good accro mode will be a solid fundation for angle mode, and it is easy and fast to implement.
Releases have been successfully tested during flights tests: I have some nice flights in FPV on a 450mm frame, both in accro and angle modes. BUT, this project is sometimes updated, mainly for code format, and unfortunately, I cannot realize flight tests for each submit. So, I cannot garantee that the last commit will allow to flight without some corrections.
Note that you may also have to tune PID according to your configuration.
I advice you to use a large frame (450mm for exemple), because it is more stable, and I did not test the software on smaller frames.
An IMU (aka MPU) is the only sensor needed for flight.
An « Inertial Measurement Unit » is a MEMS, "Microelectromechanical system", composed by a 3 axis gyroscope sensor, and 3 axis accelerometer sensor.
|Gyroscope||Measure angular speeds around each axis (°/sec)||Fast||Drift along time|
|Accelerometer||Measure linear acceleration on each axis (g)||Slow||Noizy and not usable when UAV is moving|
Accrobatic mode implements minimal stabilization algorithms to make flight possible. UAV is not able to auto-leveling, and pilot skills are required.
This mode is the required base for the angle mode, and is easier to realize: do not try to code angle mode if accro mode is not working fine.
Accrobatic mode algorithm only needs UAV angular speeds and angular speed command as entry data. Angular speeds are computed from raw gyroscope data integration.
Exemple of pitch angle speed computation using gyroscopes:
// currentPitch = previousPitch + rawSensorPitch*loopTime _pos = _pos + (accGyroRaw[0+3]/GyroSensitivity)*_loop_time;
A closed-loop system is needed to control angular speed.
This system compares angular speed command to angular speed feedback, and computes a new motor power to apply.
It is a more sophisticated flight mode than the accrobatic mode: now, UAV is able to auto-level.
Auto-leveling mode entry data is UAV attitude: the angles from the horizontal (roll, pitch, yaw) and the angular speeds.
Both angles speed and angles are required to compute reliable attitude angle feedback along time.
Both gyroscopes and accelerometers are involved.
As seen previously, Angular speeds are computed from raw gyroscope data integration.
Attitude angles are computed from accelerations:
When UAV is not moving, or if it is moving at constant speed, accelerometers measure gravity, ie vertical acceleration. Angle between UAV and the gravity vector is computed by trigonometry. Be carefull, this measure is faulty when UAV accelerates.
Exemple of pitch attitude angle computation using accelerometers:
_pos = atan(accGyroRaw/accGyroRaw))*(180/PI);
Complementary filter merges gyroscopes and accelerometers data, to get both sensors benefits, and to reduce theirs drawbacks:
The gyroscope if fast but it drifts along time.
Accelerometers are not fast enougth, they are noizy, and they are usable only when the UAV is not moving, and when UAV receive only earth acceleration.
The complementary filter mask their respective errors:
_pos = HighPassFilterCoeff*(_pos + (accGyroRaw[0+3]/GyroSensitivity)*_loop_time) + (LowPassFilterCoeff)*((atan(accGyroRaw/accGyroRaw))*(180/PI));
Note: LowPassFilterCoeff = 1 - HighPassFilterCoeff
HighPassFilterCoeff is computed from the Time constant.
Time constant is a compromise between UAV acceleration filtering, and gyroscopes drift:
timeConstant = (HighPassFilterCoeff*dt) / (1-HighPassFilterCoeff) => HighPassFilterCoeff = timeConstant / (dt + timeConstant)
Time constant in this project is set to 5 milliseconds. It implies a coefficient "HighPassFilterCoeff" of 0.9995 for a loop time of 2.49 ms.
Note: More efficient filters like the Kalman one are used in UAV, but they are more complicated to understand, and we want a very simple DIY software!
A double closed-loop system is needed to control attitude angles. It consists of a speed closed-loop [like the one in the accro mode], inside a position closed-loop.
This system compares angle command to angle feedback, and computes a new motor power to apply. Angular feedback is computed using a complementary filter to merge gyroscope dans accelerometer data.
The pilot controls each attitude angle, and if transmitter sticks are centered, command is 0°, and UAV automatically goes back to horizontal.
As exemple, my hardware configuration for a 450mm quad:
|Microcontroller board||Arduino Nano/Uno (ATmega328P MCU)|
|ESC||Afro 20A-Simonk firmware 500Hz, BEC 0.5A 1060 to 1860 us pulse width|
|Motors||Multistar 2216-800Kv 14 Poles - 222W Current max: 20A Shaft: 3mm 2-4S|
|Propellers||10x4.5 SF Props 2pc CW 2 pc CCW Rotation (Orange)|
|Battery||Zippy Flightmax 3000mAh 4S|
|Receiver||OrangeRx R617XL CPPM DSM2/DSMX 6 ch|
|IMU||MPU6050 (GY-86 breakout board)|
|Compass||HMC5883L (GY-86 breakout board)|
|Buzzer||Matek lost model beeper - Built-in MCU|
|Frame||Diatone Q450 Quad 450 V3. 450 mm wide frame choosen for better stability and higher autonomy (the lower the size, the lower the stability).|
TODO: add receiver in schematic
With minor modifications, project can be build using Arduino IDE:
Install Visual Studio code
Start VSCode and click on "Extensions" icon
Install PlatformIO IDE
PlatformIO is an open source ecosystem for IoT development.
sudo apt-get install python-pip sudo pip install --upgrade pip && sudo pip install -U platformio==3.5.2 platformio platform install atmelavr --with-package=framework-arduinoavr platformio lib install MPU6050 pio lib install "I2Cdevlib-MPU6050"
Optional, for code format:
sudo apt-get install -y clang-format
platformio upload --upload-port/ttyACM0
platformio test -e uno --verbose
The development tool "Docker" is a container platoform: it is a stand-alone, executable package of a piece of software that includes everything needed to run it: code, runtime, system tools, system libraries, settings. It isolates software from its surroundings.
|StateMachine.h||Contains current state|
|states/IState.h||Interface for the states (mother class)|
|states/AccroMode.cpp/h||Computes stabilization for accro mode|
|states/AngleMode.cpp/h||Computes stabilization for angle mode|
|states/Disarmed.cpp/h||Sets motors to idle|
|states/Safety.cpp/h||Sets motors to idle|
|hardware/InertialMeasurementUnit.cpp/h||Read roll, pitch, yaw angles from MPU6050|
|hardware/MotorsSpeedControl.cpp/h||Manage an ESC: PWM to set|
|hardware/RadioReception.h||Receives CPPM signal acquisition using INT0|
|Stabilization.cpp/h||Computes and apply new command from receiver sticks and current attitude|
|ControlLoop.cpp/h||Proportionnal, integral, derivative loop correction|
|CustomMath.h||Mathermatical functions: mean and delta max computations|
|CustomTime.h||To measure loop time and elapsed time|
|CustomSerialPrint.h||Custom serial print to enable or disable printing|
Due to security reasons, the UAV cannot start running with a flight mode enabled. Five states are defined to enable or disable flight mode safely. These states are ruled by a statemachine.
The five StateMachine states:
|Initialization||UAV must be on the ground, horizontal, and not moving. Sensors offsets are computed. Then, if receiver switch is disarmed, the system changes to the next state.|
|Disarmed||Throttle is disabled. When the receiver switch is set to a flight mode, the system changes to the corresponding state.|
|AngleMode/AccroMode||Throttle is enabled, flight can start. After 5 seconds of power idle, the system is set to "safety" state.|
|Safety||Throttle command is disabled and power is cut. To re-arm, switch to disarmed, then choose the flight mode accroMode or angleMode|
A CPPM receiver is used to fly the UAV using a remote control.
CPPM (Pulse Position Modulation) reception allows to receive all channels using only one entry pin. Each rising edge correpond to the end of the previous channel impulsion, and at the beginning of the next channel impulsion. Elapsed time between two rising edge correspond to the pulse width of a given channel.
In this projet, each pulse width is measured using INT0, and then stored in the correponding channel of an array.
Each motor is controlled by an ESC. The microcontroller drives the 4 ESC.
Motor speed is tuned by modifying the pulse width on the ESC signal wire. A pulse must be send at least every 20ms.
Max motor speed is set by a pulse of 1860ms
Motor is stopped by a pulse of 1060ms
All speeds between this two values are possible.
Each timer1 interrupt call the function "SetMotorsSpeed", which generate a falling edge on the previous motor and a rising edge on the current motor:
For security, you must set the failsafe to cut motors power when radio link is lost.
To set the failsafe:
Transmitter configuration used during the « bind » operation defines the « failsafe. »
My cheap configuration:
|Googgles||Quanum DIY FPV Goggle V2 Pro|
|Googgles battery||1000 mAh 3S|
|Receiver||Eachine RC832 Boscam FPV 5.8G 48CH Wireless AV Receiver|
|Receiver antenna||DYS FPV 5.8G Antenna 4dBi Mushroom Antenna RHCP TX RX|
|Camera||Foxeer XAT600M HS1177 600TVL CCD 2.8MM IR Mini FPV Camera IR Blocked 5-22v|
|Camera antenna||Realacc 5.8G 5dBi 50W RHCP Omnidirectional 3 Leaf Clover FPV Antenna Red|
|Video transmitter||Upgrade Aomway Mini 5.8Ghz 200mW 32CH AV Wireless Transmitter Module|
ESC Electronic Speed Control
PID It is a Proportionnal Integral Derivative control loop
IMU Inertial Measurement Unit
MPU Motion Processor Unit
FPV First Person View