flix/docs/firmware.md

Firmware overview

The firmware is a regular Arduino sketch, and it follows the classic Arduino one-threaded design. The initialization code is in the setup() function, and the main loop is in the loop() function. The sketch includes several files, each responsible for a specific subsystem.

Dataflow

The main loop is running at 1000 Hz. All the dataflow goes through global variables (for simplicity):

• t (double) — current step time, s.
• dt (float) — time delta between the current and previous steps, s.
• gyro (Vector) — data from the gyroscope, rad/s.
• acc (Vector) — acceleration data from the accelerometer, m/s².
• rates (Vector) — filtered angular rates, rad/s.
• attitude (Quaternion) — estimated attitude (orientation) of drone.
• controlRoll, controlPitch, ... (float[]) — pilot control inputs, range [-1, 1].
• motors (float[]) — motor outputs, range [0, 1].

Source files

Firmware source files are located in flix directory. The core files are:

• flix.ino — Arduino sketch main file, entry point.Includes some global variable definitions and the main loop.
• imu.ino — reading data from the IMU sensor (gyroscope and accelerometer), IMU calibration.
• rc.ino — reading data from the RC receiver, RC calibration.
• estimate.ino — attitude estimation, complementary filter.
• control.ino — control subsystem, three-dimensional two-level cascade PID controller.
• motors.ino — PWM motor output control.
• mavlink.ino — interaction with QGroundControl or pyflix via MAVLink protocol.

Utility files:

• vector.h, quaternion.h — vector and quaternion libraries.
• pid.h — generic PID controller.
• lpf.h — generic low-pass filter.

Control subsystem

Pilot inputs are interpreted in interpretControls(), and then converted to the control command, which consists of the following:

• attitudeTarget (Quaternion) — target attitude of the drone.
• ratesTarget (Vector) — target angular rates, rad/s.
• ratesExtra (Vector) — additional (feed-forward) angular rates , used for yaw rate control in STAB mode, rad/s.
• torqueTarget (Vector) — target torque, range [-1, 1].
• thrustTarget (float) — collective thrust target, range [0, 1].

Control command is processed in controlAttitude(), controlRates(), controlTorque() functions. Each function may be skipped if the corresponding target is set to NAN.

Armed state is stored in armed variable, and current mode is stored in mode variable.

Building

See build instructions in build.md.