Blackbox flight data recorderFor the Cleanflight flight controller system

Nicholas Sherlock

Introduction to quadcopters

Four sets of propellers, motors and

motor speed controllers

One radio receiver

One flight controller

- STM32 running at 72MHz

- 20kB RAM, 128kB of flash

- Sensors:

- Gyroscope

- Accelerometer

- Magnetometer

- Barometer

- GPS

- Cleanflight firmware

Cleanflight’s inheritance

Originally: MultiWii, using an 8-bit microcontroller and the

gyro/accelerometer package from a Wii remote

Baseflight, a port of MultiWii to a faster 32-bit CPU

Cleanflight, a fork of Baseflight which adds significant

features and improves code abstraction. Adds support for

new targets including faster CPUs with hardware floating

point and more RAM.

Introduction to quadcopter

flight

Target rotation rate in three axesGyroscope

measurement of 3-axis

rotation rate

Diff

3-axis error

Introduction to quadcopter

flight3-axis error

PID controller

3-axis correction Motor mixer

4 different motor

commands to be

sent to the 4

motor speed

controllers

Throttle position

PID controller

Takes the rotation rate error on 3 axes as the input and

produces 3 axes of corrections.

Three corrective terms computed for each axis:

P – Proportional. A fixed portion of the error signal.

I – Integral. Sum of the error over time.

D – Differential. The slope of the error.

Self-leveling flight modes

Accelerometer can be used to work out which way is down.

Angle flight mode:

Transmitter stick sets an angle which the quadcopter should be

sitting at.

PID controller is changed to compute corrections based on the

angle instead of the rotation rate.

Horizon flight mode:

A mixture of the acro flight mode and the angle flight mode

When the stick is near the centre, the angle flight mode is used.

When the stick is near the edge, the acro flight mode is used.

PID tuningNeed to tune the controller’s response according to your

craft’s aerodynamics, motors, props and other factors.

Each axis has a factor for tuning each of the P, I and D

corrections.

The main loopwhile (true) {

sticks = readReceiver();gyros = readGyroscopes();

corrections = runPIDController(sticks, gyros);

motorCommands = computeMotorMixer(corrections, sticks.throttle);

sendCommandsToMotors(motorCommands);}

The main loopwhile (true) {

sticks = readReceiver();gyros = readGyroscopes();

corrections = runPIDController(sticks, gyros);

motorCommands = computeMotorMixer(corrections, sticks.throttle);

sendCommandsToMotors(motorCommands);

logBlackboxState(sticks, gyro, corrections, motorCommands);}

What is logged?

Loop start time in microseconds

4-axis stick positions

3-axis gyroscope, 3-axis accelerometer

Barometer altitude

3-axis magnetometer

Battery voltage and current

P, I and D corrections for each axis

Motor commands for each motor

GPS position and altitude

Where is it logged?

Logged to an onboard

OpenLog logging device which

stores it onto a microSD card.

However maximum write

speed is currently about

14,000B/s

And we want to log 20 – 30

variables at 500Hz which is

10,000 – 15,000 readings /

second.

Log compression

Need to reduce the number of bytes per frame, so:

Create a keyframe (intraframe, “I frame”) once every 32

frames. These don’t depend on any previous frame so they

allow recovery from dropped frames.

For the other frames, store them as deltas from the previous

frame (interframes, “P frames”). Deltas are smaller in

magnitude than the original values.

Integer compression

We want the small integers left over after subtracting the

previous frame to have small encodings.

Variable byte encoding:

Use the high bit of each byte as a “more bytes follow” flag

Write the integer out in little-endian order.

7-bits 0aaa aaaa => 0aaa aaaa

8-bits baaa aaaa => 1aaa aaaa 0000 000b

14-bits 00bb bbbb baaa aaaa => 1aaa aaaa 0bbb

bbbb

Tag/group compression

Tag 2 3S32 encoding - Store a 2 tag which says how many

bits the largest of the next 3 signed 32 bit fields require to

encode.

• 2 bits per field ss11 2233

• 4 bits per field ss00 1111 2222 3333

• 6 bits per field ss11 1111 0022 2222 0033 3333

• 32 bits per field sstt tttt followed by fields of various byte

counts

Compression results

Log 1 of 1, start 00:16.376, end 09:39.730, duration 09:23.353

StatisticsI frames 7313 49.5 bytes avg 362032 bytes totalP frames 226677 22.8 bytes avg 5169085 bytes totalE frames 2 9.0 bytes avg 18 bytes totalFrames 233990 23.6 bytes avg 5531117 bytes totalData rate 415Hz 9820 bytes/s 78600 baud

1 frames failed to decode, rendering 8 loop iterations unreadable. 26 iterations are missing in total (62ms, 0.01%)

Links

Cleanflight firmware – http://github.com/cleanflight/

My YouTube channel - @thenickdude

Multicopter subreddit – http://reddit.com/r/multicopter

RCGroups rc flying forum – http://rcgroups.com/