Codemotion Rome 2015 - Building a drone from scratch with spare parts is a challenging business. To accomplish this journey, a Linux embedded stability control system is developed entirely from 0.This is a journey starting from the hardware choosing (a home WIFI router), to a stable and real flight. Unconventional implementations are one of the main topic, like using WiFi as communication between drone and pilot, HTML5 and COMET to show telemetry from the router web server, and implementing a entirely new protocol based on 802.11 Beacon Frames to prevent deauthentication attacks.

1. David Meléndez CanoDavid Meléndez Cano
@taiksontexas
Taiksonprojects.blogspot.com

2. 2
¿What is TRRP?
➔
Low (or null) cost embedded platform
➔
“Internet of things”
➔
Use whatever is liying around
➔
“Trash” can be useful
➔
DO NOT RECYCLE!!! ->>> REUSE!!!

3. Use your old (or fancy new) router as robotic platform
➔
No initial investion. You already have the router!
➔
Change the firmware-> Embedded Linux OpenWRT
Created Robots using this philosiphy
ROVER Texas Ranger: Linksys
WRT54GL
Drone ATROPOS:
La Fonera 2201

4. Round one: ROV Texas Ranger

5. Round two!: Atropos Quadcopter
➔
Fonera 2201
➔
OpenWRT
➔
Stabilization process
➔
HTML5 telemetry and control

6. A-B-C for quad drones

8. Router platform: GPIOs for I2C
Led lights are driven by SoC CPU pins

9. Router platform: GPIOs for soft I2C and serial port
GPIO---> I2C / RS232
GPIO leds
Serial port

10. Router platform: Module load
rm /etc/modules.d/59­i2c­gpio
echo "i2c­gpio­custom bus0=0,4,7 bus1=1,3,1"
>/etc/modules.d/59­i2c­gpio­custom
echo "i2c­gpio" >/etc/modules.d/60­i2c­gpio
[reboot]
i2c­detect /dev/i2c0

11. Router platform: Serial port
stty -F /dev/ttyS0 raw
stty -F /dev/ttyS0 speed 115200

12. Router platform: Gyroscope
Wii Motion Plus
Triaxial gyroscope
I2C port
•
Gives angular velocity on each axis
•
Real angle: angular velocity * time ? → not so easy

13. Router platform: Accelerometer
Wii Nunchuck
Triaxial accelerometer
Puerto I2C
G forces in each axis.

14. Router platform: Magnetometer
- HMC5881L
-Units: Gauss
-Accelerometer only cancels gyro drif for pitch & roll
-Yaw gyro drif must be cancelled with a compass (magnetometer)

15. Router platform: Reading I2C sensors
#include <linux/i2c­dev.h>
f_i2c0=open(I2C0, O_RDWR);
ioctl(f_i2c0, I2C_SLAVE, 0x52);
write(f_i2c0, buff_out, data_size);
­­­
read(f_i2c0, buffer,size);
. Ej Wii Motion+: 0x52
•
Petición de datos escribiendo al dispositivo
•
Lectura de datos en buffer

16. Imagen: “The Balance Filter” MIT
Gyroscopes
Accelerometer
Obteniendo ángulos absolutos con respecto a la Tierra

17. Mejor: Matriz de Cosenos Directores (DCM)

18. We will work with:
➔
Absolute angle: (rad) filtered value
➔
Angular velocity : (rad/sec) from gyroscopes
➔
Angullar acceleration: (rad/sec/sec) calculated from
angular velocity

19. Proportional Integral Derivative Controller

20. Proportional
error=current_value-last_value
Proportional_term=Proportional_term*error

21. Integral
accumulated_error=accumulated_error+
(error*time_between_samples)
Integral_term=Integral_term*accumulated_error

22. Derivativo
error_speed=error_error_speed
/time_between_samples
Derivative_term=Derivative_term*error_speed

23. Nested PID
P Controller
PID controller
Angle
Angular
Velocity
Router
Serial port

24. Router platform: Motor signal board (PWM)
-PIC Microcontroller receives motor speeds via router serial port
-It converts those values into 4 time-fixed digital signals (PWM)
Fonera
/dev/ttyS0 PIC16f876A
ESC
Motores

25. Router Platform: IMU (Inertial measurement Unit)
➔
Critical process that has to be executed in a time-fixed ratio
setitimer(ITIMER_REAL,&i_int,NULL);
(void)signal(SIGALRM ,timeout_real);
➔
It compites with the rest of the process for CPU time
➔
Changing priority with nice is not sufficent:
➔
Real Time scheduling → FIFO, Round Robin
#include <sched.h>
schedule.sched_priority=sched_get_priority_max(SCHED_FIFO);schedule.sched_priority=sched_get_priority_max(SCHED_FIFO);
sched_setscheduler(pthread_self(), SCHED_FIFO,&schedule);sched_setscheduler(pthread_self(), SCHED_FIFO,&schedule);

26. WATCHDOG process notifies to kernel that system is not hanged
●
It consumes SCHED_FIFO process time
●
What happens if router hangs during flight->
➔
ioctl(“/dev/watchdog”, WDIOC_KEEPALIVE, &dummywd);

27. Actuator hardware
Electronic Speed controller (ESC)
Brushless motors “Outrunner”.

28. HTTP CONTROL
Httpd.c:
[...]
if (strncmp(tptr, "cgi­bin/ajaxinput", 17) == 0) {
[...]
strcpy((char *)shmat(shmget(9998, 30, IPC_CREAT | 0666), NULL, 0),g_query);
exit(­1);
}
[...]
Pilot commands can be transmitted via AJAX or UDP packets
-AJAX → uses DOM events and sends commands accordingly
-A separate program reads USB Joypad and resends via UDP
http://atropos/cgi­bin/ajaxinput?QQZ[GAS]Z[ALABEO]Z[CABECEO]Z[GUIÑADA]Z
Router Httpd.c is tweaked to process the query faster.

29. USB Joypad control (GNU/Linux with joydev.ko)
cat /dev/input/js0
­­­­
pad=open(“/dev/input/js0”,O_RDONLY);
reads=read(pad,msg,MSG);
(msg[7]==2)
gas=((analog+127))*GAS_FACTOR;
[...]
(msg[7]==4)
yaw=­analog*YAW_FACTOR;
[...]
(msg[7]==1)
pitch=­analog*PITCH_FACTOR;
[...]
(msg[7]==0)
roll=­analog*ROLL_FACTOR;
[...]

30. What about Android?

32. HEADER
AP
“ATROPATROP”
COMMAND
Preamble
Gas
Pitch
Roll
Yaw
SEQUENCE
NUMBER
CHECKSUM
INIT.
VECTOR
Forged Beacon Frame injection
Protected with AES-128
BEACON FRAME PAYLOADBEACON FRAME PAYLOADHEADER

33. Demo?Vuelo Atropos Quadrocopter

34. Acknowledges
Vicente Polo (Albalá Ingenieros)
Jaime (Trescal)

35. Thank you!Thank you!
David Meléndez CanoDavid Meléndez Cano
@TaiksonTexas
Taiksonprojects.blogspot.com