PixRacer on Blade 130X Helicopter

9 Aug

The ultimate goal of this project is to make a fully autonomous Blade 130X helicopter using PX4 firmware. The autopilot of choice is the PixRacer, because the form-factor is relatively small and it’s the next generation pixhawk autopilot.

PixRacer

The autopilot is hard-mounted on a wooden plate, milled by my brother. The plate clamps onto to the tail-boom and a tie-wrap holds it in place. This is by far not the nicest solution, but it’s good enough for the prototype. At this point I was still not sure if it was going to work at all. The ESP8266 on top is used for communication with QGroundControl, which works out of the box.

I removed the servo-headers which are normally attached to the PixRacer. I also removed the JST connectors of the servo’s and soldered the wires directly to the PixRacer.

PixRacer mounted on Blade 130X

PixRacer mounted on Blade 130X

Power

The battery is a 2S 450mAh LiPo. The autopilot needs 5V for operation, which is provided by the ACSP5 module which I bought together with the PixRacer. Later I figured out that the servos need 3.8V, which I did not have yet. Because it’s still a prototype, I used a small 3A buck converter from ebay to provide power to the servos.

Power boards

Left: buck converter for servos. In the yellow tape: ACSP5

Motor controller

The ESC I used is a Turnigy Plush 12A. I have not yet configured it with BLHeli, but I will need a governor mode at some point. The ESC actually also has a BEC which could be used instead of the ACSP5, but I realized that too late :-p.

Turnigy Plush 12A

Turnigy Plush 12A

RC Transmitter / Receiver

I have some experience with DSMX receivers and transmitters, but for this project I decided to go with FrSky stuff. I have a Turnigy 9X transmitter with a FrSky DJT module. The receiving side is a RX-F802 which I got from banggood. It’s hard to find documentation about this module, but apparently it is similar to the RX-F801. If you put a bind plug to connect channels 1 and 3, you will get PPM output on channel 4. This plugs into the RC port of the PixRacer. It’s also important to use channel 3 for thrust on the transmitter. The receiver has a built-in failsafe mode, which centers channels 1,2 and 4 and channel 3 to low if the rc link is lost. I started with thrust on channel 1, but if you then switch off the RC and the vehicle is still armed, it will apply half throttle (and pitch backward)!

RX-F802 RC receiver

RX-F802 RC receiver

Firmware

Helicopters are not yet supported by PX4, so I had to do some programming. First I made a new airframe definition in PX4/Firmware/ROMFS/px4fmu_common/init.d

#!nsh
#
# @name Blade 130X
#
# @type Helicopter
#
# @maintainer Bart Slinger <bartslinger@gmail.com>
#

sh /etc/init.d/rc.mc_defaults

set MIXER heli_120deg

To make it visible in QGroundControl, you have to overwrite ~/.config/QGroundControl.org/PX4AirframeFactMetaData.xml with the airframes.xml generated when building PX4 firmware.

I also had to make a new mixer, which was inspired by the CCPM.main.mix mixer. However I found out that the left and right servos had longer arms, so no compensation was needed in the mixer. The ESC is on AUX1, which I control with a turn-knob on my transmitter.

Helicopter 120 degree Cyclic-Collective-Pitch Mixing (CCPM) for PX4FMU
==================================================

Output 0 - Left Servo Mixer
-----------------
Left Servo = Collective (Thurst - 3) - 0.5 * Elevator (Pitch - 1) + 0.866 * Aileron (Roll - 0)

M: 3
O:       10000   10000      0 -10000  10000
S: 0 3   10000   10000      0 -10000  10000
S: 0 1  -10000  -10000      0 -10000  10000
S: 0 0   10000   10000      0 -10000  10000


Output 1 - Front Servo Mixer
----------------

Rear Servo = Collective (Thrust - 3) + Elevator (Pitch - 1)

M: 2
O:      10000  10000      0 -10000  10000
S: 0 3  10000  10000      0 -10000  10000
S: 0 1  10000  10000      0 -10000  10000


Output 2 - Right Servo Mixer
----------------
Right Servo = Collective (Thurst - 3) - 0.5 * Elevator (Pitch - 1) - 0.866 * Aileron (Roll - 0)

M: 3
O:       10000   10000      0 -10000  10000
S: 0 3   10000   10000      0 -10000  10000
S: 0 1  -10000  -10000      0 -10000  10000
S: 0 0  -10000  -10000      0 -10000  10000


Output 3 - Tail Servo Mixer
----------------
Tail Servo = Yaw (control index = 2)

M: 1
O:      10000  10000      0 -10000  10000
S: 0 2  10000  10000      0 -10000  10000


Output 4 - Motor speed mixer
-----------------
This would be the motor speed control output from governor power demand- not sure what index to use here?

M: 1
O:      10000  10000      0 -10000  10000
S: 3 5      0  20000 -10000 -10000  10000

The final change needed is in the control law. I did manage to get it flying with the default multicopter controller, but tuning is very hard. To make it easier, I changed to control law to use a direct feed-forward from attitude-error to actuator deflections. In mc_att_control_main.cpp function MulticopterAttitudeControl::control_attitude_rates:

_att_control = _params.rate_p.emult(rates_err) + _params.rate_d.emult(_rates_prev - rates) / dt + _rates_int +
// _params.rate_ff.emult(_rates_sp - _rates_sp_prev) / dt;
_params.rate_ff.emult(_rates_sp);

The final thing is to use the kalman filter, because otherwise attitude estimation is f*cked up with the autopilot hard-mounted to the frame. The EKF2 can be selected in QGroundControl parameters section. It works surprisingly well given the massive vibrations on this thing.

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