aubreyc/coaxialrotor

A Coaxial-rotor design

Coaxial-Rotor instructions

This document presents the main steps to build a flyable coaxial-rotor UAV.

Manufactering process

The Coaxial-rotor UAV is composed of off-the-shelf electronic parts and plywood / foam-board laser cut parts. The following list of components and CAD files corresponds to a medium-size configuration shown in the following picture.

Assembled Prototype

Off the shelf

• PIXHAWK Hardware Autopilot
• RC receiver: OrangeRx R615X
• Battery : XXX
• ESC (with BEC): 2 x Mystery 30A M-30 with BEC 5V
• Motors: 2 x Multistar 2209-1200
• Propellers: 2 x
• 4 servomotors: 4 x Hextronik HXT 500

Manufactering:

Material needed

• Plywood (6mmm and 3mm)
• Foam-board (6mm)
• Laser-cut machine
• Hot glue
• Zip-ties

CAD files

This Coaxial-rotor UAV has been designed to be fully laser-cut, and assembled using hot-glue and zip-ties. The CAD files are in the CAD folder.

Plywood 5mm:

• Main Frame: Part 1 (frame_1.SLDPRT) / Part 2 (frame_2.SLDPRT)
• Motor mount: 2 x Part (motorMount.SLDPRT)
• Autopilot Plate: 2 x Part (plate.SLDPRT)
• Column plate: 4 x (columnPlate_long.SLDPRT / columnPlate_short.SLDPRT)

Plywood 2.5mm:

• Hinge: 4 x Part (hinge.SLDPRT)

Foam-board (6mm):

• Control surfaces: Part 1 (controlSurface_1_v2.SLDPRT) / Part 2 (controlSurface_2_v2.SLDPRT)

NOTE: On the control surface, the second line needs to be scored (and not cut), in order to create a hinge with the foam-board.

Laser-cut settings (Epilog Legend 36 EXT)

Foam-board	Speed	Power	Frequency
Scoring	90	100	5000
Cutting	40	100	5000

Plywood	Speed	Power	Frequency
Cutting (3mm)	25	100	500
Cutting (6mm)	8	100	500

Code base:

For the flight control, the PX4 flight stack is used for its modularity and clarity as flight control software.

The devlopment is done under linux Ubuntu 14.04 LTS. The easiest way to get started is to build a virtual machine using VirtualBox.

The PX4 development toolchain needs to be installed. Please follow the instructions: PX4 flight stack - Installing toolchain.

A lots of additional information can be found in the PX4 flight stack - Development Guide.

Coaxial-rotor controller

Using the PX4 flight stack allows to re-use most of the code base, and to adapt it for this configuration.

The coaxial-rotor uses the same controller as a quadcopter, and re-map the output of the controller (forces and torques to apply to the system) to the actuators. The mapping from the output of the attitude controller to the actuators is called the PX4 flight stack - Mixer.

For the Coaxial-rotor design, the mixer is designed as shown in the following figure:

Modifications

To support this configuration, the following modifications have been made to the base code:

• New Start up script: Start the appropriate modules for this configuration Start up - Coaxial-rotor
• New Mixer: Map the output of the attitude controller to the actuators. Mixer - Coaxial-rotor
• Low pass filter: Applied on the outputs of the controller / inputs of the mixer (aka forces and torques to apply on the system) Mixer - Coaxial-rotor
• Different attitude gains: The gains are defined as parameters in the "Attitude Control" section of the parameter list.

The changes have been incorported in the following version of the PX4 codebase (as a git fork) PX4 flight stack - Coaxial-rotor version (make sure to select the "coaxial_rotor" branch).

Clone the git to get a local copy of the code:

git clone https://github.com/AubreyC/Firmware.git

https://github.com/PX4/Firmware

NOTE: It is possible to start from the official PX4 flight stack - Github and to make the modifications manually. In order to do that add:

• Start up - Coaxial-rotor to Firmware/ROMFS/px4fmu_common/mixers
• Mixer - Coaxial-rotor to Firmware/ROMFS/px4fmu_common/init.d
• Add the low-pass filter to the Attitude Controller outputs: Firmware/blob/coaxial_rotor/src/modules/mc_att_control/mc_att_control_main.cpp

Building and configuring

Follow the instructions from the PX4 flight stack - Building the code.

Open a console and navigate to the Firmware folder of the code.

Building:

make px4fmu-v2_default

Building and uploading:

make px4fmu-v2_default upload

Once the code is uploaded on the PIXHAWK, some parameters need to be changed for the coaxial-configuration:

• SYS_AUTOSTART: set it to 15002 which corresponds to the custom start-up script.
• Attitude Gains: Gains are proposed as a start but are probably not optimal. It is recommended to tune them for better performances.

Proposed Gains:

MC_ACRO_P_MAX	360.0
MC_ACRO_R_MAX	360.0
MC_ACRO_Y_MAX	360.0
MC_PITCHRATE_D	0.0015
MC_PITCHRATE_FF	0.0000
MC_PITCHRATE_I	0.03
MC_PITCHRATE_MAX 220.0
MC_PITCHRATE_P	0.075
MC_PITCH_P	3.25
MC_PITCH_TC	0.16
MC_RATT_TH	1.00
MC_ROLLRATE_D	0.0015
MC_ROLLRATE_FF	0.0000
MC_ROLLRATE_I	0.025
MC_ROLLRATE_MAX	220.0
MC_ROLLRATE_P	0.075
MC_ROLL_P	3.25
MC_ROLL_TC	0.16
MC_YAWRATE_D	0.00
MC_YAWRATE_FF	0.0000
MC_YAWRATE_I	0.00
MC_YAWRATE_MAX	200.0
MC_YAWRATE_P	0.10
MC_YAWRAUTO_MAX	45.0
MC_YAW_FF	0.50
MC_YAW_P	1.40

NOTE: The Parameter file, from one flying prototype has been saved. This file contains the attitude gains, as well as the system start-up number for the coaxial-rotor configuration. Be carefull, this parameter file also contains all the calibration data, which are different from one system to the other.