ArduCopter custom motor mix – Vampire Hexicopter

EDIT: I’m leaving this post up here because it contains useful information, however please note that you DO NOT need to perform these steps to get the Vampire working well under Pixhawk. In fact, I found that by removing the middle motors from yaw, the copter would roll whenever trying to yaw. This is different to the results I had when using Megapirates, so perhaps something has changed in the code or how Pixhawk interprets it.

I therefore recommend you use the standard APM Hexi firmware first, and only follow the below steps if you have issues with yaw. Original post follows below: ——-

I’ve decided to install a proper Pixhawk FC in my Vampire hexicopter, to replace the Megapirates that was running on my old Multiwiicopter PARIS V5 board.

In order to get the Vampire flying nicely, you will need to compile your own hex file for the APM, and change the motor mix to accommodate the flat H motor layout.

This is because the middle motors must not be used for yaw, or the craft behaves oddly and loses / gains a lot of altitude when yawing.

Follow this guide to prepare your PC to compile the pixhawk code, and stop at step 5.

Once you’ve prepared the code, navigate to /ardupilot/libraries/AP_Motors and open the AP_MotorsHexa.cpp file. I suggest using Notepad++ to make it easy to read. In here, you’ll find the motor layouts. For the Vampire, we are interested in the X frame layout:

// X frame set-up
add_motor(AP_MOTORS_MOT_1,  90, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  2);
add_motor(AP_MOTORS_MOT_2, -90, AP_MOTORS_MATRIX_YAW_FACTOR_CCW, 5);
add_motor(AP_MOTORS_MOT_3, -30, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  6);
add_motor(AP_MOTORS_MOT_4, 150, AP_MOTORS_MATRIX_YAW_FACTOR_CCW, 3);
add_motor(AP_MOTORS_MOT_5,  30, AP_MOTORS_MATRIX_YAW_FACTOR_CCW, 1);
add_motor(AP_MOTORS_MOT_6,-150, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  4);

Here’s what the code means:

add_motor(AP_MOTORS_MOT_1,  90, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  2);

AP_MOTORS_MOT_1 - This is the motor number as outlined on the frame type page for APM.
90 - this refers to the motor's position in degrees, relative to the flight controller.
AP_MOTORS_MATRIX_YAW_FACTOR_CW - this calculates the yaw factor and applies it to the motor. Replacing this with '0' will result in no yaw being applied to the motor.
2 - the order in which motors will be tested in the 'motor test' mode. The motors should test in a clockwise direction, starting at the front or front-right motor.

Further information can be found in the AP_MotorsMatrix.cpp file.

What we want to do is replace  “AP_MOTORS_MATRIX_YAW_FACTOR_XXX” with the number 0. This will disable yaw for these motors. Referring to the motor mapping chart, we want to remove yaw mixing for motors 1 and 2. The updated file should look like this:

// X frame set-up
add_motor(AP_MOTORS_MOT_1,  90, 0, 2);
add_motor(AP_MOTORS_MOT_2, -90, 0, 5);
add_motor(AP_MOTORS_MOT_3, -30, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  6);
add_motor(AP_MOTORS_MOT_4, 150, AP_MOTORS_MATRIX_YAW_FACTOR_CCW, 3);
add_motor(AP_MOTORS_MOT_5,  30, AP_MOTORS_MATRIX_YAW_FACTOR_CCW, 1);
add_motor(AP_MOTORS_MOT_6,-150, AP_MOTORS_MATRIX_YAW_FACTOR_CW,  4);

Save the file and move onto step 6 in the Pixhawk compile guide. Use the command make px4-v2-hexa to build the hexa firmware. It can take a while – around 15 minutes for my PC.

IMPORTANT: Be sure you set the FRAME parameter to ‘1’ to tell APM to use the ‘X’ motor mix.

Enjoy!