X Configuration Observation

Hi,

I have just built a quadcopter in an X configuration. I have been having majorproblems (stability, twitchy, uncontrollable ...) trying to get it to take offon its 1st flight. So I thought I would try tuning the PID using theconfigurator software. In doing so I thought I would have a look at the motoroutput graph in response to pitch, roll and yaw. (see attached screenshotdocument).

It seemed as though my quadcopter was operating as though programmed in +configuration, despite reprogramming it and double checking the definitionsettings. The front, right, left or rear motors would respond independently,not as pairs as I would expect for the X configuration.

I have attached a document showing the graph traces from the configurator and alittle more information about my set-up.

Any thoughts and comments would be much appreciated.

Regards,
Tom

X Configuration Report.pdf

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Replies

  • It should control them as you would think they would.
    ArduCopter Quadcopter code (alpha_RC1)

    Using roll as an example, and working backwards:
    Other code was left out, but you can find these.

    The right and left motor are controlled by control_roll. You can ignore the rest.
    rightMotor = constrain(ch_throttle - control_roll + control_yaw, minThrottle, 2000);
    leftMotor = constrain(ch_throttle + control_roll + control_yaw, minThrottle, 2000);

    In acro mode:
    control_roll is controlled by err_roll and the PID settings.
    control_roll = Kp_RateRoll*err_roll + Kd_RateRoll*roll_D + Ki_RateRoll*roll_I;

    err_roll is strictly roll in + and mixed roll/pitch in x. ch_roll is the stick input.
    #ifdef FLIGHT_MODE_+
    err_roll = ((ch_roll- roll_mid) * xmitFactor) - currentRollRate;
    #endif
    #ifdef FLIGHT_MODE_X // In X mode : mix on the input
    err_roll = (((ch_roll- roll_mid)-(ch_pitch-pitch_mid)) * xmitFactor) - currentRollRate;
    #endif

    In stable mode:
    The code is backwards from acro mode.
    Strictly roll in the stable code section.
    err_roll = command_rx_roll - ToDeg(roll);
    control_roll = STABLE_MODE_KP_RATE_ROLL*err_roll;

    Elsewhere in the code:
    command_rx_roll is modified to be roll only in +, mixed roll/pitch in x.
    #ifdef FLIGHT_MODE_+
    command_rx_roll = (ch_roll-roll_mid) / STICK_TO_ANGLE_FACTOR;
    #endif
    #ifdef FLIGHT_MODE_X
    // For X mode we make a mix in the input
    float aux_roll = (ch_roll-roll_mid) / STICK_TO_ANGLE_FACTOR;
    command_rx_roll = aux_roll - aux_pitch;
    #endif
  • Probably just a trivial question, but worth trying....
    Where is your APM/IMU pointing? If you are using Alpha RC1 the the APM/IMU should always point to the front left arm even in X mode. If you are using the SVN trunk (not the ArducopterNG trunk) then the APM/IMU should point in the middle of the arms.

    This is for the current Alpha version:

    a3556749-122-alpha%20code%20frame%20setup.jpg?d=1288025577

    Cheers, Emile
  • 3D Robotics
    I think right now they're only supporting + but you should check with that team to be sure. I know they were planning to have both (plus GPS hold and waypoints) enabled when the commercial version ship in a few weeks.
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