ardupilot/libraries/AC_AttitudeControl/AC_AttitudeControl.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: t -*-

#include "AC_AttitudeControl.h"
#include <AP_HAL.h>

extern const AP_HAL::HAL& hal;

// table of user settable parameters
const AP_Param::GroupInfo AC_AttitudeControl::var_info[] PROGMEM = {

    // @Param: RATE_RP_MAX
    // @DisplayName: Angle Rate Roll-Pitch max
    // @Description: maximum rotation rate in roll/pitch axis requested by angle controller used in stabilize, loiter, rtl, auto flight modes
    // @Units: Centi-Degrees/Sec
    // @Range: 90000 250000
    // @Increment: 500
    // @User: Advanced
    AP_GROUPINFO("RATE_RP_MAX", 0, AC_AttitudeControl, _angle_rate_rp_max, AC_ATTITUDE_CONTROL_RATE_RP_MAX_DEFAULT),

    // @Param: RATE_Y_MAX
    // @DisplayName: Angle Rate Yaw max
    // @Description: maximum rotation rate in roll/pitch axis requested by angle controller used in stabilize, loiter, rtl, auto flight modes
    // @Units: Centi-Degrees/Sec
    // @Range: 90000 250000
    // @Increment: 500
    // @User: Advanced
    AP_GROUPINFO("RATE_Y_MAX",  1, AC_AttitudeControl, _angle_rate_y_max, AC_ATTITUDE_CONTROL_RATE_Y_MAX_DEFAULT),

    // @Param: SLEW_YAW
    // @DisplayName: Yaw target slew rate
    // @Description: Maximum rate the yaw target can be updated in Loiter, RTL, Auto flight modes
    // @Units: Centi-Degrees/Sec
    // @Range: 500 18000
    // @Increment: 100
    // @User: Advanced
    AP_GROUPINFO("SLEW_YAW",    2, AC_AttitudeControl, _slew_yaw, AC_ATTITUDE_CONTROL_SLEW_YAW_DEFAULT),

    // @Param: ACCEL_RP_MAX
    // @DisplayName: Acceleration Max for Roll/Pitch
    // @Description: Maximum acceleration in roll/pitch axis
    // @Units: Centi-Degrees/Sec/Sec
    // @Range: 20000 100000
    // @Increment: 100
    // @User: Advanced
    AP_GROUPINFO("ACCEL_RP_MAX", 3, AC_AttitudeControl, _accel_rp_max, AC_ATTITUDE_CONTROL_ACCEL_RP_MAX_DEFAULT),

    // @Param: ACCEL_Y_MAX
    // @DisplayName: Acceleration Max for Yaw
    // @Description: Maximum acceleration in yaw axis
    // @Units: Centi-Degrees/Sec/Sec
    // @Range: 20000 100000
    // @Increment: 100
    // @User: Advanced
    AP_GROUPINFO("ACCEL_Y_MAX",  4, AC_AttitudeControl, _accel_y_max, AC_ATTITUDE_CONTROL_ACCEL_Y_MAX_DEFAULT),

    AP_GROUPEND
};

//
// high level controllers
//

// init_targets - resets target angles to current angles
void AC_AttitudeControl::init_targets()
{
    // set earth frame angle targets to current lean angles
    _angle_ef_target.x = _ahrs.roll_sensor;
    _angle_ef_target.y = _ahrs.pitch_sensor;
    _angle_ef_target.z = _ahrs.yaw_sensor;

    // clear body frame angle errors
    _angle_bf_error.zero();

    // clear earth-frame and body-frame feed forward rates
    const Vector3f& gyro = _ins.get_gyro();
    _rate_bf_desired = gyro * AC_ATTITUDE_CONTROL_DEGX100;
    frame_conversion_bf_to_ef(_rate_bf_desired,_rate_ef_desired);
}

//
// methods to be called by upper controllers to request and implement a desired attitude
//

// angle_ef_roll_pitch_rate_ef_yaw_smooth - attempts to maintain a roll and pitch angle and yaw rate (all earth frame) while smoothing the attitude based on the feel parameter
//      smoothing_gain : a number from 1 to 50 with 1 being sluggish and 50 being very crisp
void AC_AttitudeControl::angle_ef_roll_pitch_rate_ef_yaw_smooth(float roll_angle_ef, float pitch_angle_ef, float yaw_rate_ef, float smoothing_gain)
{
    Vector3f angle_ef_error;    // earth frame angle errors
    float rate_change_limit;

    // sanity check smoothing gain
    smoothing_gain = constrain_float(smoothing_gain,1.0f,50.0f);

    float linear_angle = _accel_rp_max/(smoothing_gain*smoothing_gain);
    rate_change_limit = _accel_rp_max * _dt;
    float rate_ef_desired;
    float angle_to_target;

    // calculate earth-frame roll and pitch angle error
    angle_ef_error.x = wrap_180_cd_float(_angle_ef_target.x - _ahrs.roll_sensor);
    angle_ef_error.y = wrap_180_cd_float(_angle_ef_target.y - _ahrs.pitch_sensor);

    // calculate earth-frame feed forward roll rate using linear response when close to the target, sqrt response when we're further away
    angle_to_target = roll_angle_ef - _angle_ef_target.x;
    if (angle_to_target > linear_angle){
        rate_ef_desired = safe_sqrt(2.0f*_accel_rp_max*(fabs(angle_to_target)-(linear_angle/2.0f)));
    } else if (angle_to_target < -linear_angle){
        rate_ef_desired = -safe_sqrt(2.0f*_accel_rp_max*(fabs(angle_to_target)-(linear_angle/2.0f)));
    } else {
        rate_ef_desired = smoothing_gain*angle_to_target;
    }
    _rate_ef_desired.x = constrain_float(rate_ef_desired, _rate_ef_desired.x-rate_change_limit, _rate_ef_desired.x+rate_change_limit);

    // update earth-frame roll angle target using desired roll rate
    _angle_ef_target.x += _rate_ef_desired.x*_dt;

    // calculate earth-frame feed forward pitch rate using linear response when close to the target, sqrt response when we're further away
    angle_to_target = pitch_angle_ef - _angle_ef_target.y;
    if (angle_to_target > linear_angle){
        rate_ef_desired = safe_sqrt(2.0f*_accel_rp_max*(fabs(angle_to_target)-(linear_angle/2.0f)));
    } else if (angle_to_target < -linear_angle){
        rate_ef_desired = -safe_sqrt(2.0f*_accel_rp_max*(fabs(angle_to_target)-(linear_angle/2.0f)));
    } else {
        rate_ef_desired = smoothing_gain*angle_to_target;
    }
    _rate_ef_desired.y = constrain_float(rate_ef_desired, _rate_ef_desired.y-rate_change_limit, _rate_ef_desired.y+rate_change_limit);

    // update earth-frame pitch angle target using desired pitch rate
    _angle_ef_target.y += _rate_ef_desired.y*_dt;

    // set earth-frame feed forward rate for yaw
    rate_change_limit = _accel_y_max * _dt;
    float rate_change = yaw_rate_ef - _rate_ef_desired.z;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_ef_desired.z += rate_change;

    // calculate yaw target angle and angle error
    update_ef_yaw_angle_and_error(_rate_ef_desired.z, angle_ef_error);

    // convert earth-frame angle errors to body-frame angle errors
    frame_conversion_ef_to_bf(angle_ef_error, _angle_bf_error);

    // convert earth-frame feed forward rates to body-frame feed forward rates
    frame_conversion_ef_to_bf(_rate_ef_desired, _rate_bf_desired);

    // convert body-frame angle errors to body-frame rate targets
    update_rate_bf_targets();

    // add body frame rate feed forward
    _rate_bf_target += _rate_bf_desired;

    // body-frame to motor outputs should be called separately
}

//
// methods to be called by upper controllers to request and implement a desired attitude
//

// angle_ef_roll_pitch_rate_ef_yaw - attempts to maintain a roll and pitch angle and yaw rate (all earth frame)
void AC_AttitudeControl::angle_ef_roll_pitch_rate_ef_yaw(float roll_angle_ef, float pitch_angle_ef, float yaw_rate_ef)
{
    Vector3f    angle_ef_error;         // earth frame angle errors

    // set earth-frame angle targets for roll and pitch and calculate angle error
    _angle_ef_target.x = roll_angle_ef;
    angle_ef_error.x = wrap_180_cd_float(_angle_ef_target.x - _ahrs.roll_sensor);

    _angle_ef_target.y = pitch_angle_ef;
    angle_ef_error.y = wrap_180_cd_float(_angle_ef_target.y - _ahrs.pitch_sensor);

    // set earth-frame feed forward rate for yaw
    float rate_change_limit = _accel_y_max * _dt;
    float rate_change = yaw_rate_ef - _rate_ef_desired.z;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_ef_desired.z += rate_change;

    update_ef_yaw_angle_and_error(_rate_ef_desired.z, angle_ef_error);

    // convert earth-frame angle errors to body-frame angle errors
    frame_conversion_ef_to_bf(angle_ef_error, _angle_bf_error);

    // convert earth-frame feed forward rates to body-frame feed forward rates
    frame_conversion_ef_to_bf(_rate_ef_desired, _rate_bf_desired);

    // convert body-frame angle errors to body-frame rate targets
    update_rate_bf_targets();

    // add body frame rate feed forward
    _rate_bf_target += _rate_bf_desired;

    // body-frame to motor outputs should be called separately
}

// angle_ef_roll_pitch_yaw - attempts to maintain a roll, pitch and yaw angle (all earth frame)
//  if yaw_slew is true then target yaw movement will be gradually moved to the new target based on the SLEW_YAW parameter
void AC_AttitudeControl::angle_ef_roll_pitch_yaw(float roll_angle_ef, float pitch_angle_ef, float yaw_angle_ef, bool slew_yaw)
{
    Vector3f    angle_ef_error;

    // set earth-frame angle targets
    _angle_ef_target.x = roll_angle_ef;
    _angle_ef_target.y = pitch_angle_ef;
    _angle_ef_target.z = yaw_angle_ef;

    // calculate earth frame errors
    angle_ef_error.x = wrap_180_cd_float(_angle_ef_target.x - _ahrs.roll_sensor);
    angle_ef_error.y = wrap_180_cd_float(_angle_ef_target.y - _ahrs.pitch_sensor);
    angle_ef_error.z = wrap_180_cd_float(_angle_ef_target.z - _ahrs.yaw_sensor);

    // convert earth-frame angle errors to body-frame angle errors
    frame_conversion_ef_to_bf(angle_ef_error, _angle_bf_error);

    // convert body-frame angle errors to body-frame rate targets
    update_rate_bf_targets();

    if (slew_yaw) {
        _rate_bf_target.z = constrain_float(_rate_bf_target.z,-_slew_yaw,_slew_yaw);
    }

    // body-frame to motor outputs should be called separately
}

// rate_ef_roll_pitch_yaw - attempts to maintain a roll, pitch and yaw rate (all earth frame)
void AC_AttitudeControl::rate_ef_roll_pitch_yaw(float roll_rate_ef, float pitch_rate_ef, float yaw_rate_ef)
{
    Vector3f angle_ef_error;
    float rate_change_limit = _accel_rp_max * _dt;

    // update feed forward roll rate after checking it is within acceleration limits
    float rate_change = roll_rate_ef - _rate_ef_desired.x;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_ef_desired.x += rate_change;

    // update feed forward pitch rate after checking it is within acceleration limits
    rate_change = pitch_rate_ef - _rate_ef_desired.y;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_ef_desired.y += rate_change;

    // update feed forward yaw rate after checking it is within acceleration limits
    rate_change_limit = _accel_y_max * _dt;
    rate_change = yaw_rate_ef - _rate_ef_desired.z;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_ef_desired.z += rate_change;

    // update earth frame angle targets and errors
    update_ef_roll_angle_and_error(_rate_ef_desired.x, angle_ef_error);
    update_ef_pitch_angle_and_error(_rate_ef_desired.y, angle_ef_error);
    update_ef_yaw_angle_and_error(_rate_ef_desired.z, angle_ef_error);

    // convert earth-frame angle errors to body-frame angle errors
    frame_conversion_ef_to_bf(angle_ef_error, _angle_bf_error);

    // convert earth-frame rates to body-frame rates
    frame_conversion_ef_to_bf(_rate_ef_desired, _rate_bf_desired);

    // convert body-frame angle errors to body-frame rate targets
    update_rate_bf_targets();

    // add body frame rate feed forward
    _rate_bf_target += _rate_bf_desired;

    // body-frame to motor outputs should be called separately
}

// rate_bf_roll_pitch_yaw - attempts to maintain a roll, pitch and yaw rate (all body frame)
void AC_AttitudeControl::rate_bf_roll_pitch_yaw(float roll_rate_bf, float pitch_rate_bf, float yaw_rate_bf)
{
    Vector3f    angle_ef_error;

    // Update angle error
    if (labs(_ahrs.pitch_sensor)<_acro_angle_switch){
        _acro_angle_switch = 6000;
        // convert body-frame rates to earth-frame rates
        frame_conversion_bf_to_ef(_rate_bf_desired, _rate_ef_desired);

        // update earth frame angle targets and errors
        update_ef_roll_angle_and_error(_rate_ef_desired.x, angle_ef_error);
        update_ef_pitch_angle_and_error(_rate_ef_desired.y, angle_ef_error);
        update_ef_yaw_angle_and_error(_rate_ef_desired.z, angle_ef_error);

        // convert earth-frame angle errors to body-frame angle errors
        frame_conversion_ef_to_bf(angle_ef_error, _angle_bf_error);
    } else {
        _acro_angle_switch = 4500;
        integrate_bf_rate_error_to_angle_errors();
        frame_conversion_bf_to_ef(_angle_bf_error, angle_ef_error);
        _angle_ef_target.x = wrap_180_cd_float(angle_ef_error.x + _ahrs.roll_sensor);
        _angle_ef_target.y = wrap_180_cd_float(angle_ef_error.y + _ahrs.pitch_sensor);
        _angle_ef_target.z = wrap_360_cd_float(angle_ef_error.z + _ahrs.yaw_sensor);
        if (_angle_ef_target.y>9000){
            _angle_ef_target.x = wrap_180_cd_float(_angle_ef_target.x + 18000);
            _angle_ef_target.y = wrap_180_cd_float(18000-_angle_ef_target.x);
            _angle_ef_target.z = wrap_360_cd_float(_angle_ef_target.z + 18000);
        }
        if (_angle_ef_target.y<-9000){
            _angle_ef_target.x = wrap_180_cd_float(_angle_ef_target.x + 18000);
            _angle_ef_target.y = wrap_180_cd_float(-18000-_angle_ef_target.x);
            _angle_ef_target.z = wrap_360_cd_float(_angle_ef_target.z + 18000);
        }
    }

    // convert body-frame angle errors to body-frame rate targets
    update_rate_bf_targets();

    float rate_change, rate_change_limit;

    // update the rate feed forward with angular acceleration limits 
    rate_change_limit = _accel_rp_max * _dt;
    
    rate_change = roll_rate_bf - _rate_bf_desired.x;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_bf_desired.x += rate_change;
    
    rate_change = pitch_rate_bf - _rate_bf_desired.y;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_bf_desired.y += rate_change;

    rate_change_limit = _accel_y_max * _dt;
    
    rate_change = yaw_rate_bf - _rate_bf_desired.z;
    rate_change = constrain_float(rate_change, -rate_change_limit, rate_change_limit);
    _rate_bf_desired.z += rate_change;

    // body-frame rate commands added
    _rate_bf_target += _rate_bf_desired;

    // body-frame to motor outputs should be called separately
}

//
// rate_controller_run - run lowest level body-frame rate controller and send outputs to the motors
//      should be called at 100hz or more
//
void AC_AttitudeControl::rate_controller_run()
{
    // call rate controllers and send output to motors object
    // To-Do: should the outputs from get_rate_roll, pitch, yaw be int16_t which is the input to the motors library?
    // To-Do: skip this step if the throttle out is zero?
    _motors.set_roll(rate_bf_to_motor_roll(_rate_bf_target.x));
    _motors.set_pitch(rate_bf_to_motor_pitch(_rate_bf_target.y));
    _motors.set_yaw(rate_bf_to_motor_yaw(_rate_bf_target.z));
}

//
// earth-frame <-> body-frame conversion functions
//
// frame_conversion_ef_to_bf - converts earth frame vector to body frame vector
void AC_AttitudeControl::frame_conversion_ef_to_bf(const Vector3f& ef_vector, Vector3f& bf_vector)
{
    // convert earth frame rates to body frame rates
    bf_vector.x = ef_vector.x - _ahrs.sin_pitch() * ef_vector.z;
    bf_vector.y = _ahrs.cos_roll()  * ef_vector.y + _ahrs.sin_roll() * _ahrs.cos_pitch() * ef_vector.z;
    bf_vector.z = -_ahrs.sin_roll() * ef_vector.y + _ahrs.cos_pitch() * _ahrs.cos_roll() * ef_vector.z;
}

// frame_conversion_bf_to_ef - converts body frame vector to earth frame vector
void AC_AttitudeControl::frame_conversion_bf_to_ef(const Vector3f& bf_vector, Vector3f& ef_vector)
{
    // convert earth frame rates to body frame rates
    ef_vector.x = bf_vector.x + _ahrs.sin_roll() * (_ahrs.sin_pitch()/_ahrs.cos_pitch()) * bf_vector.y + _ahrs.cos_roll() * (_ahrs.sin_pitch()/_ahrs.cos_pitch()) * bf_vector.z;
    ef_vector.y = _ahrs.cos_roll()  * bf_vector.y - _ahrs.sin_roll() * bf_vector.z;
    ef_vector.z = (_ahrs.sin_roll() / _ahrs.cos_pitch()) * bf_vector.y + (_ahrs.cos_roll() / _ahrs.cos_pitch()) * bf_vector.z;
}

//
// protected methods
//

//
// stabilized rate controller (body-frame) methods
//

// update_ef_roll_angle_and_error - update _angle_ef_target.x using an earth frame roll rate request
void AC_AttitudeControl::update_ef_roll_angle_and_error(float roll_rate_ef, Vector3f &angle_ef_error)
{
    // calculate angle error with maximum of +- max angle overshoot
    angle_ef_error.x = wrap_180_cd(_angle_ef_target.x - _ahrs.roll_sensor);
    angle_ef_error.x  = constrain_float(angle_ef_error.x, -AC_ATTITUDE_RATE_STAB_ROLL_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_ROLL_OVERSHOOT_ANGLE_MAX);

    // To-Do: handle check for traditional heli's motors.motor_runup_complete

    // update roll angle target to be within max angle overshoot of our roll angle
    _angle_ef_target.x = angle_ef_error.x + _ahrs.roll_sensor;

    // increment the roll angle target
    _angle_ef_target.x += roll_rate_ef * _dt;
    _angle_ef_target.x = wrap_180_cd(_angle_ef_target.x);
}

// update_ef_pitch_angle_and_error - update _angle_ef_target.y using an earth frame pitch rate request
void AC_AttitudeControl::update_ef_pitch_angle_and_error(float pitch_rate_ef, Vector3f &angle_ef_error)
{
    // calculate angle error with maximum of +- max angle overshoot
    // To-Do: should we do something better as we cross 90 degrees?
    angle_ef_error.y = wrap_180_cd(_angle_ef_target.y - _ahrs.pitch_sensor);
    angle_ef_error.y  = constrain_float(angle_ef_error.y, -AC_ATTITUDE_RATE_STAB_PITCH_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_PITCH_OVERSHOOT_ANGLE_MAX);

    // To-Do: handle check for traditional heli's motors.motor_runup_complete

    // update pitch angle target to be within max angle overshoot of our pitch angle
    _angle_ef_target.y = angle_ef_error.y + _ahrs.pitch_sensor;

    // increment the pitch angle target
    _angle_ef_target.y += pitch_rate_ef * _dt;
    _angle_ef_target.y = wrap_180_cd(_angle_ef_target.y);
}

// update_ef_yaw_angle_and_error - update _angle_ef_target.z using an earth frame yaw rate request
void AC_AttitudeControl::update_ef_yaw_angle_and_error(float yaw_rate_ef, Vector3f &angle_ef_error)
{
    // calculate angle error with maximum of +- max angle overshoot
    angle_ef_error.z = wrap_180_cd(_angle_ef_target.z - _ahrs.yaw_sensor);
    angle_ef_error.z  = constrain_float(angle_ef_error.z, -AC_ATTITUDE_RATE_STAB_YAW_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_YAW_OVERSHOOT_ANGLE_MAX);

    // update yaw angle target to be within max angle overshoot of our current heading
    _angle_ef_target.z = angle_ef_error.z + _ahrs.yaw_sensor;

    // increment the yaw angle target
    _angle_ef_target.z += yaw_rate_ef * _dt;
    _angle_ef_target.z = wrap_360_cd(_angle_ef_target.z);
}

// update_rate_bf_errors - calculates body frame angle errors
//   body-frame feed forward rates (centi-degrees / second) taken from _angle_bf_error
//   angle errors in centi-degrees placed in _angle_bf_error
void AC_AttitudeControl::integrate_bf_rate_error_to_angle_errors()
{
    // roll - calculate body-frame angle error by integrating body-frame rate error
    _angle_bf_error.x += (_rate_bf_desired.x - (_ins.get_gyro().x * AC_ATTITUDE_CONTROL_DEGX100)) * _dt;
    // roll - limit maximum error
    _angle_bf_error.x = constrain_float(_angle_bf_error.x, -AC_ATTITUDE_RATE_STAB_ROLL_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_ROLL_OVERSHOOT_ANGLE_MAX);


    // pitch - calculate body-frame angle error by integrating body-frame rate error
    _angle_bf_error.y += (_rate_bf_desired.y - (_ins.get_gyro().y * AC_ATTITUDE_CONTROL_DEGX100)) * _dt;
    // pitch - limit maximum error
    _angle_bf_error.y = constrain_float(_angle_bf_error.y, -AC_ATTITUDE_RATE_STAB_PITCH_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_PITCH_OVERSHOOT_ANGLE_MAX);


    // yaw - calculate body-frame angle error by integrating body-frame rate error
    _angle_bf_error.z += (_rate_bf_desired.z - (_ins.get_gyro().z * AC_ATTITUDE_CONTROL_DEGX100)) * _dt;
    // yaw - limit maximum error
    _angle_bf_error.z = constrain_float(_angle_bf_error.z, -AC_ATTITUDE_RATE_STAB_YAW_OVERSHOOT_ANGLE_MAX, AC_ATTITUDE_RATE_STAB_YAW_OVERSHOOT_ANGLE_MAX);

    // To-Do: handle case of motors being disarmed or g.rc_3.servo_out == 0 and set error to zero
}

// update_rate_bf_targets - converts body-frame angle error to body-frame rate targets for roll, pitch and yaw axis
//   targets rates in centi-degrees taken from _angle_bf_error
//   results in centi-degrees/sec put into _rate_bf_target
void AC_AttitudeControl::update_rate_bf_targets()
{
    // stab roll calculation
    _rate_bf_target.x = _p_angle_roll.kP() * _angle_bf_error.x;
    // constrain roll rate request
    if (_flags.limit_angle_to_rate_request) {
        _rate_bf_target.x = constrain_float(_rate_bf_target.x,-_angle_rate_rp_max,_angle_rate_rp_max);
    }

    // stab pitch calculation
    _rate_bf_target.y = _p_angle_pitch.kP() * _angle_bf_error.y;
    // constrain pitch rate request
    if (_flags.limit_angle_to_rate_request) {
        _rate_bf_target.y = constrain_float(_rate_bf_target.y,-_angle_rate_rp_max,_angle_rate_rp_max);
    }

    // stab yaw calculation
    _rate_bf_target.z = _p_angle_yaw.kP() * _angle_bf_error.z;
    // constrain yaw rate request
    if (_flags.limit_angle_to_rate_request) {
        _rate_bf_target.z = constrain_float(_rate_bf_target.z,-_angle_rate_y_max,_angle_rate_y_max);
    }
}

//
// body-frame rate controller
//

// rate_bf_to_motor_roll - ask the rate controller to calculate the motor outputs to achieve the target rate in centi-degrees / second
float AC_AttitudeControl::rate_bf_to_motor_roll(float rate_target_cds)
{
    float p,i,d;            // used to capture pid values for logging
    float current_rate;     // this iteration's rate
    float rate_error;       // simply target_rate - current_rate

    // get current rate
    // To-Do: make getting gyro rates more efficient?
    current_rate = (_ins.get_gyro().x * AC_ATTITUDE_CONTROL_DEGX100);

    // calculate error and call pid controller
    rate_error = rate_target_cds - current_rate;
    p = _pid_rate_roll.get_p(rate_error);

    // get i term
    i = _pid_rate_roll.get_integrator();

    // update i term as long as we haven't breached the limits or the I term will certainly reduce
    if (!_motors.limit.roll_pitch || ((i>0&&rate_error<0)||(i<0&&rate_error>0))) {
        i = _pid_rate_roll.get_i(rate_error, _dt);
    }

    // get d term
    d = _pid_rate_roll.get_d(rate_error, _dt);

    // constrain output and return
    return constrain_float((p+i+d), -AC_ATTITUDE_RATE_RP_CONTROLLER_OUT_MAX, AC_ATTITUDE_RATE_RP_CONTROLLER_OUT_MAX);

    // To-Do: allow logging of PIDs?
}

// rate_bf_to_motor_pitch - ask the rate controller to calculate the motor outputs to achieve the target rate in centi-degrees / second
float AC_AttitudeControl::rate_bf_to_motor_pitch(float rate_target_cds)
{
    float p,i,d;            // used to capture pid values for logging
    float current_rate;     // this iteration's rate
    float rate_error;       // simply target_rate - current_rate

    // get current rate
    // To-Do: make getting gyro rates more efficient?
    current_rate = (_ins.get_gyro().y * AC_ATTITUDE_CONTROL_DEGX100);

    // calculate error and call pid controller
    rate_error = rate_target_cds - current_rate;
    p = _pid_rate_pitch.get_p(rate_error);

    // get i term
    i = _pid_rate_pitch.get_integrator();

    // update i term as long as we haven't breached the limits or the I term will certainly reduce
    if (!_motors.limit.roll_pitch || ((i>0&&rate_error<0)||(i<0&&rate_error>0))) {
        i = _pid_rate_pitch.get_i(rate_error, _dt);
    }

    // get d term
    d = _pid_rate_pitch.get_d(rate_error, _dt);

    // constrain output and return
    return constrain_float((p+i+d), -AC_ATTITUDE_RATE_RP_CONTROLLER_OUT_MAX, AC_ATTITUDE_RATE_RP_CONTROLLER_OUT_MAX);

    // To-Do: allow logging of PIDs?
}

// rate_bf_to_motor_yaw - ask the rate controller to calculate the motor outputs to achieve the target rate in centi-degrees / second
float AC_AttitudeControl::rate_bf_to_motor_yaw(float rate_target_cds)
{
    float p,i,d;            // used to capture pid values for logging
    float current_rate;     // this iteration's rate
    float rate_error;       // simply target_rate - current_rate

    // get current rate
    // To-Do: make getting gyro rates more efficient?
    current_rate = (_ins.get_gyro().z * AC_ATTITUDE_CONTROL_DEGX100);

    // calculate error and call pid controller
    rate_error  = rate_target_cds - current_rate;
    p = _pid_rate_yaw.get_p(rate_error);

    // separately calculate p, i, d values for logging
    p = _pid_rate_yaw.get_p(rate_error);

    // get i term
    i = _pid_rate_yaw.get_integrator();

    // update i term as long as we haven't breached the limits or the I term will certainly reduce
    if (!_motors.limit.yaw || ((i>0&&rate_error<0)||(i<0&&rate_error>0))) {
        i = _pid_rate_yaw.get_i(rate_error, _dt);
    }

    // get d value
    d = _pid_rate_yaw.get_d(rate_error, _dt);

    // constrain output and return
    return constrain_float((p+i+d), -AC_ATTITUDE_RATE_YAW_CONTROLLER_OUT_MAX, AC_ATTITUDE_RATE_YAW_CONTROLLER_OUT_MAX);

    // To-Do: allow logging of PIDs?
}


//
// throttle functions
//

 // set_throttle_out - to be called by upper throttle controllers when they wish to provide throttle output directly to motors
 // provide 0 to cut motors
void AC_AttitudeControl::set_throttle_out(int16_t throttle_out, bool apply_angle_boost)
{
    if (apply_angle_boost) {
        _motors.set_throttle(get_angle_boost(throttle_out));
    }else{
        _motors.set_throttle(throttle_out);
        // clear angle_boost for logging purposes
        _angle_boost = 0;
    }

    // update compass with throttle value
    // To-Do: find another method to grab the throttle out and feed to the compass.  Could be done completely outside this class
    //compass.set_throttle((float)g.rc_3.servo_out/1000.0f);
}

// get_angle_boost - returns a throttle including compensation for roll/pitch angle
// throttle value should be 0 ~ 1000
int16_t AC_AttitudeControl::get_angle_boost(int16_t throttle_pwm)
{
    float temp = _ahrs.cos_pitch() * _ahrs.cos_roll();
    int16_t throttle_out;

    temp = constrain_float(temp, 0.5f, 1.0f);

    // reduce throttle if we go inverted
    temp = constrain_float(9000-max(labs(_ahrs.roll_sensor),labs(_ahrs.pitch_sensor)), 0, 3000) / (3000 * temp);

    // apply scale and constrain throttle
    // To-Do: move throttle_min and throttle_max into the AP_Vehicles class?
    throttle_out = constrain_float((float)(throttle_pwm-_motors.throttle_min()) * temp + _motors.throttle_min(), _motors.throttle_min(), 1000);

    // record angle boost for logging
    _angle_boost = throttle_out - throttle_pwm;

    return throttle_out;
}