ardupilot/libraries/AP_Motors/AP_MotorsMulticopter.cpp

/*
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

/*
 *       AP_MotorsMulticopter.cpp - ArduCopter multicopter motors library
 *       Code by Randy Mackay and Robert Lefebvre. DIYDrones.com
 *
 */

#include "AP_MotorsMulticopter.h"
#include <AP_HAL/AP_HAL.h>

extern const AP_HAL::HAL& hal;

// parameters for the motor class
const AP_Param::GroupInfo AP_MotorsMulticopter::var_info[] = {
    // 0 was used by TB_RATIO
    // 1,2,3 were used by throttle curve
    // 5 was SPIN_ARMED

    // @Param: YAW_HEADROOM
    // @DisplayName: Matrix Yaw Min
    // @Description: Yaw control is given at least this pwm in microseconds range
    // @Range: 0 500
    // @Units: PWM
    // @User: Advanced
    AP_GROUPINFO("YAW_HEADROOM", 6, AP_MotorsMulticopter, _yaw_headroom, AP_MOTORS_YAW_HEADROOM_DEFAULT),

    // 7 was THR_LOW_CMP

    // @Param: THST_EXPO
    // @DisplayName: Thrust Curve Expo
    // @Description: Motor thrust curve exponent (from 0 for linear to 1.0 for second order curve)
    // @Range: 0.25 0.8
    // @User: Advanced
    AP_GROUPINFO("THST_EXPO", 8, AP_MotorsMulticopter, _thrust_curve_expo, AP_MOTORS_THST_EXPO_DEFAULT),

    // @Param: SPIN_MAX
    // @DisplayName: Motor Spin maximum
    // @Description: Point at which the thrust saturates expressed as a number from 0 to 1 in the entire output range
    // @Values: 0.9:Low, 0.95:Default, 1.0:High
    // @User: Advanced
    AP_GROUPINFO("SPIN_MAX", 9, AP_MotorsMulticopter, _spin_max, AP_MOTORS_SPIN_MAX_DEFAULT),

    // @Param: BAT_VOLT_MAX
    // @DisplayName: Battery voltage compensation maximum voltage
    // @Description: Battery voltage compensation maximum voltage (voltage above this will have no additional scaling effect on thrust).  Recommend 4.4 * cell count, 0 = Disabled
    // @Range: 6 35
    // @Units: V
    // @User: Advanced
    AP_GROUPINFO("BAT_VOLT_MAX", 10, AP_MotorsMulticopter, _batt_voltage_max, AP_MOTORS_BAT_VOLT_MAX_DEFAULT),

    // @Param: BAT_VOLT_MIN
    // @DisplayName: Battery voltage compensation minimum voltage
    // @Description: Battery voltage compensation minimum voltage (voltage below this will have no additional scaling effect on thrust).  Recommend 3.5 * cell count, 0 = Disabled
    // @Range: 6 35
    // @Units: V
    // @User: Advanced
    AP_GROUPINFO("BAT_VOLT_MIN", 11, AP_MotorsMulticopter, _batt_voltage_min, AP_MOTORS_BAT_VOLT_MIN_DEFAULT),

    // @Param: BAT_CURR_MAX
    // @DisplayName: Motor Current Max
    // @Description: Maximum current over which maximum throttle is limited (0 = Disabled)
    // @Range: 0 200
    // @Units: A
    // @User: Advanced
    AP_GROUPINFO("BAT_CURR_MAX", 12, AP_MotorsMulticopter, _batt_current_max, AP_MOTORS_BAT_CURR_MAX_DEFAULT),

    // 13, 14 were used by THR_MIX_MIN, THR_MIX_MAX

    // @Param: PWM_TYPE
    // @DisplayName: Output PWM type
    // @Description: This selects the output PWM type, allowing for normal PWM continuous output, OneShot or brushed motor output
    // @Values: 0:Normal,1:OneShot,2:OneShot125,3:Brushed
    // @User: Advanced
    // @RebootRequired: True
    AP_GROUPINFO("PWM_TYPE", 15, AP_MotorsMulticopter, _pwm_type, PWM_TYPE_NORMAL),

    // @Param: PWM_MIN
    // @DisplayName: PWM output miniumum
    // @Description: This sets the min PWM output value in microseconds that will ever be output to the motors, 0 = use input RC3_MIN
    // @Units: PWM
    // @Range: 0 2000
    // @User: Advanced
    AP_GROUPINFO("PWM_MIN", 16, AP_MotorsMulticopter, _pwm_min, 0),

    // @Param: PWM_MAX
    // @DisplayName: PWM output maximum
    // @Description: This sets the max PWM value in microseconds that will ever be output to the motors, 0 = use input RC3_MAX
    // @Units: PWM
    // @Range: 0 2000
    // @User: Advanced
    AP_GROUPINFO("PWM_MAX", 17, AP_MotorsMulticopter, _pwm_max, 0),

    // @Param: SPIN_MIN
    // @DisplayName: Motor Spin minimum
    // @Description: Point at which the thrust starts expressed as a number from 0 to 1 in the entire output range.  Should be higher than MOT_SPIN_ARM.
    // @Values: 0.0:Low, 0.15:Default, 0.3:High
    // @User: Advanced
    AP_GROUPINFO("SPIN_MIN", 18, AP_MotorsMulticopter, _spin_min, AP_MOTORS_SPIN_MIN_DEFAULT),

    // @Param: SPIN_ARM
    // @DisplayName: Motor Spin armed
    // @Description: Point at which the motors start to spin expressed as a number from 0 to 1 in the entire output range.  Should be lower than MOT_SPIN_MIN.
    // @Values: 0.0:Low, 0.1:Default, 0.2:High
    // @User: Advanced
    AP_GROUPINFO("SPIN_ARM", 19, AP_MotorsMulticopter, _spin_arm, AP_MOTORS_SPIN_ARM_DEFAULT),

    // @Param: BAT_CURR_TC
    // @DisplayName: Motor Current Max Time Constant
    // @Description: Time constant used to limit the maximum current
    // @Range: 0 10
    // @Units: s
    // @User: Advanced
    AP_GROUPINFO("BAT_CURR_TC", 20, AP_MotorsMulticopter, _batt_current_time_constant, AP_MOTORS_BAT_CURR_TC_DEFAULT),

    // @Param: THST_HOVER
    // @DisplayName: Thrust Hover Value
    // @Description: Motor thrust needed to hover expressed as a number from 0 to 1
    // @Range: 0.2 0.8
    // @User: Advanced
    AP_GROUPINFO("THST_HOVER", 21, AP_MotorsMulticopter, _throttle_hover, AP_MOTORS_THST_HOVER_DEFAULT),

    // @Param: HOVER_LEARN
    // @DisplayName: Hover Value Learning
    // @Description: Enable/Disable automatic learning of hover throttle
    // @Values: 0:Disabled, 1:Learn, 2:LearnAndSave
    // @User: Advanced
    AP_GROUPINFO("HOVER_LEARN", 22, AP_MotorsMulticopter, _throttle_hover_learn, HOVER_LEARN_AND_SAVE),

    // @Param: SAFE_DISARM
    // @DisplayName: Motor PWM output disabled when disarmed
    // @Description: Disables motor PWM output when disarmed
    // @Values: 0:PWM enabled while disarmed, 1:PWM disabled while disarmed
    // @User: Advanced
    AP_GROUPINFO("SAFE_DISARM", 23, AP_MotorsMulticopter, _disarm_disable_pwm, 0),

    // @Param: YAW_SV_ANGLE
    // @DisplayName: Yaw Servo Max Lean Angle
    // @Description: Yaw servo's maximum lean angle
    // @Range: 5 80
    // @Units: deg
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO_FRAME("YAW_SV_ANGLE", 35, AP_MotorsMulticopter,  _yaw_servo_angle_max_deg, 30, AP_PARAM_FRAME_TRICOPTER),

    // @Param: SPOOL_TIME
    // @DisplayName: Spool up time
    // @Description: Time in seconds to spool up the motors from zero to min throttle. 
    // @Range: 0 2
    // @Units: s
    // @Increment: 0.1
    // @User: Advanced
    AP_GROUPINFO("SPOOL_TIME",   36, AP_MotorsMulticopter,  _spool_up_time, AP_MOTORS_SPOOL_UP_TIME_DEFAULT),

    // @Param: BOOST_SCALE
    // @DisplayName: Motor boost scale
    // @Description: This is a scaling factor for vehicles with a vertical booster motor used for extra lift. It is used with electric multicopters that have an internal combusion booster motor for longer endurance. The output to the BoostThrottle servo function is set to the current motor thottle times this scaling factor. A higher scaling factor will put more of the load on the booster motor. A value of 1 will set the BoostThrottle equal to the main throttle.
    // @Range: 0 5
    // @Increment: 0.1
    // @User: Advanced
    AP_GROUPINFO("BOOST_SCALE",  37, AP_MotorsMulticopter,  _boost_scale, 0),

    // 38 RESERVED for BAT_POW_MAX
    
    AP_GROUPEND
};

// Constructor
AP_MotorsMulticopter::AP_MotorsMulticopter(uint16_t loop_rate, uint16_t speed_hz) :
    AP_Motors(loop_rate, speed_hz),
    _spool_mode(SHUT_DOWN),
    _spin_up_ratio(0.0f),
    _lift_max(1.0f),
    _throttle_limit(1.0f),
    _throttle_thrust_max(0.0f),
    _disarm_safety_timer(0)
{
    AP_Param::setup_object_defaults(this, var_info);

    // disable all motors by default
    memset(motor_enabled, false, sizeof(motor_enabled));

    // setup battery voltage filtering
    _batt_voltage_filt.set_cutoff_frequency(AP_MOTORS_BATT_VOLT_FILT_HZ);
    _batt_voltage_filt.reset(1.0f);

    // default throttle range
    _throttle_radio_min = 1100;
    _throttle_radio_max = 1900;
};

// output - sends commands to the motors
void AP_MotorsMulticopter::output()
{
    // update throttle filter
    update_throttle_filter();

    // calc filtered battery voltage and lift_max
    update_lift_max_from_batt_voltage();

    // run spool logic
    output_logic();

    // calculate thrust
    output_armed_stabilizing();

    // apply any thrust compensation for the frame
    thrust_compensation();
    
    // convert rpy_thrust values to pwm
    output_to_motors();

    // output any booster throttle
    output_boost_throttle();
};

// output booster throttle, if any
void AP_MotorsMulticopter::output_boost_throttle(void)
{
    if (_boost_scale > 0) {
        float throttle = constrain_float(get_throttle() * _boost_scale, 0, 1);
        SRV_Channels::set_output_scaled(SRV_Channel::k_boost_throttle, throttle*1000);        
    }
}
    

// sends minimum values out to the motors
void AP_MotorsMulticopter::output_min()
{
    set_desired_spool_state(DESIRED_SHUT_DOWN);
    _spool_mode = SHUT_DOWN;
    output();
}

// update the throttle input filter
void AP_MotorsMulticopter::update_throttle_filter()
{
    if (armed()) {
        _throttle_filter.apply(_throttle_in, 1.0f/_loop_rate);
        // constrain filtered throttle
        if (_throttle_filter.get() < 0.0f) {
            _throttle_filter.reset(0.0f);
        }
        if (_throttle_filter.get() > 1.0f) {
            _throttle_filter.reset(1.0f);
        }
    } else {
        _throttle_filter.reset(0.0f);
    }
}

// return current_limit as a number from 0 ~ 1 in the range throttle_min to throttle_max
float AP_MotorsMulticopter::get_current_limit_max_throttle()
{
    // return maximum if current limiting is disabled
    if (_batt_current_max <= 0) {
        _throttle_limit = 1.0f;
        return 1.0f;
    }

    // remove throttle limit if disarmed
    if (!_flags.armed) {
        _throttle_limit = 1.0f;
        return 1.0f;
    }

    // calculate the maximum current to prevent voltage sag below _batt_voltage_min
    float batt_current_max = MIN(_batt_current_max, _batt_current + (_batt_voltage-_batt_voltage_min)/_batt_resistance);

    float batt_current_ratio = _batt_current/batt_current_max;

    float loop_interval = 1.0f/_loop_rate;
    _throttle_limit += (loop_interval/(loop_interval+_batt_current_time_constant))*(1.0f - batt_current_ratio);

    // throttle limit drops to 20% between hover and full throttle
    _throttle_limit = constrain_float(_throttle_limit, 0.2f, 1.0f);

    // limit max throttle
    return get_throttle_hover() + ((1.0-get_throttle_hover())*_throttle_limit);
}

// apply_thrust_curve_and_volt_scaling - returns throttle in the range 0 ~ 1
float AP_MotorsMulticopter::apply_thrust_curve_and_volt_scaling(float thrust) const
{
    float throttle_ratio = thrust;
    // apply thrust curve - domain 0.0 to 1.0, range 0.0 to 1.0
    float thrust_curve_expo = constrain_float(_thrust_curve_expo, -1.0f, 1.0f);
    if (fabsf(thrust_curve_expo) < 0.001) {
        // zero expo means linear, avoid floating point exception for small values
        return thrust;
    }
    if(!is_zero(_batt_voltage_filt.get())) {
        throttle_ratio = ((thrust_curve_expo-1.0f) + safe_sqrt((1.0f-thrust_curve_expo)*(1.0f-thrust_curve_expo) + 4.0f*thrust_curve_expo*_lift_max*thrust))/(2.0f*thrust_curve_expo*_batt_voltage_filt.get());
    } else {
        throttle_ratio = ((thrust_curve_expo-1.0f) + safe_sqrt((1.0f-thrust_curve_expo)*(1.0f-thrust_curve_expo) + 4.0f*thrust_curve_expo*_lift_max*thrust))/(2.0f*thrust_curve_expo);
    }

    return constrain_float(throttle_ratio, 0.0f, 1.0f);
}

// update_lift_max from battery voltage - used for voltage compensation
void AP_MotorsMulticopter::update_lift_max_from_batt_voltage()
{
    // sanity check battery_voltage_min is not too small
    // if disabled or misconfigured exit immediately
    if((_batt_voltage_max <= 0) || (_batt_voltage_min >= _batt_voltage_max) || (_batt_voltage < 0.25f*_batt_voltage_min)) {
        _batt_voltage_filt.reset(1.0f);
        _lift_max = 1.0f;
        return;
    }

    _batt_voltage_min = MAX(_batt_voltage_min, _batt_voltage_max * 0.6f);

    // contrain resting voltage estimate (resting voltage is actual voltage with sag removed based on current draw and resistance)
    _batt_voltage_resting_estimate = constrain_float(_batt_voltage_resting_estimate, _batt_voltage_min, _batt_voltage_max);

    // filter at 0.5 Hz
    float batt_voltage_filt = _batt_voltage_filt.apply(_batt_voltage_resting_estimate/_batt_voltage_max, 1.0f/_loop_rate);

    // calculate lift max
    float thrust_curve_expo = constrain_float(_thrust_curve_expo, -1.0f, 1.0f);
    _lift_max = batt_voltage_filt*(1-thrust_curve_expo) + thrust_curve_expo*batt_voltage_filt*batt_voltage_filt;
}

float AP_MotorsMulticopter::get_compensation_gain() const
{
    // avoid divide by zero
    if (_lift_max <= 0.0f) {
        return 1.0f;
    }

    float ret = 1.0f / _lift_max;

#if AP_MOTORS_DENSITY_COMP == 1
    // air density ratio is increasing in density / decreasing in altitude
    if (_air_density_ratio > 0.3f && _air_density_ratio < 1.5f) {
        ret *= 1.0f / constrain_float(_air_density_ratio,0.5f,1.25f);
    }
#endif
    return ret;
}

int16_t AP_MotorsMulticopter::calc_thrust_to_pwm(float thrust_in) const
{
    thrust_in = constrain_float(thrust_in, 0.0f, 1.0f);
    return get_pwm_output_min() + (get_pwm_output_max()-get_pwm_output_min()) * (_spin_min + (_spin_max-_spin_min)*apply_thrust_curve_and_volt_scaling(thrust_in));
}

int16_t AP_MotorsMulticopter::calc_spin_up_to_pwm() const
{
    return get_pwm_output_min() + constrain_float(_spin_up_ratio, 0.0f, 1.0f) * _spin_min * (get_pwm_output_max()-get_pwm_output_min());
}
// get minimum or maximum pwm value that can be output to motors
int16_t AP_MotorsMulticopter::get_pwm_output_min() const
{
    // return _pwm_min if both PWM_MIN and PWM_MAX parameters are defined and valid
    if ((_pwm_min > 0) && (_pwm_max > 0) && (_pwm_max > _pwm_min)) {
        return _pwm_min;
    }
    return _throttle_radio_min;
}

// get maximum pwm value that can be output to motors
int16_t AP_MotorsMulticopter::get_pwm_output_max() const
{
    // return _pwm_max if both PWM_MIN and PWM_MAX parameters are defined and valid
    if ((_pwm_min > 0) && (_pwm_max > 0) && (_pwm_max > _pwm_min)) {
        return _pwm_max;
    }
    return _throttle_radio_max;
}

// set_throttle_range - sets the minimum throttle that will be sent to the engines when they're not off (i.e. to prevents issues with some motors spinning and some not at very low throttle)
// also sets throttle channel minimum and maximum pwm
void AP_MotorsMulticopter::set_throttle_range(int16_t radio_min, int16_t radio_max)
{
    // sanity check
    if (radio_max <= radio_min) {
        return;
    }

    _throttle_radio_min = radio_min;
    _throttle_radio_max = radio_max;

    hal.rcout->set_esc_scaling(get_pwm_output_min(), get_pwm_output_max());
}

// update the throttle input filter.  should be called at 100hz
void AP_MotorsMulticopter::update_throttle_hover(float dt)
{
    if (_throttle_hover_learn != HOVER_LEARN_DISABLED) {
        // we have chosen to constrain the hover throttle to be within the range reachable by the third order expo polynomial.
        _throttle_hover = constrain_float(_throttle_hover + (dt/(dt+AP_MOTORS_THST_HOVER_TC))*(get_throttle()-_throttle_hover), AP_MOTORS_THST_HOVER_MIN, AP_MOTORS_THST_HOVER_MAX);
    }
}

// run spool logic
void AP_MotorsMulticopter::output_logic()
{
    if (_flags.armed) {
        _disarm_safety_timer = 100;
    } else if (_disarm_safety_timer != 0) {
        _disarm_safety_timer--;
    }

    // force desired and current spool mode if disarmed or not interlocked
    if (!_flags.armed || !_flags.interlock) {
        _spool_desired = DESIRED_SHUT_DOWN;
        _spool_mode = SHUT_DOWN;
    }

    if (_spool_up_time < 0.05) {
        // prevent float exception
        _spool_up_time.set(0.05);
    }

    switch (_spool_mode) {
        case SHUT_DOWN:
            // Motors should be stationary.
            // Servos set to their trim values or in a test condition.

            // set limits flags
            limit.roll_pitch = true;
            limit.yaw = true;
            limit.throttle_lower = true;
            limit.throttle_upper = true;

            // make sure the motors are spooling in the correct direction
            if (_spool_desired != DESIRED_SHUT_DOWN) {
                _spool_mode = SPIN_WHEN_ARMED;
                break;
            }

            // set and increment ramp variables
            _spin_up_ratio = 0.0f;
            _throttle_thrust_max = 0.0f;
            break;

        case SPIN_WHEN_ARMED: {
            // Motors should be stationary or at spin when armed.
            // Servos should be moving to correct the current attitude.

            // set limits flags
            limit.roll_pitch = true;
            limit.yaw = true;
            limit.throttle_lower = true;
            limit.throttle_upper = true;

            // set and increment ramp variables
            float spool_step = 1.0f/(_spool_up_time*_loop_rate);
            if (_spool_desired == DESIRED_SHUT_DOWN){
                _spin_up_ratio -= spool_step;
                // constrain ramp value and update mode
                if (_spin_up_ratio <= 0.0f) {
                    _spin_up_ratio = 0.0f;
                    _spool_mode = SHUT_DOWN;
                }
            } else if(_spool_desired == DESIRED_THROTTLE_UNLIMITED) {
                _spin_up_ratio += spool_step;
                // constrain ramp value and update mode
                if (_spin_up_ratio >= 1.0f) {
                    _spin_up_ratio = 1.0f;
                    _spool_mode = SPOOL_UP;
                }
            } else {    // _spool_desired == SPIN_WHEN_ARMED
                float spin_up_armed_ratio = 0.0f;
                if (_spin_min > 0.0f) {
                    spin_up_armed_ratio = _spin_arm / _spin_min;
                }
                _spin_up_ratio += constrain_float(spin_up_armed_ratio-_spin_up_ratio, -spool_step, spool_step);
            }
            _throttle_thrust_max = 0.0f;
            break;
        }
        case SPOOL_UP:
            // Maximum throttle should move from minimum to maximum.
            // Servos should exhibit normal flight behavior.

            // initialize limits flags
            limit.roll_pitch = false;
            limit.yaw = false;
            limit.throttle_lower = false;
            limit.throttle_upper = false;

            // make sure the motors are spooling in the correct direction
            if (_spool_desired != DESIRED_THROTTLE_UNLIMITED ){
                _spool_mode = SPOOL_DOWN;
                break;
            }

            // set and increment ramp variables
            _spin_up_ratio = 1.0f;
            _throttle_thrust_max += 1.0f/(_spool_up_time*_loop_rate);

            // constrain ramp value and update mode
            if (_throttle_thrust_max >= MIN(get_throttle(), get_current_limit_max_throttle())) {
                _throttle_thrust_max = get_current_limit_max_throttle();
                _spool_mode = THROTTLE_UNLIMITED;
            } else if (_throttle_thrust_max < 0.0f) {
                _throttle_thrust_max = 0.0f;
            }
            break;

        case THROTTLE_UNLIMITED:
            // Throttle should exhibit normal flight behavior.
            // Servos should exhibit normal flight behavior.

            // initialize limits flags
            limit.roll_pitch = false;
            limit.yaw = false;
            limit.throttle_lower = false;
            limit.throttle_upper = false;

            // make sure the motors are spooling in the correct direction
            if (_spool_desired != DESIRED_THROTTLE_UNLIMITED) {
                _spool_mode = SPOOL_DOWN;
                break;
            }

            // set and increment ramp variables
            _spin_up_ratio = 1.0f;
            _throttle_thrust_max = get_current_limit_max_throttle();
            break;

        case SPOOL_DOWN:
            // Maximum throttle should move from maximum to minimum.
            // Servos should exhibit normal flight behavior.

            // initialize limits flags
            limit.roll_pitch = false;
            limit.yaw = false;
            limit.throttle_lower = false;
            limit.throttle_upper = false;

            // make sure the motors are spooling in the correct direction
            if (_spool_desired == DESIRED_THROTTLE_UNLIMITED) {
                _spool_mode = SPOOL_UP;
                break;
            }

            // set and increment ramp variables
            _spin_up_ratio = 1.0f;
            _throttle_thrust_max -= 1.0f/(_spool_up_time*_loop_rate);

            // constrain ramp value and update mode
            if (_throttle_thrust_max <= 0.0f){
                _throttle_thrust_max = 0.0f;
            }
            if (_throttle_thrust_max >= get_current_limit_max_throttle()) {
                _throttle_thrust_max = get_current_limit_max_throttle();
            } else if (is_zero(_throttle_thrust_max)) {
                _spool_mode = SPIN_WHEN_ARMED;
            }
            break;
    }
}

// passes throttle directly to all motors for ESC calibration.
//   throttle_input is in the range of 0 ~ 1 where 0 will send get_pwm_output_min() and 1 will send get_pwm_output_max()
void AP_MotorsMulticopter::set_throttle_passthrough_for_esc_calibration(float throttle_input)
{
    if (armed()) {
        uint16_t pwm_out = get_pwm_output_min() + constrain_float(throttle_input, 0.0f, 1.0f) * (get_pwm_output_max() - get_pwm_output_min());
        // send the pilot's input directly to each enabled motor
        for (uint16_t i=0; i < AP_MOTORS_MAX_NUM_MOTORS; i++) {
            if (motor_enabled[i]) {
                rc_write(i, pwm_out);
            }
        }
    }
}

// output a thrust to all motors that match a given motor mask. This
// is used to control tiltrotor motors in forward flight. Thrust is in
// the range 0 to 1
void AP_MotorsMulticopter::output_motor_mask(float thrust, uint8_t mask)
{
    for (uint8_t i=0; i<AP_MOTORS_MAX_NUM_MOTORS; i++) {
        if (motor_enabled[i]) {
            int16_t motor_out;
            if (mask & (1U<<i)) {
                motor_out = calc_thrust_to_pwm(thrust);
            } else {
                motor_out = get_pwm_output_min();
            }
            rc_write(i, motor_out);
        }
    }
}

// save parameters as part of disarming
void AP_MotorsMulticopter::save_params_on_disarm()
{
    // save hover throttle
    if (_throttle_hover_learn == HOVER_LEARN_AND_SAVE) {
        _throttle_hover.save();
    }
}