ardupilot/APMrover2/AP_MotorsUGV.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/>.
 */

#include <AP_HAL/AP_HAL.h>
#include <AP_Math/AP_Math.h>
#include "SRV_Channel/SRV_Channel.h"
#include "AP_MotorsUGV.h"
#include "Rover.h"

extern const AP_HAL::HAL& hal;

// parameters for the motor class
const AP_Param::GroupInfo AP_MotorsUGV::var_info[] = {
    // @Param: PWM_TYPE
    // @DisplayName: Motor Output PWM type
    // @Description: This selects the output PWM type as regular PWM, OneShot, Brushed motor support using PWM (duty cycle) with separated direction signal, Brushed motor support with separate throttle and direction PWM (duty cyle)
    // @Values: 0:Normal,1:OneShot,2:OneShot125,3:BrushedWithRelay,4:BrushedBiPolar
    // @User: Advanced
    // @RebootRequired: True
    AP_GROUPINFO("PWM_TYPE", 1, AP_MotorsUGV, _pwm_type, PWM_TYPE_NORMAL),

    // @Param: PWM_FREQ
    // @DisplayName: Motor Output PWM freq for brushed motors
    // @Description: Motor Output PWM freq for brushed motors
    // @Units: kHz
    // @Range: 1 20
    // @Increment: 1
    // @User: Advanced
    // @RebootRequired: True
    AP_GROUPINFO("PWM_FREQ", 2, AP_MotorsUGV, _pwm_freq, 16),

    // @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", 3, AP_MotorsUGV, _disarm_disable_pwm, 0),

    // @Param: THR_MIN
    // @DisplayName: Throttle minimum
    // @Description: Throttle minimum percentage the autopilot will apply. This is useful for handling a deadzone around low throttle and for preventing internal combustion motors cutting out during missions.
    // @Units: %
    // @Range: 0 20
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("THR_MIN", 5, AP_MotorsUGV, _throttle_min, 0),

    // @Param: THR_MAX
    // @DisplayName: Throttle maximum
    // @Description: Throttle maximum percentage the autopilot will apply. This can be used to prevent overheating an ESC or motor on an electric rover
    // @Units: %
    // @Range: 30 100
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("THR_MAX", 6, AP_MotorsUGV, _throttle_max, 100),

    // @Param: SLEWRATE
    // @DisplayName: Throttle slew rate
    // @Description: Throttle slew rate as a percentage of total range per second. A value of 100 allows the motor to change over its full range in one second.  A value of zero disables the limit.  Note some NiMH powered rovers require a lower setting of 40 to reduce current demand to avoid brownouts.
    // @Units: %/s
    // @Range: 0 1000
    // @Increment: 1
    // @User: Standard
    AP_GROUPINFO("SLEWRATE", 8, AP_MotorsUGV, _slew_rate, 100),

    // @Param: THST_EXPO
    // @DisplayName: Thrust Curve Expo
    // @Description: Thrust curve exponent (-1 to +1 with 0 being linear)
    // @Range: -1.0 1.0
    // @User: Advanced
    AP_GROUPINFO("THST_EXPO", 9, AP_MotorsUGV, _thrust_curve_expo, 0.0f),

    // @Param: VEC_THR_BASE
    // @DisplayName: Vector thrust throttle base
    // @Description: Throttle level above which steering is scaled down when using vector thrust.  zero to disable vectored thrust
    // @Units: %
    // @Range: 0 100
    // @User: Advanced
    AP_GROUPINFO("VEC_THR_BASE", 10, AP_MotorsUGV, _vector_throttle_base, 0.0f),

    AP_GROUPEND
};

AP_MotorsUGV::AP_MotorsUGV(AP_ServoRelayEvents &relayEvents) :
        _relayEvents(relayEvents)
{
    AP_Param::setup_object_defaults(this, var_info);
}

void AP_MotorsUGV::init()
{
    // setup servo ouput
    setup_servo_output();

    // setup pwm type
    setup_pwm_type();

    // set safety output
    setup_safety_output();
}

// setup output in case of main CPU failure
void AP_MotorsUGV::setup_safety_output()
{
    if (_pwm_type == PWM_TYPE_BRUSHED_WITH_RELAY) {
        // set trim to min to set duty cycle range (0 - 100%) to servo range
        SRV_Channels::set_trim_to_min_for(SRV_Channel::k_throttle);
        SRV_Channels::set_trim_to_min_for(SRV_Channel::k_throttleLeft);
        SRV_Channels::set_trim_to_min_for(SRV_Channel::k_throttleRight);
    }

    if (_disarm_disable_pwm) {
        // throttle channels output zero pwm (i.e. no signal)
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttle, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttleLeft, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttleRight, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
    } else {
        // throttle channels output trim values (because rovers will go backwards if set to MIN)
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttle, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttleLeft, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
        SRV_Channels::set_safety_limit(SRV_Channel::k_throttleRight, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
    }

    // stop sending pwm if main CPU fails
    SRV_Channels::set_failsafe_limit(SRV_Channel::k_throttle, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
    SRV_Channels::set_failsafe_limit(SRV_Channel::k_throttleLeft, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
    SRV_Channels::set_failsafe_limit(SRV_Channel::k_throttleRight, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
}

// setup servo output ranges
void AP_MotorsUGV::setup_servo_output()
{
    // k_steering are limited to -45;45 degree
    SRV_Channels::set_angle(SRV_Channel::k_steering, SERVO_MAX);

    // k_throttle are in power percent so -100 ... 100
    SRV_Channels::set_angle(SRV_Channel::k_throttle, 100);

    // skid steering left/right throttle as -1000 to 1000 values
    SRV_Channels::set_angle(SRV_Channel::k_throttleLeft,  1000);
    SRV_Channels::set_angle(SRV_Channel::k_throttleRight, 1000);

    // k_motor1, k_motor2 and k_motor3 are in power percent so -100 ... 100
    SRV_Channels::set_angle(SRV_Channel::k_motor1, 100);
    SRV_Channels::set_angle(SRV_Channel::k_motor2, 100);
    SRV_Channels::set_angle(SRV_Channel::k_motor3, 100);
}

// set steering as a value from -4500 to +4500
//   apply_scaling should be set to false for manual modes where
//   no scaling by speed or angle should be performed
void AP_MotorsUGV::set_steering(float steering, bool apply_scaling)
{
    _steering = constrain_float(steering, -4500.0f, 4500.0f);
    _scale_steering = apply_scaling;
}

// set throttle as a value from -100 to 100
void AP_MotorsUGV::set_throttle(float throttle)
{
    // only allow setting throttle if armed
    if (!hal.util->get_soft_armed()) {
        return;
    }

    // check throttle is between -_throttle_max and  +_throttle_max
    _throttle = constrain_float(throttle, -_throttle_max, _throttle_max);
}

// get slew limited throttle
// used by manual mode to avoid bad steering behaviour during transitions from forward to reverse
// same as private slew_limit_throttle method (see below) but does not update throttle state
float AP_MotorsUGV::get_slew_limited_throttle(float throttle, float dt) const
{
    if (_slew_rate <= 0) {
        return throttle;
    }

    const float throttle_change_max = MAX(1.0f, (float)_slew_rate * dt);
    return constrain_float(throttle, _throttle_prev - throttle_change_max, _throttle_prev + throttle_change_max);
}

/*
  work out if skid steering is available
 */
bool AP_MotorsUGV::have_skid_steering() const
{
    if (SRV_Channels::function_assigned(SRV_Channel::k_throttleLeft) &&
        SRV_Channels::function_assigned(SRV_Channel::k_throttleRight)) {
        return true;
    }
    return false;
}

// returns true if omni rover
bool AP_MotorsUGV::is_omni_rover() const
{
    if (SRV_Channels::function_assigned(SRV_Channel::k_motor1) &&
        SRV_Channels::function_assigned(SRV_Channel::k_motor2) &&
        SRV_Channels::function_assigned(SRV_Channel::k_motor3)) {
        return true;
    }
    return false;
}

void AP_MotorsUGV::output(bool armed, float ground_speed, float dt)
{
    // soft-armed overrides passed in armed status
    if (!hal.util->get_soft_armed()) {
        armed = false;
        _throttle = 0.0f;
    }

    // sanity check parameters
    sanity_check_parameters();

    // clear and set limits based on input (limit flags may be set again by output_regular or output_skid_steering methods)
    set_limits_from_input(armed, _steering, _throttle);

    // slew limit throttle
    slew_limit_throttle(dt);

    // output for regular steering/throttle style frames
    output_regular(armed, ground_speed, _steering, _throttle);

    // output for omni style frames
    output_omni(armed, _steering, _throttle);

    // output for skid steering style frames
    output_skid_steering(armed, _steering, _throttle);

    // send values to the PWM timers for output
    SRV_Channels::calc_pwm();
    SRV_Channels::cork();
    SRV_Channels::output_ch_all();
    SRV_Channels::push();
}

// test steering or throttle output as a percentage of the total (range -100 to +100)
// used in response to DO_MOTOR_TEST mavlink command
bool AP_MotorsUGV::output_test_pct(motor_test_order motor_seq, float pct)
{
    // check if the motor_seq is valid
    if (motor_seq > MOTOR_TEST_THROTTLE_RIGHT) {
        return false;
    }
    pct = constrain_float(pct, -100.0f, 100.0f);

    switch (motor_seq) {
        case MOTOR_TEST_THROTTLE: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttle)) {
                return false;
            }
            output_throttle(SRV_Channel::k_throttle, pct);
            break;
        }
        case MOTOR_TEST_STEERING: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_steering)) {
                return false;
            }
            SRV_Channels::set_output_scaled(SRV_Channel::k_steering, pct * 45.0f);
            break;
        }
        case MOTOR_TEST_THROTTLE_LEFT: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttleLeft)) {
                return false;
            }
            output_throttle(SRV_Channel::k_throttleLeft, pct);
            break;
        }
        case MOTOR_TEST_THROTTLE_RIGHT: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttleRight)) {
                return false;
            }
            output_throttle(SRV_Channel::k_throttleRight, pct);
            break;
        }
        default:
            return false;
    }
    SRV_Channels::calc_pwm();
    SRV_Channels::cork();
    SRV_Channels::output_ch_all();
    SRV_Channels::push();
    return true;
}

// test steering or throttle output using a pwm value
bool AP_MotorsUGV::output_test_pwm(motor_test_order motor_seq, float pwm)
{
    // check if the motor_seq is valid
    if (motor_seq > MOTOR_TEST_THROTTLE_RIGHT) {
        return false;
    }
    switch (motor_seq) {
        case MOTOR_TEST_THROTTLE: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttle)) {
                return false;
            }
            SRV_Channels::set_output_pwm(SRV_Channel::k_throttle, pwm);
            break;
        }
        case MOTOR_TEST_STEERING: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_steering)) {
                return false;
            }
            SRV_Channels::set_output_pwm(SRV_Channel::k_steering, pwm);
            break;
        }
        case MOTOR_TEST_THROTTLE_LEFT: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttleLeft)) {
                return false;
            }
            SRV_Channels::set_output_pwm(SRV_Channel::k_throttleLeft, pwm);
            break;
        }
        case MOTOR_TEST_THROTTLE_RIGHT: {
            if (!SRV_Channels::function_assigned(SRV_Channel::k_throttleRight)) {
                return false;
            }
            SRV_Channels::set_output_pwm(SRV_Channel::k_throttleRight, pwm);
            break;
        }
        default:
            return false;
    }
    SRV_Channels::calc_pwm();
    SRV_Channels::cork();
    SRV_Channels::output_ch_all();
    SRV_Channels::push();
    return true;
}

//  returns true if checks pass, false if they fail.  report should be true to send text messages to GCS
bool AP_MotorsUGV::pre_arm_check(bool report) const
{
    // check if both regular and skid steering functions have been defined
    if (SRV_Channels::function_assigned(SRV_Channel::k_throttleLeft) &&
        SRV_Channels::function_assigned(SRV_Channel::k_throttleRight) &&
        SRV_Channels::function_assigned(SRV_Channel::k_throttle) &&
        SRV_Channels::function_assigned(SRV_Channel::k_steering)) {
        if (report) {
            gcs().send_text(MAV_SEVERITY_CRITICAL, "PreArm: regular AND skid steering configured");
        }
        return false;
    }
    // check if only one of skid-steering output has been configured
    if (SRV_Channels::function_assigned(SRV_Channel::k_throttleLeft) != SRV_Channels::function_assigned(SRV_Channel::k_throttleRight)) {
        if (report) {
            gcs().send_text(MAV_SEVERITY_CRITICAL, "PreArm: check skid steering config");
        }
        return false;
    }
    // check if only one of throttle or steering outputs has been configured
    if (SRV_Channels::function_assigned(SRV_Channel::k_throttle) != SRV_Channels::function_assigned(SRV_Channel::k_steering)) {
        if (report) {
            gcs().send_text(MAV_SEVERITY_CRITICAL, "PreArm: check steering and throttle config");
        }
        return false;
    }
    // check if only one of the omni rover outputs has been configured
    if ((SRV_Channels::function_assigned(SRV_Channel::k_motor1)) != (SRV_Channels::function_assigned(SRV_Channel::k_motor2)) ||
        (SRV_Channels::function_assigned(SRV_Channel::k_motor1)) != (SRV_Channels::function_assigned(SRV_Channel::k_motor3)) ||
        (SRV_Channels::function_assigned(SRV_Channel::k_motor2)) != (SRV_Channels::function_assigned(SRV_Channel::k_motor3))) {
        if (report) {
            gcs().send_text(MAV_SEVERITY_CRITICAL, "PreArm: check motor 1, motor2 and motor3 config");
        }
    }
    return true;
}

// sanity check parameters
void AP_MotorsUGV::sanity_check_parameters()
{
    _throttle_min = constrain_int16(_throttle_min, 0, 20);
    _throttle_max = constrain_int16(_throttle_max, 30, 100);
    _vector_throttle_base = constrain_float(_vector_throttle_base, 0.0f, 100.0f);
}

// setup pwm output type
void AP_MotorsUGV::setup_pwm_type()
{
    switch (_pwm_type) {
    case PWM_TYPE_ONESHOT:
        hal.rcout->set_output_mode(0xFFFF, AP_HAL::RCOutput::MODE_PWM_ONESHOT);
        break;
    case PWM_TYPE_ONESHOT125:
        hal.rcout->set_output_mode(0xFFFF, AP_HAL::RCOutput::MODE_PWM_ONESHOT125);
        break;
    case PWM_TYPE_BRUSHED_WITH_RELAY:
    case PWM_TYPE_BRUSHED_BIPOLAR:
        hal.rcout->set_output_mode(0xFFFF, AP_HAL::RCOutput::MODE_PWM_BRUSHED);
        /*
         * Group 0: channels 0 1
         * Group 1: channels 4 5 6 7
         * Group 2: channels 2 3
         */
        // TODO : See if we can seperate frequency between groups
        hal.rcout->set_freq((1UL << 0), static_cast<uint16_t>(_pwm_freq * 1000));  // Steering group
        hal.rcout->set_freq((1UL << 2), static_cast<uint16_t>(_pwm_freq * 1000));  // Throttle group
        break;
    default:
        // do nothing
        break;
    }
}

// output to regular steering and throttle channels
void AP_MotorsUGV::output_regular(bool armed, float ground_speed, float steering, float throttle)
{
    // output to throttle channels
    if (armed) {
        if (_scale_steering) {
            // vectored thrust handling
            if (have_vectored_thrust()) {
                if (fabsf(throttle) > _vector_throttle_base) {
                    // scale steering down linearly as throttle increases above _vector_throttle_base
                    steering *= constrain_float(_vector_throttle_base / fabsf(throttle), 0.0f, 1.0f);
                }
            } else {
                // scale steering down as speed increase above 1m/s
                if (fabsf(ground_speed) > 1.0f) {
                    steering *= (1.0f / fabsf(ground_speed));
                } else {
                    // regular steering rover at low speed so set limits to stop I-term build-up in controllers
                    if (!have_skid_steering()) {
                        limit.steer_left = true;
                        limit.steer_right = true;
                    }
                }
                // reverse steering output if backing up
                if (is_negative(ground_speed)) {
                    steering *= -1.0f;
                }
            }
        } else {
            // reverse steering direction when backing up
            if (is_negative(throttle)) {
                steering *= -1.0f;
            }
        }
        output_throttle(SRV_Channel::k_throttle, throttle);
    } else {
        // handle disarmed case
        if (_disarm_disable_pwm) {
            SRV_Channels::set_output_limit(SRV_Channel::k_throttle, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
        } else {
            SRV_Channels::set_output_limit(SRV_Channel::k_throttle, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
        }
    }

    // always allow steering to move
    SRV_Channels::set_output_scaled(SRV_Channel::k_steering, steering);
}

// output for omni style frames
void AP_MotorsUGV::output_omni(bool armed, float steering, float throttle)
{
    if (!is_omni_rover()) {
        return;
    }
    if (armed) {
        // scale throttle and steering to a 1000 to 2000 range for motor throttle calculations
        const float scaled_throttle = (throttle - (100)) * (2000 - 1000) / (-100 - (100)) + 1000;
        const float scaled_steering = (steering - (-4500)) * (2000 - 1000) / (4500 - (-4500)) + 1000;

        // calculate desired vehicle speed and direction
        // 1500 is a place-holder value for lateral movement input
        const float magnitude = safe_sqrt((scaled_throttle*scaled_throttle)+(1500*1500));
        const float theta = atan2f(scaled_throttle,1500);

        // calculate X and Y vectors using the following the equations: vx = cos(?) * magnitude and vy = sin(?) * magnitude
        const float Vx = -(cosf(theta)*magnitude);
        const float Vy = -(sinf(theta)*magnitude);

        // calculate output throttle for each motor and scale it back to a -100 to 100 range
        // calculations are done using the following equations: MOTOR1 = <20>vx, MOTOR2 = 0.5 * v <20> v(3/2) * vy, MOTOR 3 = 0.5 * vx + v(3/2) * vy
        // safe_sqrt((3)/2) used because the motors are 120 degrees apart in the frame, this setup is mandatory
        const int16_t motor_1 = (((-Vx) + scaled_steering) - (2500)) * (100 - (-100)) / (3500 - (2500)) + (-100);
        const int16_t motor_2 = ((((0.5*Vx)-((safe_sqrt(3)/2)*Vy)) + scaled_steering) - (1121)) * (100 - (-100)) / (2973 - (1121)) + (-100);
        const int16_t motor_3 = ((((0.5*Vx)+((safe_sqrt(3)/2)*Vy)) + scaled_steering) - (-1468)) * (100 - (-100)) / (383 - (-1468)) + (-100);

        // send pwm value to each motor
        output_throttle(SRV_Channel::k_motor1, motor_1);
        output_throttle(SRV_Channel::k_motor2, motor_2);
        output_throttle(SRV_Channel::k_motor3, motor_3);
    } else {
        // handle disarmed case
        if (_disarm_disable_pwm) {
            SRV_Channels::set_output_limit(SRV_Channel::k_motor1, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
            SRV_Channels::set_output_limit(SRV_Channel::k_motor2, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
            SRV_Channels::set_output_limit(SRV_Channel::k_motor3, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
        } else {
            SRV_Channels::set_output_limit(SRV_Channel::k_motor1, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
            SRV_Channels::set_output_limit(SRV_Channel::k_motor2, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
            SRV_Channels::set_output_limit(SRV_Channel::k_motor3, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
        }
    }
}

// output to skid steering channels
void AP_MotorsUGV::output_skid_steering(bool armed, float steering, float throttle)
{
    if (!have_skid_steering()) {
        return;
    }

    // handle simpler disarmed case
    if (!armed) {
        if (_disarm_disable_pwm) {
            SRV_Channels::set_output_limit(SRV_Channel::k_throttleLeft, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
            SRV_Channels::set_output_limit(SRV_Channel::k_throttleRight, SRV_Channel::SRV_CHANNEL_LIMIT_ZERO_PWM);
        } else {
            SRV_Channels::set_output_limit(SRV_Channel::k_throttleLeft, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
            SRV_Channels::set_output_limit(SRV_Channel::k_throttleRight, SRV_Channel::SRV_CHANNEL_LIMIT_TRIM);
        }
        return;
    }

    // skid steering mixer
    float steering_scaled = steering / 4500.0f; // steering scaled -1 to +1
    float throttle_scaled = throttle / 100.0f;  // throttle scaled -1 to +1

    // apply constraints
    steering_scaled = constrain_float(steering_scaled, -1.0f, 1.0f);
    throttle_scaled = constrain_float(throttle_scaled, -1.0f, 1.0f);

    // check for saturation and scale back throttle and steering proportionally
    const float saturation_value = fabsf(steering_scaled) + fabsf(throttle_scaled);
    if (saturation_value > 1.0f) {
        steering_scaled = steering_scaled / saturation_value;
        throttle_scaled = throttle_scaled / saturation_value;
    }

    // add in throttle and steering
    const float motor_left = throttle_scaled + steering_scaled;
    const float motor_right = throttle_scaled - steering_scaled;

    // send pwm value to each motor
    output_throttle(SRV_Channel::k_throttleLeft, 100.0f * motor_left);
    output_throttle(SRV_Channel::k_throttleRight, 100.0f * motor_right);
}

// output throttle value to main throttle channel, left throttle or right throttle.  throttle should be scaled from -100 to 100
void AP_MotorsUGV::output_throttle(SRV_Channel::Aux_servo_function_t function, float throttle)
{
    // sanity check servo function
    if (function != SRV_Channel::k_throttle && function != SRV_Channel::k_throttleLeft && function != SRV_Channel::k_throttleRight && function != SRV_Channel::k_motor1 && function != SRV_Channel::k_motor2 && function != SRV_Channel::k_motor3) {
        return;
    }

    // constrain and scale output
    throttle = get_scaled_throttle(throttle);

    // set relay if necessary
    if (_pwm_type == PWM_TYPE_BRUSHED_WITH_RELAY) {
        // find the output channel, if not found return
        const SRV_Channel *out_chan = SRV_Channels::get_channel_for(function);
        if (out_chan == nullptr) {
            return;
        }
        const int8_t reverse_multiplier = out_chan->get_reversed() ? -1 : 1;
        bool relay_high = is_negative(reverse_multiplier * throttle);

        switch (function) {
            case SRV_Channel::k_throttle:
            case SRV_Channel::k_throttleLeft:
            case SRV_Channel::k_motor1:
                _relayEvents.do_set_relay(0, relay_high);
                break;
            case SRV_Channel::k_throttleRight:
            case SRV_Channel::k_motor2:
                _relayEvents.do_set_relay(1, relay_high);
                break;
            case SRV_Channel::k_motor3:
                _relayEvents.do_set_relay(2, relay_high);
                break;
            default:
                // do nothing
                break;
        }
        // invert the output to always have positive value calculated by calc_pwm
        throttle = reverse_multiplier * fabsf(throttle);
    }

    // output to servo channel
    switch (function) {
        case SRV_Channel::k_throttle:
        case SRV_Channel::k_motor1:
        case SRV_Channel::k_motor2:
        case SRV_Channel::k_motor3:
            SRV_Channels::set_output_scaled(function,  throttle);
            break;
        case SRV_Channel::k_throttleLeft:
        case SRV_Channel::k_throttleRight:
            SRV_Channels::set_output_scaled(function,  throttle * 10.0f);
            break;
        default:
            // do nothing
            break;
    }
}

// slew limit throttle for one iteration
void AP_MotorsUGV::slew_limit_throttle(float dt)
{
    const float throttle_orig = _throttle;
    _throttle = get_slew_limited_throttle(_throttle, dt);
    if (throttle_orig > _throttle) {
        limit.throttle_upper = true;
    } else if (throttle_orig < _throttle) {
        limit.throttle_lower = true;
    }
    _throttle_prev = _throttle;
}

// set limits based on steering and throttle input
void AP_MotorsUGV::set_limits_from_input(bool armed, float steering, float throttle)
{
    // set limits based on inputs
    limit.steer_left = !armed || (steering <= -4500.0f);
    limit.steer_right = !armed || (steering >= 4500.0f);
    limit.throttle_lower = !armed || (throttle <= -_throttle_max);
    limit.throttle_upper = !armed || (throttle >= _throttle_max);
}

// scale a throttle using the _throttle_min and _thrust_curve_expo parameters.  throttle should be in the range -100 to +100
float AP_MotorsUGV::get_scaled_throttle(float throttle) const
{
    // exit immediately if throttle is zero
    if (is_zero(throttle)) {
        return throttle;
    }

    // scale using throttle_min
    if (_throttle_min > 0) {
        if (is_negative(throttle)) {
            throttle = -_throttle_min + (throttle * ((100.0f - _throttle_min) / 100.0f));
        } else {
            throttle = _throttle_min + (throttle * ((100.0f - _throttle_min) / 100.0f));
        }
    }

    // skip further scaling if thrust curve disabled or invalid
    if (is_zero(_thrust_curve_expo) || (_thrust_curve_expo > 1.0f) || (_thrust_curve_expo < -1.0f)) {
        return throttle;
    }

    // calculate scaler
    const float sign = (throttle < 0.0f) ? -1.0f : 1.0f;
    const float throttle_pct = constrain_float(throttle, -100.0f, 100.0f) / 100.0f;
    return 100.0f * sign * ((_thrust_curve_expo - 1.0f) + safe_sqrt((1.0f - _thrust_curve_expo) * (1.0f - _thrust_curve_expo) + 4.0f * _thrust_curve_expo * fabsf(throttle_pct))) / (2.0f * _thrust_curve_expo);
}