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Fixed Wing: Compensate minimum sink rate based on weight ratio and air density #22049

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Fixed Wing: Compensate minimum sink rate based on weight ratio and air density #22049

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d6a770a
FixedWingPositionControl: compensate minimum sink rate for weight and…
RomanBapst Sep 6, 2023
f7d7a71
addressed review comments
RomanBapst Sep 11, 2023
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40 changes: 19 additions & 21 deletions src/modules/fw_pos_control/FixedwingPositionControl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -94,16 +94,21 @@ FixedwingPositionControl::init()
return true;
}

float FixedwingPositionControl::getMaximumClimbRate()
float FixedwingPositionControl::getWeightRatio()
{

float weight_factor = 1.0f;

if (_param_weight_base.get() > 0.0f && _param_weight_gross.get() > 0.0f) {
weight_factor = math::constrain(_param_weight_base.get() / _param_weight_gross.get(), MIN_WEIGHT_RATIO,
if (_param_weight_base.get() > FLT_EPSILON && _param_weight_gross.get() > FLT_EPSILON) {
weight_factor = math::constrain(_param_weight_gross.get() / _param_weight_base.get(), MIN_WEIGHT_RATIO,
MAX_WEIGHT_RATIO);
}

return weight_factor;
}

float FixedwingPositionControl::getMaximumClimbRate()
{
float climbrate_max = _param_fw_t_clmb_max.get();

const float density_min = _param_density_min.get();
Expand All @@ -116,7 +121,12 @@ float FixedwingPositionControl::getMaximumClimbRate()
climbrate_max = _param_fw_t_clmb_max.get() + density_gradient * delta_rho;
}

return climbrate_max * weight_factor;
return climbrate_max / getWeightRatio();
}

float FixedwingPositionControl::getMinimumSinkRate()
{
return _param_fw_t_sink_min.get() * sqrtf(getWeightRatio() * CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / _air_density);
}

int
Expand Down Expand Up @@ -147,7 +157,7 @@ FixedwingPositionControl::parameters_update()

// TECS parameters
_tecs.set_max_climb_rate(getMaximumClimbRate());
_tecs.set_max_sink_rate(_param_fw_t_sink_max.get());
_tecs.set_max_sink_rate(getMinimumSinkRate());
_tecs.set_min_sink_rate(_param_fw_t_sink_min.get());
_tecs.set_speed_weight(_param_fw_t_spdweight.get());
_tecs.set_equivalent_airspeed_trim(_param_fw_airspd_trim.get());
Expand Down Expand Up @@ -451,19 +461,12 @@ FixedwingPositionControl::adapt_airspeed_setpoint(const float control_interval,
load_factor_from_bank_angle = 1.0f / cosf(_att_sp.roll_body);
}

float weight_ratio = 1.0f;

if (_param_weight_base.get() > FLT_EPSILON && _param_weight_gross.get() > FLT_EPSILON) {
weight_ratio = math::constrain(_param_weight_gross.get() / _param_weight_base.get(), MIN_WEIGHT_RATIO,
MAX_WEIGHT_RATIO);
}

// Here we make sure that the set minimum airspeed is automatically adapted to the current load factor.
// The minimum airspeed is the controller limit (FW_AIRSPD_MIN, unless either in takeoff or landing) that should
// resemble the vehicles stall speed (CAS) with a 1g load plus some safety margin (as no controller tracks perfectly).
// Stall speed increases with the square root of the load factor: V_stall ~ sqrt(load_factor).
// The load_factor is composed of a term from the bank angle and a term from the weight ratio.
calibrated_min_airspeed *= sqrtf(load_factor_from_bank_angle * weight_ratio);
calibrated_min_airspeed *= sqrtf(load_factor_from_bank_angle * getWeightRatio());

// Aditional option to increase the min airspeed setpoint based on wind estimate for more stability in higher winds.
if (_airspeed_valid && _wind_valid && _param_fw_wind_arsp_sc.get() > FLT_EPSILON) {
Expand Down Expand Up @@ -2292,6 +2295,7 @@ FixedwingPositionControl::Run()
if (_vehicle_air_data_sub.update(&air_data)) {
_air_density = PX4_ISFINITE(air_data.rho) ? air_data.rho : _air_density;
_tecs.set_max_climb_rate(getMaximumClimbRate());
_tecs.set_min_sink_rate(getMinimumSinkRate());
}

if (_vehicle_land_detected_sub.updated()) {
Expand Down Expand Up @@ -2508,13 +2512,6 @@ float FixedwingPositionControl::calculateTrimThrottle(float throttle_min,
throttle_trim = _param_fw_thr_trim.get() + slope_above_trim * (airspeed_sp - _param_fw_airspd_trim.get());
}

float weight_ratio = 1.0f;

if (_param_weight_base.get() > FLT_EPSILON && _param_weight_gross.get() > FLT_EPSILON) {
weight_ratio = math::constrain(_param_weight_gross.get() / _param_weight_base.get(), MIN_WEIGHT_RATIO,
MAX_WEIGHT_RATIO);
}

float air_density_throttle_scale = 1.0f;

if (PX4_ISFINITE(_air_density)) {
Expand All @@ -2525,7 +2522,8 @@ float FixedwingPositionControl::calculateTrimThrottle(float throttle_min,
}

// compensate trim throttle for both weight and air density
return math::constrain(throttle_trim * sqrtf(weight_ratio) * air_density_throttle_scale, throttle_min, throttle_max);
return math::constrain(throttle_trim * sqrtf(getWeightRatio()) * air_density_throttle_scale, throttle_min,
throttle_max);
}

void
Expand Down
17 changes: 17 additions & 0 deletions src/modules/fw_pos_control/FixedwingPositionControl.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -488,7 +488,24 @@ class FixedwingPositionControl final : public ModuleBase<FixedwingPositionContro
*/
float get_terrain_altitude_takeoff(float takeoff_alt);

/**
* @brief Return the maximum climb rate achievable given the estimated air density and the vehicle weight.
* @return Maximum climbrate [m/s].
*/
float getMaximumClimbRate();

/**
* @brief Return the minimum sink rate achievable given the estimated air density and the vehicle weight.
* @return Minimum sink rate [m/s].
*/
float getMinimumSinkRate();

/**
* @brief Return the ratio of actual vehicle weight to vehicle base weight.
* @return Weight ratio.
*/
float getWeightRatio();

/**
* @brief Maps the manual control setpoint (pilot sticks) to height rate commands
*
Expand Down