/*
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 .
*/
/*
Sailboat simulator class
see explanation of lift and drag explained here: https://en.wikipedia.org/wiki/Forces_on_sails
To-Do: add heel handling by calculating lateral force from wind vs gravity force from heel to arrive at roll rate or acceleration
*/
#include "SIM_Sailboat.h"
#include
#include
#include
namespace SITL {
#define STEERING_SERVO_CH 0 // steering controlled by servo output 1
#define MAINSAIL_SERVO_CH 3 // main sail controlled by servo output 4
Sailboat::Sailboat(const char *home_str, const char *frame_str) :
Aircraft(home_str, frame_str),
steering_angle_max(35),
turning_circle(1.8)
{
}
// calculate the lift and drag as values from 0 to 1
// given an apparent wind speed in m/s and angle-of-attack in degrees
void Sailboat::calc_lift_and_drag(float wind_speed, float angle_of_attack_deg, float& lift, float& drag) const
{
const uint16_t index_width_deg = 10;
const uint8_t index_max = ARRAY_SIZE(lift_curve) - 1;
// check extremes
if (angle_of_attack_deg <= 0.0f) {
lift = lift_curve[0];
drag = drag_curve[0];
} else if (angle_of_attack_deg >= index_max * index_width_deg) {
lift = lift_curve[index_max];
drag = drag_curve[index_max];
} else {
uint8_t index = constrain_int16(angle_of_attack_deg / index_width_deg, 0, index_max);
float remainder = angle_of_attack_deg - (index * index_width_deg);
lift = linear_interpolate(lift_curve[index], lift_curve[index+1], remainder, 0.0f, index_width_deg);
drag = linear_interpolate(drag_curve[index], drag_curve[index+1], remainder, 0.0f, index_width_deg);
}
// apply scaling by wind speed
lift *= wind_speed;
drag *= wind_speed;
}
// return turning circle (diameter) in meters for steering angle proportion in the range -1 to +1
float Sailboat::get_turn_circle(float steering) const
{
if (is_zero(steering)) {
return 0;
}
return turning_circle * sinf(radians(steering_angle_max)) / sinf(radians(steering * steering_angle_max));
}
// return yaw rate in deg/sec given a steering input (in the range -1 to +1) and speed in m/s
float Sailboat::get_yaw_rate(float steering, float speed) const
{
if (is_zero(steering) || is_zero(speed)) {
return 0;
}
float d = get_turn_circle(steering);
float c = M_PI * d;
float t = c / speed;
float rate = 360.0f / t;
return rate;
}
// return lateral acceleration in m/s/s given a steering input (in the range -1 to +1) and speed in m/s
float Sailboat::get_lat_accel(float steering, float speed) const
{
float yaw_rate = get_yaw_rate(steering, speed);
float accel = radians(yaw_rate) * speed;
return accel;
}
/*
update the sailboat simulation by one time step
*/
void Sailboat::update(const struct sitl_input &input)
{
// update wind
update_wind(input);
// in sailboats the steering controls the rudder, the throttle controls the main sail position
float steering = 2*((input.servos[STEERING_SERVO_CH]-1000)/1000.0f - 0.5f);
// calculate mainsail angle from servo output 4, 0 to 90 degrees
float mainsail_angle_bf = constrain_float((input.servos[MAINSAIL_SERVO_CH]-1000)/1000.0f * 90.0f, 0.0f, 90.0f);
// calculate apparent wind in earth-frame (this is the direction the wind is coming from)
// Note than the SITL wind direction is defined as the direction the wind is travelling to
// This is accounted for in these calculations
Vector3f wind_apparent_ef = wind_ef + velocity_ef;
const float wind_apparent_dir_ef = degrees(atan2f(wind_apparent_ef.y, wind_apparent_ef.x));
const float wind_apparent_speed = safe_sqrt(sq(wind_apparent_ef.x)+sq(wind_apparent_ef.y));
const float wind_apparent_dir_bf = wrap_180(wind_apparent_dir_ef - degrees(AP::ahrs().yaw));
// set RPM and airspeed from wind speed, allows to test RPM and Airspeed wind vane back end in SITL
rpm1 = wind_apparent_speed;
airspeed_pitot = wind_apparent_speed;
// calculate angle-of-attack from wind to mainsail
float aoa_deg = MAX(fabsf(wind_apparent_dir_bf) - mainsail_angle_bf, 0);
// calculate Lift force (perpendicular to wind direction) and Drag force (parallel to wind direction)
float lift_wf, drag_wf;
calc_lift_and_drag(wind_apparent_speed, aoa_deg, lift_wf, drag_wf);
// rotate lift and drag from wind frame into body frame
const float sin_rot_rad = sinf(radians(wind_apparent_dir_bf));
const float cos_rot_rad = cosf(radians(wind_apparent_dir_bf));
const float force_fwd = fabsf(lift_wf * sin_rot_rad) - (drag_wf * cos_rot_rad);
// how much time has passed?
float delta_time = frame_time_us * 1.0e-6f;
// speed in m/s in body frame
Vector3f velocity_body = dcm.transposed() * velocity_ef;
// speed along x axis, +ve is forward
float speed = velocity_body.x;
// yaw rate in degrees/s
float yaw_rate = get_yaw_rate(steering, speed);
gyro = Vector3f(0,0,radians(yaw_rate));
// update attitude
dcm.rotate(gyro * delta_time);
dcm.normalize();
// hull drag
float hull_drag = sq(speed) * 0.5f;
if (!is_positive(speed)) {
hull_drag *= -1.0f;
}
// accel in body frame due acceleration from sail and deceleration from hull friction
accel_body = Vector3f(force_fwd - hull_drag, 0, 0);
accel_body /= mass;
// add in accel due to direction change
accel_body.y += radians(yaw_rate) * speed;
// now in earth frame
Vector3f accel_earth = dcm * accel_body;
// we are on the ground, so our vertical accel is zero
accel_earth.z = 0;
// work out acceleration as seen by the accelerometers. It sees the kinematic
// acceleration (ie. real movement), plus gravity
accel_body = dcm.transposed() * (accel_earth + Vector3f(0, 0, -GRAVITY_MSS));
// new velocity vector
velocity_ef += accel_earth * delta_time;
// new position vector
position += velocity_ef * delta_time;
// update lat/lon/altitude
update_position();
time_advance();
// update magnetic field
update_mag_field_bf();
}
} // namespace SITL