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Code Issues Releases Activity

OP-1352 redesigned course calculation to take complete wind vector into account

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This commit is contained in:
Corvus Corax 2014-05-17 16:52:22 +02:00
parent 9306cbc7c5
commit 5544e9c984
2 changed files with 157 additions and 86 deletions
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233
flight/modules/FixedWingPathFollower/fixedwingpathfollower.c
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@ -63,6 +63,7 @@
#include "taskinfo.h"
#include <pios_struct_helper.h>
#include "sin_lookup.h"
#include "paths.h"
#include "CoordinateConversions.h"
@ -85,6 +86,7 @@ static void updatePathVelocity();
static uint8_t updateFixedDesiredAttitude();
static void updateFixedAttitude();
static void airspeedStateUpdatedCb(UAVObjEvent *ev);
static bool correctCourse(float *C, float *V, float *F, float s);
/**
* Initialise the module, called on startup
@ -316,8 +318,10 @@ static void updatePathVelocity()
// scale to correct length
float l = sqrtf(velocityDesired.North * velocityDesired.North + velocityDesired.East * velocityDesired.East);
if (l > 0.0f) {
velocityDesired.North *= groundspeed / l;
velocityDesired.East *= groundspeed / l;
}
float downError = altitudeSetpoint - positionState.Down;
velocityDesired.Down = downError * fixedwingpathfollowerSettings.VerticalPosP;
@ -372,8 +376,7 @@ static uint8_t updateFixedDesiredAttitude()
AirspeedStateData airspeedState;
SystemSettingsData systemSettings;
float groundspeedState;
float groundspeedDesired;
float groundspeedProjection;
float indicatedAirspeedState;
float indicatedAirspeedDesired;
float airspeedError;
@ -384,13 +387,9 @@ static uint8_t updateFixedDesiredAttitude()
float descentspeedError;
float powerCommand;
float airspeedVector[2];
float fluidMovement[2];
float heading;
float headingError;
float correctedHeading;
float course;
float courseComponent[2];
float correctedCourse;
float courseError;
float courseCommand;
@ -405,60 +404,47 @@ static uint8_t updateFixedDesiredAttitude()
SystemSettingsGet(&systemSettings);
/**
* Calculate where we are heading and why (wind issues)
* Compute speed error and course
*/
heading = RAD2DEG(atan2f(velocityState.East, velocityState.North));
headingError = heading - attitudeState.Yaw;
if (headingError < -180.0f) {
headingError += 360.0f;
}
if (headingError > 180.0f) {
headingError -= 360.0f;
}
// Error condition: wind speed is higher than airspeed. We are forced backwards!
// missing sensors for airspeed-direction we have to assume within reasonable error
// that measured airspeed is always the component in forward pointing direction
// this vector is normalized
airspeedVector[0] = cos_lookup_deg(attitudeState.Yaw);
airspeedVector[1] = sin_lookup_deg(attitudeState.Yaw);
// Current ground speed projected in forward direction
groundspeedProjection = velocityState.North * airspeedVector[0] + velocityState.East * airspeedVector[1];
// note that airspeedStateBias is ( calibratedAirspeed - groundspeedProjection ) at the time of measurement,
// but thanks to accelerometers, groundspeedProjection reacts faster to changes in direction
// than airspeed and gps sensors alone
indicatedAirspeedState = groundspeedProjection + indicatedAirspeedStateBias;
// fluidMovement is a vector describing the aproximate movement vector in surrounding fluid (2d)
fluidMovement[0] = velocityState.North - (indicatedAirspeedState * airspeedVector[0]);
fluidMovement[1] = velocityState.East - (indicatedAirspeedState * airspeedVector[1]);
courseComponent[0] = velocityDesired.North - fluidMovement[0];
courseComponent[1] = velocityDesired.East - fluidMovement[1];
indicatedAirspeedDesired = boundf(sqrtf(courseComponent[0] * courseComponent[0] + courseComponent[1] * courseComponent[1]),
fixedwingpathfollowerSettings.HorizontalVelMin,
fixedwingpathfollowerSettings.HorizontalVelMax);
// if we could fly at arbitrary speeds, we'd just have to move into courseComponent and we'd be fine
// unfortunately we bound by min and max speed, so we need to calculate the correct course to meet
// at least the velocityDesired vector direction at our current speed
bool valid = correctCourse(courseComponent, (float *)&velocityDesired.North, fluidMovement, indicatedAirspeedDesired);
// Error condition: wind speed too high, we can't go where we want anymore
fixedwingpathfollowerStatus.Errors.Wind = 0;
if ((headingError > fixedwingpathfollowerSettings.Safetymargins.Wind ||
headingError < -fixedwingpathfollowerSettings.Safetymargins.Wind) &&
if ((!valid) &&
fixedwingpathfollowerSettings.Safetymargins.Wind > 0.5f) { // alarm switched on
// we are flying backwards
fixedwingpathfollowerStatus.Errors.Wind = 1;
result = 0;
}
/**
* Compute speed error (required for thrust and pitch)
*/
// Current ground speed
groundspeedState = sqrtf(velocityState.East * velocityState.East + velocityState.North * velocityState.North);
// assume groundspeed is negative if we are flying backwards (otherwise increasing airspeed would reduce groundspeed)
if (fabsf(headingError) > 90.0f) {
groundspeedState = -groundspeedState;
}
// note that airspeedStateBias is ( calibratedAirspeed - groundSpeed ) at the time of measurement,
// but thanks to accelerometers, groundspeed reacts faster to changes in direction
// than airspeed and gps sensors alone
indicatedAirspeedState = groundspeedState + indicatedAirspeedStateBias;
// fluidMovement is a vector describing the aproximate movement vector in surrounding fluid (2d)
fluidMovement[0] = indicatedAirspeedState * cosf(DEG2RAD(attitudeState.Yaw));
fluidMovement[1] = indicatedAirspeedState * sinf(DEG2RAD(attitudeState.Yaw));
// Desired ground speed
groundspeedDesired = sqrtf(velocityDesired.North * velocityDesired.North + velocityDesired.East * velocityDesired.East);
// take negative speeds into account (if we are supposed to go the opposite way)
// this has two advantages:
// 1. it reduces speed to minimum for tight turns -- reducing speed = turn quicker - especially since we pull up to reduce speed ;)
// 2. in the unlikely case that we can fly backwards in strong headwind, we will - leads to awesome position hold ;)
if ((velocityDesired.North * fluidMovement[0] + velocityDesired.East * fluidMovement[1]) < 0.0f) { // difference >90 degrees
groundspeedDesired = -groundspeedDesired;
}
indicatedAirspeedDesired = boundf(groundspeedDesired + indicatedAirspeedStateBias,
fixedwingpathfollowerSettings.HorizontalVelMin,
fixedwingpathfollowerSettings.HorizontalVelMax);
// Airspeed error
airspeedError = indicatedAirspeedDesired - indicatedAirspeedState;
@ -489,16 +475,10 @@ static uint8_t updateFixedDesiredAttitude()
result = 0;
}
if (indicatedAirspeedState < 1e-6f) {
// prevent division by zero, abort without controlling anything. This guidance mode is not suited for takeoff or touchdown, or handling stationary planes
// also we cannot handle planes flying backwards, lets just wait until the nose drops
fixedwingpathfollowerStatus.Errors.Lowspeed = 1;
return 0;
}
/**
* Compute desired thrust command
*/
// compute saturated integral error thrust response. Make integral leaky for better performance. Approximately 30s time constant.
if (fixedwingpathfollowerSettings.PowerPI.Ki > 0) {
powerIntegral = boundf(powerIntegral + -descentspeedError * dT,
@ -595,26 +575,25 @@ static uint8_t updateFixedDesiredAttitude()
/**
* Compute desired roll command
*/
// Calculate wind corrected heading angle - this approach avoids oscillation at high airspeed but low groundspeed situations (headwind)
correctedHeading = RAD2DEG(atan2f(velocityState.East + fluidMovement[1], velocityState.North + fluidMovement[0]));
course = RAD2DEG(atan2f(velocityDesired.East, velocityDesired.North));
if (groundspeedDesired >= 0.0f || groundspeedDesired <= fixedwingpathfollowerSettings.HorizontalVelMin - indicatedAirspeedStateBias) {
courseComponent[0] = 2.0f *indicatedAirspeedState *cosf(DEG2RAD(course));
courseComponent[1] = 2.0f *indicatedAirspeedState *sinf(DEG2RAD(course));
} else { // small negative groundspeeds can be achieved if flying slowly into head wind - this allows hovering on the spot ;)
courseComponent[0] = -groundspeedDesired *cosf(DEG2RAD(course));
courseComponent[1] = -groundspeedDesired *sinf(DEG2RAD(course));
}
correctedCourse = RAD2DEG(atan2f(courseComponent[1] + fluidMovement[1], courseComponent[0] + fluidMovement[0]));
courseError = correctedCourse - correctedHeading;
courseError = RAD2DEG(atan2f(courseComponent[1], courseComponent[0])) - attitudeState.Yaw;
if (courseError < -180.0f) {
courseError += 360.0f;
courseError += 360;
}
if (courseError > 180.0f) {
courseError -= 360;
}
// overlap calculation. Theres a dead zone behind the craft where the
// counter-yawing of some craft while rolling could render a desired right
// turn into a desired left turn. Making the turn direction based on
// current roll angle keeps the plane committed to a direction once chosen
if (courseError < -180.0f + (fixedwingpathfollowerSettings.ReverseCourseOverlap * 0.5f)
&& attitudeState.Roll > 0.0f) {
courseError += 360.0f;
}
if (courseError > 180.0f - (fixedwingpathfollowerSettings.ReverseCourseOverlap * 0.5f)
&& attitudeState.Roll < 0.0f) {
courseError -= 360.0f;
}
@ -668,10 +647,100 @@ static void airspeedStateUpdatedCb(__attribute__((unused)) UAVObjEvent *ev)
AirspeedStateGet(&airspeedState);
VelocityStateGet(&velocityState);
float groundspeed = sqrtf(velocityState.East * velocityState.East + velocityState.North * velocityState.North);
float airspeedVector[2];
float yaw;
AttitudeStateYawGet(&yaw);
airspeedVector[0] = cos_lookup_deg(yaw);
airspeedVector[1] = sin_lookup_deg(yaw);
// vector projection of groundspeed on airspeed vector to handle both forward and backwards movement
float groundspeedProjection = velocityState.North * airspeedVector[0] + velocityState.East * airspeedVector[1];
indicatedAirspeedStateBias = airspeedState.CalibratedAirspeed - groundspeed;
// note - we do fly by Indicated Airspeed (== calibrated airspeed)
// however since airspeed is updated less often than groundspeed, we use sudden changes to groundspeed to offset the airspeed by the same measurement.
// warning - deliberately messed up airspeed sensor value to see if course calculation is coping with crappy sensor
// do not let this pass the review ;)
indicatedAirspeedStateBias = airspeedState.CalibratedAirspeed - groundspeedProjection;
// note - we do fly by Indicated Airspeed (== calibrated airspeed) however
// since airspeed is updated less often than groundspeed, we use sudden
// changes to groundspeed to offset the airspeed by the same measurement.
// This has a side effect that in the absence of any airspeed updates, the
// pathfollower will fly using groundspeed.
}
/**
* Function to correct course C based on airspeed s, fluid movement F and desired movement vector V
*/
static bool correctCourse(float *C, float *V, float *F, float s)
{
// approach:
// let Sc be a circle around origin marking possible movement vectors
// of the craft with airspeed s
// let Vl be a line through the origin along movement vector V
// let Wl be a line parallel to Vl where for any point v on line Vl
// there is a point w on WL with w = v - F
// then any intersecting point between Sc and Wl is a course which
// results in a movement vector k*V
// if there is no intersection point, S is insufficient to compensate
// for F and we better fly in direction of V (thus having wind drift
// but at least making progress orthogonal to wind)
s = fabsf(s);
float f = sqrtf(F[0] * F[0] + F[1] * F[1]);
// normalize V
float v = sqrtf(V[0] * V[0] + V[1] * V[1]);
if (v < 1e-6f) {
// if we aren't supposed to move, turn into the wind (this allows hovering)
C[0] = -F[0];
C[1] = -F[1];
return fabsf(f - s) < 1e-3f; // returns true if a hover is actually intended
}
float Vn[2] = { V[0] / v, V[1] / v };
// project F on V
float fp = F[0] * Vn[0] + F[1] * Vn[1];
// find component of F orthogonal to V (distance between V and W)
float Fo[2] = { F[0] - (fp * Vn[0]), F[1] - (fp * Vn[1]) };
float fo2 = Fo[0] * Fo[0] + Fo[1] * Fo[1];
// find k where k * Vn = C - Fo
// S is the hypothenuse in any rectangular triangle formed by k * Vn and Fo
// so k^2 + fo^2 = s^2 (since |Vn|=1)
float k2 = s * s - fo2;
if (k2 <= -1e-3f) {
// there is no solution, we will be drifted off either way
// fallback: fly stupidly towards V and hope for the best
C[0] = V[0];
C[1] = V[1];
return false;
} else if (k2 <= 1e-3f) {
// there is one solution: -Fo
C[0] = -Fo[0];
C[1] = -Fo[1];
return true;
}
// now we have two possible solutions k positive and k negative
// which one is better?
// two criteria:
// 1. we MUST move in the right direction, if k leads to -v its invalid
// 2. we should minimize the speed error
float k = sqrt(k2);
float C1[2] = { -k * Vn[0] - Fo[0], -k * Vn[1] - Fo[1] };
float C2[2] = { k *Vn[0] - Fo[0], k * Vn[1] - Fo[1] };
// project each solution on Vn to find length
float vp1 = (C1[0] + F[0]) * Vn[0] + (C1[1] + F[1]) * Vn[1];
float vp2 = (C2[0] + F[0]) * Vn[0] + (C2[1] + F[1]) * Vn[1];
if (vp1 >= 0.0f && fabsf(v - vp1) < fabsf(v - vp2)) {
C[0] = C1[0];
C[1] = C1[1];
return true;
}
C[0] = C2[0];
C[1] = C2[1];
if (vp2 >= 0.0f) {
return true;
} else {
return false;
}
}

2
shared/uavobjectdefinition/fixedwingpathfollowersettings.xml
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@ -12,6 +12,8 @@
<field name="CourseFeedForward" units="s" type="float" elements="1" defaultvalue="3.0"/>
<!-- how many seconds to plan the flight vector ahead when initiating necessary heading changes - increase for planes with sluggish response -->
<field name="ReverseCourseOverlap" units="deg" type="float" elements="1" defaultvalue="20.0"/>
<!-- how big the overlapping area behind the plane is, where, if the desired course lies behind, the current bank angle will determine if the plane goes left or right -->
<field name="HorizontalPosP" units="(m/s)/m" type="float" elements="1" defaultvalue="0.05"/>
<!-- proportional coefficient for correction vector in path vector field to get back on course - reduce for fast planes to prevent course oscillations -->