ardupilot/libraries/APM_Control/AP_PitchController.cpp

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/*
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.
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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.
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You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
APM_Control: ROLL and PITCH controllers These changes reduce height variation in turns and improve robustness. the specific changes are: 1) Linked roll and pitch integrator protection to the final output value so that if final output is on upper limit, the integrator is prevented from increasing and vice-versa. This improves wind-up protection. 2) Modified rate feedback in roll and pitch controllers to use body rates rather than Euler or earth rates. 3) Changed the roll to pitch compensation to use measured roll angle and estimated airspeed to calculate the component of turn rate (assuming a level coordinated turn) around the pitch axis. This a mathematically correct calculation and will work over a range of bank angles and aircraft with minimal (if any) tuning required. 4) The integrator in the roll and pitch loop is clamped when the estimated speed is below the minimum FBW speed 5) The noise filter in the pitch and roll loop has been changed to use a FOH discretisation. This gives improved noise rejection and less phase loss when compared to the previous filter that used a ZOH or equivalent discretisation. This has been flown on the rascal in the SITL and on a X-8 with limited flight testing. Initial results have been encouraging with reduced height variation in turns. Compare to standard PIDS, the revised pitch and roll controllers allow the use of rate feedback (effectively the same as the old D term) without beating the servos to death. The bank angle compensation in the pitch loop works effectively over a much larger range of bank angles and requires minimal tuning compared to the old calculation. YAW CONTROLLER Currently testing the a 3-loop acceleration autopilot topology for the yaw loop with feed forward yaw rate for turn compensation. This 3-loop topology is commonly used in tactical skid to to turn missiles and is easy to tune. The following block diagram shows the general signal flow Note that the acceleration measurement has to pass through an integrator before it gets to the actuator. This is a important feature as it eliminates problems of high frequency noise and potential coupling with structural modes associated with direct feedback of measured acceleration to actuator. The high pass filter has been inserted to compensate for airspeed and bank angle measurement errors which will cause steady state errors in the calculation of the turn yaw rate. The yaw controller flies SITL well, but hasn't been flight tested yet. It can be configured either as a simple yaw damper, or the acceleration and integral term can be turned on to allow feedback control of lateral acceleration/sideslip. TO DO: Need to reduce number of tuning parameters and provide consistent naming Need to provide guidance on tuning these loops with definitions for all the gain terms. Need to check signs and units into and out of lateral loops. DESIGN DECISIONS PENDING: 1) Can we remove the noise filters? Provided the mpu6k noise filter is running they are of limited benefit given the 25Hz Nyquist frequency 2) If we do remove them and rely on the mpu6k noise filter, what is the apprporiate default cutoff frequency for plane use. 20Hz is probably OK for most setups, but some noisy/high vibration setups would require as low as 10Hz 3) The inverted flight logic looks like a crash waiting to happen. It's problematic to test and even if implemented correctly would still crash a plane with poor inverted flight capability. We should either implement it properly and fully tested or delete it.
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// Initial Code by Jon Challinger
// Modified by Paul Riseborough
#include <AP_HAL/AP_HAL.h>
#include "AP_PitchController.h"
#include <AP_AHRS/AP_AHRS.h>
extern const AP_HAL::HAL& hal;
const AP_Param::GroupInfo AP_PitchController::var_info[] = {
// @Param: 2SRV_TCONST
// @DisplayName: Pitch Time Constant
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// @Description: Time constant in seconds from demanded to achieved pitch angle. Most models respond well to 0.5. May be reduced for faster responses, but setting lower than a model can achieve will not help.
// @Range: 0.4 1.0
// @Units: s
// @Increment: 0.1
// @User: Advanced
AP_GROUPINFO("2SRV_TCONST", 0, AP_PitchController, gains.tau, 0.5f),
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// index 1 to 3 reserved for old PID values
// @Param: 2SRV_RMAX_UP
// @DisplayName: Pitch up max rate
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// @Description: Maximum pitch up rate that the pitch controller demands (degrees/sec) in ACRO mode.
// @Range: 0 100
// @Units: deg/s
// @Increment: 1
// @User: Advanced
AP_GROUPINFO("2SRV_RMAX_UP", 4, AP_PitchController, gains.rmax_pos, 0.0f),
// @Param: 2SRV_RMAX_DN
// @DisplayName: Pitch down max rate
// @Description: This sets the maximum nose down pitch rate that the controller will demand (degrees/sec). Setting it to zero disables the limit.
// @Range: 0 100
// @Units: deg/s
// @Increment: 1
// @User: Advanced
AP_GROUPINFO("2SRV_RMAX_DN", 5, AP_PitchController, gains.rmax_neg, 0.0f),
// @Param: 2SRV_RLL
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// @DisplayName: Roll compensation
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// @Description: Gain added to pitch to keep aircraft from descending or ascending in turns. Increase in increments of 0.05 to reduce altitude loss. Decrease for altitude gain.
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// @Range: 0.7 1.5
// @Increment: 0.05
// @User: Standard
AP_GROUPINFO("2SRV_RLL", 6, AP_PitchController, _roll_ff, 1.0f),
// index 7, 8 reserved for old IMAX, FF
// @Param: _RATE_P
// @DisplayName: Pitch axis rate controller P gain
// @Description: Pitch axis rate controller P gain. Converts the difference between desired roll rate and actual roll rate into a motor speed output
// @Range: 0.08 0.35
// @Increment: 0.005
// @User: Standard
// @Param: _RATE_I
// @DisplayName: Pitch axis rate controller I gain
// @Description: Pitch axis rate controller I gain. Corrects long-term difference in desired roll rate vs actual roll rate
// @Range: 0.01 0.6
// @Increment: 0.01
// @User: Standard
// @Param: _RATE_IMAX
// @DisplayName: Pitch axis rate controller I gain maximum
// @Description: Pitch axis rate controller I gain maximum. Constrains the maximum motor output that the I gain will output
// @Range: 0 1
// @Increment: 0.01
// @User: Standard
// @Param: _RATE_D
// @DisplayName: Pitch axis rate controller D gain
// @Description: Pitch axis rate controller D gain. Compensates for short-term change in desired roll rate vs actual roll rate
// @Range: 0.001 0.03
// @Increment: 0.001
// @User: Standard
// @Param: _RATE_FF
// @DisplayName: Pitch axis rate controller feed forward
// @Description: Pitch axis rate controller feed forward
// @Range: 0 3.0
// @Increment: 0.001
// @User: Standard
// @Param: _RATE_FLTT
// @DisplayName: Pitch axis rate controller target frequency in Hz
// @Description: Pitch axis rate controller target frequency in Hz
// @Range: 2 50
// @Increment: 1
// @Units: Hz
// @User: Standard
// @Param: _RATE_FLTE
// @DisplayName: Pitch axis rate controller error frequency in Hz
// @Description: Pitch axis rate controller error frequency in Hz
// @Range: 2 50
// @Increment: 1
// @Units: Hz
// @User: Standard
// @Param: _RATE_FLTD
// @DisplayName: Pitch axis rate controller derivative frequency in Hz
// @Description: Pitch axis rate controller derivative frequency in Hz
// @Range: 0 50
// @Increment: 1
// @Units: Hz
// @User: Standard
// @Param: _RATE_SMAX
// @DisplayName: Pitch slew rate limit
// @Description: Sets an upper limit on the slew rate produced by the combined P and D gains. If the amplitude of the control action produced by the rate feedback exceeds this value, then the D+P gain is reduced to respect the limit. This limits the amplitude of high frequency oscillations caused by an excessive gain. The limit should be set to no more than 25% of the actuators maximum slew rate to allow for load effects. Note: The gain will not be reduced to less than 10% of the nominal value. A value of zero will disable this feature.
// @Range: 0 200
// @Increment: 0.5
// @User: Advanced
AP_SUBGROUPINFO(rate_pid, "_RATE_", 11, AP_PitchController, AC_PID),
AP_GROUPEND
};
AP_PitchController::AP_PitchController(const AP_Vehicle::FixedWing &parms)
: aparm(parms)
{
AP_Param::setup_object_defaults(this, var_info);
rate_pid.set_slew_limit_scale(45);
}
/*
AC_PID based rate controller
*/
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int32_t AP_PitchController::_get_rate_out(float desired_rate, float scaler, bool disable_integrator, float aspeed)
{
const float dt = AP::scheduler().get_loop_period_s();
const AP_AHRS &_ahrs = AP::ahrs();
const float eas2tas = _ahrs.get_EAS2TAS();
bool limit_I = fabsf(_last_out) >= 45;
float rate_y = _ahrs.get_gyro().y;
float old_I = rate_pid.get_i();
rate_pid.set_dt(dt);
bool underspeed = aspeed <= 0.5*float(aparm.airspeed_min);
if (underspeed) {
limit_I = true;
}
// the P and I elements are scaled by sq(scaler). To use an
// unmodified AC_PID object we scale the inputs and calculate FF separately
//
// note that we run AC_PID in radians so that the normal scaling
// range for IMAX in AC_PID applies (usually an IMAX value less than 1.0)
rate_pid.update_all(radians(desired_rate) * scaler * scaler, rate_y * scaler * scaler, limit_I);
if (underspeed) {
// when underspeed we lock the integrator
rate_pid.set_integrator(old_I);
}
// FF should be scaled by scaler/eas2tas, but since we have scaled
// the AC_PID target above by scaler*scaler we need to instead
// divide by scaler*eas2tas to get the right scaling
const float ff = degrees(rate_pid.get_ff() / (scaler * eas2tas));
if (disable_integrator) {
rate_pid.reset_I();
}
// convert AC_PID info object to same scale as old controller
_pid_info = rate_pid.get_pid_info();
auto &pinfo = _pid_info;
const float deg_scale = degrees(1);
pinfo.FF = ff;
pinfo.P *= deg_scale;
pinfo.I *= deg_scale;
pinfo.D *= deg_scale;
// fix the logged target and actual values to not have the scalers applied
pinfo.target = desired_rate;
pinfo.actual = degrees(rate_y);
// sum components
float out = pinfo.FF + pinfo.P + pinfo.I + pinfo.D;
// remember the last output to trigger the I limit
_last_out = out;
if (autotune != nullptr && autotune->running && aspeed > aparm.airspeed_min) {
// let autotune have a go at the values
autotune->update(pinfo, scaler, angle_err_deg);
}
// output is scaled to notional centidegrees of deflection
return constrain_int32(out * 100, -4500, 4500);
}
/*
Function returns an equivalent elevator deflection in centi-degrees in the range from -4500 to 4500
A positive demand is up
Inputs are:
1) demanded pitch rate in degrees/second
2) control gain scaler = scaling_speed / aspeed
3) boolean which is true when stabilise mode is active
4) minimum FBW airspeed (metres/sec)
5) maximum FBW airspeed (metres/sec)
*/
int32_t AP_PitchController::get_rate_out(float desired_rate, float scaler)
{
float aspeed;
if (!AP::ahrs().airspeed_estimate(aspeed)) {
// If no airspeed available use average of min and max
aspeed = 0.5f*(float(aparm.airspeed_min) + float(aparm.airspeed_max));
}
return _get_rate_out(desired_rate, scaler, false, aspeed);
}
/*
get the rate offset in degrees/second needed for pitch in body frame
to maintain height in a coordinated turn.
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Also returns the inverted flag and the estimated airspeed in m/s for
use by the rest of the pitch controller
*/
float AP_PitchController::_get_coordination_rate_offset(float &aspeed, bool &inverted) const
{
float rate_offset;
float bank_angle = AP::ahrs().roll;
// limit bank angle between +- 80 deg if right way up
if (fabsf(bank_angle) < radians(90)) {
bank_angle = constrain_float(bank_angle,-radians(80),radians(80));
inverted = false;
} else {
inverted = true;
if (bank_angle > 0.0f) {
bank_angle = constrain_float(bank_angle,radians(100),radians(180));
} else {
bank_angle = constrain_float(bank_angle,-radians(180),-radians(100));
}
}
const AP_AHRS &_ahrs = AP::ahrs();
if (!_ahrs.airspeed_estimate(aspeed)) {
// If no airspeed available use average of min and max
aspeed = 0.5f*(float(aparm.airspeed_min) + float(aparm.airspeed_max));
}
if (abs(_ahrs.pitch_sensor) > 7000) {
// don't do turn coordination handling when at very high pitch angles
rate_offset = 0;
} else {
rate_offset = cosf(_ahrs.pitch)*fabsf(ToDeg((GRAVITY_MSS / MAX((aspeed * _ahrs.get_EAS2TAS()) , MAX(aparm.airspeed_min, 1))) * tanf(bank_angle) * sinf(bank_angle))) * _roll_ff;
}
if (inverted) {
rate_offset = -rate_offset;
}
return rate_offset;
}
// Function returns an equivalent elevator deflection in centi-degrees in the range from -4500 to 4500
// A positive demand is up
// Inputs are:
// 1) demanded pitch angle in centi-degrees
// 2) control gain scaler = scaling_speed / aspeed
// 3) boolean which is true when stabilise mode is active
// 4) minimum FBW airspeed (metres/sec)
// 5) maximum FBW airspeed (metres/sec)
//
int32_t AP_PitchController::get_servo_out(int32_t angle_err, float scaler, bool disable_integrator)
{
// Calculate offset to pitch rate demand required to maintain pitch angle whilst banking
// Calculate ideal turn rate from bank angle and airspeed assuming a level coordinated turn
// Pitch rate offset is the component of turn rate about the pitch axis
float aspeed;
float rate_offset;
bool inverted;
if (gains.tau < 0.05f) {
gains.tau.set(0.05f);
}
rate_offset = _get_coordination_rate_offset(aspeed, inverted);
APM_Control: ROLL and PITCH controllers These changes reduce height variation in turns and improve robustness. the specific changes are: 1) Linked roll and pitch integrator protection to the final output value so that if final output is on upper limit, the integrator is prevented from increasing and vice-versa. This improves wind-up protection. 2) Modified rate feedback in roll and pitch controllers to use body rates rather than Euler or earth rates. 3) Changed the roll to pitch compensation to use measured roll angle and estimated airspeed to calculate the component of turn rate (assuming a level coordinated turn) around the pitch axis. This a mathematically correct calculation and will work over a range of bank angles and aircraft with minimal (if any) tuning required. 4) The integrator in the roll and pitch loop is clamped when the estimated speed is below the minimum FBW speed 5) The noise filter in the pitch and roll loop has been changed to use a FOH discretisation. This gives improved noise rejection and less phase loss when compared to the previous filter that used a ZOH or equivalent discretisation. This has been flown on the rascal in the SITL and on a X-8 with limited flight testing. Initial results have been encouraging with reduced height variation in turns. Compare to standard PIDS, the revised pitch and roll controllers allow the use of rate feedback (effectively the same as the old D term) without beating the servos to death. The bank angle compensation in the pitch loop works effectively over a much larger range of bank angles and requires minimal tuning compared to the old calculation. YAW CONTROLLER Currently testing the a 3-loop acceleration autopilot topology for the yaw loop with feed forward yaw rate for turn compensation. This 3-loop topology is commonly used in tactical skid to to turn missiles and is easy to tune. The following block diagram shows the general signal flow Note that the acceleration measurement has to pass through an integrator before it gets to the actuator. This is a important feature as it eliminates problems of high frequency noise and potential coupling with structural modes associated with direct feedback of measured acceleration to actuator. The high pass filter has been inserted to compensate for airspeed and bank angle measurement errors which will cause steady state errors in the calculation of the turn yaw rate. The yaw controller flies SITL well, but hasn't been flight tested yet. It can be configured either as a simple yaw damper, or the acceleration and integral term can be turned on to allow feedback control of lateral acceleration/sideslip. TO DO: Need to reduce number of tuning parameters and provide consistent naming Need to provide guidance on tuning these loops with definitions for all the gain terms. Need to check signs and units into and out of lateral loops. DESIGN DECISIONS PENDING: 1) Can we remove the noise filters? Provided the mpu6k noise filter is running they are of limited benefit given the 25Hz Nyquist frequency 2) If we do remove them and rely on the mpu6k noise filter, what is the apprporiate default cutoff frequency for plane use. 20Hz is probably OK for most setups, but some noisy/high vibration setups would require as low as 10Hz 3) The inverted flight logic looks like a crash waiting to happen. It's problematic to test and even if implemented correctly would still crash a plane with poor inverted flight capability. We should either implement it properly and fully tested or delete it.
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// Calculate the desired pitch rate (deg/sec) from the angle error
angle_err_deg = angle_err * 0.01;
float desired_rate = angle_err_deg / gains.tau;
// limit the maximum pitch rate demand. Don't apply when inverted
// as the rates will be tuned when upright, and it is common that
// much higher rates are needed inverted
if (!inverted) {
if (gains.rmax_neg && desired_rate < -gains.rmax_neg) {
desired_rate = -gains.rmax_neg;
} else if (gains.rmax_pos && desired_rate > gains.rmax_pos) {
desired_rate = gains.rmax_pos;
}
}
if (inverted) {
desired_rate = -desired_rate;
}
// Apply the turn correction offset
desired_rate = desired_rate + rate_offset;
/*
when we are past the users defined roll limit for the aircraft
our priority should be to bring the aircraft back within the
roll limit. Using elevator for pitch control at large roll
angles is ineffective, and can be counter productive as it
induces earth-frame yaw which can reduce the ability to roll. We
linearly reduce pitch demanded rate when beyond the configured
roll limit, reducing to zero at 90 degrees
*/
const AP_AHRS &_ahrs = AP::ahrs();
float roll_wrapped = labs(_ahrs.roll_sensor);
if (roll_wrapped > 9000) {
roll_wrapped = 18000 - roll_wrapped;
}
const float roll_limit_margin = MIN(aparm.roll_limit_cd + 500.0, 8500.0);
if (roll_wrapped > roll_limit_margin && labs(_ahrs.pitch_sensor) < 7000) {
float roll_prop = (roll_wrapped - roll_limit_margin) / (float)(9000 - roll_limit_margin);
desired_rate *= (1 - roll_prop);
}
return _get_rate_out(desired_rate, scaler, disable_integrator, aspeed);
}
void AP_PitchController::reset_I()
{
_pid_info.I = 0;
rate_pid.reset_I();
}
/*
convert from old to new PIDs
this is a temporary conversion function during development
*/
void AP_PitchController::convert_pid()
{
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AP_Float &ff = rate_pid.ff();
if (ff.configured_in_storage()) {
return;
}
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float old_ff=0, old_p=1.0, old_i=0.3, old_d=0.08;
int16_t old_imax = 3000;
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bool have_old = AP_Param::get_param_by_index(this, 1, AP_PARAM_FLOAT, &old_p);
have_old |= AP_Param::get_param_by_index(this, 3, AP_PARAM_FLOAT, &old_i);
have_old |= AP_Param::get_param_by_index(this, 2, AP_PARAM_FLOAT, &old_d);
have_old |= AP_Param::get_param_by_index(this, 8, AP_PARAM_FLOAT, &old_ff);
have_old |= AP_Param::get_param_by_index(this, 7, AP_PARAM_FLOAT, &old_imax);
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if (!have_old) {
// none of the old gains were set
return;
}
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const float kp_ff = MAX((old_p - old_i * gains.tau) * gains.tau - old_d, 0);
rate_pid.ff().set_and_save(old_ff + kp_ff);
rate_pid.kI().set_and_save_ifchanged(old_i * gains.tau);
rate_pid.kP().set_and_save_ifchanged(old_d);
rate_pid.kD().set_and_save_ifchanged(0);
rate_pid.kIMAX().set_and_save_ifchanged(old_imax/4500.0);
}
/*
start an autotune
*/
void AP_PitchController::autotune_start(void)
{
if (autotune == nullptr) {
autotune = new AP_AutoTune(gains, AP_AutoTune::AUTOTUNE_PITCH, aparm, rate_pid);
if (autotune == nullptr) {
if (!failed_autotune_alloc) {
GCS_SEND_TEXT(MAV_SEVERITY_ERROR, "AutoTune: failed pitch allocation");
}
failed_autotune_alloc = true;
}
}
if (autotune != nullptr) {
autotune->start();
}
}
/*
restore autotune gains
*/
void AP_PitchController::autotune_restore(void)
{
if (autotune != nullptr) {
autotune->stop();
}
}