Navigation PID tuning (FW)

This page is a work in progress until this message is removed

The aim of this page is to separate the tuning of the FW Navigation PIDs from the Navigation modes page. Its goal is also to have FW Nav PID tuning separate from MC Nav PID tuning. To de-clutter and make for easier reading.

Tip

There is also a companion wiki page GPS and Compass setup, that is essential reading beforehand. The Installation location, Setup and Calibration of the GNSS/Magnetometer module, to provide the best navigation performance achievable, is of absolute importance. Accounting for this detail can make your build a greater success, especially when using a magnetometer.

ALTITUDE PID TUNING
POSITION PID TUNING
RANGE-FINDER TUNING

Caution

The Main Stabilization PID_FF, RATES and LEVEL controller MUST be tuned well before attempting to tune the navigation controllers. This is because the output from the navigation controllers are place into action through the main stabilization PID controllers. With incorrectly tuned FeedForward, noticeable impacting navigation performance.
Take note of how the Rate/FF Auto-tune process is performed.

Tuning Altitude Controller - Z axis:

Inability to obtain an accurate target altitude or climb rate can be caused by a number of reasons:

Poor GNSS satellite accuracy and EPV position data - Ensure you have a HDOP less than 1.2 for best precision. And never above 1.8. See here
Main stabilization PID_FF, RATES and LEVEL is poorly tuned. Or incorrectly setup control surface throws and/or C.G. - Setup hardware and Tune main PID's first.
Insufficient motor thrust - The airplanes thrust to weight ratio is too low.
High accelerometer vibrations from the motor(s) or prop(s).
Insufficient POS_Z_P, POS_Z_I or too much POS_Z_D and/or FW_FF_PITCH - Only if all the previous conditions are satisfied.

Make sure these hardware conditions are addressed first, before you even attempt to tune the navigation VEL PID's and climb rate settings.

ALTITUDE POS(`VEL`) PID Tuning:

Altitude is always referred too as the vertical or (Z) axis.

Tuning of an airplanes Z axis controller should be done on a day no colder than 5°C, for the best outcome. Due to the effect temperature has on the Baro and IMU.

The following settings can be accessed using the Configurator Tuning tab and Advanced Tuning tab. Or the CLI and CMS OSD stick menu's.

nav_fw_pos_z_p - Controls velocity to acceleration. Increasing the gain will provide a stronger elevator/pitch2throttle response to reach the required altitude target.
nav_fw_pos_z_i - Attempts to compensate for climb rate fluctuations cause by turbulence, thermals etc. Use sparingly. It can only do so much on a fixedwing platform.
nav_fw_pos_z_d - Attempts to smooth the POS(VEL)_Z_P and POS(VEL)_Z_I rate of response. Too much damping can weaken the response, leading to oscillations around the altitude target. Even becoming more exaggerated by an incorrectly tuned fw_ff_pitch response.
nav_fw_pos_z_ff - Attempts to provide a faster control response to meet the require target, based on altitude and gyro rate data. Depending on the aircraft's build specifics, lowering POS_Z_D and increasing POS_Z_FF can help.
nav_fw_alt_control_response - Alters the altitude control response as the airplane gets closer to reaching the altitude target.
fw_ff_pitch - Passes the angular rate target directly to the servo mixer, bypassing the gyro PID loop stabilization.
nav_fw_auto_climb_rate- Maximum climb/descent rate in [cm/s], the airplane is allowed to reach in modes that control altitude.

These settings should also be configured or tweaked to suit your aircraft. This will assist the Velocity Z controller.

Tuning Position Controller - XY axis:

Inability to obtain an accurate horizontal position can be caused by a number of reasons:

Poor GNSS satellite accuracy and EPH position data - Ensure you have a HDOP less than 1.2 for best precision. And never above 1.8. See here
Main stabilization PID_FF, RATES and LEVEL is poorly tuned. Or incorrectly setup control surface throws and/or C.G.- Setup hardware and Tune main PID's first.
Poorly Installed, Aligned or Calibrated magnetometer (compass) - If a magnetometer is used, read here to provide the best results.
Insufficient POS_XY_P, POS_XY_I or POS_HDG_P if nav_use_fw_yaw_control = ON - Only if all the previous conditions are satisfied.
High accelerometer vibrations from the motor(s) or prop(s). - Can lead to attitude and heading reference drift.

Make sure these hardware conditions are addressed first, before you even attempt to tune the navigation POS, VEL and HEADING PID's.

Horizontal POS + HEADING PID Tuning:

Tuning of an airplanes XY axis controllers should be done on a day no colder than 5°C, for the best outcome. This is due to the effect temperature has on the IMU.

When tuning the Nav XY controllers, you require a means to reference the WP markers, to real-time heading and turning positions.
This is best done by logging flight controller data. And gauging the changes with one or all of the following software: MWP Tools , INAV Blackbox Explorer or Blackbox Tools

The following settings can be accessed using the Configurator Tuning tab and Advanced Tuning tab. Or the CLI and CMS OSD stick menu's.

Position XY gains:

nav_fw_pos_xy_p - Controls how fast the airplane will attempt to use the roll and yaw axis to align its trajectory with the target position. This includes the strength it will apply to turning and re-aligning with the track after a turn.
nav_fw_pos_xy_i - Increasing this gain can compensation for trajectory drift caused by the wind. But should be used sparingly on elevon aircraft.
nav_fw_pos_xy_d - Increasing this gain can help smooth the P gain response. But if increased too much, it can cause over-shooting of the target trajectory alignment.
nav_fw_cruise_thr - Should be set to the airplanes optimal cruise speed. Keeping the average airspeed less than 75km/h, will obtain the highest target position accuracy.

Heading gains:

nav_fw_heading_p - Sets the strength the IMU heading target will hold. Heading data is updated from the GNSS course and/or mag bearing.
nav_use_fw_yaw_control- When enabled, it allows the use of the yaw heading controller settings below, for a fixedwing. It can be used with elevon aircraft via turn assist, but will not experience the full benefits of an airplane that has yaw control.
nav_fw_pos_hdg_p - Sets the strength the heading trajectory target is tracking.
nav_fw_pos_hdg_i - When used sparingly, it can filter-out heading target drift.
nav_fw_pos_hdg_d - Can smooth abrupt heading irregularity. But better suited to aircraft that have a means of yaw control.
heading_hold_rate_limit - Limits the yaw induced rotation rate the HEADING_HOLD controller can request from PID controller inner loop. It's independent from manual yaw rate and only active when HEADING_HOLD NAV flight modes are in use.
These settings can also influence WP tracking accuracy.

In my experience. Enabling the nav_fw_pos_hdg controller combined with an airplane that has yaw control. i.e. A Rudder, Differential thrust or Vectored thrust, when those yaw gains are tuned. Always provides the most accurate turn response in a waypoint mission or RTH Trackback.

Tuning Rangefinder:

The Rangefinder (surface) gains should only be tuned after the XYZ navigation PID's are optimally tuned.

Rangefinder - Lidar or Sonar altitude terrain settings and sensor weights:

rangefinder_median_filter - Enables a 3-point median filter to helps smooth out altitude variations in the readout.
nav_max_terrain_follow_alt - Maximum allowable distance above the ground for altitude tracking in [CM], that directly maps throttle to altitude.
inav_max_surface_altitude - Maximum allowable altitude [CM] for the vertical position estimators validity check over a set time period.
inav_w_z_surface_p - Weight applied to the Rangefinders estimated altitude. When a rangefinder is present, within its operational distance above the ground.
inav_w_z_surface_v - Weight applied to the Rangefinders estimated climb rate. When a rangefinder is present, within its operational distance above the ground.

Optical-flow is not available for fixedwing use, due to the speed airplanes travel across the ground. And the inherent inaccuracy of Optical Flow under such conditions.