QuadPlane VTOL Tuning

This section provides an overview of how to tune various QuadPlane VTOL parameters.

Overview

The default parameters controlling the VTOL motors PID loops should allow most frames to initially hover uncontrollably, if the motors’ mechanics are setup and aligned correctly and escs calibrated.

The most important parameters controlling stability are the Roll/Pitch/Yaw PIDS. For altitude control, the vertical position controller’s parameters and Motor Thrust Scaling parameters, and for navigation/loiter the Loiter controllers’s parameters

Normally, it’s best to start by tuning the Rate Roll/Pitch P in QSTABILIZE mode then move onto tuning altitude hold in QHOVER mode, then QLOITER (which often needs no tuning) and finally the waypoint navigation performance in Auto mode.

Filter tuning

QuadPlanes are often affected by vibration and tuning the various software filters available is critical to achieving an optimum overall tune. A guide on tuning the various notch filters available can be found on the Notch Filtering wiki page. A optimum tune will be obtained on the D terms, below, if the filtering has been optimized first. However, in many circumstances reasonable performance can be obtained with just PID tuning the P terms.

Roll/Pitch tuning

The Q_A_RAT_RLL_x and Q_A_RAT_PIT_x Roll/Pitch Rate parameters which convert the desired rotation rate into a motor output are the most important.

The Q_A_ANG_RLL_P and Q_A_ANG_PIT_P Roll/Pitch P converts the desired angle into a desired rotation rate which is then fed to the Rate controller.

  • A higher value will make the QuadPlane more responsive to roll/pitch inputs, a lower value will make it smoother
  • If set too high, the QuadPlane will oscillate on the roll and/or pitch axis
  • If set too low the QuadPlane will become sluggish to inputs

An objective view of the overall Roll and Pitch performance can be seen by graphing the dataflash log’s ATT message’s DesRoll vs Roll and DesPit vs Pitch. The “Roll” (i.e. actual roll) should closely follow the “DesRoll” while in stabilized modes. Pitch should similarly closely follow DesPit.

Alternatively you may wish to try tuning both the rate and angle parameters using the QAUTOTUNE mode.

Yaw Tuning

The Angle Yaw and Rate Yaw parameters control the yaw response. With QuadPlanes, these often need tuning to get the desired YAW response, since configurations vary widely.

Similar to roll and pitch, if either Q_A_RAT_YAW_P or Q_A_ANG_YAW_P is too high the QuadPlane’s heading will oscillate. If they are too low, the QuadPlane may be unable to maintain its heading.

The Q_A_ANG_YAW_P is the gain on the error between the autopilots desired heading and actual heading which is fed into the Rate controller to demand a rotation rate. The Q_A_RAT_YAW_P is the gain applied to the difference between demanded rotation rate and actual.

The Q_YAW_RATE_MAX parameter controls how quickly QuadPlane rotates based on a pilot’s yaw input in stabilized modes.

Altitude Tuning

The QHOVER (altitude hold) related tuning parameters are related to the vertical position controller and the motor thrust scaling, which linearizes the throttle to motor thrust response to improve the position controllers response.

The Q_P_POSZ_P parameter is used to convert the altitude error (the difference between the desired altitude and the actual altitude) to a desired climb or descent rate. A higher rate will make it more aggressively attempt to maintain it’s altitude but if set too high leads to a jerky throttle response.

The Q_P_VELZ_P (which normally requires no tuning) converts the desired climb or descent rate into a desired acceleration up or down.

The Q_P_ACCZ_P ,:ref:Q_P_ACCZ_I<Q_P_ACCZ_I> ,:ref:Q_P_ACCZ_D<Q_P_ACCZ_D> PID gains convert the acceleration error (i.e the difference between the desired acceleration and the actual acceleration) into a motor output. The 1:2 ratio of P to I (i.e. I is twice the size of P) should be maintained if you modify these parameters. These values should never be increased but for very powerful QuadPlane VTOL motors you may get better response by reducing both by 50% (i.e P to 0.5, I to 1.0).

Loiter Tuning

Generally if Roll and Pitch are tuned correctly, the GPS and compass are set-up and performing well and vibration levels are acceptable, Loiter does not require much tuning but please see the Loiter Mode page for more details on tunable parameters including the horizontal speed.

In-flight Tuning

Many parameters can be tuned while in flight, see Transmitter Based Tuning

Video introduction to PIDs

PIDs (Proportional - Integral - Derivative) are the method used by our firmware to continuously stabilize the vehicle

  • Proportional = Immediate Correction: The further off you are the bigger the correction you make.
  • Integral = Over time or steady state correction: If we are failing to make progress add additional correction.
  • Derivative = Take it Easy correction: Is the correction going to fast? if it is slow it down (dampen) it a bit to avoid overshoot.

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