PID Attenuation and scaling

This topic covers methods used to dynamically adjust your models PID gains, to prevent control oscillations as the models airspeed changes throughout the flight.
Multiple methods are used to accomplish this. Depending on whether it's a multicopter or fixedwing platform.

• Multicopter TPA - Throttle PID Attenuation

• Fixedwing APA - AirSpeed PID Attenuation and boost (INAV 9.0)

• Fixedwing TPA and Pitch Angle - Throttle and Pitch Angle PID Attenuation and boost (INAV 9.0)

• Fixedwing TPA - Throttle PID Attenuation and boost (Pre INAV 9.0)

Multicopter TPA

Settings :

• TPA_Rate - percentage of PID attenuation that will occur when the throttle is increased above the TPA_breakpoint.

• TPA_Breakpoint - the throttle micro-second value in the curve at which TPA_Rate will begin to be applied. Below that point the PIDs are not attenuated at all.

How to use it?

• Firstly, set TPA_Rate = 15 as a starting point.
  The PID's should be tuned in the throttle range your copter will comfortably cruise at - e.g. 1300 - 1600uS based on thrust to weight ratio and bank angle.
• But once the throttle is moved higher than this point, you may start seeing oscillations. So begin increasing the TPA_breakpoint to a throttle value just prior to the onset of those oscillations.
• Then slowly increase the TPA_Rate value until the oscillations in the higher throttle range are gone.
  It may require increasing even more on powerful freestyle or race quads.

Note - On reverse motor 3D installs, TPA is not recommended.

Example of multicopter TPA curve

Note

Fixedwing dynamic PIDFF adjustment is broken into multiple methods, based on changes made in INAV 9.0.
It is important to understand the operation of both methods. Due to TPA and Pitch angle becoming a fall-back if the APA airspeed hardware source fails.

Fixedwing APA

INAV 9.0

Settings :

• Fw_reference_airspeed - Is the Airspeed at which your PIDs, Rates and Feedforwards should be optimally tuned, to provide a strong stabilization response.

• apa_pow - Sets how aggressively the gains will be dynamically adjusted from the base PIDFF tune.
  Increasing its value from the default of 120, will boost the gains more aggressively below the fw_reference_airspeed, and attenuate them more aggressively above the fw_reference_airspeed.
  While decreasing apa_pow will make the gain adjustments less aggressive. Meaning the PIDFF gain boost will be weaker, as will its attenuation strength, leaving the based gain tune less altered by airspeed changes.

Tip

Setting apa_pow = 0 will disable this function and revert to using the TPA and Pitch Angle method instead.
If either GNSS data or the Pitot sensor data becomes untrusted. It will also revert back to the TPA and Pitch Angle method.

FUNCTION :

This method uses airspeed data to dynamically adjusts the PIDFF gains.
Airspeed can be obtained from either a Pitot airspeed sensor. Or Virtual airspeed, derived from GNSS data requiring a 3D satellite fix..
As with the throttle TPA_rate setting, APA_pow also uses a similar scaling limit factor of [2x boost] 200% gain increase - [0.3 attenuation] 70% gain reduction.

How to use this?

• If you have not done so; firstly tune your airplanes rates and Feedforwards with AutoTune, using the default PID gains. But if you already have a workable tune for your airplane, use that.

• Enter the airspeed value your airplane will comfortably cruise at, into the fw_reference_airspeed setting.

• Start incrementally increasing your PID gains, while holding the approximate cruise airspeed you entered above. Tuning can be done easier via inflight tuning.

• Once this is complete. You will notice that the stabilization automatically becomes tighter when the airspeed reduces below the fw_reference_airspeed, and control surface oscillations are prevented as the airspeed increases above that point.

• However, if you do encounter control surface oscillations at higher airspeeds, this is when you can increase the value of apa_pow. It will allow the gains to become more aggressively attenuated at higher speeds to prevent this occurrence. Only make adjustments of no more than 10 at a time.

• Also keep in mind that control surface throws as well as higher airspeeds will influence the need to adjust apa_pow. If you have larger control surface throws, it may also require increasing.
  But if your airplane is very draggy and can't make it past 120km in a full throttle dive. You can reduce apa_pow to provide a tighter stabilization response over the planes whole speed range.

• fw_tpa_time_constant is not used in airspeed based dynamic PID adjustment.

Fixedwing TPA and Pitch Angle

This method uses the throttle position, combine with the airplanes climb or dive angle, to determine the optimal PIDFF gain adjustment required.

INAV 9.0

Settings :

• TPA_Rate - the amount of scaling apply to the PIDs. 100% TPA_Rate allows the base PID tune to be scaled by a factor of [2x boost] 200% gain increase - [0.5 attenuation] 50% gain reduction.

• TPA_breakpoint - is the point in the throttle curve when the base PID tune is not boosted or attenuated.

• nav_fw_pitch2thr - is used to calculate the collective effect throttle and pitch will exert to dynamically adjust the PIDFF gains.
  The ideal value for this function is 10. With 10 being the default value. Adjusting it too far outside this range may significantly reduce or increase TPA/pitch angle effectiveness.
  NOTE : This setting also influences the navigation climb throttle. So only adjust it in small amounts either side of the default.

• fw_tpa_time_constant - is a smoothing and time delay constant, reflecting the non-instantaneous response of the airplane, based on relative drag, inertia and thrust. This filter works upon a throttle approximation or the forward speed, and pitch angle gravity induced speed changes.

FUNCTION :

The Throttle position and Climb/Dive angle are used together to provide more accurate attenuation or boost control over the gains.
The attenuation/boost strength is determined by how high you set TPA_rate.
Because this method can account for the pitch angle. It can override the conventional throttle based gain adjustments, according to the effect gravity has on the airplane in a climb or a dive. i.e. The airplane will either speed up in a fall, or slow down in a climb; which throttle position alone can not account for.

Example :
If your airplane is flying level at 80% throttle, the gains will have some attenuation applied to them at that airspeed to prevent control oscillations. But if you then pull back on the elevator stick, so the airplane starts climbing at high angle. The vertical airspeed will generally start to wash-off on airplanes that aren't over powered. Leading to a transition from gain attenuation to gain boost. This will tighten the gains as the airplane slows closer to a stall, even when the throttle is higher.

The same applies if the airplane is placed in a 90° downward dive. When using the old method, having the throttle low in this case, would cause the gains to boost. But because this method knows that the airplane is in a dive, it will start attenuating the gains, preventing oscillations, because the airplanes free-fall speed is increasing, irrespective of the throttle.

How to use this?

• Tune your PIDs to the throttle range you intend to fly your airplane. nav_fw_cruise_thr is ideal. Set that value as the TPA_breakpoint. e.g. 1400 - 1480uS

• If you haven't already tuned your airplane. Make sure you complete an AutoTune with the default PID gains. Otherwise you can use the tune you already have.

• Now you can start increasing the P and D gains around the throttle value you have TPA_breakpoint set. You should also increase the TPA_Rate from the default, to a conservative value of 40% when you're tuning the base gains. So it allows some dynamic PID adjustment to occur. Otherwise a tight base gain tune around the TPA_breakpoint will instantly cause oscillation if you increase the throttle/speed by too much.

• Now when your flying at lower throttle, your airplane should feel a bit tighter in its stabilization response. And when flying at higher throttle (up to full throttle), the control surfaces should not oscillate.
  Once you have the base PID gains at the TPA_breakpoint throttle value tuned as tight as you are comfortable with. Push the throttle up toward full and let the speed increase. If oscillations start to occur. Incrementally increase the TPA_rate until they are gone.

Caution

Be cautious when increasing the TPA_rate on airplanes that have a thrust to weight ratio greater than 1.3 : 1. Such airplanes can climb vertically at a high speed. Which means high gain boost will occur under this condition. Make sure you run a vertical climb test, to look for control oscillations when increasing the TPA_rate during the tuning process.

• Due to drag also effecting how fast a given airplane will gain or lose speed. fw_tpa_time_constant may require adjustment to account for the time it takes for the speed to ramp up or ramp down, based on climb or dive pitch angle.

Fixedwing TPA

Pre INAV 9.0

Settings :

• TPA_Rate - the amount of scaling apply to the PIDs. 100% TPA_Rate allows the base PID tune to be scaled by a limiting factor of [2x boost] 200% - [0.5 attenuation] 50%.

• TPA_breakpoint - is the point in the throttle curve when the base PID tune is not boosted or attenuated.

• fw_tpa_time_constant - is a smoothing and time delay constant, reflecting the non-instantaneous speed/throttle response of an airplane, based on drag and inertia.

FUNCTION :

The Throttle stick position is used to not only attenuate the PID gains in the higher throttle range, above the TPA_Breakpoint. But also boosts them in the lower throttle range, below the TPA_breakpoint, for tighter stabilization control when flying or gliding at lower speeds with minimal or no throttle.

How to use this?

• Tune your PIDs to the throttle range you intend to fly your airplane. nav_fw_cruise_thr is ideal. Set that value as the TPA_breakpoint. e.g. 1400 - 1480uS

• Once your P and D gains are tuned, and before you add any TPA_Rate. You may notice when flying at lower throttle, your airplane handles more loosely. And when flying at higher throttle (up to full throttle) control surfaces may begin to oscillate.
  You can now start increasing the TPA_Rate value until those oscillations are gone or minimal in the higher throttle/speed range. This will also translate to better handling at lower throttle/speeds, by boosting the PID gains.

Note

The above method had its limitations. It can not attenuate the PIDs at lower throttle values if the airplane is placed into a dive, causing the air-speed to increase. This could lead to control surface oscillations.

Example of airplane TPA curve