Every aspect of flight dynamics is controlled by the selected "PID controller". This is an algorithm which is responsible for reacting to your stick inputs and keeping the craft stable in the air by using the gyroscopes and/or accelerometers (depending on your flight mode).
The "PIDs" are a set of tuning parameters which control the operation of the PID controller. The optimal PID settings are different on every craft, so if you can't find someone with your exact setup who will share their settings with you, some trial and error is required to find the best performing PID settings.
A video on how to recognise and correct different flight problems caused by PID settings is available here:
Basically, the goal of the PID controller is to bring the craft's rotation rate in all three axes to the rate that you're commanding with your sticks. An error is computed which is the difference between your target rotation rate and the actual one measured by the gyroscopes, and the controller tries to bring this error to zero.
The P term controls the strength of the correction that is applied to bring the craft toward the target angle or rotation rate. If the P term is too low, the craft will be difficult to control as it won't respond quickly enough to keep itself stable. If it is set too high, the craft will rapidly oscillate/shake as it continually overshoots its target.
The I term corrects small, long term errors. If it is set too low, the craft's attitude will slowly drift. If it is set too high, the craft will oscillate (but with slower oscillations than with P being set too high).
The D term attempts to increase system stability by monitoring the rate of change in the error. If the error is rapidly converging to zero, the D term causes the strength of the correction to be backed off in order to avoid overshooting the target.
TPA and TPA Breakpoint
TPA stands for Throttle PID Attenuation and according to AlexYork.net:
"TPA basically allows an aggressively tuned multi-rotor (one that feels very locked in) to reduce its PID gains when throttle is applied beyond the TPA threshold/breakpoint in order to eliminate fast oscillations.."
Note that TPA is set via CLI or on the PID TUNING tab of the GUI.
tpa_breakpoint is set via CLI
Also note that TPA and
tpa_breakpoint may not be used with certain PID controllers. Check the description on the individual controller.
TPA applies a PID value reduction in relation to full throttle. It is used to apply dampening of PID values as full throttle is reached.
TPA = % of dampening that will occur at full throttle.
tpa_breakpoint = the point in the throttle curve at which TPA will begin to be applied.
An Example: With TPA = 50 (or .5 in the GUI) and
tpa_breakpoint = 1500 (assumed throttle range 1000 - 2000)
- At 1500 on the throttle channel, the PIDs will begin to be dampened.
- At 3/4 throttle (1750), PIDs are reduced by approximately 25% (half way between 1500 and 2000 the dampening will be 50% of the total TPA value of 50% in this example)
- At full throttle (2000) the full amount of dampening set in TPA is applied. (50% reduction in this example)
- TPA can lead into increase of rotation rate when more throttle applied. You can get faster flips and rolls when more throttle applied due to coupling of PID's and rates. Only the PID controllers MWREWRITE and LUX are using a linear TPA implementation, where no rotation rates are affected when TPA is being used.
How and Why to use this?
If you are getting oscillations starting at say 3/4 throttle, set
tpa_breakpoint = 1750 or lower (remember, this is assuming your throttle range is 1000-2000), and then slowly increase TPA until your oscillations are gone. Usually, you will want
tpa_breakpoint to start a little sooner than when your oscillations start so you'll want to experiment with the values to reduce/remove the oscillations.
Cleanflight 1.x had experimental pid controllers, for cleanflight 2.0 there is only one.
PID controller "LUXFloat"
This is a new floating point based PID controller. MW23 and MWREWRITE use integer arithmetic, which was faster in the days of the slower 8-bit MultiWii controllers, but is less precise.
This controller has code that attempts to compensate for variations in the looptime, which should mean that the PIDs don't have to be retuned when the looptime setting changes.
It is the first PID Controller designed for 32-bit processors and not derived from MultiWii.
The strength of the auto-leveling correction applied during Angle mode is controlled by the LEVEL "P" PID term which is labeled "Angle", "Strength" in the GUI (prior to version v1.13.0 the parameter
level_angle was used). This can be used to tune the auto-leveling strength in Angle mode compared to Horizon mode. The default is 50.
The strength of the auto-leveling correction applied during Horizon mode is set by the LEVEL "I" PID term which is labeled "Horizon", "Strength" in the GUI (prior to version v1.13.0 the parameter
level_horizon was used). The default is also 50.
The transition between self-leveling and acro behavior in Horizon mode is controlled by the LEVEL "D" term which is labeled "Horizon", "Transition" in the GUI (prior to version of v1.13.0 the parameter
sensitivity_horizon parameter was used) . This sets the percentage of your stick travel that should have self-leveling applied to it, so smaller values cause more of the stick area to fly using only the gyros.
For example, at a setting of "100" for sensitivity horizon, 100% self-leveling strength will be applied at center stick, 50% self-leveling will be applied at 50% stick, and no self-leveling will be applied at 100% stick. If sensitivity is decreased to 75, 100% self-leveling will be applied at center stick, 50% will be applied at 63% stick, and no self-leveling will be applied at 75% stick and onwards.
See below for descriptions of the Horizon Mode Commands.
RC rate, Pitch and Roll Rates (P/R rate before they were separated), and Yaw rate
An overall multiplier on the RC stick inputs for pitch, roll, and yaw.
This basically sets the baseline stick sensitivity.
Pitch and Roll rates
This is an multiplier on overall stick sensitivity, like RC rate, but for roll and pitch independently. Stablility (to outside factors like turbulence) is not reduced at stick extremes. A zero value is no increase in stick sensitivity over that set by RC rate above. Higher values increases stick sensitivity across the entire stick movement range.
It acts as a stick sensitivity multiplier, as explained above.
gyro_lpf sets the hardware gyro low pass filter value. If 0 or 256 the gyro uses the least hardware filtering available (256Hz) and the internal sampling rate is the fastest possible (8kHz) with the least possible delay. The lower the number the stronger the filtering. Stronger filtering reduces noise in the gyro signal before that data gets into the PID calculations. Stronger filtering adds delays that can be associated with wobble and reduced responsiveness. Filtering is needed because motor/frame noise can cause overheating of motors especially when amplified by Dterm in quads with low mass and fast braking ESCs. If 188 or lower are chosen, the gyro sampling is internally at 1kHz and delays are greater. Faster sampling is good because things are slightly more responsive but can cause aliasing noise. Setting to 188 allows syncing of the FC to the gyro at 1kHz (if
gyro_sync is enabled and available in the code) which reduces aliasing a lot.
gyro_soft_lpf is an IIR (Infinite Impulse Response) software low-pass filter that can be configured to any desired frequency. If set to a value above zero it is active. It works after the hardware filter on the gyro (in the FC code) and further reduces noise. The two filters in series have twice the cut rate of one alone. There's not a lot of sense running
gyro_soft_lpf at a value above
gyro_lpf. If used, it is typically set about half the hardware filter rate to enhance the cut of higher frequencies before the PID calculations. Frequencies above 100Hz are of no interest to us from a flight control perspective - they can and should be removed from the signal before it gets to the PID calculation stage.
dterm_cut_hz is an IIR software low-pass filter that can be configured to any desired frequency. It works after the gyro_cut filters and specifically filters only the D term data. D term data is frequency dependent, the higher the frequency, the greater the computed D term value. This filter is required if despite the gyro filtering there remains excessive D term noise. Typically it needs to be set quite low because D term noise is a major problem with typical IIR filters. If set too low the phase shift in D term reduces the effectiveness of D term in controlling stop wobble, so this value needs some care when varying it. Again blackbox recording is needed to properly optimise the value for this filter.
Horizon Mode Commands
The CLI commands
horizon_tilt_expert_mode control the effect the current inclination has on self-leveling in the Horizon flight mode. (The current inclination is the number of degrees of pitch or roll that the vehicle is away from level, whichever is greater).
horizon_tilt_effect: Controls the effect the current inclination (tilt) has on self-leveling in the Horizon flight mode. Larger values result in less self-leveling (more "acro") as the tilt of the vehicle increases. The default value of 75 provides good performance when doing large loops and fast-forward flight. With a value of 0 the strength of the self-leveling would be solely dependent on the stick position.
horizon_tilt_expert_mode OFF|ON: Sets the performance mode for 'horizon_tilt_effect'
OFF = leveling always active when sticks centered:
This is the "safer" mode because the self-leveling is always active when the sticks are centered. So, when the vehicle is upside down (180 degrees) and the sticks are then centered, the vehicle will immediately be self-leveled to upright and flat. (Note that after this kind of very-fast 180-degree self-leveling, the heading of the vehicle can be unpredictable.)
ON = leveling can be totally off when inverted:
In this mode, the inclination (tilt) of the vehicle can fully "override" the self-leveling. In this mode, when the 'horizon_tilt_effect' parameter is set to around 75, and the vehicle is upside down (180 degrees) and the sticks are then centered, the vehicle is not self-leveled. This can be desirable for performing more-acrobatic maneuvers and potentially for 3D-mode flying.
The 'horizon_tilt_effect' and 'horizon_tilt_expert_mode' values are separate for each profile.