This article uses an intuitive example to explain Betaflight’s Absolute Control: When the aircraft cannot follow the Setpoint 100%, it will leave a tracking error. After the aircraft body rotates in the three-dimensional space, this error should also be converted to the new aircraft system coordinates in order to continue to express the pilot’s original control intention. The video describes Absolute Control as performing a vector rotation on the three-axis error and helping to complete the incompletely executed action by correcting the Setpoint. It is based on the 2021 code and the author's personal understanding. Parameter names, implementations and whether they are still supported must be based on the current Betaflight source code and documentation.
The error direction problem that Absolute Control wants to solve
After the pilot gives the joystick command, the PID controller strives to make the angular velocity follow the Setpoint. However, due to the limitations of inertia, thrust, disturbance and control capabilities, the actual response will always leave errors. If the body attitude subsequently changes, the old error cannot remain intact on the original axis because the Roll, Pitch, and Yaw directions of the body system have rotated together.
The video uses a two-dimensional road U-turn as an analogy: the vehicle originally needs to return to the left to the center line, but after the U-turn, the same world direction is corrected, and it may become right in the vehicle body coordinates. Without rotating the error, the controller may continue to compensate in the wrong direction. Real flight is a three-dimensional angular velocity problem. The analogy simplifies the mathematics, but captures the core of coordinate transformation.
From I-Term Rotation to three-axis error rotation
According to the historical explanation of the video, I-Term Rotation mainly deals with the impact of Yaw changes on the Roll/Pitch integral error direction; Absolute Control treats the three axes equally, and the attitude change of one axis may change the expression of the errors of the other two axes, so the complete error vector is rotated.
This can be understood as a more general coordinate transformation idea, rather than simply adding an extra gain to a certain axis. The video also speculates that old functions may be replaced in the future, but that is the state at the time of recording, not the current version. When checking the configuration today, you should first confirm that the target firmware still contains the relevant commands.
How it affects the Setpoint given by the pilot
The author compares this function to a pair of "auxiliary hands": the general direction of the pilot is still determined by the joystick. The controller captures the unfinished tracking error, rotates the error to the new direction of the aircraft, and uses it to correct the Setpoint after limitations and calculations, allowing the aircraft to continue to make up for the previously incomplete parts.
This metaphor does not mean that the firmware will predict the pilot's intentions, nor does it guarantee that any aircraft will be more obedient. The effectiveness of Absolute Control depends on error estimation, limiting, filtering and the entire set of PID behavior; improper configuration can alter the feel of control. Potentially valuable for precision movements, but must be verified through logs and repeatable testing.
Understand the four types of parameters instead of blindly copying values
The video uses "get abs" in the CLI to view relevant parameters, focusing on Absolute Control Gain. Gain is similar to the intensity coefficient: when it is 0, it is equivalent to turning off. When the value increases, the proportion of the error brought into the correction increases. Absolute Control Limit and Error Limit are used to constrain the scope of action and prevent infinite amplification of errors.
The author made it clear that he did not explain Absolute Control Cutoff thoroughly, and also invited control and signal professionals to add to it. This article therefore does not concoct an explanation for Cutoff, nor does it provide a general numerical value. Version differences may change default values, units, and commands; you should save the diff before adjusting parameters, and read the source code or official instructions of the corresponding firmware version.
Secure, fallbackable verification method
Back up a full CLI diff to confirm motor, filtering, PID and mechanical status are stable before establishing a shutdown/current value baseline in the same map or controlled flight path. Only change one relevant parameter at a time, and compare Setpoint and Gyro tracking, abnormal oscillations, and flying feel using the same Rates, blades, batteries, and movements.
Production aircraft should not be downgraded to reproduce video if the current firmware has removed or rebuilt Absolute Control. The value of the historical tutorial is in understanding error vector rotation; the specific implementation should be subject to the current version. Any real-machine verification should start with small actions and retain the configuration for immediate fallback.
Operation steps
- First understand why the error requires rotation
Match the coordinate changes after the body rotates with the two-dimensional U-turn analogy, and then read the parameters.
- Distinguish between Absolute Control and old I-Term Rotation
According to the historical context of the video, the former handles more general three-axis error vector rotation, while the latter has a narrower range.
- Confirm whether the current firmware supports
Back up the CLI diff and query the relevant commands in the corresponding version; do not assume that the 2021 parameters still exist.
- Let’s start with the meaning of Gain
Understand that 0 means off, and increasing means stronger error correction, but does not directly copy the video or other people's values.
- Preserve the version boundaries of Limit and Cutoff
Check the current documentation and source code; the video author did not fully explain Cutoff, and this article does not add speculation.
- Verify with single variable and log
Keep hardware, rates and test actions consistent, change only one item at a time and compare tracking, oscillation and feel.
FAQ
Is Absolute Control autonomous driving?
No. It still works around the pilot's Setpoint, making coordinate rotation and limited corrections to unfinished tracking errors as explained in the video.
Why does the error direction change after the machine body rotates?
Because Roll, Pitch, and Yaw are machine system coordinates. After the body direction is changed, the same world direction correction will be projected to different axes in the new machine system.
The larger the Gain, the more accurate the aircraft will be?
This cannot be inferred. Greater Gain means stronger error correction, which may also change the feeling of control or expose other problems, which must be verified in small steps and with logs.
What should the Absolute Control Cutoff be set to?
The author of the video clearly did not explain it clearly, and this article does not provide guess values. Please check the source code, documentation and default configuration of the corresponding Betaflight version.
Does current Betaflight still have these parameters?
Not necessarily. Video released in 2021, features, commands, and defaults may have been refactored or removed and should be confirmed in the current target firmware.
Full timeline transcript
Transcripts are arranged according to video time, making it easy to quickly locate the explanation content. Transcript language: Simplified Chinese.
What are we going to talk about today? Today we are going to talk about the absolute control of Betaflight, that is, the function of Absolute Control is rarely mentioned on the Internet, but it does not mean that this function is not important or that this function is not easy to use. In my opinion, this function is actually very, very easy to use, and it is especially important for this Racing or when you are Free Freestyle in some relatively narrow places.
Let's first take a look at this function. What problem does it mainly solve? We know that when we fly the plane, after turning the joystick, the plane needs to track our setpoint, right? But often, this plane cannot achieve 100% of this track. Our setpoint will always have some errors, right? What about these errors? After the plane makes various movements, these errors should be rotated accordingly.
Better reflects our desire to control at the time. The paragraph I just said is a little difficult to understand. Let's use an example. The airplane moves in a three-dimensional space, so it is a bit difficult to analyze. Here we use motion in a two-dimensional space instead to simplify this analysis. Let’s assume that Lao Wang just bought a Laoye drone. The Laoye drone can only travel on a two-dimensional road.
Then our traffic police stipulated that when we drive this old drone, we can only drive on this center line and must drive near this center line to be safe. Now this Lao Wang accidentally ran to the other end. So at this time, he thought that I have to go to the middle to step on this step for safety, right? So he started to drive slowly towards the middle, right? But he was still driving halfway before he finished driving.
I received a call at this time, and I saw, eh, Xiaoli is calling. Xiaoli said, Lao Wang, Lao Wang, Xiaoli needs you today. When Lao Wang heard this, he said, "Hey, it's easy to say no." What should we do? Turn around. Now Lao Wang has made a U-turn, right? Now Lao Wang has made a U-turn. When we were here just now, Lao Wang had something, something, nothing wrong. When we were here just now, our Lao Wang needed to go here, to the left, right? We need to go to the left to return to the center line. Now turn around here.
The hanging head needs to go inside. Although it is our left side for us, what does this place become to Lao Wang? This place becomes its far right, right? If it still goes to the left, if it continues to go to the left, will it get farther and farther away from the center line? So after it hangs its head, what does the error of walking to the left become? Now it becomes the need to go to the right. This place is a little difficult to understand, I'll do it again.
At the beginning, when Lao Wang didn't receive Xiaoli's call, he only needed to go left and gradually return to the center line. After receiving a call from Xiaoli, asking him to go home, Xiaoli and Lao Wang made a U-turn. After the U-turn, the left error has now been transformed. It needs to be turned to the right in order to return to the center line. Otherwise, it will become farther and farther away from the center line. Then this upstool control absolute control is used to solve this problem.
In more academic terms, it is the actual rotation of the error. If you have followed Betaflight for a long time, you will know that there was a function called Item Rotation. Item Rotation is mainly used to solve similar problems. It is just that Item Rotation only rotates the impact of Yaw transformation on Raw and Page.
Apps Control This function treats the three axes equally. Changes in each axis will cause undifferentiated changes in the other two axes, and then it will perform real rotation on them. So Apps Control can be understood as an enhancement of Item Rotation. So Item Rotation may actually be removed from the library in the near future. There is just no one doing this job right now.
You can think about this particle carefully. Although this particle has made a lot of reductions, one is to change the three-dimensional space into a two-dimensional space, and the other is to reduce the angular velocity to this position, but in fact its basic principles are similar.
For us, when we fly an airplane, there will always be some errors. Even though this PID algorithm tries to reduce this error, it will leave some errors every time we make a movement. These remaining errors will eventually be absolutely controlled by us. This function will be replenished and used after replenishment. Rotate and then re-rotate.
After doing some calculations on the material difference, it is finally applied to our setpoint. What does it mean? It is equivalent to your two hands flying the airplane, right? But in fact, there is another pair of Betaflight little hands there to help you move your joystick, and complete the action that you have just not completed on the airplane by moving the joystick with its little hands. I don’t know why everyone understands this. That’s it
Your big hands are controlling the aircraft to ensure that the main direction of the aircraft is correct. Then there is a little secretary beside you to complete the actions of the aircraft and then move the joystick. The joystick I am talking about is the setpoint method. The setpoint method allows the aircraft to continue to execute the things that have just been executed. So why is this function suitable for pilots who require high operational accuracy?
After understanding this function, let's take a look at how to set this function in Betaflight. We open the bell line and type in ZABS. We can see that it has four parameters that can be adjusted. The most important parameter is the Absolute Control Gain. What does Gain mean? It is just a gain. What does this gain mean? This gain is actually
It is equivalent to a well-known volume knob. If you turn the sound of the TV up or down, the gain you adjust is the gain of the sound. It is actually a coefficient. The error will be expressed as this coefficient. If this coefficient is 0, then you are actually turning off the function of this app. Then the larger the coefficient, the greater the coefficient?
The error it will bring you will be larger. This means that the overall operating intensity of its Absolute Control in the project will be higher. The smaller it is, the lower its overall operating intensity will be. In addition, there are several other parameters that will set the Absolute Control Limit and the Error Limit. In fact, they set two upper and lower lines. This is not particularly important. There is another parameter, which is Absolute Control Cutoff.
What this parameter does is a bit complicated. I am not confident that I can explain it clearly in this video, but I know that there are many friends in my audience who were born in industrial control or signal system related majors. If you know this function, please add it in this barrage area or comment area so that we can all learn and make progress together. Well, I will make so much noise with you today. If you think this video is helpful to you.
Please don’t forget to connect three times with one click. I’m Horank. See you next time!
