From more than a month from now Aug 2014, I posted a blog titled ZERO_PID Tune for Multicopter, It was about an algorithm I developed that allows quadcopter pilot to reset Gyro PIDs to ZERO, and fly! Using this algorithm, the quadcopter was able to generate valid values for P & I, and can successfully fly, the demo was on Multiwii code. Since then I was working on enhancing this algorithm, as I discovered some opportunities for improvment.
Although as you can see in the video that quadcopter really flies, there was a devil in the details. P-factor & I-factor would saturate if you fly long enough – I discovered this later few days after the video-. Although quadcopter was safely taking off, but flying for a long time -1 to 2 min- in this mode will make P & I saturate especially if you play with sticks hard.
Another issue I have discovered in the first version was calculating I-factor assuming it is a percentage of P-factor. Although it was calculated separately from P-factor, it ended up as a percentage of P-factor.
Although my background is engineering, but I am not a big fan of mathJ, so I started to simplify the problem as much as possible, and avoid complex math. The model depends on angular velocity of gyros only. It assumes there is no interference between different axes –which is true theoretically at least- as actual Gyro MEMS has slight interactions between axes.
First, let us start by a quad arm with a motor on it as in the figure, initially the motor is running, but you do not know if it is running fast enough to generate exact thrust to keep the arm with angular velocity zero. Again let’s assume that the motor is not producing enough thrust, so when reading the gyro we will get V1 = v1. With single value you cannot judge, you need to read two values, so you wait until the next IMU loop and read V2=v2.
Now assume that both V1 & V2 have the same signs. This means the same direction, which is falling down –counter clockwise in the figure-.
Condition#1: If V2 > V1 : this means the arm is accelerating and the quad in danger of flip. So let us reduce the P value by one if the difference is high enough.
Condition#2: If V2 < V1 : this means the arm is decelerating and the quad is trying to adjust itself to reach angular velocity zero. However there is a danger here that the deceleration is so fast so that it will not only stop the arm but will move in the other direction and start oscillations. So let’s decrease P value by one if the difference is high enough.
Please note that by design, quad stability is active stability, i.e. you need to monitor and adjust to keep it stable, so whatever the P value is, it will oscillate, the idea here is the oscillation amplitude should be minimal.
Condition#3: What if V1 & V2 has different signs, i.e. our quad arm has reversed its rotation direction. This case is ignored as it will always happen due to oscillations as I mentioned in the previous paragraph. And we rely on condition 2 to minimize oscillations.
What if the arm –in the figure- rotation direction is clockwise, in this case we are calculating the right arm that is falling down. That is why we use abs(Error) . So we always consider the falling arm.
ZERO_PID algorithm can be written now as follows:
void G_Tune (Error) {
If (Sign(Error) == Sign (OldError))
{
If ((abs(Error) - abs (OldError)) > High_Enough _1) // we are falling down.
{
P = P + 1;
}
If ((abs(Error) - abs (OldError)) < - High_Enough_2 ) // we may oscillate.
{
P = P - 1;
}
}
OldError = Error;
}
Simple :) and yet it works. Well almost :)
As we can see the above algorithm changes P-factor only. In the first version I used a parallel condition for I-factor that increases and decreases it with 0.1 steps. Later I discovered this was not the best approach.
Challenges
Algorithm idea looks simple; however we need to consider some points that affect the performance of the algorithm.
As I mentioned in the beginning of this article, I was calculating I as a percentage of P. Well in an article about sensors & PIDs I mentioned that I component in PID for gyro is used for restoring, it acts as a memory, because it is based on many values not the most recent one as P or the difference as D. I think about I component as the DC current in a signal or as a trim in your TX. Try to fly in Acro mode with I factor equal to Zero, you will find that if you tilt your try then take your hand of the sticks it will not get back, if you add some values to I-factor, it will tend to restore itself. And this is obvious because the I-component –i.e. the integration value not the I factor value- that was accumulated to overcome your stick need to be reduced back to zero. To have this effect I used a simple average variable to calculate the average value of gyro readings in a time window, and if the value is not Zero –higher or lower than a certain range- I is incremented otherwise it is decremented.
I attached a working code, you can use it to fly in ACRO mode only, as I used other PID parameters to as ZERO_PIDs parameters in order to study the effects above.
LEVEL_P: represents High_Enough _1
LEVEL _I: represents High_Enough_2
ALT_P: represents time_skip counts.
ALT_I: represents number of samples used for averaging Error for calculating I factor.
Note: if you put value ALT_I = 0.01 this means actually 10, as the minimum value is 0.001 which is 1. Same for other values, for example LEVEL_P = 1.0 means 10 and value 0.1 means 1 as there is no 0.001 and this is the minimum value for P, so take care when you play with values.
These figures are taken from MultiWii EZ-GUI for 3 flights:
as we can see High_Enough_1 = 10 & High_Enough_2 = 15 works very nice. time_skip here is 10 and values that are read for averaging to calculate I-factor is 100.
as I increased High_Enough_2 to 20 P values started to raise, especially in the third trial where I played more aggressive with quad compared to the calm first two flights.
Final Notice
Still everything is experimental, and try it on your OWN RISK, try to fly smoothly and dont take off suddenly, let it leave ground easy.
Latest Code is Here
Thank you very much for publishing your code and the explanation, you rock!
welcome ... Thank you :)
With the final frontier being automatic calibration of the loiter PIDs, the whole thing may one day calibrate itself.
Is not this how autotune in the APM works ? Watching the error and increase/decreasing until the error is reduced to some acceptably low value ?
No there are complete different things.
chk this"
The AUTOTUNE mode is a flight mode that flies in the same way as FBWA, but uses changes in flight attitude input by the pilot to learn the key values for roll and pitch tuning. So the pilot uses their transmitter mode switch to switch to AUTOTUNE mode and then flies the plane for a few minutes. While flying the pilot needs to input as many sharp attitude changes as possible so that the autotune code can learn how the aircraft responds."
As the name suggest it is tuning so you should have initial PID and fly in stabilize mode, and autotuning will adjust PIDs more sharply, after injecting some attitudes and check response.
Zero_PID is different, you need no preset PIDs just put them all to ZEROs, and fly in ACRO mode, and it will fly and suggest new numbers.
Actually I believe that they complete each others, I believe ZERO_PIDs will give good initial estimates for GYRO, then comes Auto_Tuning to update the values in stabilize mode.
Thanks
I've always wondered if this type of tuning is happening automatically in the background on some of the commercial flight controllers. I know a few people who swear that as they fly performance improves. Unfortunately there's no way to find out given they are closed systems.
Can this be used for camera gimbals as well?
Of course! But don't tell them..
ok hehe!
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