Rudder P vs I Gain

[Trex3940]

Hi Guys,

I know I've out similar posts up before regarding the understanding of these parameters, however I just need a more refined simplified understanding which someone can hopefully provide.

Ive kinda understood the basics of rudder P (proportional) gain to be a setting adjusting how fast a response the heli is receiving but can someone explain further?

Someone has told me I (Integral) gain is an adjustment of how locked in the rudder is during a manoeuvre, more like a rudder gyro gain, is this correct?

if I gain is adjusting locked-in feel, what is standard gyro gain used for and what settings do I adjust??


Any help will be much appreciated,

thanks

[waveydavey]

I don't think you can attribute the locked in feel to the I gain. The I gain can be viewed as being the error between what you've asked of the gyro and how long the error has been active but this works with the P gain. If the error between what you've asked for and what you're getting from the heli is say 10 degrees per second and the proportional gain is for example 0.5 then the calculated error response would be 5 degrees per second. Now the clever bit is trying to speed the response up of getting that error to zero and this is where the I gain comes in but too fast and you'll go past the error being at zero so you need something to dampen that response down and that is where the D gain comes into it.

Not sure if that helps, take a look at a great FBL article written by Jim Davie in the tech room, it's a great description of the PID and how flybarless controllers work. More about the swash but same principle

RcHeliaddict - MHW FBL Article

[Grumpy]

As I understand it...

P-gain is instantaneous, this is the response to momentary movement. Increasing it means that the tail will respond more strongly to any instantaneous movement in the yaw axis.

I-gain takes into account 'history'. It compares current position with previous position. If it 'sees' a difference then it tries to correct it and move the tail back to where it used to be. increasing means it responds more strongly to the difference it 'sees'. This is why i-gain is called 'rudder locking' because it has the effect of locking the tail in one position and preventing it from drifting over time.

[waveydavey]

The P I and D in most controllers work together to control the error from what you've asked for IE stick position and the rate the heli responds. The stick position and the fact the heli won't react immediately leads to an error IE the difference between requested and actual. It's the summation of these errors that give you the output to the control surface.

I gain is the error over time IE the time it should take the controller to correct the error. So if this was a high gain I suppose it would make the tail seem more locked.

Not sure that with any rate command system the controller is looking at where it was and where it should be it's only concerned with am I rotating at ? degrees per second, if not what do I have to do to get there.

[Mike Sanders]

take a look at a great FBL article written by Jim Davie

I have just read that. Phew!

I am wiser as a result but as the magician used to say...not a lot!

[waveydavey]

I have just read that. Phew!

I am wiser as a result but as the magician used to say...not a lot!

I must have read it 10 times Mike, still go back to it

[myxiplx]

The best resource I've ever found on this are the series of podcasts by RC Heli Nation, they discussed PID tuning in a few episodes. They did one early on about governor gain, and then episodes 133 and 134 really went into FBL tuning. I've been doing my best to write up a summary of what I've learnt, but haven't had the time to really listen to everything again to check that I'm actually right.

The first podcast is here, and it's well worth a listen:
RCHN V 2.0 EP133: PID 101 » RC Heli Nation

But here's my understanding of what the gains do, and why you sometimes need to tune them:

• Main gain
  • This is usually a combination of P, I and D gains, set in a particular ratio by the manufacturer.
  • You always want to get the main gain close first before doing any individual PID fine tuning. All the different gains interact, so if you change this later you will likely lose the benefit of any tuning you have done.
  
  
• P gain
  • Proportional gain. Basically means the bigger the error, the harder it will try to correct. Higher P gain makes the helicopter more responsive, it makes cyclic more snappy,or the tail return to position more quickly.
  • Too high P gain will cause overshoot, and a fast wag. It's easy to illustrate with a tail example: If the P gain is too high it rushes to get where you want, but can't stop in time and overshoots. It then rushes back the other way, but overshoots again. End result: A fast wagging tail, or a fast wobble on the cyclic.
  • Too low P gain gives a 'mushy' feel. Basically it will be slower to respond, and you could have a slow wobble.
  
  
• I gain
  • Integral gain, this basically looks back at the error over time.
  • This is basically what locks it into place, it works with P gain but it's main effect is to eliminate drift or hold position in hard manoeuvres.
  • So while P gain makes the tail (or cyclic) more responsive. The main effect of I gain is to hold it in place.
  • If your tail drifts off position or you have a slow wag that won't go away, increase your I gain. (do a hard pitch pump, even if the tail blows out it should return to exactly the same spot each time).
  • However I believe that if you increase I gain too far you will get more overshoot.
  
  
• D gain
  • Differential gain. This attempts to look ahead and predict what's about to happen. ie. It tries to stop things like overshoot.
  • D gain improves stop speed.
  • The major problem with D gain is that it's *extremely* sensitive to vibrations, and can amplify them to the point the helicopter becomes uncontrollable. Because of that some manufacturers don't let you play with D gain, and others heavily restrict how much you can tune it.
  • You can increase D gain to get more crisp stops, but if you start to see a shudder or shake on stops drop it back down.

The tricky thing with gains is that while you can learn the basics, there's no easy way to tune them. Every physical setup is different, you just have to learn how tuning works and apply it each time.

The nice thing is that modern FBL units have pretty good defaults, so you don't always need to go too far into this stuff.

This is the sequence I'm working on now for tuning:

1. Disable any governor (it'll make tail tuning a nightmare otherwise)
2. Base setting.
   1. To begin with, find some decent middle ground settings, so the heli hovers ok.
   2. Give a quick jab of cyclic and drop the main gain if you see any shuddering.
   3. Similarly, give it a quick stab on the tail, and drop the tail gain if you see a fast wag.
   
   
3. Nitro tuning.
   1. If you have a nitro engine, the next thing you need to do is to tune the engine and get that running well and holding headspeed.
   
   
4. Tail tuning.
   1. Next is the tail, problems with the tail can make it appear that cyclic isn't holding, so you have to tune this first.
   2. Ensure anything like tail precomp is disabled, these can mask problems, or make it harder to tune.
   3. Start with the main gain, raise it until you start to see tail wag, then drop it a little.
   4. If you want to do PID tuning, move to that next, but it's optional and really only needed for advanced fine tuning.
      1. From a hover, do a hard pitch pump. If the tail doesn't return to exactly the same spot, increase your I gain.
      2. Do a fast piro left and right and see how it stops. If the stops are slow, increase D gain a little. If you overshoot or get wag, decrease your P gain a little.
      
      
   5. Finally, with the tail tuned you can re-activate features like tail precomp to help the tail hold better during heavy collective.
   
   
5. Cyclic tuning
   1. In a hover, give a quick jab of aileron or elevator. Just as with tail tuning, you want to increase the gain so it's nice and responsive, then when you start to see a quick shudder or wobble drop the gain down a point or two.
   2. I still need to work on writing up the more advanced PID cyclic tuning.
   
   
6. Governor
   1. With everything else set, activate the governor.
   2. Too high a governor gain will give you a 'choppy' sound, basically it's overshooting and running the motor too fast / too slow / too fast / etc...
   3. A high gain can also cause the tail to wag lose hold. If you start to see tail issues after enabling governor, drop your governor gain a few points.

I'm working to add more details to this, with suggestions on what to look for when flying, and how to apply that to PID tuning, but I've got a lot more work to do before I'm confident with what I'm saying there.

I hope you find the above useful, and if anybody has any feedback or suggestions based on that I'd love to hear them.

[scallybert]

Really briefly...

Bear in mind that gyros measure angular velocity (rate of rotation), not position.

Proportional is effectively damping. It's an input proportional to the error. Not dissimilar to a viscous damper on a door - but it's active (so can put energy into the system, such as when you get oscillations).

Integral integrates the error. Roughly the input is increased over time in relation to the error. This can take account of slop or offsets. However, since the integral of speed is position, it also gives you heading hold.

[soluble fish]

thanks - useful and makes sense. will give the podcast a listen.
thanks again (",)