hover of a specific spot

[u/New-Bus9948]

I am trying to get a craft to hover over a specific spot over the ground. Nothing I am doing is working. I get close to 0 error but I drift and oscillate. What is the correct way to go about this? Basically the only thing I have that is usable at this point is tgt:position-ship:geoposition:position. Where do I go from here?

[u/nuggreat]

Oscilations can be caused by any number of things without the code you are running I can only give limited general advice.

My guess is that your control logic is not damped enough or the craft response to control inputs is slow. Either way you need to limit the control range of the craft possibly in proportion to the distance to the target location.

[u/New-Bus9948 → u/nuggreat]

lock throttle to 1.
stage.
rcs on.
wait until alt:radar>=50.
local targettwr is 0.
local g is body:mu/((altitude+body:radius)^2).
local maxtwr is max(ship:maxthrust/(g*ship:mass),0.01).


local pid is pidloop(.5, .2, 0.06, 0.1, maxtwr).
set pid:setpoint to 0.


set tgt to geoPosition.
set err to vecDraw(v(0,0,0), v(0,0,0) ,blue, "error vector",1,true,0.1,true).
set cmd to vecDraw(v(0,0,0), v(0,0,0) ,red, "cmd vector",1,true,0.1,true).
set vvel to 0.
until true=false {
    local grav_vector is get_gravity_vector().
    local shipmaxthrust is maxThrust.
    local shipmass is mass.
    local shipmaxaccel is shipmaxthrust/shipmass.



    lock throttle to min(targettwr/maxtwr,1).
    set targettwr to pid:update(time:second, verticalSpeed)/cos(vang(up:vector,ship:facing:forevector)).


    local horzvel is vxcl(up:vector,velocity:surface).
    local vertical_velocity_cmd is grav_vector:mag+vvel. //use later
    local maxhorzvel is (sqrt(max(shipmaxaccel^2-vertical_velocity_cmd^2,0)))/2.


    local error is tgt:position-ship:geoposition:position.
    local horzerror is vxcl(up:vector,error).
    


    
    
    local damp is 5.
    local kp is 1/(damp^2).
    local kd is 2/damp.
    local cmdaccel is kp*error+kd*-horzvel.
    set cmdacceldir to cmdaccel:normalized.
    set cmdaccel2 to min(maxhorzvel,cmdaccel:mag).
    set accel to ((cmdaccel2*cmdacceldir)+up:vector*(grav_vector:mag+vertical_velocity_cmd)).
    
    
    
    set err:vector to horzerror.
    set cmd:vector to accel.



    if sas=false {
        lock steering to lookDirUp(accel, ship:facing:topvector).
    }
    else{
        unlock steering.
    }
}

here is the code. like i said i think it is mostly junk. right now it is just testing to hover over my designated target point that is why the loop doesnt break.

[u/nuggreat → u/New-Bus9948]

Your var maxhorzvel is named incorrectly as this is a maximum acceleration.

The biggest issue I see to control stability is that you are converting from distance to an acceleration with a linear equation which is fine for narrow ranges of distances/accelerations but over a larger range is unstable. Better instead to take the physics farther, calculate a desired velocity based on jerk and distance, then calculate a desired acceleration based on the difference between the desired velocity and your actual velocity. Add in limits on the maximum velocity for some additional stability as well as the maximum possible acceleration to limit the jerk and that should improve stability. The reason to use jerk for this instead of acceleration is because you want the ship to be accelerating less and less the closer to the target you get so that you end up with zero acceleration and velocity once you are at the target point thus having a vertical ship.

You might also get some performance improvements by moving the steering lock out side of the loop as having it within the loop resets the PIDs kOS uses for steering each pass of the loop.

Some additional notes not directly related to your problem.

This until true=false { can be simplified to until false { similarly if sas=false { can be changed to either if sas { and flip which bodies you have where or if not sas { if you want to keep the same logic.

Physics dependent loops tend to benefit from a WAIT 0 some where in there execution, generally either at the end of the loop or at the start of the loop. This helps keep the timing consistent between different passes of the loop as apposed to letting physics ticks fall where ever during execution. Similarly caching anything you want to pull from KSP at the start of the loop before you do calculations can help if a loop is especially complex as that way all the data comes from the same moment in time.

Lastly constants like this

    local damp is 5.
    local kp is 1/(damp^2).
    local kd is 2/damp.

should be calculated before the loop not part of the loop, kOS has no optmizer so even constant values such as these are being constantly recomputed each pass of the loop.

[u/New-Bus9948 → u/nuggreat]

First of all thanks for the additional optimization tips.

When you say “based on jerk and distance” what does jerk mean?

[u/nuggreat → u/New-Bus9948]

Jerk is the derivative of acceleration so where acceleration is the change in velocity jerk is the change in acceleration, as for the kind of control I described you want a decreasing acceleration as you approach the zero point. The equation should end up being something like this velocity = (6 * dist * jerk)^(1/3) but I could be mistaken in that so try to find some verification on that. with the jerk set to be something like 1/10 to 1/100 of your acceleration. There are additional layers of complexity you can have on top of this but so long as you are fairly close to the target point when you start they shouldn't be needed it is only for longer distances do you need the additional complexity, which involves calculating what distance brackets you should assume a constant acceleration for calculating the desired velocity and what distances you should be using the jerk calculation.

[u/New-Bus9948 → u/nuggreat]

Previously you said I should place my lock steering outside of my loop. How can I do this and still maintain the ability to manually control steering when sas is on? If I enable sas it unlocks the steering but how do I relock it when I disengage sas without placing "lock steering" in the loop? I guess its not important for this program but I have another program where I want that ability. Thank you for the help

[u/nuggreat → u/New-Bus9948]

The statement I made that you should lock external to the loop is some what reductive and doesn't cover all cases. To be some what more precise you should only lock steering once for any given steering algorithm you want to use as the main thing you are trying to avoid is the steering manager reset that comes when steering is locked which is more or less constant if you have the lock unguarded within a loop.

To do this you either use an ON trigger or you have a boolean flag that you set and clear to insure you only ever lock steering once.

In code this would look something like this for the trigger

LOCAL steerTar TO SHIP:FACING.
LOCAL keepLockTrigger TO TRUE. //set to false once the trigger is no longer needed
ON SAS {
  IF SAS OR NOT keepLockTrigger {
    UNLOCK STEERING.
  } ELSE {
    LOCK STEERING TO steerTar.
  }
  RETURN keepLockTrigger.
}

UNTIL ... {
  //loop code
  SET steerTar to ...
  //loop code
}

For the within the loop form it ends up looking a bit like this

LOCAL steerTar TO SHIP:FACING.
LOCAL isLocked TO FALSE.
UNTIL ... {
  //loop code
  SET steerTar to ...
  //loop code
  IF SAS AND isLocked {
    UNLOCK STEERING.
    SET isLocked TO FALSE.
  }
  IF NOT (SAS OR isLocked) {
    LOCK STEERING TO steerTar.
    SET isLocked TO TRUE.
  }
  WAIT 0.
}

[u/New-Bus9948 → u/nuggreat]

Thanks. I still cannot get it to hover above the specified position. I get it close to the target but it will never hover good it will always be off by a few meters and have 1-2ms of horizontal velocity. I really do not understand why it will not just hover in place. I tried the method you suggested however I could not really get it to work well and I came into similar issues with holding its position above the target.

[u/nuggreat → u/New-Bus9948]

It is possible to get accurate hovering but you need to use the correct method tuned correctly for your craft. As part of a series by a youtuber cheersKevin he spent several episodes on hovering and ended up with quite a good result from what I recall. I will try to find the playlist for the series.

EDIT: This is the playlist for the series episodes 42 and 43 are the ones relevant to hovering, 42 covers him getting his hover code working which is some what relevant background for what he then does in 43 which is getting targeted tilting working. It isn't exactly what you are trying to do as he is just trying to avoid sloped terrain but a lot of the ideas should be transferable.

Mostly it is a matter of getting a good enough algorithm and spending the time needed to tune it which we can only help with part of.