Poor SAS performance

[michelcolman]

When I started using SAS, I was a bit surprised how poorly it hunts for the intended target, often overshooting it, especially on craft with poor attitude control. I often have to help it and actually counteract its inputs, slowing it down as it approaches the target while SAS is still trying to accelerate towards it. It looks like the system's gains are tuned for high performance attitude control (working well using vectored engine thrust, with RCS or on very light craft with sufficiently large reaction wheels) but are woefully wrong when control is more limited. I can see it continue to accelerate towards the target while it's totally obvious it's going to overshoot and it ought to be slowing down.

It would be nice if SAS would take the available torque into account so it stopped accelerating towards the target and started decelerating a lot earlier to avoid overshooting.

[Evanitis]

Umm... SAS's function is to hold a specific heading, and it does that pretty well if the craft has sufficient ways to control. It shouldn't be able to land at a specific target, not to mention making a rendezvous.

[michelcolman]

When I used the word "target", I meant whatever target direction the SAS system has been tuned to. Prograde, normal, maneuver, whatever. When it's not already pointing that way and, for example, you switch it to hold prograde, it starts turning towards prograde but then overshoots it horribly, comes back, overshoots again, etc, until finally stabilizing. Almost invariably, I actually need to counteract its inputs. While it's rotating to the right and approaching the symbol, I have to push "A" to slow it down before it gets there (or cheat by activating and deactivating time warp when it passes through the desired direction). Otherwise, on a relatively large rocket using standard pod control wheels, it takes forever to stabilize.

[Evanitis]

Yeah, I see. I tend to use MJ's SmartA.S.S., that feels a bit better in this respect... but does the same if the craft is huge and has poor controls. Guess it's not that easy to precisely compute. I kinda' do what you do - corect the controls manually, or do it in full manual. Though I always blame my craft when I need to do so - I could have included sufficient RCS or reaction wheels.

[Frozen_Heart]

The SAS is pretty poor. It can't hold a heading even if the player can with ease. Just drifts away.

[NathanKell]

We're aware the PID has trouble. When we have time we'll be working on it.

[Snark]

8 hours ago, NathanKell said:

We're aware the PID has trouble. When we have time we'll be working on it.

Thanks, good to hear!

One other issue I have with it, quite aside from the PID tuning:  it doesn't seem to be following great-circle paths.

That is, suppose I'm pointing in direction A (and am stationary), and then I tell SAS to point in direction B (prograde, retrograde, maneuver node, whatever).  I would expect the ship to rotate in a "straight line" from A to B.  That is, if you imagine the navball crosshairs scribing a trail across the navball's surface as the ship rotates, I'd expect it to be a great-circle arc on the surface of the navball.

Instead, what the navball seems to do is to follow some sort of "do X, then Y" behavior.  I've never observed it closely enough to work out exactly what it's doing, but I'd guess it's something like "rotate to make angle #1 match, then angle #2", or something like that.  As if a plane were navigating from Seattle to Miami by heading due southeast until the latitude matches, then sharply turning to head due east to the target-- something like that.

 

[FullMetalMachinist]

15 minutes ago, Snark said:

I'd guess it's something like "rotate to make angle #1 match, then angle #2", or something like that.

It might be doing the equivalent of a 'dogleg' movement. When some older CNC milling machines do a rapid movement, they can't calculate the correct angle to move straight from coordinate x1, y1 to x2, y2. So instead they move in a 45 degree angle until it hits the first position (x or y, whichever is closer), then goes straight to the second. 

[Snark]

19 minutes ago, FullMetalMachinist said:

It might be doing the equivalent of a 'dogleg' movement. When some older CNC milling machines do a rapid movement, they can't calculate the correct angle to move straight from coordinate x1, y1 to x2, y2. So instead they move in a 45 degree angle until it hits the first position (x or y, whichever is closer), then goes straight to the second. 

That kind of motion is certainly what it feels like.

[Crzyrndm]

I would have actually said it just has a lot of trouble with being close to 90 degrees out from it's target, as it's passing through about 100-80 degrees of separation that it suddenly dives off in a seemingly random direction (probably indicates it's using Euler angles or Vectors instead of Quaternions). Even with Quat's, if you do the calculations in single precision the roll error gets really jumpy around 90 degrees (found that one out when I was calculating the shortest roll distance for my SAS mod)

Another oddity to note with the current SAS PID modules, the module used for the direction following has damping even when it's only under proportional control. I'd guess this is so the roll controller actually does something without any fixed target, but it does make tuning pitch/yaw for some torque ratio's nigh on impossible.

Edited January 26, 2016 by Crzyrndm

[magnemoe]

On 24.1.2016 at 0:30 PM, Evanitis said:

Yeah, I see. I tend to use MJ's SmartA.S.S., that feels a bit better in this respect... but does the same if the craft is huge and has poor controls. Guess it's not that easy to precisely compute. I kinda' do what you do - corect the controls manually, or do it in full manual. Though I always blame my craft when I need to do so - I could have included sufficient RCS or reaction wheels.

Yes, mechjeb has an better controller who don't overshoot so much. I found that using SAS keep prograde during aerobrake tend to drain batteries hard. 
 

[Francois424]

Remote tech computer is pretty nice as well... as long as you have a connection (or a human at the helm?  been a while since I played with that mod).

Good news Nathan, thanks for the heads-up !