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Docking Arm Flails Smaller Ship Around on Surface of Mun/Minmus


Rogue Phoenix
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I've built and deployed a refueling station - mines ore, converts it to fuel. On this station I have a robotic arm with docking ports (regular, Jr) at the end. Its purpose is to act like the fuel nozzle at a gas station for cars. Dock to a spacecraft that's landed nearby, and pump fuel into it. 

The problem is, whenever they connect, the other ship flails about wildly, sometimes breaking parts and often winding up at some weird angle that makes it difficult or impossible to launch. 

Reducing the docking force helps, and slllllooooooowwwwwwwwly moving the arm into the place where they dock sometimes does the trick. But more often, some kind of feedback loop between the landing suspension of and the docking arm makes it flail. If the to-be-fueled craft is light enough - and it often is, particularly before loaded with fuel - it gets thrown about like a hooked fish. 

How do I reduce or eliminate this behavior? 

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Here is another tip, which has worked well for me-

Turn down the Spring Rate and Damping Rate on your suspension.  Turn them down to zero, or as low as you can.  You may need to enable advanced tweakables to do this.   Do that for wheels and landing gear, anything with a spring in it.

That will make your vehicle handle differently.   If that works, and if needed, then slowly try docking with increased spring or damper rates.

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Hi @Rogue Phoenix

I see you have 4 posts. I am unsure if I should welcome you.

Yes, I am sure. Welcome to the forum.

 

Might not be your problem but I have found that when I dock a ship with a space station there can a Kraken attack.

Especially when the station is complex (Iss). My solution is to have the SAS off on the station when close and just about to dock turn the ships SAS off as well.

It is as if the two SAS are on and commanding two incompatible things As opposed to becoming one SAS on one ship.

 

M E

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I assume you are using breaking ground robotics? If so, immediately after docking shut off all SAS. Especially if COM is not, well centered. Also, lower the motor power and increase the dampening settings beforehand. And when I say increase I mean a LOT if the docking masses are large.

 

Then after successful dock, lock the motors before moving to another vessel. When all is settled, reactivate SAS. So far this has worked for me.

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+1 to the turn off SAS tip others said.

 Another helpful tip is if you are using autostruts, make sure they are set to grandparent, not root or heaviest.  When you dock craft with autostruts to root or heaviest part, the root or heaviest part can change and the autostruts reset to attach to the new root/heaviest part, creating forces that can make a mess of your craft.

 

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@Rogue Phoenix welcome.

Yes this is a problem I have too.

For surface bases I 'think' it is to do with 'flex' of springs etc on landing legs, wheels and robotic parts .   

When you dock or connect with a claw the whole thing becomes one vessel, and the 'physics' kicks in to adjust from two seperate vessels to one, causing things to move and flex.  Which causes a viscious cycle of jumping and flexing.  Also, as you add or remove fuel you change weight distribution, which causes movement due to landing legs etc compressing.

Adjusting spring and damper settings may help (if you can do it fast enough).

I found that having the 'base' or 'station' not resting on anything moveble (landing legs, wheels, robotic parts etc) helps, and not using any robotic parts on the connection mechanism is safest.  The only point of 'springiness' on my surface refuelling system are the wheels on the harvesing rovers.   And I connect as slowly as possible to avoid adding any tension.

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An explanation why SAS can sometimes become the problem and not a solution is that it generates control input from the orientation of the control point, and not the whole of the craft. So if this control point (the "control from here" point) is at the other side of a flexible connection from the reaction wheels, then the control inputs from SAS will bend this connection and not really re-orient the control point. But the advice stays the same: switch off SAS.

For the actual docking I second @Dientus' advice: lower the power on the docking arm servos to the minimum value needed to move the arm itself. That way it is more likely that the arm flexes instead of throwing around your craft.

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On 6/16/2021 at 11:23 AM, Dientus said:

I assume you are using breaking ground robotics? If so, immediately after docking shut off all SAS. Especially if COM is not, well centered. Also, lower the motor power and increase the dampening settings beforehand. And when I say increase I mean a LOT if the docking masses are large.

 

Then after successful dock, lock the motors before moving to another vessel. When all is settled, reactivate SAS. So far this has worked for me.

yes.  Ran some tests which I'll post soon.

What does locking the motors do? I set up a KAL-1000 specifically to lock all the arm components (there are a lot of them). In the VAB, I locked everything, then ran the program (on a separate KAL-1000) to stow the arm, expecting nothing to happen. The arm stowed. Priority on both controllers was 3.

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7 hours ago, Rogue Phoenix said:

yes.  Ran some tests which I'll post soon.

What does locking the motors do? I set up a KAL-1000 specifically to lock all the arm components (there are a lot of them). In the VAB, I locked everything, then ran the program (on a separate KAL-1000) to stow the arm, expecting nothing to happen. The arm stowed. Priority on both controllers was 3.

Last I knew using KAL-1000 to lock motors was dodgey. If the part is sensed as moving the parts won't lock. I think that might still be the case but do not know since I lock individually in flight and just haven't tested in quite awhile.

 

In my experience, locking and shutting off motors seem to make the part much less 'wobbly' and leaving them that way while switching to another vessel has so far avoided the kracken.

 

As an example how I use robotic parts, I have a space station that has 4 very large solar panel arrays made up of g11 hinge, servo, structural pieces and rows of gigantor xl panels. When I dock certain large ships, they would destroy my sollar array so i unlock and turn on, fold the arrays up out of the way, relock and shutoff,  then dock. I have three similar space stations in orbit around Duna, Mun, and Kerbin. So far I have yet to ever experience a kracken attack with any.

 

Hope this helps.

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I've had exactly the same problem with a little rover that I'm using a  hinge/docking port arm to deploy and stow.  To my knowledge, I was not using autostruts on that craft.  Like @Dientus said, turning off SAS immediately after docking was what fixed it for me.  Adjusting  stuff about the motors no doubt could work as well, as does turning off all the reaction wheels in the smaller craft, but those maneuvers are more cumbersome and not really doable after the fact when everything is flopping about. The SAS code has never been all that good at damping rather than amplifying oscillations around wobbly joints like docking ports, especially if what's on one side has a lot more control authority than what's on the other. If you think of it as riding a playground swing, it tends to stick its legs out when it should be pulling them in and vice versa, especially if the lag time between applied force and motion is different between different reaction wheel modules.

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