Hard joint behaves like rubber band [SOLVED]

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Re: Hard joint behaves like rubber band

Postby Dave Gravel » Thu Mar 28, 2019 4:51 pm

Yes now I see.
I have already get similar gab with the joint, but currently only when I connect 2 joints acting on the same body.
Edited: Video showing the gab problem when two joints acting on the same body, Maybe it can help a other user with similar problem.
https://www.youtube.com/watch?v=qQJ6R9Nk8Bg

In the video the left gear with the long shaft in the middle is one compound construction.
The right gear is a other compound construction without shaft.
The left gear is linked with a hinge on the H base, And the right gear is linked by a hinge to the left gear with the shaft.
This situation cause gaps on the two gear body linked with the two hinges.

For fix this problem I have remove the hinge on the right gear.
I have add the right gear objects inside the left gear compound construction.
On this way the left gear the shaft and the right gear become only one object with only one hinge.
This have fix the gabs problem with my demo and my wrong construction method.
Sorry about my english.
Last edited by Dave Gravel on Thu Mar 28, 2019 6:30 pm, edited 3 times in total.
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Re: Hard joint behaves like rubber band

Postby misho » Thu Mar 28, 2019 4:58 pm

The thing is, this doesn't happen (or, it happens with a far less effect) when the joined bodies are applied a torque (not a linear force) and they rotate around each other. The centripetal force doesn't pull them apart like the linear force does.

You can easily reproduce this in sandBoxDemo by replacing the code in MishoRocketTest.cpp with what I posted... Anyway - let's see what Julio says...
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Re: Hard joint behaves like rubber band

Postby Julio Jerez » Thu Mar 28, 2019 5:16 pm

what Dave say is true, In newton is not legal to have tow bodies connected by more than one joint with bilateral row. Basically each joint can only have up to 6 mutually independent rows.
contact are treated specially by adding a regularizer to the contact row.
This is why I say I was confused when you were talking about contacts.

But this is not the case, you repro case is quite clear and reproduce the but.
I committed the repro case so hat we are on the same page. I suspect what it is will debug it later.
and I will debug the later.
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Re: Hard joint behaves like rubber band

Postby misho » Thu Mar 28, 2019 5:18 pm

I just did a quick check - In this diagram:

Image

On the left, the GREEN object is pulling the blue object upwards, creating a gap between them

On the right, the BLUE object is pushing the green object upwards using the exact same thrust amount. This time, the objects overlap (their volumes intersect) by about 1/10th of what the gap is in the above situation.
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Re: Hard joint behaves like rubber band

Postby Julio Jerez » Thu Mar 28, 2019 6:05 pm

wow the acceleration is 17700 g, :shock: that scared my to the point I thigh was not going to work, but guess what, it was an obvious silly bug that will only when asymmetric forces are applied.
please sync and try again

the error is now on the order of this
d(30.000013) v(-0.000057 88.545127 0.000000)
d(30.000013) v(0.000006 88.545151 -0.000000)

the error is 1e-5 m/s hikes :mrgreen: :lol:

make sure you run in double, or else the residual value will cause the bodies to drift apart.
this is one case where double makes a difference.
using double should work in all cases.
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Re: Hard joint behaves like rubber band

Postby misho » Thu Mar 28, 2019 6:34 pm

Awesome - FIXED!!! :mrgreen: I'm SO glad this was a simple fix!

The parachutes are now behaving way better, as well!! Although I am getting a crash inside the engine when I deploy the largest parachute. Can you take a look at my post in "General Discussion" forum and let me know if I have the right idea? Thx!!

Julio Jerez wrote:wow the acceleration is 17700 g, :shock:


LOL What?? either my numbers (which I got from NASA) are way off, or they will be scooping the crew through a straw :mrgreen:
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Re: Hard joint behaves like rubber band [SOLVED]

Postby Julio Jerez » Thu Mar 28, 2019 7:36 pm

upps I got that wrong
force = 1779288.64
mass of one body = 10386
a = 1779288.64 / 10386 = 171 g

but when the two bodies are jointed the mass is abut double so the total
G is about 80 to 90.

still quite hefty for an astronaut to withstand.
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Re: Hard joint behaves like rubber band [SOLVED]

Postby misho » Fri Mar 29, 2019 12:05 am

Julio Jerez wrote:the total
G is about 80 to 90.

still quite hefty for an astronaut to withstand.


Yes it is - that would kill them. By the looks of it (I made a video if it a few months ago), it does not seem as if it would be that much. Jet fighter ejection seats are between 12-14Gs, and they look far more violent than the Launch Abort System...
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