[ODE] Disabling axis of rotations on a body

[Alex Hetu]

Although your last sentence looked like alien language to me, i think you're
basically suggesting a solid way to disable torque on certain axis, am i
right? (sorry i'm not really good with maths). I agree that "correcting" the
thing manually isn't good enough. In fact, it's an horrible way of doing it.
It doesn't really work well for what i'm trying to do anyway; a box with a
minimum of friction will kinda shakes (not much, but just enough to be
noticed) when pushes on a floor.

I'll try the amotor trick Martin C.Martin suggested... if i can figure out
how to properly set up a amotor joint first.. |:)

Anybody with some code sample out there?

thx



-----Original Message-----
From: ode-admin@q12.org [mailto:ode-admin@q12.org]On Behalf Of Bob
Dowland
Sent: Thursday, July 10, 2003 5:40 AM
To: ode@q12.org
Subject: RE: [ODE] Disabling axis of rotations on a body


> If you truly want to disable an axis of rotation you should
> just manually
> set it after each step. For car wheels its important that
> they are always in
> the right rotation, and i found no way other than forcing the
> rotation to be
> correct.

My goodness! (No offence to Alex) I can see this working okay in the case of
wheels but in general it would be a shame if this were the only way to go.
In particular in a situ where it is possible to have a number of
interactions between objects (dynamic and static) "manually correcting"
things can be very dangerous, often leading to (at best) unphysical
behaviour and (at worst) producing states that the integrators etc end up
throwing up on.

I'm not sure if this is possible in ODE so sorry if this is a completely
silly suggestion but... in some systems it is possible to make dynamic
objects behave like static or kinematic objects (ie non-physically animated)
by just giving them infinite mass (particularly easy when systems store
reciprocal mass rather than mass itself). ie, in the linear case,
(translational motion) where F/m = a, making 1/m = 0 produces a = 0. I'm
wondering whether a similar effect for torques could be got by setting the
appropriate coefficent of the rest-frame inertia matrix (which is hopefully
diagonal) so that it's inverse say "inv_I" is essentially zero about the
required axis, then wdot = inv_I.Ldot giving kth coord of wdot = zero
whenever kkth coordinate of inv_I is zero.

Just a thought... :)


> -----Original Message-----
> From: Alex Hetu [mailto:alexhetu@videotron.ca]
> Sent: 23 July 2003 20:54
> To: Erwin de Vries; ode@q12.org
> Subject: RE: [ODE] Disabling axis of rotations on a body
>
>
> Hm. yes i was thinking about doing that, I guess it would also require
> reseting the torque too.... it's not hard obviously, and i
> guess that's what
> i'll use for now, but it sure ain't clean. :)
>
> Martin C.Martin suggested adding code to allow a AMotor joint
> to support the
> static environment as body2. It's a very good idea i think.
> But i wish ODE
> would provide a simple joint to do that though, or maybe just some
> parameters that could be set on the body maybe.
>
> Thx guys
>
>
> -----Original Message-----
> From: ode-admin@q12.org [mailto:ode-admin@q12.org]On Behalf
> Of Erwin de
> Vries
> Sent: Wednesday, July 09, 2003 3:18 PM
> To: ode@q12.org
> Subject: Re: [ODE] Disabling axis of rotations on a body
>
>
> If you truly want to disable an axis of rotation you should
> just manually
> set it after each step. For car wheels its important that
> they are always in
> the right rotation, and i found no way other than forcing the
> rotation to be
> correct.
>
> Erwin
>
> ----- Original Message -----
> From: "Alex Hetu" <alexhetu@videotron.ca>
> To: <ode@q12.org>
> Sent: Wednesday, July 23, 2003 20:47
> Subject: [ODE] Disabling axis of rotations on a body
>
>
> > Hi,
> >
> > I'd like to know if it is possible to disable certain axis
> of rotation on
> a
> > body and still move it around in the world and make it
> collide with other
> > bodies without ever rotating around those disabled axis
> (ex: a tall box or
> > cylinder that can only rotate on itself around the Y-axis
> and which can
> > never fall on its sides...). Or maybe it could be possible
> to actually
> > disable all rotations of a body?
> >
> > I looked for joints or constraints, but joints don't really apply on
> single
> > bodies... and i wish to find a CLEAN solution. Am I missing
> something?
> >
> > thx!
> > Alex Hetu
> >
> > _______________________________________________
> > ODE mailing list
> > ODE@q12.org
> > http://q12.org/mailman/listinfo/ode
> >
> >
>
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