[ODE] Re: ODE Digest, Vol 13, Issue 23

[Pete Baron]

Hi Devrim,

if you set the low stop and the high stop as the same value for a joint then
you are effectively saying "this joint doesn't slide, it's fixed at the
location between the low stop and the high stop".  This has been used in a
number of driving simulations to set the steering on a Joint2 type
connection, but my experience with it is that it causes some very peculiar
motions (eg. the steering works fine turning left [the stops are moving
towards smaller numbers] but gets very jerky when turning right [the stops
are increasing towards larger positive numbers]).  It may be that you are
encountering bugs of a similar nature with your slider joints.  I fixed the
steering problems by using forces to turn the front wheels of my cars, and
it works great.
Try setting the slider low stop to the first object's Z coord (or less), and
high stop to the second object's Z coord (or more), then see if the boxes
explode still.  If they do then this is not the problem in your situation.
If they don't, then try moving the boxes together by using applied forces
instead of the (highly artificial) situation with low-stop/high-stop ranges
(don't forget that the low stop and high stop are relative to the joint
which is defined as an offset from one of your objects I believe).
I had a look at the low/high stop code in ODE for my car simulation, and as
far as I could see the Joint2 enforces it's limits by manually moving the
objects back within the legitimate range - and we all know already (from the
manuals) that just moving stuff around is bad for ODE stability.
I would like to see the joint limit code changed to enforce the limits using
counter-acting forces, but I must admit that the physics involved is way
beyond my puny understanding of the field!

Hope this helps,

Pete Baron

sibaroni at hotmail.com
http://home.btconnect.com/pete/homepage.html


----- Original Message ----- 
From: "Devrim Erdem" <devrim.erdem at gmail.com>
To: <ode at q12.org>
Sent: Thursday, September 16, 2004 7:42 AM
Subject: [ODE] Re: Can not get slider working


> Hello all,
>
> When a question stays unanswered, it is probably because it is a bad
> question. So could anyone tell me how I could improve my question :) ?
>
> Thanks.
>
>
> On Tue, 14 Sep 2004 22:54:00 +0200, Devrim Erdem <devrim.erdem at gmail.com>
wrote:
> > Hello,
> >
> > I am trying to connect two boxes with a slider in z axis.
> >
> > Box1 is at z=0.5
> > Box2 is at z=2.5
> >
> > They are attached with a slider joint.
> >
> > Low stop and high stop params of joint are both set to 2 ( for testing
> > purposes ).
> >
> > When the sim starts, bodies are pulled to each other ( box1 takes off
> > ) and rotate around z axises.
> >
> > What could be wrong with this setup ?
> >
> > Thanks in advance,
> >
> > Devrim.
> >
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