Re: [Pyode-user] collision woes

[Marc K. <mki...@ga...>]

Ethan,

Regardless of the delay, I really appreciate the response.  Your solution
was exactly correct and cleared up my collision problems.

I was confused about the initial orientation given to created cylinder
bodies.  Also, although rotation about the center of mass for each body
makes the most sense, I was confused by the fact that all coordinates (both
used for setting and returning a body's state) are in a global frame.  In
any case, works great now, thank you.

To explain my last point more, I'm interested in getting forces that result
from collisions, but when I tried to use "setFeedback" with my contact
joints, it threw the following error:

ODE INTERNAL ERROR 2: Bad argument(s) in dJointSetFeedback()
Aborted

I'd seen this in other threads and thought this meant that getting reaction
forces from the contact was not possible.  Also, I assumed that because the
geometry has no "mass", that getting contact forces from the collision with
a body and a geometry was also not possible.  So I made an extra body for my
obstacle that is rigidly fixed to the environment with a fixed joint that
will give forces induced by the collision somewhere along the body.  If
there is a better/other way, I'd be happy to hear about it.

Thanks again for the help and course correction.

Sincerely,

Marc Killpack
Healthcare Robotics Lab - Georgia Tech



On Sun, Jan 16, 2011 at 4:45 AM, Ethan Glasser-Camp <gl...@cs...>wrote:

> Hi! Sorry about the delay -- it's hard for me to find free time these days.
> I'm not really familiar with vpython so I may be vastly misinterpreting the
> function of your program.
>
>
> On 01/11/2011 03:17 PM, Marc Killpack wrote:
>
>> I've been trying to make a simple robot simulator which includes
>> collisions.  I'm getting weird
>> behavior though and wondered if I'm just making a dumb mistake.  I want
>> some objects that are
>> fixed in the environment.  From other examples and from archived questions
>> on the list it seems like
>> the best way to do this is using geometry objects that are not associated
>> with a body.  The only
>> collision that works so far is if I collide with the floor plane that is
>> defined as a fixed geometry.
>> I've attached my file that uses vpython for visualization and I had the
>> following questions if anyone
>> is willing:
>>
>> 1)If I set a tall square in place, different sizes will result in
>> collisions while smaller sizes do not, why?
>> (Try changing width and length of square to 0.1 in line 56 of .py file to
>> reproduce this)
>>
>
> It looks like body1, your robot arm, is not being rotated. If I understand
> correctly, this means that the capsule is extending along the Z-axis,
> whereas you are drawing it as extending along the X-axis. That is to say,
> your joint is not swinging the capsule like the arm of a turnstile, but more
> like the head of a hammer. I think the box obstacle is sufficiently wide
> that the capsule hits it anyhow, but if it is only 0.1, it fits "inside" the
> the radius of the hammer swing. To fix this, I put:
>
> body1.setRotation([0, 0, 1, 0, 1, 0, -1, 0, 0])
>
> Just after your body1.setPosition() call, before adding the geom. (A 90
> degree rotation around the Y axis, to exchange the X and Z axes, to make
> your visualization line up with the simulation.) Now a collision always
> happens.
>
>
>  Also, if I change the bounciness in the collision callback function (line
>> 28), it seems to have no effect
>> on the collision, why?
>>
>
> I'm guessing the motor on the joint is sufficiently powerful to keep
> pressing the body against the obstacle. Try changing MaxForce to 0 after the
> first time step (at the end of the while loop), so that the body has an
> initial velocity. You'll definitely see a bounce, and it will definitely be
> different for different bouncinesses.
>
>
>  2)I can place a geometry object that is a capped cylinder in the
>> environment but how do I orient it?  I tried
>> setting the rotation, but still couldn't get a collision to occur.  Change
>> "OBSTACLE = 2" on line 9 to see this.
>>
>
> This looks right to me, and once I added the rotation of the body (as per
> point 1), this caused collisions.
>
>
>  3)If I want forces from the collision, it seems that I have to have the
>> obstacle be associated with a body
>> as well, is that true?  If so, what kind of hinge or connection should I
>> use?  I tried a fixed joint to the
>> environment but that also did weird things.
>>
> I don't think this is true. Can you explain further?
>
> Ethan
>
>

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