Re: [Pyode-user] Questions about inertia tensors

[Bill C. <we...@pr...>]

On 5/21/2012 1:32 PM, Will Bosworth wrote:
> Hi Bill,
>
> I do not understand your initial assumption not to use joints. Fixed
> joints are very straightforward to implement, and I believe are stiff
> to the extent that the global ERP and CFM integrator parameters are
> stiff. What documentation suggests not to use joints? I'd like to see
> it.
>
> Based on what I know about your problem and ODE, if in your shoes i
> would stick to a one-to-one mass and body implementation and use fixed
> joints to sew the complex body together. I am pretty sure that bodies
> can be rotated with body.setRotation(r), for instance as in
> http://monsterden.net/software/ragdoll-pyode-tutorial (string search
> setRotation there).
>
> best,
> Will
>
>
>
> On Sat, May 19, 2012 at 8:23 PM, Bill Call<we...@pr...>  wrote:
>> Hi :
>>
>> I'm trying to build a rigid-body simulation using PyODE/PyOpenGL. Since
>> I'm  just doing rigid-body,  it is my understanding that I have no need
>> for joints; I just need to add masses to my body at the appropriate
>> translations/rotations.
>>
>> I have a number of cylindrical masses in my body.  These cylinders have
>> fixed positions and rotations within the body, but are not necessarily
>> aligned with the X, Y or Z axis.  PyODE does not bind the rotateMass()
>> function from ODE; it only allows me to specify an X, Y or Z orientation
>> for cylindrical mass (at creation time). This means that I am unable to
>> rotate my cylindrical masses to the correct orientation.
>>
>> My guess is that this would not be a problem if each of these masses had
>> their own Body; I could just rotate the body after the mass was bound.
>> Doing doing it that way might work, but it would mean connecting the
>> cylinders to the main body via joints, and what little documentation
>> there is seems to contraindicate the use of joints for rigid-body
>> simulations.
>>
>> My questions are as follows:
>>
>> 1) Am I correct in my assumption that I must rotate/translate the mass
>> tensor to reflect the true location/orientation of my cylindrical masses
>> within the containing body?  It seems (to me) to be intuitively obvious
>> that I do, but I don't put much stock in my intuition regarding this
>> subject.
>>
>> 2) If the answer to (1) is "yes", then can anybody point me a rotation
>> implementation?  If "no", then would someone be willing to explain why not?
>>
>> I have already spent a lot of time Googling about for this information,
>> but I am not sufficiently grounded in the subject to really understand
>> the answers I have been able to find on my own; not well enough to write
>> a function to rotate inertial tensors, anyway.
>>
>> Thanks in Advance,
>>
>>
>> Bill Call
>>
>>
>>
>>
>>
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Will:

Following up on the whole "why not use fixed joints?" question, I was 
able to find the original data. The most telling comment comes from the 
ODE Wiki itself:

http://ode-wiki.org/wiki/index.php?title=Manual:_Joint_Types_and_Functions#Fixed

and reads as follows:

"The fixed joint maintains a fixed relative position and orientation 
between two bodies, or between a body and the static environment. Using 
this joint is almost never a good idea in practice, except when 
debugging. If you need two bodies to be glued together it is better to 
represent that as a single body."

I can understand the point: the ERP and CFM parameters are always going 
to leave some wiggle room.  If you want it rigid, make it a single body. 
I can testify that this works quite well. In my case, though I am trying 
to simulate a quad-copter, and I just want to apply torque to the motor 
bodies and let ODE compute the net effect. I don't see how I can do that 
with a single body unless I want to do the math myself.

Regards,


Bill C.