Re: [simspark-devel] new problem of new server

[Joschka B. <jbo...@un...>]

Hi Yuan,

On Mon, 2007-06-18 at 20:34 +0800, Yuan Xu wrote:
> Hi all,
> 
> It seems there is still a problem in joint stop ;-(
> While the robot have a fall, because of collide, some joint may out of
> the range,
> it seems that ODE add powerful external force to restrict it, then the
> robot will crash...
> I have experimented this many times ;-(

The problem might be, that we have set the joints to be too restrictive.
The ODE manual [1] lists two parameters that can be used to affect the
joint constraints: the Error Reduction Paramter (ERP) and the Constraint
Force Mixing (CFM), cf. sections 3.7 -- 3.8.2 of the manual. 

These parameters can be set globally, as we do e.g. for the CFM in
lib/spark/spark.rb (line 353), or they can be set locally for individual
joints which we don't do right now.

Increasing the global CFM is recommended for numerical stability, but it
may also result in softer joints in general, including the contact
joints. So if the parameter is increased too much, the ball and the
robots might sink into the ground! It's difficult, but it might be worth
playing around with the global CFM and ERP parameters, and maybe also
try to set local CFM and ERP values for the individual joints (or joint
stops). As a start, one could try sth. like

world.setCFM(0.01)
world.setERP(0.9)

in spark.rb, and see if it changes the stability of the simulation in a
positive way.

Yuan, do you think you could do some tests with this?

Cheers,
Joschka

[1] http://ode.org/ode-latest-userguide.html


> 2007/6/16, Yuan Xu <xuy...@gm...>:
> > Hi Jan,
> >
> > You are right!
> > After change 0 to 1, the server works well.
> > It is great that we do not disable this feature ;-)
> >
> 
> 
>