[ros-users] Pan Tilt Unit in Simulation

[John Hsu]

HI Marc,

On Sun, Sep 26, 2010 at 7:47 AM, MightTower <marcjoeris at uni-koblenz.de>wrote:

>
> Hmm ok now i can control the JointPosition controller, but i cant tell him
> that i want to turn around the axis 90 degrees exactly (from the last
> position),



the joint angle commands are absolute, if you want relative position
commands you'll have to do it yourself (i.e. get current position, send
current + relative position).



> after 1 command it did turn around and around all the time.


are you talking about oscillatory behavior of the joint?  As presented in
the tutorial, it's a frictionless system, and actual dynamics are being
enforced by the physics engine.  You are right that damping (or pid tuning)
is necessary to stop the oscillations, ideally you want critical damping for
your system.


> I tried to wrote my own controller but there the same thing i cant stop it
> once it started to turn.
> Is it possible to tell the default controller this things or what i have to
> do to for this?
> In the pendulums tutorials its the same once started they didnt stop. Ok i
> can use damping but this dont helps me to turn eg. 90 degrees exactly.
>

viscous damping will not affect the asymptotic position of your joint.  You
can easily plot it to see the result (see attached),

rxplot
"/test_controller/state/process_value,/test_controller/state/set_point"

The plot is for the single link example with added joint damping of 2
(everything metric units).

If you don't want to deal with the dynamics of the system, alternatively,
you can set the pose of the links directly.  For that approach, you can
write a plugin to call set pose directly.  You'll have to do a tiny bit of
math to compute the desired global pose of the links.

John



> Thanks again.
> Marc
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