[ODE] simulating a gripper in ODE

[Stanisław Szymczyk]

Hello

In Gazebo robot simulator there is an ActivMedia Pioneer2 Gripper
simulated in ODE by using two boxes (serving as paddles) connected to
another box (gripper's base) with slider joints. Paddles are opened
and closed by setting joints motors desired velocity to some opposite
constant values. The real thing looks like that:
http://roboti.cs.siue.edu/kids/Pictures/gripper.jpg.

I noticed that when I grab something with the gripper and the robot is
moving, gripper paddles and grabbed object slides back and forth
perpendicularly to sliders axes. I guess the reason is pretty obvious,
gripper paddles are trying to move with velocities v and -v, gripped
object is blocking their way, so dParamFMax force is applied to that
object from opposite directions. When there is no external force, the
resulting force vector is null, so the gripper is doing just fine.
When the robot turns, some additional force is applied to gripped
object due to inertia, so one of the paddles "wins" and is pushing
gripped object and the other paddle.

I was trying to fix the gripper by:

1. Moving paddles with gripped object back to center position by
manipulating dParamFMax accordingly to slider joints positions, so
when one of the paddles pushes the other due to inertia, the other
will push back with increased slider joint dParamFMax. The result was
so-so: paddles were no more sliding, but unfortunately they were in
small continuous oscillatory motion.

2. Setting paddles slider joints motion range to 0 (dParamLoStop =
dParamHiStop) and "moving" gripper paddles by incrementally changing
both values simultaneously (dParamLoStop = dParamHiStop = desired
position). This resulted in nice, stiff gripper paddles, but I
couldn't figure out the when to stop moving.

Here comes the question: how to fix the gripper? I'm just starting
with ODE, so I guess you will have some more brilliant ideas. Is there
any other gripper design possible? Is there a way to make "stiff"
slider joints, without possibility of changing joint position by
external force?

Thanks for your time.

Stanislaw Szymczyk