Collision without forces

[pea]

Thanks a lot for your reply. I guess the proximity sensors are not exactly what I need, I really wanted to solve the problem with mere collision detection.
I now figured out how to build an arm of an excavator with joints and motors and tried to grab my object. When I am moving my joints with "joint.setMotorAngle" and density and maxTorque settings which seem to work well for the movement I get severe problems when I am colliding with the ground object and the error: ODE Message 3: LCP internal error.
The collision with the floor leads to jerky movements or makes the whole excavator disappear.
Does anybody have any advice regarding the tuning of the values of the physics engine (like density, hardness, friction, maxTorque) in this case for me?
I spent two hours on adjusting all kinds of parameters but I could not find any configuration which seems to help with the collision problem.
I attached the Vizard file and .ive scene to my posting.
Moreover, I have no idea what I can do to overcome the error messages that appear on collisions, like:

ODE Message 3: LCP internal error, s <= 0 (s=-8.0282e+005)
** WARNING: Physics engine returned invalid data for node 3

I would be really, really grateful for any help or advice on my problem.