Is it possible to immobilize one part of a robot?

[omichel]

I need to fix, e.g., pin-point one part of my robot to the static environment.
Can I do this in Webots?

[omichel]

To immobilize one part of the robot, you need to fix the part to the static environment. This must be done with a physics plugin. You can add a physics plugin with the File / New / New Physics Plugin... menu item. In the plugin code, you must simply add an ODE fixed joint between the dBodyID of the robot part and the static environment. This can be implemented like this:

#include <ode/ode.h>
#include <plugins/physics.h>

void webots_physics_init() {

  // get body of the robot part
  dBodyID body = dWebotsGetBodyFromDEF("MY_ROBOT_PART");

  // get the matching world
  dWorldID world = dBodyGetWorld(body);

  // the joint group ID is 0 to allocate the joint normally
  dJointID joint = dJointCreateFixed(world, 0);

  // attach robot part to the static environment: 0
  dJointAttach(joint, body, 0);

  // fix now: remember current position and rotation
  dJointSetFixed(joint);
}

void webots_physics_step() {
  // nothing to do
}

void webots_physics_cleanup() {
  // nothing to do
}

You will find the description of Webots physics plugin API here. You will find the description about the ODE functions on this page.