This package contains the robot model that is used by the realtime
  controllers
  inside controller
  manager. This robot model focuses on controlling the robot
  mechanism in a realtime control loop, and therefore it only contains
  the components of a robot that are relevant in realtime: the robot
  joints (with encoders, transmisisons and actuators) and the
  kinematic/dynamic model of the robot.
 
  The pr2_mechanism_model package is well tested and is released with a stable API.
PR2 Mechanism Model