Hi Andreas,

Since the PR2 accepts force/torque commands, the Gazebo simulation of
the PR2 also accepts only force/torque commands.  The pid part of the
yaml configuration is our preferred way of specifying PID gains to the
controller.  It is up to the controller to parse the pid parameters
and to make use of them in its update loop, but it can rely on other
objects for assistance (in most cases, the Pid class in
control_toolbox:
http://www.ros.org/doc/api/control_toolbox/html/classcontrol__toolbox_1_1Pid.html
).

For a simple example of how to use PID feedback to command positions,
see the JointPositionController:

http://code.ros.org/svn/wg-ros-pkg/stacks/pr2_controllers/tags/pr2_controllers-1.0.2/robot_mechanism_controllers/include/robot_mechanism_controllers/joint_position_controller.h
http://code.ros.org/svn/wg-ros-pkg/stacks/pr2_controllers/tags/pr2_controllers-1.0.2/robot_mechanism_controllers/src/joint_position_controller.cpp

Regards,
-Stu

On Fri, May 7, 2010 at 4:45 AM, Andreas Vogt
<andreasv@informatik.uni-bremen.de> wrote:
> Hi,
>
> Can I control my joints only with forces/torques or is there any
> possibility to control the position?
> Why is there a PID Controller in the yaml available if I must implement
> a kind of force controller in the update() method?
>
> YAML:
>  my_controller_ns:
>    type: MyControllerPlugin
>    wheel1: joint_c-w1
>  pid:
>    p: 10.0
>    i: 0.0
>    d: 0.0
>    i_clamp: 1
>
> void MyControllerClass::update(){
>
>    double desired_pos = init_pos_ + amplitude_ *
> sin(ros::Time::now().toSec());
>     double current_pos = joint_state_->position_;
>     joint_state_->commanded_effort_ = -10 * (current_pos - desired_pos);
>
> }
>
> --
>
>  Andreas Vogt
>  Logistics and Production Robotics
>
>  DFKI Bremen
>  Robotics Innovation Center
>  Robert-Hooke-Straße 5
>  28359 Bremen, Germany
>
>  Phone: +49 (0)421 218-64140
>  Fax:   +49 (0)421 218-64150
>  E-Mail: andreas.vogt@dfki.de
>
>  Weitere Informationen: http://www.dfki.de/robotik
>  -----------------------------------------------------------------------
>  Deutsches Forschungszentrum fuer Kuenstliche Intelligenz GmbH
>  Firmensitz: Trippstadter Straße 122, D-67663 Kaiserslautern
>  Geschaeftsfuehrung: Prof. Dr. Dr. h.c. mult. Wolfgang Wahlster
>  (Vorsitzender) Dr. Walter Olthoff
>  Vorsitzender des Aufsichtsrats: Prof. Dr. h.c. Hans A. Aukes
>  Amtsgericht Kaiserslautern, HRB 2313
>  Sitz der Gesellschaft: Kaiserslautern (HRB 2313)
>  USt-Id.Nr.:    DE 148646973
>  Steuernummer:  19/673/0060/3
>
> _______________________________________________
> ros-users mailing list
> ros-users@code.ros.org
> https://code.ros.org/mailman/listinfo/ros-users
>



-- 
Stuart Glaser
sglaser -at- willowgarage -dot- com
www.willowgarage.com