Note: pr2_gripper_sensor_action/Tutorials/Grab and Release an Object Using pr2_gripper_sensor_action.
(!) Please ask about problems and questions regarding this tutorial on Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags.

Force Control with the PR2 Gripper

Description: This tutorial teaches the user how to use the pr2_gripper_sensor_action package to do force control with the PR2 gripper.

Tutorial Level: INTERMEDIATE

Overview and prerequisites

This tutorial will show you to use the basic pr2_gripper_sensor_action commands of find_contact force_servo and gripper_action to open the gripper, then quickly find contact with an object, and move into a force control mode to hold the object with a user-specified amount of force.

Using any of the pr2_gripper_sensor_action commands requires three components:

  • a controller that directly sends commands to the joint motors;
  • an action node that translates your desired actions to low-level commands for the controller;
  • the higher level program that sends your commands to the action node.

The first two of these components are available in ROS as part of the pr2_gripper_sensor_action, pr2_gripper_sensor_controller, and pr2_gripper_sensor_msgs packages. In this tutorial, we will show you how to use them by writing the third component, the higher level program.

Building pr2_gripper_sensor_action

You'll need to build pr2_gripper_sensor_action, if you haven't already.

rosdep install pr2_gripper_sensor_action
rosmake pr2_gripper_sensor_action

After that you can bring up a robot on the hardware.

Package setup

In order to create a ROS node that sends goals to the gripper_sensor_action, the first thing we'll need to do is create a package. To do this we'll use the handy roscreate-pkg command where we want to create the package directory:

roscreate-pkg my_gripper_project roscpp actionlib pr2_gripper_sensor_action

After this is done we'll need to roscd to the package we created, since we'll be using it as our workspace.

roscd my_gripper_project

Launching the controller and action nodes

Currently the pr2_gripper_sensor_controller and pr2_gripper_sensor_action nodes are not brought up by default when you launch the robot. To write a program that uses these resources you will first need to launch the controller and action nodes for both the left and right gripper by running the following launch script:

roslaunch pr2_gripper_sensor_action pr2_gripper_sensor_actions.launch

Creating the Action Client

Now we will make an action client node that grabs and releases an object.

Put the following into src/my_gripper_project.cpp:

   1 #include <ros/ros.h>
   2 #include <pr2_controllers_msgs/Pr2GripperCommandAction.h>
   3 #include <pr2_gripper_sensor_msgs/PR2GripperFindContactAction.h>
   4 #include <pr2_gripper_sensor_msgs/PR2GripperForceServoAction.h>
   5 #include <actionlib/client/simple_action_client.h>
   7 // Our Action interface type, provided as a typedef for convenience
   8 typedef actionlib::SimpleActionClient<pr2_gripper_sensor_msgs::PR2GripperFindContactAction> ContactClient;
   9 // Our Action interface type, provided as a typedef for convenience                   
  10 typedef actionlib::SimpleActionClient<pr2_controllers_msgs::Pr2GripperCommandAction> GripperClient;
  11 // Our Action interface type, provided as a typedef for convenience
  12 typedef actionlib::SimpleActionClient<pr2_gripper_sensor_msgs::PR2GripperForceServoAction> ForceClient;
  15 class Gripper{
  16 private:
  17   GripperClient* gripper_client_;  
  18   ContactClient* contact_client_;
  19   ForceClient* force_client_;
  21 public:
  22   //Action client initialization
  23   Gripper(){
  25     //Initialize the client for the Action interface to the gripper controller
  26     //and tell the action client that we want to spin a thread by default
  27     gripper_client_ = new GripperClient("r_gripper_sensor_controller/gripper_action", true);
  28     contact_client_  = new ContactClient("r_gripper_sensor_controller/find_contact",true);
  29     force_client_  = new ForceClient("r_gripper_sensor_controller/force_servo",true);
  31     //wait for the gripper action server to come up 
  32     while(!gripper_client_->waitForServer(ros::Duration(5.0))){
  33       ROS_INFO("Waiting for the r_gripper_sensor_controller/gripper_action action server to come up");
  34     }
  36     while(!contact_client_->waitForServer(ros::Duration(5.0))){
  37       ROS_INFO("Waiting for the r_gripper_sensor_controller/find_contact action server to come up");
  38     }
  40     while(!force_client_->waitForServer(ros::Duration(5.0))){
  41       ROS_INFO("Waiting for the r_gripper_sensor_controller/force_servo action server to come up");
  42     }
  43   }
  45   ~Gripper(){
  46     delete gripper_client_;
  47     delete contact_client_;
  48     delete force_client_;
  49   }
  51   //Open the gripper
  52   void open(){
  53     pr2_controllers_msgs::Pr2GripperCommandGoal open;
  54     open.command.position = 0.08;
  55     open.command.max_effort = -1.0;
  57     ROS_INFO("Sending open goal");
  58     gripper_client_->sendGoal(open);
  59     gripper_client_->waitForResult();
  60     if(gripper_client_->getState() == actionlib::SimpleClientGoalState::SUCCEEDED)
  61       ROS_INFO("The gripper opened!");
  62     else
  63       ROS_INFO("The gripper failed to open.");
  64   }
  67   //Hold somethign with a constant force in the gripper
  68   void hold( double holdForce){
  69     pr2_gripper_sensor_msgs::PR2GripperForceServoGoal squeeze;
  70     squeeze.command.fingertip_force = holdForce;   // hold with X N of force
  72     ROS_INFO("Sending hold goal");
  73     force_client_->sendGoal(squeeze);
  74     force_client_->waitForResult();
  75     if(force_client_->getState() == actionlib::SimpleClientGoalState::SUCCEEDED)
  76       ROS_INFO("Stable force was achieved");
  77     else
  78       ROS_INFO("Stable force was NOT achieved");
  79   }
  82   //Find two contacts and go into force control mode
  83   void findTwoContacts(){
  84     pr2_gripper_sensor_msgs::PR2GripperFindContactGoal findTwo;
  85     findTwo.command.contact_conditions = findTwo.command.BOTH;  // close until both fingers contact
  86     findTwo.command.zero_fingertip_sensors = true;   // zero fingertip sensor values before moving
  89     ROS_INFO("Sending find 2 contact goal");
  90     contact_client_->sendGoal(findTwo);
  91     contact_client_->waitForResult(ros::Duration(5.0));
  92     if(contact_client_->getState() == actionlib::SimpleClientGoalState::SUCCEEDED)
  93     {
  94       ROS_INFO("Contact found. Left: %d, Right: %d", contact_client_->getResult()->data.left_fingertip_pad_contact, 
  95            contact_client_->getResult()->data.right_fingertip_pad_contact);
  96       ROS_INFO("Contact force. Left: %f, Right: %f", contact_client_->getResult()->data.left_fingertip_pad_force, 
  97            contact_client_->getResult()->data.right_fingertip_pad_force);
  98     }
  99     else
 100       ROS_INFO("The gripper did not find a contact or could not maintain contact force.");
 101   }
 102 };
 104 int main(int argc, char** argv){
 105   ros::init(argc, argv, "simple_gripper");
 107   Gripper gripper;
 110   sleep(2.0);
 112   gripper.findTwoContacts();
 113   gripper.hold(10.0);   // hold with 10 N of force
 115   return 0;
 116 }


Add the following line to the CMakeLists.txt:

rosbuild_add_executable(my_gripper_project src/my_gripper_project.cpp)

and make the binary by typing 'make' in the my_gripper_project directory.


  • Bring up a robot.
  • Make sure the pr2_gripper_sensor controller and action nodes are launched (see above)
  • Start the ROS console (rxconsole) to view any errors that occur.

  • Run your new node that you created:

rosrun my_gripper_project my_gripper_project

The robot should now wait a few seconds, grab an object, wait a few more seconds, and release upon object contact with the environment.

Wiki: pr2_gripper_sensor_action/Tutorials/Force Control with the PR2 Gripper (last edited 2010-09-01 02:30:41 by JoeRomano)