Measuring force of tip using Jacobian function in KDL

edit

Hi, I understand from here that you can use the Jacobian function from the KDL library to calculate a the torque to input for a desired force at the tip. i.e. Torque = Jac(j,i)*Force

However, does anyone know a way to do the opposite, to measure the actual force exerted at the end tip by reading the respective measured joint torques. i.e. Measured Force = [some function?]*measured torque at joints.

I tried using the pr2_mechanism_model::JointState to get the measured effort of each joint and use back the Jacobian Transform to get the measured force,

     for (unsigned int i = 0 ; i < 6 ; i++)
  {
    F_measured_(i) = 0;
    for (unsigned int j = 0 ; j < kdl_chain_.getNrOfJoints() ; j++)
      F_measured_ += J_(i,j) * tau_measured_(j);
  }

However, when I can't compile it as the type for the JointState's measured force(double) is different from KDL::Wrench.

In member function ‘virtual void my_controller_ns::MyCartControllerClass::update()’:
  /home/c/fuerte_workspace/sandbox/my_controller_pkg/src/my_cart_controller_file.cpp:112:47: error: no match for ‘operator+=’ in ‘((my_controller_ns::MyCartControllerClass*)this)->my_controller_ns::MyCartControllerClass::F_measured_ += (((my_controller_ns::MyCartControllerClass*)this)->my_controller_ns::MyCartControllerClass::J_.KDL::Jacobian::operator()(i, j) * ((my_controller_ns::MyCartControllerClass*)this)->my_controller_ns::MyCartControllerClass::tau_measured_.KDL::JntArray::operator()(j, 0u))’
  /home/c/fuerte_workspace/sandbox/my_controller_pkg/src/my_cart_controller_file.cpp:112:47: note: candidate is:
  /opt/ros/fuerte/stacks/orocos_kinematics_dynamics/orocos_kdl/install_dir/include/kdl/frames.inl:209:9: note: KDL::Wrench& KDL::Wrench::operator+=(const KDL::Wrench&)
  /opt/ros/fuerte/stacks/orocos_kinematics_dynamics/orocos_kdl/install_dir/include/kdl/frames.inl:209:9: note:   no known conversion for argument 1 from ‘double’ to ‘const KDL::Wrench&’

Can anyone advise me on how do I go about measuring the force of the tip of pr2's arm using the Jacobian function in KDL? Thank you in advance.

EDIT: Thanks Peter, I appreciate any help as I am not very familiar with coding or robotics. Anyway, I realised I made a very silly mistake. The code should be

  for (unsigned int i = 0 ; i < 6 ; i++)
  {
    F_measured_(i) = 0;
    for (unsigned int j = 0 ; j < kdl_chain_.getNrOfJoints() ; j++)
      F_measured_(i) += J_(i,j) * tau_measured_(j);
  }

I forgot the I wrote "F_measured_" instead of "F_measured_(i)" earlier. It can compile now but not sure if it actually works. Will let you know my results afterwards.