从ros parameter server 获取URDF

// get the general robot description, the lwr class will take care of parsing what’s useful to itself
std::string urdf_string = getURDF(lwr_nh, “/robot_description”);

// Get the URDF XML from the parameter server
std::string getURDF(ros::NodeHandle &model_nh_, std::string param_name)
{
  std::string urdf_string;
  std::string robot_description = "/robot_description";

  // search and wait for robot_description on param server
  while (urdf_string.empty())
  {