how to get the pressure of the models link which touch the ground

edit

I have solved it

1) urdf I add ft_sensor in the foot

 <xacro:macro name="gazebo_pressure" params="prefix parent num x y z">
    <link name="${prefix}_foot_sensor_m${num}" >
    <inertial>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <mass
        value="0.01" />
      <inertia
        ixx="0.000"
        ixy="0.000"
        ixz="0.000"
        iyy="0.000"
        iyz="0.000"
        izz="0.000" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <box size="0.025 0.025 0.002"/>
      </geometry>
      <material
        name="">
        <color
          rgba="0.75294 0.75294 0.75294 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <box size="0.025 0.025 0.002"/>
      </geometry>
    </collision>
  </link>
<joint 
    name="${prefix}_foot_sensor_m${num}_j"
    type="revolute">
    <origin
      xyz="${x} ${y} ${z}"
      rpy="0 0 0" />
    <parent
      link="${parent}" />
    <child
      link="${prefix}_foot_sensor_m${num}" />
    <axis
      xyz="0 1 0" />
    <limit effort="0" velocity="0" lower="-0.001" upper="0.001" />
  </joint>
  <gazebo reference="${prefix}_foot_sensor_m${num}_j">
    <provideFeedback>true</provideFeedback>
  </gazebo>

  <gazebo>
    <plugin name="ft_sensor" filename="libgazebo_ros_ft_sensor.so">
      <always_on>true</always_on>
      <updateRate>100.0</updateRate>
      <topicName>/leap_one/${prefix}_foot_sensor_m${num}</topicName>
      <jointName>${prefix}_foot_sensor_m${num}_j</jointName>
    </plugin>
  </gazebo>

  <gazebo reference="${prefix}_foot_sensor_m${num}">
        <material>Gazebo/Blue</material>
        <mu1>9000</mu1>
        <mu2>9000</mu2>
        <kp>1000000.0</kp>
        <kd>10.0</kd>
        <minDepth>0.001</minDepth>
        <maxContacts>100</maxContacts>
    </gazebo>

  </xacro:macro>

  <xacro:gazebo_pressure prefix="left"  parent="left_m6" num="0" x="-0.03" y="-0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="left"  parent="left_m6" num="1" x="0.03" y="-0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="left"  parent="left_m6" num="2" x="-0.03" y="0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="left"  parent="left_m6" num="3" x="0.03" y="0.03" z="-0.034" />

  <xacro:gazebo_pressure prefix="right"  parent="right_m6" num="0" x="-0.03" y="-0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="right"  parent="right_m6" num="1" x="0.03" y="-0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="right"  parent="right_m6" num="2" x="-0.03" y="0.03" z="-0.034" />
  <xacro:gazebo_pressure prefix="right"  parent="right_m6" num="3" x="0.03" y="0.03" z="-0.034" />

2) modify ros source code clone gazebo_ros_pkgs repository in github add one line in the gazebo_plugins/src/gazebo_ros_ft_sensor.cpp

////////////////////////////////////////////////////////////////////////////////
// Update the controller
void GazeboRosFT::UpdateChild()
{
#if GAZEBO_MAJOR_VERSION >= 8
  common::Time cur_time = this->world_->SimTime();
#else
  common::Time cur_time = this->world_->GetSimTime();
#endif

  // rate control
  if (this->update_rate_ > 0 &&
      (cur_time-this->last_time_).Double() < (1.0/this->update_rate_))
  {
**+++      if(cur_time.Double() >= 5) // ignore it when reset_simulation** 
      return;
  }

  if (this->ft_connect_count_ == 0)
    return;

  physics::JointWrench wrench;
  ignition::math::Vector3d torque;
  ignition::math::Vector3d force;

  // FIXME: Should include options for diferent frames and measure directions
  // E.g: https://bitbucket.org/osrf/gazebo/raw/default/gazebo/sensors/ForceTorqueSensor.hh
  // Get force ...