Update README.md

README.md

+Talos_data
+==========
+
+## Introduction
+
 This repository is dedicated to use the TALOS robot with the stack-of-tasks.
 
 Specific data were generated to account to some specific problems when generating/planning motion
@@ -9,3 +14,78 @@ Some dummy data were added (rotor inertia) to help the dynamic regularization.
 
 For initial and data validated by PAL-Robotics please see the http://github.com/pal-robotics/talos_robot
 repository.
+
+## Flexibilities
+
+To start the flexibilities:
+```
+roslaunch talos_data talos_gazebo.launch enable_leg_passive:=true
+```
+
+You should see in the robot model two new joints:
+```
+leg_left_1_link_passive
+leg_right_1_link_passive
+```
+and two new joints
+```
+leg_left_1_joint_passive
+leg_right_1_joint_passive
+```
+
+They are updated by the plugin in the src directory:
+```
+src/SpringPlugin.cc
+```
+
+This plugin is specified in the file:
+```
+urdf/leg/leg.urdf.xacro
+```
+by the following lines
+```
+    <gazebo>
+      <plugin filename="libSpringPlugin.so" name="spring_leg_${prefix}_1_joint_passive">
+        <joint_spring>leg_${prefix}_1_joint_passive</joint_spring>
+        <kp>970.0</kp>
+        <kd>0.0</kd>
+      </plugin>
+    </gazebo>
+```
+
+You can create new joints by expanding the robot kinematic tree
+and adding the same lines in the xacro files.
+
+The spring constants are modified by the associated fields
+*kp* and *kd*
+The name of the joint to which this apply is specified by the field
+*joint_spring*
+
+The plugin is generating a torque based on the actuator state using:
+$$\tau = - kp q - kd \dot{q}$$ where $q$ is the actuator position and $\dot{q}$
+is the actuator velocity.
+
+This might be changed in the future to cope with a different equilibrium point.
+
+Finally to have the state of the joint you need to add the following lines:
+```
+    <transmission name="leg_${prefix}_1_passive_trans">
+      <type>transmission_interface/SimpleTransmission</type>
+      <actuator name="leg_${prefix}_1_motor" >
+        <mechanicalReduction>1.0</mechanicalReduction>
+      </actuator>
+      <joint name="leg_${prefix}_1_joint_passive">
+        <hardwareInterface>hardware_interface/PositionJointInterface</hardwareInterface>
+        <hardwareInterface>hardware_interface/EffortJointInterface</hardwareInterface>
+      </joint>
+    </transmission>
+```
+
+This is telling to ros_control that the passive joint has Position and Effort interfaces.
+
+
+
+
+
+
+