KDL for a 21 dof robot

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Your question has very limited information on what you actually want to achieve (calculate forward kinematics, inverse kinematics, inverse dynamics, wrenches, twists etc.?). I´d recommend looking at (and possible using) MoveIt!. It provides advanced capabilities or motion planning, but also basic kinematics functionality described here. If you´re more interested in the low level control side of things, this cartesian controller implementation for ros_control demonstrates how you can read a URDF file and initialize KDL objects from it.

As you might have noted, the recommended first step is to generate a URDF model or your robot, as this makes use of KDL (and many other tools in the ROS ecosystem) much easier.