How to define(in URDF) a joint to stay at the desired position even when external torques are applied.

edit

In answer to two of your comments, and your original question.

Increasing gains can increase the torque applied, however the URDF joint definition may define force limits. If these limits are defined and are being reached then increasing the PID gains not result in the behaviour you expect. It should be noted that tuning a PID controller to behave the way the want is a complex process worthy of significant study itself.

Regarding defining a parameter twice, don't do it. This is bad practice and makes for terrible code to maintain and debug. My advice would be to keep the controller configurations just in the YAML and remove them from the launch file.

To summarise this with reference to your original question: You want to check the URDF doesn't define torque limits for the joint and increase or remove the limits if they're there. Then you'll want to tune your controllers PID gains to increase the correctional force the joint applies. There is a method described here to use the dynamic reconfigure interface to tune the gains while a simulation is running, this is far easier than re-running the simulation with different gains many times.

Hope this helps.