When should i create my own controller?

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Hi! I have a question about controller : I created my own robot (humanoid) with 10 joints for the legs and i use the default position controller. Now i want to publish new values for all 10 joints and all joints should move on the same time (for walking). So i created a python file for doing this. It works sometimes.... I think its random which joint is moving first. So i have to start the simulation a few times before it works how i want. Do you know how to solve this? And when is it a good idea to create a own controller? Thanks!! :)