The code I’m using for the balancing robot is based on a sketch that is all over the internet in various guises.  I can’t find any mention of the original author.  The version I am working on was part of a project by Aalto University in Helsinki. It is the only one that tried to use the encoders on the dc motors for additional stability.  I say tried because their project ultimately failed to produce a stable robot.  I can but try,  at least it’s a starting point.  The Aalto University project can be found here.