I'm guessing it's a rather untuned PD controller. Inverted pendulums can be controlled pretty stably if you use even just PD. X-travel targeting can be accomplished by adding in the Integral.
We did this in my controls lab. We used LQR and PID control (though the manual only lists LQR). There's some good math behind it all though. The lab does discuss adding in an Integral controller though.
https://engineering.purdue.edu/AAE/A...11/lab3/manual
These labs were always scary because we had to calculate all of our gains prior to actually testing and we had to put our gains to the test in lab (only 1 chance to get it right)