All of the examples of keeping a double/triple inverted pendulum balanced using a PID controller I've seen seem to be on a cart. Like this one https://www.youtube.com/watch?v=cyN-CRNrb3E
How come the PID controller always controls a cart rather than a servo that holds the first pendulum? The second/third pendulum could be connected loosely on the first pendulum and the PID controller controls the first pendulum. Is it because servos tend to be too slow or are there other reasons?