Here's an example subsystem from our 2014 robot which used some fingers on the end of servos to keep the ball centered over our catapult: Subsystem;
https://github.com/Team2168/2014_Mai...allTapper.java
Commands:
https://github.com/Team2168/2014_Mai...ommands/tapper
And an example from 2016 for controlling hood position:
Subsystem:
https://github.com/Team2168/2016_Mai...ooterHood.java
Command:
https://github.com/Team2168/2016_Mai...odToAngle.java
The code is a little rough around he edges, but it's fairly well commented and it's all been used in match, so should work unless the Servo class has been changed.