[Kevin Sevcik]

As other have said, look into PID control. I'd personally recommend just trying P or PD control first and see if that gets you the desired results. Adding the I term to the system can introduce a lot of instabilities and problems if you're not careful. If you feel you must have it, keep your I gain as low as possible and look into adding "Integral Anti-Windup" into your control equation. The I term tracks errors over time and if your system simply can't meet your commanded value for some reason, then the I term can build up to a huge value, overflow your variable, etc. None of these things are good, so keep things simple at first and stick with P or PD control.