http://blog.csdn.net/huangkangying/article/details/78129148
https://zh.wikipedia.org/wiki/PID%E6%8E%A7%E5%88%B6%E5%99%A8
http://www.dfrobot.com.cn/community/thread-14783-1-1.html
http://robotsforroboticists.com/pid-control/
http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/