A new PI and PID control design method and its application to active queue management of TCP flows

Abstract

PID controllers are continuing to be used in many control applications due to their simple structures. Design of such controllers for unstable systems with time delays is an active research area. Recently, stabilizing PI and PD controllers for a class of unstable MIMO (multi-input multi-output) systems with input/output delays have been investigated and allowable controller gain intervals for such controllers have been maximized. Motivated by these studies, this thesis proposes a new method for tuning the parameters of PI, PD and PID controllers for integrating processes with time delays. The method is based on selecting the centers of the maximized gain intervals as the controller gains for the purpose of obtaining optimal controllers. As an application of this method, controllers for AQM (Active Queue Management) of TCP (Transmission Control Protocol) flows have been designed. AQM is a congestion control method used in computer networks to increase link utilization with less queueing delays. The fluid flow model of TCP’s congestion avoidance mode based on delay differential equations supplies the mathematical background for modelling the AQM as a feedback control system and designing different control schemes accordingly. Firstly, the proposed controller design method has been applied to AQM for the case of time invariant time delay and secondly the method has been supported with switching control technique to obtain optimum system performance in the case of time varying time delay. The performance of the designed controllers for both cases has been illustrated by packet level simulations in ns-2.