Browsing by Subject "PID controllers."
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Item Open Access A new PI and PID control design method and its application to active queue management of TCP flows(Bilkent University, 2007) Üstebay, DenizPID 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.Item Open Access PID controller design for first order unstable time delay systems(Bilkent University, 2009) Arslan, Gül EzgiIn this thesis, problem of designing P, PI and PD-like controllers for switched first order unstable systems with time delay is studied. For each type of controller, the problem is solved in two steps. First, the set of stabilizing controllers for the class of plants considered is determined using different approaches. Then, an appropriate controller inside this set is chosen such that the feedback systems satisfies a desired property, which is for example gain and phase margin maximization or the dwell time minimization. In the first part, we focus on PI controllers and tune the PI controller parameters in order to maximize the gain and phase margins. The observations in this part show that a P controller is adequate to maximize gain and phase margins. Then, we move on to the problem of tuning P, PI and PD-like (first order stable) controller parameters such that the switched feedback system is stabilized and the dwell time (minimum required time between consequent switchings to ensure stability) is minimized. For this purpose, a dwell-time based stability condition of [39] is used for the class of switched time delay systems. We show that a proportional controller can be found with this method, but a PI controller is not feasible. Finally, we focus on the design of PD-like controllers for switched first order unstable systems with time delays. The proposed method finds the values of PD-like (first order stable) controller parameters which minimize an upper bound of the dwell time. The conservatism analysis of this method is done by time domain simulations. The results show that the calculated upper bound for the dwell time is close to the lower bound of the dwell time observed by simulations. In addition, we compare the obtained PD-like controller results with some alternative PD and first order controller design techniques proposed in the literature.