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I would like to know more about PID controller.And the advantage and disadvantage of PID controller. Thanks.

-- poe (wypoe@zwallet.com), February 27, 2004

I will answer this question comparising CDM (coefficient diagram method)controller: The most considerable advantages of CDM can be listed as follows: 1. The design procedure is quiet easily understandable, systematic and useful. Therefore, the coefficients of the CDM controller polynomials can be determined more easily than those of the PID controller. This creates the possibility of an easy realization for a new designer to control any kind of system. 2. There are explicit relations between the performance parameters specified before the design and the coefficients of the controller polynomials. For this reason, the designer can easily realize many control systems having different performance properties for a given control problem in a wide range of freedom. 3. It is needed to develop different tuning methods for the processes with various properties in PID control. But it is sufficient to use a single design procedure in CDM technique. This is an outstanding advantage. 4. It is particularly hard to design robust controllers realizing the desired performance properties for unstable, integrating and oscillatory processes having poles near the imaginary axis. It has been reported that successful designs can be achieved even in these cases by using CDM.

Ref: Hamamci S.E., Koksal M., "Robust controller design for TITO systems with Coefficient Diagram Method", CCA 2003 IEEE Conference on Control Applications, Istanbul, June 23-25, 2003.

------------------------------------------------------------------- The Coefficient Diagram Method (CDM), recently developed and introduced by Prof. Manabe (http://www.cityfujisawa.ne.jp/~manabes/) in 1991. CDM is an algebraic approach applied to polynomial loop in the parameter space, where a special diagram called coefficient diagram is used as the vehicle to carry the necessary information, and as the criteria of good design (Manabe,1998). The performance of the closed loop system is monitored on coefficient diagram. The simplicity of the controller structure made it very powerful for systems with uncertainties such as robotic manipulator (Hamamci(http://web.inonu.edu.tr/~shamamci/) and Ucar, 2002).

Generally, the problem of a control system design consists of choosing a proper controller considering the system dynamics, which is to be controlled, and desired performance specifications. There are three main theory for a design procedure: Conventional Control Theory, Modern Control Theory and Algebraic Approach. The main difference among these theories is the design approach used to obtain the controller and the mathematical expressions used to represent the system . Classical Control Methods, such as Frequency Response Method and Root-Locus Method, use the transfer function for the system representation. However, this representation can lead to undesired results because of pole-zero cancellations due to uncontrollable or unobservable situations. Modern Control Methods, like Pole-Placement, Optimal Control (LQR) and Hinf , use state-space representation. This representation, especially as the plant degree gets larger, involves complex calculations which require the use of a computer. Algebraic methods like Pole-placement direct method and CDM use polynomial expressions. In this representation, since the numerator and denominator of the transfer function are considered independently from each other, better results can be achieved against pole-zero cancellations. In this approach, the type and degree of the controller polynomials and characteristic polynomial of the closed-loop system are defined at the beginning. Considering the design specifications, coefficients of the polynomials are found later in the design procedure. In algebraic methods, CDM is the one which gives the most proper results with the the easiest procedure. In CDM, design specifications are equivalent time constant , stability indices and stability limits. These parameters have certain relations which will be explained later with the controller polynomials (Manabe and Kim, 2000).

REFERENCES 1. S. Manabe, "Coefficient Diagram Method", 14th IFAC Symp. on Automatic Control in Aerospace, Seoul,1998. 2. Hamamci S.E, Ucar A, "A Model Based Control for Uncertain Systems", Transaction Institute of Measurement and Control, v.24, no.4, pp.331-345, 2002. 3. S. Manabe and Y.C. Kim, "Recent development of Coefficient Diagram Method", ASSC'2000 3rd Asian Control Conference, Shanghai, 2000.

Dr. Serdar Ethem HAMAMCI ---------------------------------------- Inonu University Engineering Faculty Electrical&Electronics Eng. Dept. 44280 Malatya/TURKEY e-mail: shamamci@inonu.edu.tr

-- serdar ethem hamamci (shamamci@inonu.edu.tr), April 01, 2004.