M. Chidambaram

Direct synthesis-based controller design for integrating processes with time delay

Using the direct synthesis method, a PID controller in series with a lead/lag compensator is designed for control of open loop integrating processes with time delay. Set-point weighting is considered for reducing the undesirable overshoot. Guidelines are provided for selection of the desired closed loop tuning parameter in the direct synthesis method and set point weighting parameter. The method gives significant load disturbance rejection performances. Illustrative examples are considered to show the performances of the proposed method. Significant improvement is obtained when compared to recently reported methods.

Centralized PI controllers for interacting multivariable processes by synthesis method.

In this article, two methods of designing a centralized control system for multi-input, multi-output (MIMO) processes are presented. Centralized proportional-integral (PI) controllers are designed based on a direct synthesis method. The inverse of the process transfer function matrix in the direct synthesis method is approximated based on the relative gain array concept. The method is further improved by using a relative normalized gain array, and an equivalent transfer function for each element in the process transfer function matrix is derived for the closed-loop control system. The transpose of the effective transfer function is used to approximate the inverse of the process transfer function matrix. The simulation studies demonstrate the effectiveness of this method. The proposed centralized controllers reduce the interactions better than recently reported decentralized controllers do. A centralized controller designed based on a relative normalized gain array (RNGA) gives a better performance than a centralized controller designed based on a relative gain array (RGA).

Limitations of a PI controller for a first-order nonlinear process with dead time.

A nonlinear process, the conical tank level process whose parameters vary with respect to the process variable, is considered. The time constant and gain of the chosen process vary as a function of level. A deliberate dead time is introduced in the system by delaying the valve opening. The limitations of the conventional PI controller tuned using Ziegler-Nichols settings for the chosen process are brought out. The servo and regulatory responses through simulation and experimentation for various magnitudes of set-point changes and load changes at various operating points with the controller tuned only at a chosen nominal operating point are obtained and analyzed. Regulatory responses for output load changes are studied. A benchmark system is presented to understand clearly how variations of system parameters affect the performance of the controller. A gain scheduled controller is proposed to handle the control problem. Present work illustrates the simplicity and effectiveness of the gain scheduling.

Analytical design of modified Smith predictor in a two-degrees-of-freedom control scheme for second order unstable processes with time delay

A modified form of Smith predictor is designed for controlling unstable second order plus time delay (USOPTD) processes, with/without a zero. USOPTD process transfer functions arise in modeling of many chemical and biological systems, such as isothermal continuous stirred tank reactors (CSTR) carrying out autocatalytic reactions, crystallizers and non-isothermal CSTRs. The modified Smith predictor scheme requires the design of three controllers. Synthesis method is used for the design of the three controllers, and analytical formulas are given for these controllers. The method has two tuning parameters, and guidelines are provided for selecting the tuning parameters. Robust disturbance rejection performances are achieved. Different examples are considered, and simulation studies are given, to show the effectiveness of the proposed method. A significant improvement is obtained, when compared with recently reported methods.

Design of controller using variable transformations for a nonlinear process with dead time.

In this work, a globally linearized controller (GLC) for a first-order nonlinear system with dead time is proposed. This is similar to the GLC proposed by Ogunnalke [Ind. Eng. Chem. Process. Des. Dev. 25, 241-248 (1986)] for nonlinear systems without dead time. Two methods are proposed. One is based on the Smith prediction from the model in the transformed domain and the other is based on Newtons extrapolation method. The simulation study is made on the conical tank level process and the results are compared with those obtained using a conventional PI controller and the Smith PI controller based on the transfer function model about the operating point 39%. Finally, experimental results on the laboratory conical tank level process are also given.

AN IMPROVED RELAY AUTO TUNING OF PID CONTROLLERS FOR CRITICALLY DAMPED SOPTD SYSTEMS

Using a single-symmetric relay feedback test, a method is proposed to identify all three parameters of a stable second-order plus time delay (SOPTD) model with equal time constants [kp exp(-Ds)/(τs + 1)2]. The conventional analysis of the relay auto-tune method gives 27% error in the calculation of ku, for a large D/τ. In the present work, a method is proposed to improve the accuracy in the ku calculation by incorporating higher order harmonics. Three simulation examples are given for SOPTD and higher order time delay transfer function models. The estimated model parameters of the SOPTD model are compared with those of the Li et al. (1991) method and those of the Thyagarajan and Yu (2003) method. The open-loop performance of the identified model is compared with that of the actual system. The proposed method gives performance close to that of the actual system. Simulation results are also given for a nonlinear bioreactor system. The closed-loop performance of the model identified by the proposed method is close to that of the actual system.