Somanath Majhi

Brief Obtaining controller parameters for a new Smith predictor using autotuning

The paper extends recent work on a modified Smith predictor strategy, which leads to significant improvements in its regulatory capacities for reference inputs and disturbances. High-order or long dead time stable, integrating and unstable plants are modelled as lower-order plant models with a longer time delay. The controllers are designed so that the delay-free component of the output is tuned to be either a first- or second-order response if there are no modelling errors in the assumed plant transfer function. Plant model transfer functions and the controller parameters are estimated using exact analysis from the peak amplitude and frequency of the process output obtained from a single-relay feedback test. Illustrative examples show the simplicity and superiority of the proposed controller design method over previously published approaches both for the setpoint response and for the load disturbance rejection.

Relay based PI-PD design for stable and unstable FOPDT processes

Abstract A simple identification method based on relay feedback test is proposed for stable and unstable first order plus time delay (FOPDT) processes. Initially, a relay in parallel with a proportional controller is applied to an inner feedback controller-stabilized process on the usual manner to induce limit cycle output. From the measurements made on the limit cycle output, two parameters of the FOPDT process model are estimated. The steady state gain of the FOPDT process is assumed to be known a priori. Then, these parameters are used to design a two degree of freedom proportional-integral–proportional-derivative (PI–PD) controller based on the loop phase margin and gain margin criteria. Simulation examples illustrate the effectiveness and the simplicity of the proposed identification and control method.

A new control scheme for PID load frequency controller of single-area and multi-area power systems

A new control structure with a tuning method to design a PID load frequency controller for power systems is presented. Initially, the controller is designed for single area power system, then it is extended to multi-area case. The controller parameters are obtained by expanding controller transfer function using Laurent series. Relay based identification technique is adopted to estimate power system dynamics. Robustness studies on stability and performance are provided, with respect to uncertainties in the plant parameters. The proposed scheme ensures that overall system remains asymptotically stable for all bounded uncertainties and for system oscillations. Simulation results show the feasibility of the approach and the proposed method improves the load disturbance rejection performance significantly even in the presence of the uncertainties in plant parameters.

PID controller tuning for integrating processes.

Minimizing the integral squared error (ISE) criterion to get the optimal controller parameters results in a PD controller for integrating processes. The PD controller gives good servo response but fails to reject the load disturbances. In this paper, it is shown that satisfactory closed loop performances for a class of integrating processes are obtained if the ISE criterion is minimized with the constraint that the slope of the Nyquist curve has a specified value at the gain crossover frequency. Guidelines are provided for selecting the gain crossover frequency and the slope of the Nyquist curve. The proposed method is compared with some of the existing methods to control integrating plant transfer functions and in the examples taken it always gave better results for the load disturbance rejection whilst maintaining satisfactory setpoint response. For ease of use, analytical expressions correlating the controller parameters to plant model parameters are also given.

Limitations of PID controllers

The objective of the paper is to outline the limitations of classical PID controllers acting on the error signal for control of a plant modelled as a linear transfer function. It is shown how a modified form of controller, a PI-PD controller, which corresponds to PI control of a plant transfer function changed by the PD feedback can produce improved control in several situations. This implementation avoids the derivative kick problem associated with derivative action in the forward path, which still exists when a filter is included. Further, the PD in the inner feedback loop enables placement of the open loop poles in appropriate positions thereby providing good control for open loop system transfer functions having resonances, unstable, or integrating poles. The simulation results show significantly improved performance of the proposed control method particularly for the latter processes.

A new Smith predictor and controller for unstable and integrating processes with time delay

A new Smith predictor for control of an unstable process with time delay and a process with an integrator and long dead-time is proposed. The controller decouples the setpoint response from the load response. The design approach is based on standard forms of the closed loop system response and on the Nyquist stability analysis. Simulation results show high performance for the setpoint response and load disturbance rejection.

PI/PID controller design based on IMC and percentage overshoot specification to controller setpoint change.

In this work, the normalized Internal Model Control (IMC) filter time constant is designed to achieve a specified value of the maximum sensitivity for stable first and second order plus time delay process models, respectively. Since a particular value of the maximum sensitivity results in an almost constant percentage overshoot to controller setpoint change, an empirical relationship between the normalized IMC filter time constant and percentage overshoot is presented. The main advantage of the proposed method is that only a user-defined overshoot is required to design a PI/PID controller. Simulation examples are given to demonstrate the value of the proposed method.

Modified Smith predictor based cascade control of unstable time delay processes.

An improved cascade control structure with a modified Smith predictor is proposed for controlling open-loop unstable time delay processes. The proposed structure has three controllers of which one is meant for servo response and the other two are for regulatory responses. An analytical design method is derived for the two disturbance rejection controllers by proposing the desired closed-loop complementary sensitivity functions. These two closed-loop controllers are considered in the form of proportional-integral-derivative (PID) controller cascaded with a second order lead/lag filter. The direct synthesis method is used to design the setpoint tracking controller. By virtue of the enhanced structure, the proposed control scheme decouples the servo response from the regulatory response in case of nominal systems i.e., the setpoint tracking controller and the disturbance rejection controller can be tuned independently. Internal stability of the proposed cascade structure is analyzed. Kharitonovs theorem is used for the robustness analysis. The disturbance rejection capability of the proposed scheme is superior as compared to existing methods. Examples are also included to illustrate the simplicity and usefulness of the proposed method.

Enhanced cascade control for a class of integrating processes with time delay

Unlike self-regulating processes, cascade control strategies for control of integrating processes with time delay are limited. A novel series cascade control structure to enhance the closed loop performance is proposed for integrating and time delay processes. The proposed controller structure has only two controllers and a setpoint filter. The inner loop controller is designed based on IMC approach and the primary setpoint filter is based on optimal performance index. The primary load disturbance rejection controller, a PID controller in series with a lead/lag compensator, is designed on the basis of the desired closed-loop complementary sensitivity function. The robustness analysis is carried out using Kharitonovs theorem. Simulation results demonstrate the efficacy of the proposed method by showing satisfactory nominal and robust performances.

Automatic generation control of an interconnected hydrothermal power system

This paper deals with automatic generation control of an interconnected hydrothermal system considering reheat type turbine in thermal plant area and low head turbine in hydro plant area using optimal controllers. The dynamical response of the load frequency control problem in an interconnected power system under consideration is improved with a practical viewpoint by designing the optimal output feedback controller. The optimal control methods are proposed and their dynamic responses are compared. The optimal control is determined by minimizing a performance index under the proposed output feedback conditions. The results indicate that the controllers exhibit better performance. In fact, the control systems designed on these methods satisfy the load frequency control requirements with a reasonable dynamic response.