Transactions of the Institute of Measurement and Control current issue

PI control of first-order lag plus time-delay plants: root locus design for optimal stability

Cogan, B., de Paor, A. M., Quinn, A. — Thu, 24 Sep 2009 06:20:38 PDT

We present a procedure for the design of a PI controller for a general first-order lag plus time-delay plant. We derive two equations that allow a designer to calculate the PI controller parameters using only the plant parameters. The central idea of this design procedure is to use the root locus method to select controller parameters that put the system’s rightmost eigenvalue as far to the left as possible in the (, ) plane. When the system is operating at this point, it is said to be operating at a point of optimum stability in the root locus sense.

Robust performance design of single-input fuzzy control system for plant with time delay

Yordanova, S. — Thu, 24 Sep 2009 06:20:38 PDT

Fuzzy logic has recently gained popularity in the development of robust non-linear control convenient for real-time applications, satisfying the high-performance requirements of the control systems by dealing with data imprecision and noise. There are, however, problems in designing a unique controller that will provide system stability and desirable performance in the presence of plant model uncertainties and time delays. The aim of this investigation is to develop a simple approach for the design of an incremental PI-like single-input fuzzy controller (SI FC) that ensures a robust performance in the closed-loop system for the control of industrial plants with time delays and model uncertainties. The main results are: 1) derivation of a robust performance criterion of the SI FC system in the frequency domain, employing the stabilization of the system linear part, the linearization of the fuzzy unit and the Popov stability approach; 2) development of an SI FC tuning algorithm; 3) application of the design approach for the control of air temperature in a furnace using MATLAB^TM, and assessment of the advantages over an ordinary PI controller. The designed SI FC preserves system stability and performance for a high range of plant model uncertainties.

Composite disturbance-observer-based control and variable structure control for non-linear systems with disturbances

Wei, X., Zhang, H., Guo, L. — Thu, 24 Sep 2009 06:20:38 PDT

A novel type of control scheme combining the disturbance-observer-based control (DOBC) with variable structure control (VSC) is proposed for a class of continuous non-linear systems subject to disturbances. The disturbances are supposed to include two parts. One in the input channel is generated by an exogenous system with uncertainty, which can represent the harmonic signals with modelling perturbations. The other is supposed to have the bounded H₂ norm. The disturbance observers based on regional pole placement and D-stability theory are presented, which can be designed separately from the controller design. By integrating DOBC with VSC laws, the disturbances can be rejected and the desired dynamic performances can be guaranteed for continuous systems in finite time with known and unknown non-linear dynamics, respectively. Simulations for a flight control system are given to demonstrate the effectiveness of the results and compare the proposed results with the previous schemes.

Time scaling in PID controller tuning

Atherton, D. P., Boz, A. F. — Thu, 24 Sep 2009 06:20:38 PDT

The paper examines the problem of tuning the parameters of a PID controller for the frequently assumed first-order plus dead time plant. The original contribution is to look at the problem from a time-scaling viewpoint so that the controller parameters can be given as a function of the time delay to time constant ratio. This enables the controller parameter results of different rules to be easily compared and reveals those that are not consistent under time scaling.

Sliding mode controlled bioreactor using a time-varying sliding surface

Tokat, S. — Thu, 24 Sep 2009 06:20:39 PDT

Bioreactor processes based on substrate concentration have both highly non-linear behaviour and parameter uncertainties mostly bounded with physical and biological constraints, which make them a subject area of sliding mode control. This work deals with sliding mode control of a fermentation process in a continuous stirred bioreactor. A new sliding surface is defined using angular information as a function of time that provides adjustable continuous movement of the sliding surface. Computer simulations are presented to show the effectiveness and efficiency of the proposed method and to make a quantitative comparison with the conventional sliding mode controller having a constant sliding surface. The excess amount of substrate concentration in the mixture causes undesired by-product information. Therefore, the substrate concentration should not stay above its desired value for a long time. It is shown in the simulations that the performance of the proposed method, especially in regard to the settling time of the substrate concentration and reaching time, is improved. It is also shown that different system state trajectories could be obtained with the proposed method by adjusting the defined parameters of the angular information.