Open-loop Instability Research Articles

Open-Loop Instability of the Bridge Rectifier with Input Filter

The instability characteristics of the bridge rectifier with input filter are described. A stability criterion based on the perturbational analysis is proposed. According to this criterion, the steady-stale circuit variables during stable operation are employed to examine the stability of the rectifier. Thereby, a simple and flexible stability method is developed. The method is applied to six-pulse and two-pulse bridge circuits with LC type input filter. The accuracy of the results is verified by experiment. The effects of converter output ripple, input voltage distortion, and type of gating circuit on the stability of the converter are examined experimentally and justified theoretically. Finally, the stability condition is determined explicitly in terms of the circuit parameters.

An Investigation on the Methodology and Technique of Model Reduction

Abstract In this paper an attempt is made to search for new computational methods of model reduction which will overcome the chief drawbacks of the current methods, such as the possibility of open-loop instability and the large error in the middle-frequency range. The principle of an optimal allotment of the informational resources in n reduced-order model and the technique of separate treatments for middle-frequency and low-frequency ranges are presented. Two methods based upon these concepts are offered. Results of computation show convincingly that these new methods of model reduction are superior to the current methods from the point-of-view of a better similitude to the original model over a wide range of frequency. They are also simple to use and versatile in application.

Design of a 3×3 multivariable feedback system with large plant uncertainty†

A recent paper presented a quantitative synthesis theory based on Schauder's fixed point theorem, for uncertain multiple input-output feedback systems. This paper is devoted to practical design execution, by means of a detailed 3×3 problem with large plant uncertainty, including open-loop instability and some non-minimum-phase plant elements. The outstanding features of the theory are : 1. The quantitative nature of the assigned tolerances on the nine closed-loop system response functions. In this problem four are specified basically non-interacting and the other five interacting. 2, The design problem is translated into separate quantitative single-loop problems, each with plant uncertainty, external disturbances and closed-loop tolerances derived from the original problem. The solutions for these single-loop problems are guaranteed to solve the original multivariable problem, whose system characteristic equation need never be examined. 3. There is developed a systematic means of (a) optimization of the loops, wherein loops may be improved at no expense to any other loops, until equilibrium is achieved and this is no longer possible, (b) if desired, trade-offs between the loops where some may be sacrificed for the improvement of others. Analytical trade-off relations are established for the higher frequency range. The problem was simulated with excellent design verification.

Some Effects of Parameter Variations on the Lateral-Directional Stability of Aircraft

Methods of analyzing the effects of aerodynamic uncertainties on the stability of lateral-directional motions are presented. Truncated and residualized reduced-order models of the motion are developed. Together with covariance analysis, these models provide simplified estimates of the probability of instability. A Space Shuttle entry flight condition is examined with this technique, and the predicted probability of open-loop instability is found to be low (< 0.0001). Monte Carlo stability analysis illustrates that closed-loop control can increase the probability of instability as a consequence of uncertainty in the control derivatives.

STABILITY INVESTIGATION OF FREQUENCY-CONTROLLED THREE-PHASE INDUCTION MOTOR

Abstract The paper deals with open-loop instability of a three-phase induction motor fed from a sine-wave source with variable frequency and zero internal impedance. An analytic method is presented for determining the upper bound of the instability range of the motor as function of its various parameters. The whole range of instability is examined by eigenvalue analysis about given working points. Digital simulation of the motor behavior is presented.

Damped EMI Filters for Switching Regulators

The phenomenon of open-loop instability between the powerline EMI filter and the switching-regulator power supply is investigated [1]. As a first step to the filter-design problem, simple modifications are made to the filter chosen from EMI considerations alone to assure stability as well.

Phase-controlled circulating current cycloconvertor-induction-motor drive using rotating intergroup reactor

The paper describes the use of an induction motor stator as an intergroup reactor in a circulating-current cyclo-convertor. A double-layer stator winding formed into electrically separate thyristor group circuits, magnetically coupled in the stator slots, provides a conventional rotating field in the airgap. This enables the double wound stator machine to act as a reactor maintaining circulating current operation in the cycloconvertor and as a torque producing motor. Standard induction motors may be additionally incorporated into the system as in a conventional cycloconvertor drive. A closed-loop speed control system is incorporated that maintains constant speed independent of load on the divided-winding reactor machine. Any additional induction motors then operate technically under open-loop conditions, but open-loop instability is avoided by the inclusion of the reactor machine in the loop.

Thermodynamics

Fuzzy logic control frequently exhibits superior performance to classical linear controllers even for ‘hard’, mathematically well defined plants, as described in this paper. The case-study of a highly nonlinear exothermic continuous stirred tank reactor, which poses a multivariable control problem with two interacting loops and open-loop instability, is used. The behaviour of the fuzzy logic controller is compared with that of a PID controller. A smooth, easily tuneable gain-schedule is designed to handle offset-like problems with a fuzzy controller. It is analytically shown that such a gain-schedule is the simpler, intuitive equivalent of a manipulation of the corresponding fuzzy membership functions. The fuzzy controller structure chosen is a parsimonious one, with the choice of Gaussian bell-shaped membership functions generating a smooth input/output surface with nontrivial inferencing spanning the entire input space. This provides a clear, non-heuristic reason to select Gaussian over triangular shapes for membership functions. The gain-scheduled fuzzy controller shows excellent control performance, significantly outperforming the PID controllers in both servo and regulatory modes. The disturbance rejection behaviour of the modified fuzzy controller is observed to be particularly good.