Adaptive control: Myths and realities

It was found that all currently existing globally stable adaptive algorithms have three basic properties in common: positive realness of the error equation, square-integrability of the parameter adjustment law and, need for sufficient excitation for asymptotic parameter convergence. Of the three, the first property is of primary importance since it satisfies a sufficient condition for stabillity of the overall system, which is a baseline design objective. The second property has been instrumental in the proof of asymptotic error convergence to zero, while the third addresses the issue of parameter convergence. Positive-real error dynamics can be generated only if the relative degree (excess of poles over zeroes) of the process to be controlled is known exactly; this, in turn, implies perfect modeling. This and other assumptions, such as absence of nonminimum phase plant zeros on which the mathematical arguments are based, do not necessarily reflect properties of real systems. As a result, it is natural to inquire what happens to the designs under less than ideal assumptions. The issues arising from violation of the exact modeling assumption which is extremely restrictive in practice and impacts the most important system property, stability, are discussed

Feedback system design with an uncertain plant

A method is developed to design a fixed-parameter compensator for a linear, time-invariant, SISO (single-input single-output) plant model characterized by significant structured, as well as unstructured, uncertainty. The controller minimizes the H(infinity) norm of the worst-case sensitivity function over the operating band and the resulting feedback system exhibits robust stability and robust performance. It is conjectured that such a robust nonadaptive control design technique can be used on-line in an adaptive control system

Guaranteed robust fault detection and isolation techniques for small satellites

The paper presents two generic fault detection and isolation (FDI) techniques which have shown remarkable robustness when applied to the SIMULINK model of a small satellite for thruster failures. While fundamentally different in their design approach, they both generate ʽstructured residualsʼ which accurately capture the failure mode. The diagnosis criterion in both methods relies on residuals direction rather than magnitude, which avoids the delays and expense of setting accurate thresholds for residuals magnitudes. Most importantly, this fact can account for the enhanced robustness to disturbances and sensor noise, as well as to significant parametric variations. Extensive Monte Carlo simulations are presented validating the robust performance of the two algorithms

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS Evaluation of Approaches to Active Compressor Surge Stabilization

A 345 E. 47 St., New York, N.Y. 10017 The Society shalt not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. M Discussion is printed only if the paper Is published In an ASME Journal. Papers are available ]^C ® from ASME for fifteen months after the meeting. Printed in USA. 92-GT-182 Evaluation ABSTRACT m* dimensionless mass of gas in plenum; m* = ppvp V me dimensionless slope of compressor speed line; mC P ad'vao Recent work has shown that compression systems can be 11e dimensionless slope of equivalent compressor speed line actively stabilized against the instability known as surge, thereby realizing a significant gain in system mass flow range. Ideally, this formed by compressor in series with close-coupled valve; surge stabilization requires only a single sensor and a single actuator mCe = a/a0 (Tcby a suitable control law. Almost all research to date has mT dimensionless slope of throttle pressure drop versus flow been aimed at proof of concept studies of this technique, using various characteristic; mT = 1/(ate/a v) actuators and sensor combinations. In contrast, the work reported mTe dimensionless slope of equivalent throttle characteristic for herein can be regarded as a step towards developing active control into parallel combination of throttle and plenum bleed valve; a practical technique. In this context, the paper presents the first rtrre = 1/(a(1+cb)/a v ) systematic definition of the influence of sensor and actuator selection MT impeller tip Mach number, MT = UT/a a on increasing the range of stabilized compressor performance. The results show that proper choice of sensor as well as actuator crucially P* absolute pressure affects the ability to stabilize these systems, and that, overall, those Pp plenum pressure ratio; pp = Pp / P a actuators which are most closely coupled to the compressor (as Q heat release rate opposed to the plenum or throttle) appear most effective. In addition, Q* dimensionless heat release rate; Q* = Q / p aUTAcCp Ta the source of the disturbances driving the system (for example, R gas constant for ideal gas; R = Cp -C., unsteady compressor pressure rise or unsteady combustor heat s Laplace transform variable release) has a strong influence on control effectiveness, as would be expected for a controls problem of this type. This paper both AToc total temperature rise across compressor delineates general methodologies for the evaluation of active AToc dimensionless total temperature rise across compressor; compressor stabilization strategies and quantifies the performance of AT.*c = AT. / Ta several approaches which might be implemented in gas turbine t time engines