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  • CYBERNETICS  (19)
  • 1985-1989  (9)
  • 1980-1984  (10)
  • 1
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    Some design guidelines for discrete-time adaptive controllers (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: There have been many algorithms proposed for adaptive control which will provide globally asymptotically stable controllers if some stringent conditions on the plant are met. The conditions on the plant cannot be met in practice as all plants will contain high frequency unmolded dynamics therefore, blind implementation of the published algorithms can lead to disastrous results. This paper uses a linearization analysis of a non-linear adaptive controller to demonstrate analytically design guidelines which aleviate some of the problems associated with adaptive control in the presence of unmodeled dynamics.
    Keywords: CYBERNETICS
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  • 2
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    Beyond singular values and loop shapes (1985)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: The status of singular value loop-shaping as a design paradigm for multivariable feedback systems is reviewed. It shows that this paradigm is an effective design tool whenever the problem specifications are spacially round. The tool can be arbitrarily conservative, however, when they are not. This happens because singular value conditions for robust performance are not tight (necessary and sufficient) and can severely overstate actual requirements. An alternate paradign is discussed which overcomes these limitations. The alternative includes a more general problem formulation, a new matrix function mu, and tight conditions for both robust stability and robust performance. The state of the art currently permits analysis of feedback systems within this new paradigm. Synthesis remains a subject of research.
    Keywords: CYBERNETICS
    Type: NASA-CR-177018 , NAS 1.26:177018 , LIDS-P-1504
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  • 3
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    Robustness of continuous-time adaptive control algorithms in the presence of unmodeled dynamics (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper examines the robustness properties of existing adaptive control algorithms to unmodeled plant high-frequency dynamics and unmeasurable output disturbances. It is demonstrated that there exist two infinite-gain operators in the nonlinear dynamic system which determines the time-evolution of output and parameter errors. The pragmatic implications of the existence of such infinite-gain operators is that: (1) sinusoidal reference inputs at specific frequencies and/or (2) sinusoidal output disturbances at any frequency (including dc), can cause the loop gain to increase without bound, thereby exciting the unmodeled high-frequency dynamics, and yielding an unstable control system. Hence, it is concluded that existing adaptive control algorithms as they are presented in the literature referenced in this paper, cannot be used with confidence in practical designs where the plant contains unmodeled dynamics because instability is likely to result. Further understanding is required to ascertain how the currently implemented adaptive systems differ from the theoretical systems studied here and how further theoretical development can improve the robustness of adaptive controllers.
    Keywords: CYBERNETICS
    Type: IEEE Transactions on Automatic Control (ISSN 0018-9286); AC-30; 881-889;
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  • 4
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    Robustness of adaptive control algorithms in the presence of unmodeled dynamics (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper reports the outcome of an exhaustive analytical and numerical investigation of stability and robustness properties of a wide class of adaptive control algorithms in the presence of unmodeled dynamics and output disturbances. The class of adaptive algorithms considered are those commonly referred to as model-reference adaptive control algorithms, self-tuning controllers, and dead-beat adaptive controllers; they have been developed for both continuous-time systems and discrete-time systems. The existing adaptive control algorithms have been proven to be globally asymptotically stable under certain assumptions, the key ones being (1) that the number of poles and zeroes of the unknown plant are known, and (2) that the primary performance criterion is related to good command following. These theoretical assumptions are too restrictive from an engineering point of view. Real plants always contain unmodeled high-frequency dynamics and small delays, and hence no upper bound on the number of the plant poles and zeroes exists. Also real plants are always subject to unmeasurable output additive disturbances, although these may be guide small. Hence, it is important to critically examine the stability robustness properties of the existing adaptive algorithms when some of the theoretical assumptions are removed; in particular, their stability and performance properties in the presence of unmodeled dynamics and output disturbances. Previously announced in STAR as N83-16061
    Keywords: CYBERNETICS
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  • 5
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    Angles of multivariable root loci (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A generalized eigenvalue problem is demonstrated to be useful for computing the multivariable root locus, particularly when obtaining the arrival angles to finite transmission zeros. The multivariable root loci are found for a linear, time-invariant output feedback problem. The problem is then employed to compute a closed-loop eigenstructure. The method of computing angles on the root locus is demonstrated, and the method is extended to a multivariable optimal root locus.
    Keywords: CYBERNETICS
    Type: LIDS-P-1147 , IEEE Transactions on Automatic Control; AC-27; Dec. 198
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  • 6
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    Multivariable feedback design - Concepts for a classical/modern synthesis (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper presents a practical design perspective on multivariable feedback control problems. It reviews the basic issue - feedback design in the face of uncertainties - and generalizes known single-input, single-output (SISO) statements and constraints of the design problem to multiinput, multioutput (MIMO) cases. Two major MIMO design approaches are then evaluated in the context of these results.
    Keywords: CYBERNETICS
    Type: IEEE Transactions on Automatic Control; AC-26; Feb. 198
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  • 7
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    Asymptotic Eigenstructures (1980)
    In:  CASI
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    Publication Date: 2019-06-27
    Description: The behavior of the closed loop eigenstructure of a linear system with output feedback is analyzed as a single parameter multiplying the feedback gain is varied. An algorithm is presented that computes the asymptotically infinite eigenstructure, and it is shown how a system with high gain, feedback decouples into single input, single output systems. Then a synthesis algorithm is presented which uses full state feedback to achieve a desired asymptotic eigenstructure.
    Keywords: CYBERNETICS
    Type: NASA-CR-162900 , LIDS-P-982
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  • 8
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    Design of feedback control systems for stable plants with saturating actuators (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A systematic control design methodology is introduced for multi-input/multi-output stable open loop plants with multiple saturations. This new methodology is a substantial improvement over previous heuristic single-input/single-output approaches. The idea is to introduce a supervisor loop so that when the references and/or disturbances are sufficiently small, the control system operates linearly as designed. For signals large enough to cause saturations, the control law is modified in such a way as to ensure stability and to preserve, to the extent possible, the behavior of the linear control design. Key benefits of the methodology are: the modified compensator never produces saturating control signals, integrators and/or slow dynamics in the compensator never windup, the directional properties of the controls are maintained, and the closed loop system has certain guaranteed stability properties. The advantages of the new design methodology are illustrated in the simulation of an academic example and the simulation of the multivariable longitudinal control of a modified model of the F-8 aircraft.
    Keywords: CYBERNETICS
    Type: NASA-CR-182613 , NAS 1.26:182613 , LIDS-P-1756
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  • 9
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    Conic sectors for sampled-data feedback systems (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The conic-sector analysis of the closed-loop stability and robustness of a multivariable-analog-system controller based on sampled-data feedback compensation is investigated. Conic sectors and sampled-data feedback systems are defined, and the existence of a conic sector containing a sampled-data operator is established mathematically. An example is presented to prove that the conic sector is computable and gives sufficient conditions of closed-loop stability. A procedure for determining sampled-data-operator gain is also derived.
    Keywords: CYBERNETICS
    Type: Systems and Control Letters (ISSN 0167-6911); 3; 77-82
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  • 10
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    A method for designing robust multivariable feedback systems (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A new methodology is developed for the synthesis of linear, time-invariant (LTI) controllers for multivariable LTI systems. The aim is to achieve stability and performance robustness of the feedback system in the presence of multiple unstructured uncertainty blocks; i.e., to satisfy a frequency-domain inequality in terms of the structured singular value. The design technique is referred to as the Causality Recovery Methodology (CRM). Starting with an initial (nominally) stabilizing compensator, the CRM produces a closed-loop system whose performance-robustness is at least as good as, and hopefully superior to, that of the original design. The robustness improvement is obtained by solving an infinite-dimensional, convex optimization program. A finite-dimensional implementation of the CRM was developed, and it was applied to a multivariate design example.
    Keywords: CYBERNETICS
    Type: NASA-CR-182459 , NAS 1.26:182459 , LIDS-P-1749
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