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  • 1
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    Some design guidelines for discrete-time adaptive controllers (1985)
    In:  Other Sources
    Details
    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
    Details
    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
    Format: application/pdf
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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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    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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  • 5
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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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  • 6
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    A frequency-domain estimator for use in adaptive control systems (1987)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The paper presents a frequency-domain estimator which can identify both a nominal model of a plant as well as a frequency-domain bounding function on the modeling error associated with this nominal model. This estimator, which is called a robust estimator, can be used in conjunction with a robust control-law redesign algorithm to form a robust adaptive controller.
    Keywords: CYBERNETICS
    Type: 1987 American Control Conference; Jun 10, 1987 - Jun 12, 1987; Minneapolis, MN; United States
    Format: text
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  • 7
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    The LQR/LTR procedure for multivariable feedback control design (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: This paper provides a tutorial overview of the linear quadratic Gaussian with loop transfer recovery (LQG/LTR) design procedure for linear multivariable feedback systems. LQR/LTR is interpreted as the solution of a specific weighted H-squared tradeoff between transfer functions in the frequency domain. Properties of this solution are examined for both minimum-phase and nonminimum-phase systems. This leads to a formal weight augmentation procedure for the minimum-phase case which permits essentially arbitrary specification of system sensitivity functions in terms of the weights. While such arbitrary specifications are not possible for nonminimum-phase problems, a direct relationship between weights and sensitivities is developed for nonminimum-phase SISO and certain nonminimum-phase MIMO cases which guides the weight selection process.
    Keywords: CYBERNETICS
    Type: IEEE Transactions on Automatic Control (ISSN 0018-9286); AC-32; 105-114
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  • 8
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    Design of feedback control systems for stable plants with saturating actuators (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    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 this 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: IEEE Conference on Decision and Control; Dec 07, 1988 - Dec 09, 1988; Austin, TX; United States
    Format: text
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  • 9
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    A method for designing robust multivariable feedback systems (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    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: IEEE Conference on Decision and Control; Dec 07, 1988 - Dec 09, 1988; Austin, TX; United States
    Format: text
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