Peter M. Young

A formula for computation of the real stability radius

This paper presents a readily computable formula for the real stability radius with respect to an arbitrary stability region in the complex plane.

Computational complexity of /spl mu/ calculation

The structured singular value /spl mu/ measures the robustness of uncertain systems. Numerous researchers over the last decade have worked on developing efficient methods for computing /spl mu/. This paper considers the complexity of calculating /spl mu/ with general mixed real/complex uncertainty in the framework of combinatorial complexity theory. In particular, it is proved that the /spl mu/ recognition problem with either pure real or mixed real/complex uncertainty is NP-hard. This strongly suggests that it is futile to pursue exact methods for calculating /spl mu/ of general systems with pure real or mixed uncertainty for other than small problems. >

Auger lifetime enhancement in InAs–Ga1−xInxSb superlattices

We have experimentally and theoretically investigated the Auger recombination lifetime in InAs–Ga1−xInxSb superlattices. Data were obtained by analyzing the steady‐state photoconductive response to frequency‐doubled CO2 radiation, at intensities varying by over four orders of magnitude. Theoretical Auger rates were derived, based on a k⋅p calculation of the superlattice band structure in a model which employs no adjustable parameters. At 77 K, both experiment and theory yield Auger lifetimes which are approximately two orders of magnitude longer than those in Hg1−xCdxTe with the same energy gap. This finding has highly favorable implications for the application of InAs–Ga1−xInxSb superlattices to infrared detector and nonlinear optical devices.

Long wavelength InAs/InGaSb infrared detectors: Optimization of carrier lifetimes

The performance characteristics of type‐II InAs/InxGa1−xSb superlattices for long and very long‐wave infrared detection are discussed. This system promises benefits in this wavelength range over conventional technology based on Hg1−xCdxTe, in part because of suppressed band‐to‐band Auger recombination rates which lead to improved values of detectivity. The formalism for calculating Auger rates in superlattices is developed and the physical origin of Auger suppression in these systems is discussed. Accurate K⋅p band structures are used to obtain radiative, electron–electron, hole–hole, and band‐to‐band Auger rules, as well as shallow trap level assisted Auger recombination rates for photodiodes. Theoretical limits for high temperature operation of ideal photovoltaic detectors are presented and compared with HgCdTe.

mu analysis with real parametric uncertainty

The authors give a broad overview, from a LFT (linear fractional transformation)/ mu perspective, of some of the theoretical and practical issues associated with robustness in the presence of real parametric uncertainty, with a focus on computation. Recent results on the properties of mu in the mixed case are reviewed, including issues of NP completeness, continuity, computation of bounds, the equivalence of mu and its bounds, and some direct comparisons with Kharitonov-type analysis methods. In addition, some advances in the computational aspects of the problem, including a branch-and-bound algorithm, are briefly presented together with the mixed mu problem may have inherently combinatoric worst-case behavior, practical algorithms with modes computational requirements can be developed for problems of medium size (<100 parameters) that are of engineering interest.<<ETX>>

Minority carrier lifetimes in ideal InGaSb/InAs superlattices

Calculations of band‐to‐band Auger and radiative recombination lifetimes of the recently proposed InxGa1−xSb/InAs superlattices (SL) show them to be promising infrared detectors. Several superlattices with energy gaps in the 5–11 μm range exhibit suppressed p‐type Auger recombination rates due to a large light hole–heavy hole splitting. The p‐type Auger lifetime at 77 K of an 11 μm InxGa1−xSb/InAs SL is found to be, respectively, three and five orders of magnitude longer than those of bulk and superlattice HgCdTe with the same energy gap. The n‐type lifetimes are comparable.

Computation of mu with real and complex uncertainties

The robustness analysis of system performance is one of the key issues in control theory, and one approach is to reduce this problem to that of computing the structured singular value, mu . When real parametric uncertainty is included, then mu must be computed with respect to a block structure containing both real and complex uncertainties. It is shown that mu is equivalent to a real eigenvalue maximization problem, and a power algorithm is developed to solve this problem. The algorithm has the property that mu is (almost) always an equilibrium point of the algorithm, and that whenever the algorithm converges a lower bound for mu results. This scheme has been found to have fairly good convergence properties. Each iteration of the scheme is very cheap, requiring only such operations as matrix-vector multiplications and vector inner products, and the method is sufficiently general to handle arbitrary numbers of repeated real scalars, repeated complex scalars, and full complex blocks.<<ETX>>

A tighter bound for the echo state property

This letter provides a brief explanation of echo state networks (ESNs) and provides a rigorous bound for guaranteeing asymptotic stability of these networks. The stability bounds presented here could aid in the design of echo state networks that would be applicable to control applications where stability is required

Nonconservative Lagrangian mechanics: a generalized function approach

We reexamine the problem of having nonconservative equations of motion arise from the use of a variational principle. In particular, a formalism is developed that allows the inclusion of fractional derivatives. This is done within the Lagrangian framework by treating the action as a Volterra series. It is then possible to derive two equations of motion, one of these is an advanced equation and the other is retarded.

Control Design for Variations in Structural Natural Frequencies

The lightly damped nature of flexible structures can lead to controllers that are highly sensitive to modeling errors in structural natural frequencies and damping levels. An approach to directly incorporating these modeling errors into the control design process is presented that leads to less sensitive and more robust controllers with improved performance/This approach is used to synthesize controllers for the NASA Langley Mini-Mast experimental structure. The resulting designs obtain good performance in the presence of significant modeling errors in the structural natural frequencies.