Research Papers

Absolute Stability Analysis Using the Liénard Equation: A Study Derived From Control of Fuel Cell Ultracapacitor Hybrids

[+] Author and Article Information
William Nowak

Rochester Institute of Technology,
Rochester, NY 14623

Daniel Geiyer

Department of Mechanical and Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816

Tuhin Das

Department of Mechanical and Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816
e-mail: Tuhin.Das@ucf.edu

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received April 4, 2015; final manuscript received December 9, 2015; published online January 12, 2016. Assoc. Editor: Junmin Wang.

J. Dyn. Sys., Meas., Control 138(3), 031007 (Jan 12, 2016) (10 pages) Paper No: DS-15-1152; doi: 10.1115/1.4032318 History: Received April 04, 2015; Revised December 09, 2015

Load-following in solid oxide fuel cells (SOFCs), hybridized with an ultracapacitor for energy storage, refers to an operating mode where the fuel cell's generated power follows the variable power demand, delivering the total demanded power at steady-state. Implementing this operating mode presents a rich set of problems in dynamical systems and control. This paper focuses on state-of-charge (SOC) control of the ultracapacitor during load-following, under transient constraints, and in the presence of an unknown nonlinearity. The problem is generalized to stabilization of a plant containing a cascaded connection of a driver and a driven dynamics, where the former is nonlinear and largely unknown. Closed-loop stability of the system is studied as a Lur'e problem and via energy-based Lyapunov equations, but both impose conservative conditions on the nonlinearity. An alternate approach is developed, where the closed-loop dynamics are formulated as a class of Liénard equations. The corresponding analysis, which is based on the nonlinear characteristics of the Liénard equation, yields more definitive and less conservative stability criteria. Additional conditions that lead to limit cycles are also derived, and a bifurcation pattern is revealed. The generality of the proposed approach indicates applicability to a variety of nonlinear systems.

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Fig. 1

Schematic diagram of the hybrid SOFC system

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Fig. 2

SOFC/UC hybrid as a cascaded system

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Fig. 3

Generalized form of Fig. 2

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Fig. 4

A Lur'e problem formulation derived from Fig. 3

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Fig. 5

Circle criterion simulations: (b) with fb(e) and (c) with fc(e)

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Fig. 7

Variation OF λ1,2 with a, indicating Hopf bifurcation

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Fig. 6

Feedback system with driven dynamic equation generalized to Eq. (38)

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Fig. 8

Simulations to verify Theorems 3 and 4

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Fig. 10

HIL test stand and experimental results

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Fig. 9

Simulation verifying the stability result of Eq. (55)



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