Randi Potekin, Ph.D.Sandia National Laboratories
Friday, April 17th, 2025, 3:00 - 4:00 PM
Jett Hall, room 109
Title: Seminar Title: Intermodulation Nonlinear System Identification
ABSTRACT: This work proposes a new technique for open-loop nonlinear system identification, in which a single-degree-of-freedom (SDOF) nonlinear mechanical oscillator is resonantly excited by two closely spaced frequencies. Several forms of nonlinearity are considered including cubic stiffness, cubic damping, and clearance gaps. In the frequency domain, the oscillator’s response is dominated by a narrow-band frequency comb symmetric about the center frequency and spaced by the difference frequency. The corresponding time-domain representation is fast oscillations at the center frequency, with a slow amplitude and phase modulation at half the difference frequency. Using simulated data, I show how to reconstruct the nonlinear virial and energy dissipation, as functions of amplitude, from the response. In the small amplitude limit of the dynamics, the nonlinear virial is a measure of the maximum potential energy stored in the nonlinear elastic force-oscillator interaction per cycle. The nonlinear energy dissipation represents the energy dissipated per fast oscillation, by the nonlinear dissipative forces. Moreover, first-order averaging of the oscillator’s equation of motion mathematically relates the nonlinear effective resonant frequency and damping coefficient to the amplitude-dependent nonlinear virial and energy dissipation. For certain types of nonlinearities, such as clearance gaps, the nonlinear force itself can be reconstructed without prior knowledge of the nonlinear functional form. This new technique can be interpreted as an extension of established atomic force microscopy techniques to a wider class of nonlinear systems.
BIO: Dr. Randi Potekin received her BS, MS, and PhD from the University of Illinois at Urbana-Champaign, where she studied linear and nonlinear dynamics and vibrations. Since then, she has worked as a structural dynamics analyst in the aerospace and defense industries. She is currently at Sandia National Laboratories supporting the Mechanical Environments Engineering Group within the Division of Engineering Sciences.
