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Describing Function Analysis of Control Systems with Common Nonlinearities

October 26, 2017 @ 11:00 am - 12:15 pm

Co-sponsored by: WIE

This is an introduction to the describing functions and its use in analyzing some common nonlinearities. Linear frequency response methods that ignore these nonlinearities fail to predict limit cycle oscillations. The describing function method discussed herein predicts them. Books, at least as far back as 1955, and various contemporary online descriptions contain discussionss of the describing function. This is an account of using the describing function to predict limit cycles. Frequency response methods work well with single input single output linear feedback systems even those that have some form of nonlinearity such as a slightly nonlinear gain. These generally yield to approximation by linear elements. There are, however, some common nonlinearities containing abrupt transitions that fail to yield to linearizing approximations. These often cause limit cycle oscillations. Three such nonlinearities are dead zone, saturation, and the infinite gain limiter. The describing function is based on these assumptions:  

  • Circulation of higher harmonics through the loop attenuates them so only a miniscule amount of higher harmonic energy returns to the nonlinearity
  • Nonlinearity produces no subharmonics or DC
  • Nonlinearity in the loop exists in one transfer function.

Speaker(s): Lecturer: Mr. Alan Lipsky,


11:00 am – 11:10am Introduction

11:10am – 12:00pm Mr. Lipsky’s Lecture 

12:00pm – 12:10pm Questions and Answers

Room: 247
Bldg: Lupton Hall
SUNY College at Farmingdale
Farmingdale, New York


October 26, 2017
11:00 am - 12:15 pm



IEEE Region 1 Website