**Contents**

NyquistGui (Nyquist Graphical User Interface) is a Matlab program designed to help to understand Nyquist Plots. It is not meant to be an accurate, flexible tool for creating Nyquist diagrams; if that is what you are looking for, use Matlab's "nyquist" command. The gui has two basic "modes," one for learning about the process of mapping from one domain to another (i.e., from the "s" domain to the "L(s)" domain). Once you have a good understanding of the mapping process, you can choose to learn about using the Nyquist plot to check for system stability. A brief (4 minute)video follows that shows how to start the program, and describes its major controls.

To run NyquistGUI, you need two files,

- NyquistGui.m: The Matlab program
- NyquistGui.fig: The figure that makes up the gui
- or, NyquistGui.zip: A zip archive with both files.

You should put these files in a directory in the Matlab path so that Matlab can find them. After that you just need to enter "NyquistGui" at the command prompt.

The system has several know limitations:

- There is no dynamic scaling of axes, so you must choose systems with poles that fit within the axes. Again - this is a tool for learning, not for design. If you need an accurate plot with scalable axes, use Matlab's "nyquist" command.
- System time delays are ignored.
- In the case of a system that is marginally stable (i.e., it passes through the -1+j0 point), the angles drawn on the "L(s)" plot may be inaccurate, particularly if all poles of the closed loop system are on the jω axis.
- Any poles of L(s) that are very near the imaginary axis are placed on the axis.

© Copyright 2005-2013 Erik Cheever This page may be freely used for educational purposes.

Erik Cheever Department of Engineering Swarthmore College