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Why we need Bode Plot for a given system? We already have Nyquist and other plots to know about the character of the system, why BODE then?
asked Dec 23, 2015 in Electrical by Arka (109 points) | 54 views

1 Answer

+1 vote

As we know, Bode plots, Nyquist plots and Nichols chart are frequency domain plots which are used to analyse system behaviour, in "Classical Control Theory." Thus what ever is discussed in this article is pivoting around the Classical Control Theory.

In a very crude way, one of the main reasons why bode plots are required even though other plots are available, is that bode plots not only help us in analysing the system behaviour, stability and specifications but is also used in designing
of control systems more commonly. Whereas calculation and determination of relative stability based on Nyquist plots and 
the Nyquist Stability Criterion are more common. However, the information from both these plots and are simultaneously used to get an overall idea of Control System 
Design. 
 
Some of the advantages of Bode Plots(in general as well as in comparison to other plots) are-
1.The logarithmic magnitude bode plot has an added advantage of converting multiplication of magnitudes into addition.
This point when analysed form the design point of view, can be realised as a big advantage to have, as while designing 
controllers for a system, the respective gains contributed by them can taken into account by making shifts along the 
logarithmic magnitude axis.
2.In contrary to Nyquist Plot, where amplitude(mod(G(s)H(s))) and phase angle (G(s)H(s)) are displayed simultaneously, with frequency as a parameter, and Nichols Chart, where the logarithmic amplitude on Y-Axis and phase information on X-axis; Bode plots have separate plots for both amplitude and phase, making design activities to be easily visualised and analysed.
3.Moreover, popular software like MATLAB, etc. have different tools for Control System Design in both the Time and Frequency Domain. SISO tool is one of them, which is in MATLAB, allows design, "tuning" and "analysis" options in both domains. But as a matter of fact, design and tuning options support Root-Locus and Open-Loop Bode Plots where as its the analysis plots that support Nyquist plots along with others. Snapshots are provided as example-

Graphical Tuning:

https://drive.google.com/file/d/0B7JVOE2j0p2LZUZLMnY4dHZYS2c/view?usp=sharing

Analysis:

https://drive.google.com/file/d/0B7JVOE2j0p2LeFZEMXJMRVI4MFU/view?usp=sharing

answered Jan 10, 2016 by Pranjal7 (6 points)