[Linear System] What is the order of the system?

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Discussion Overview

The discussion revolves around determining the order of a linear system based on Bode plots generated in MATLAB. Participants analyze discrepancies between the magnitude and phase plots, exploring whether the system can be classified as first-order or second-order.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that the magnitude plot indicates a first-order system, while the phase plot suggests a second-order system, raising questions about the reliability of the plots.
  • Another participant argues that if the phase approaches -180°, the system cannot be accurately modeled as first-order, proposing that a second-order response with two break points might fit better.
  • A different participant points out that for a second-order system, the phase should approach -180° at high frequencies, implying potential measurement errors in the phase plot.
  • One participant requests additional information about the data collection process, expressing skepticism about the accuracy of the plots and suggesting that the attenuation appears inconsistent with first or second-order behavior.
  • There is a call for the original circuit schematic and measurement equipment details to better understand the context of the data presented.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the Bode plots, with no consensus on the order of the system or the validity of the measurements. Multiple competing perspectives on the system's behavior are present.

Contextual Notes

Participants note potential measurement errors and inconsistencies in the data, but do not resolve these issues. The discussion highlights the need for further information to clarify the situation.

Who May Find This Useful

Individuals interested in control systems, signal processing, or those working with Bode plots in MATLAB may find this discussion relevant.

EdajMay
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Hi,

I plotted two Bode plots on MATLAB, and I'm wondering what's the system's order, because the magnitude plot looks like a first order (-20 dB/ dec) whether the phase plot looks like a second order (-180 degrees).

I'm leaning towards a first-order system since the magnitude is more precise.

However, what could explain the difference between the two plots? Is it possible to obtain those plots?

Thank you
 
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If the phase can approach -180° then it's not well approximated as a first-order response. You are trying to fit your points around a curve with a single break point, but could you do better with two break points—a second-order response but where the poles do not co-incide?
 
The phase plot looks rather strange because - for a second-order system - the phase must APPROACH the 180 deg line for infinite frequencies.
Most probably, a measurement error.
 
EdajMay said:
Hi,

I plotted two Bode plots on MATLAB, and I'm wondering what's the system's order, because the magnitude plot looks like a first order (-20 dB/ dec) whether the phase plot looks like a second order (-180 degrees).

I'm leaning towards a first-order system since the magnitude is more precise.

However, what could explain the difference between the two plots? Is it possible to obtain those plots?

Thank you
How did you obtain this data to plot? I agree with the others that it looks like there are errors in your data. The attenuation looks to be -25 or -30dB/decade, but without another decade higher plotted, it's hard to tell. And the phase plot looks all wrong for either a 1st or 2nd order system.

Can you show the schematic of the circuit you measured, and show the equipment used to make the measurements? Can you share the raw data?
 

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