what is mean by absolute stability and relative stability..i think only two types of system will...

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what is mean by absolute stability and relative stability..i think only two types of system will exit ..if syem stable that means stable sytem other wise unstable system so why we use absolute and relative ..how we can define it through phase margin and gain margin....pls help

Robert · Accepted Answer

Microsoft Word - Absolute stability means whether system is stable or unstable
Absolute stability means whether system is stable or unstable. For example, if you find that the real part of one of your 
poles is positive (right‐half plane), then your system is unstable. 
Relative Stability gives the degree of stability or how close it is to instability.  
Gain Margin: Greater will the gain margin greater will be the stability of the system. It refers to the amount of gain, 
which can be increased or decreased without making the system unstable. It is usually expressed in dB.  
The vertical distance between the    curve and the   at the frequency where . 
Phase Margin: Greater will the phase margin greater will be the stability of the system. It refers to the phase which can 
be increased or decreased without making the system unstable. It is usually expressed in phase. 
 The phase of   exceeds   when . 
The phase crossover frequency, wpc , is the frequency where phase shift is equal to ‐180o. The gain crossover frequency, 
wgc, is the frequency where the amplitude ratio is 1, or when log modulus is equal to 0. 
Example: Phase margin is the phase difference from ‐180 degrees for the loop transfer function. As long as the phase 
margin is positive,

what is mean by absolute stability and relative stability..i think only two types of system will exit ..if syem stable that means stable sytem other wise unstable system so why we use absolute and...

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