MECH431_topic3_root_locus

# MECH431_topic3_root_locus - K G(s Chapter 6 Root-locus...

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K G(s) + - MECH 431 Root Locus Analysis Slide 1 Chapter 6 Root-locus Analysis

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K G(s) + - MECH 431 Root Locus Analysis Slide 2 Introduction • The Root Locus (RL) is the locus (i.e., location) of roots of the characteristic equation of the closed-loop system as a specific parameter (usually, gain K ) is varied from 0 to infinity. • The RL shows the contributions of each open-loop pole or zero to the locations of the closed-loop poles.
K G(s) + - MECH 431 Root Locus Analysis Slide 3 Introduction • The RL method is valuable when designing a linear control system since it indicates the manner in which the open-loop poles and zeros should be modified so that the response meets system performance requirements. • By using the RL, it is possible to determine the value of the loop gain K that makes the damping ratio of the dominant closed loop poles as prescribed.

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K G(s) + - MECH 431 Root Locus Analysis Slide 4 Angle and magnitude conditions • Given: + - G(s) C(s) H(s) R(s) • To find the poles of the characteristic equation: Angle: Magnitude: • Therefore:
K G(s) + - MECH 431 Root Locus Analysis Slide 5 Angle and magnitude conditions • The values of s that satisfy both the angle and magnitude conditions are the roots of the characteristic equation (i.e., closed loop poles). • A locus of the points in the complex domain satisfying the angle condition alone is the root locus. • The roots of the characteristic equation corresponding to a given gain value are found from the magnitude condition.

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K G(s) + - MECH 431 Root Locus Analysis Slide 6 Angle and magnitude conditions • The closed loop transfer function becomes: • Given: If K 0: poles of closed loop = poles open loop If K : poles of the closed loop = zeros of open loop
K G(s) + - MECH 431 Root Locus Analysis Slide 7 Angle and magnitude conditions • For example s A 1 -p 1 A 3 -p 3 A 4 -p 4 A 2 -p 2 B 1 -z 1 * Note the symmetry about the real axis.

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K G(s) + - MECH 431 Root Locus Analysis Slide 8 Example 6-1 • Given: • Sketch the root-locus plot • Determine the value of K such that the damping ratio of a pair of dominant complex-conjugate closed- loop poles is 0.5.
K G(s) + - MECH 431 Root Locus Analysis Slide 9 Example 6-1 • Given: • The angle condition gives:

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K G(s) + - MECH 431 Root Locus Analysis Slide 10 Example 6-1 • Given: • The magnitude condition gives:
K G(s) + - MECH 431 Root Locus Analysis Slide 11 Example 6-1 1) Determine the root loci on the real axis : The first step is to locate the open loop poles (i.e., K =0): 0 -1 -2

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