Introduction
Root locus, a graphical presentation of the closed-loop poles as a system parameter is
varied, is a powerful method of analysis and design for stability and transient response
(Evans, 1948; 1950). Feedback control systems are difficult to comprehend from a
qualitative point of view, and hence they rely heavily uponmathematics. The root locus
covered in this chapter is a graphical technique that gives us the qualitative description
of a control system’s performance that we are looking for and also serves as a powerful
quantitative tool that yields more information than the methods already discussed.
Up to this point, gains and other system parameters were designed to yield a
desired transient response for only first- and second-order systems. Even though the
root locus can be used to solve the same kind of problem, its real power lies in its
ability to provide solutions for systems of order higher than 2. For example, under
the right conditions, a fourth-order system’s parameters can be designed to yield a
given percent overshoot and settling time using the concepts learned in Chapter 4.
The root locus can be used to describe qualitatively the performance of a
system as various parameters are changed. For example, the effect of varying gain
upon percent overshoot, settling time, and peak time can be vividly displayed. The
qualitative description can then be verified with quantitative analysis.
Besides transient response, the root locus also gives a graphical representation
of a system’s stability.We can clearly see ranges of stability, ranges of instability, and
the conditions that cause a system to break into oscillation.
Before presenting root locus, let us review two concepts that we need for the
ensuing discussion: (1) the control system problem and (2) complex numbers and
their representation as vectors.
IntroductionRoot locus, a graphical presentation of the closed-loop poles as a system parameter isvaried, is a powerful method of analysis and design for stability and transient response(Evans, 1948; 1950). Feedback control systems are difficult to comprehend from aqualitative point of view, and hence they rely heavily uponmathematics. The root locuscovered in this chapter is a graphical technique that gives us the qualitative descriptionof a control system’s performance that we are looking for and also serves as a powerfulquantitative tool that yields more information than the methods already discussed.Up to this point, gains and other system parameters were designed to yield adesired transient response for only first- and second-order systems. Even though theroot locus can be used to solve the same kind of problem, its real power lies in itsability to provide solutions for systems of order higher than 2. For example, underthe right conditions, a fourth-order system’s parameters can be designed to yield agiven percent overshoot and settling time using the concepts learned in Chapter 4.The root locus can be used to describe qualitatively the performance of asystem as various parameters are changed. For example, the effect of varying gainupon percent overshoot, settling time, and peak time can be vividly displayed. Thequalitative description can then be verified with quantitative analysis.Besides transient response, the root locus also gives a graphical representationof a system’s stability.We can clearly see ranges of stability, ranges of instability, andthe conditions that cause a system to break into oscillation.Before presenting root locus, let us review two concepts that we need for theensuing discussion: (1) the control system problem and (2) complex numbers andtheir representation as vectors.
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