What notation represents open-loop poles on the root locus plot?

• A. Circles (O)
• B. Dots
• C. Crosses (X)
• D. Squares

Answer: (C)

In control theory and specifically in root locus plots, open-loop poles are represented by crosses (X). This notation is standard because it visually distinguishes poles from zeros, which are typically represented by circles or other symbols. Open-loop poles are critical points in the root locus as they indicate the locations in the complex plane where the system's transfer function becomes infinite. The behavior of the system as parameters, like gain, change can be analyzed effectively when the positions of these poles are marked clearly.

This representation helps engineers and students quickly identify the stability and response characteristics of the system by observing where the open-loop poles are located relative to the zeros and the imaginary axis, which is essential in assessing the overall system dynamics.