Performance limitations of the servomechanism problem when the number of tracking/disturbance poles increases

In this paper, we study the cheap control problem and determine what some of the inherent system limitations are in achieving high performance for LTI systems. In particular, we observe that a fundamental difficulty in designing a high performance controller for a system may occur, which is related to the infinite transmission zero structure of the system. A continuous measure, called the Toughness Index, is introduced to characterize such limitations. We then apply these results to the robust servomechanism problem (RSP), and show that the Toughness Index of the RSP becomes worst as the number of tracking/disturbance poles to be tracked/regulated increases. This implies that high performance control in the RSP cannot be obtained for a large number of tracking/disturbance poles, even for minimum phase systems.