Aiming at the excitation control in multi-machine power systems， the paper proposes the COI-tracking concept in which the angle of each generator tracks the angle COI of the system with a certain distance and the frequency of each generator tracks the frequency COI of the system. Compared to the currently used concept in which every generator is regulated towards the pre-fault operation point， the main characteristic of the COI-tracking concept is that it combines all generators together and makes them synchronous by the dynamic COI signals and the power system will settle in an acceptable frequency， which is not restricted to the power frequency. Based on the same model of the generator， two control system models are firstly established with the two control concepts respectively. Then， two robust controllers are designed using the same method. Finally， based on a 2-area 4-machine power system， dynamic simulations are carried out to make a comparison between the effects of AVR+PSS excitation system and the two nonlinear robust controllers in improving transient stability of the system.