Frequency estimation is a common problem in many practical applications which include the problem of disturbance rejection with unknown frequency. These kinds of disturbance is identifiable thanks to its narrow frequency band so it can be cancelled. In order to achieve this goal, the disturbance frequency should be estimated then this frequency estimate should be given to a disturbance cancellation algorithm to reject sinusoidal disturbance. There diverse methods which has already proposed for frequency estimation and are different in the terms of stability and response speed. These methods includes phased-locked loop, second order generalized integrator-frequency locked loop, induction motor frequency estimation, Kalman filter, and internal model principal. In addition to the estimation of disturbance frequency, these algorithms produce their input signal and its orthogonal signals which can be used for the rejection of disturbance. One of the challenging issues with which we are confronted is the condition that the plant is unknown. In this circumstance we should use plant's estimate. Furthermore, in practical applications, the plants roughly have time delay. Overlooking this delay can reduce efficiency of the control system and even make it unstable. Thus, in disturbance rejection problem, delay should be considered and the problem should be solved for unknown time-delay plant. In this research, the method of second order generalized integrator-frequency locked loop is used for frequency estimation. Firstly, the problem is solved for known plant without delay. Then the problem is extended for unknown plant without delay which uses an adaptive estimator for plant's parameter's estimation. Using frequency and plant's parameters estimates, a control law is proposed to cancel sinusoidal disturbance. Simulations depicts the efficiency of this algorithm. Afterwards, the problem is extended unknown time-delay plants and two methods are used to solve this problem. In first method, regardless of the fact that plant has time delay, plant's parameters are estimated and this algorithm is not different from the condition which plant does not have time delay. Simulations demonstrate that this forfeits its efficiency by rising the amount of time delay. In second method, a delay estimator is added to the algorithm and the parameters of a plant's part which has no time delay is estimated. Using parameter estimates, a control law is proposed to reject sinusoidal disturbance. {Key Words} : Sinusoidal Disturbance Cancellation , Frequency Estimation , Time-Delay Estimation , Second Order Generalized Integrator, Frequency Locked Loop, Averaging, Time-Delay Systems
