In this thesis, the proof-mass actuator is studied for vibration suppression of a flexible structure. While these actuators have a favorable force-to-weight ratio, the finite travel of the proof-mass, called the stroke length, imposes restrictions on the use of the actuator. This restriction implies that the actuator has a finite operating region in terms of initial conditions on the state. This operating region, along with the amount of vibration suppression potential, defines the performance of the actuator. To increase the performance, nonlinear control is proposed. These control laws monitor the position and velocity of the proof-mass and apply a large restoring force whenever the proof-mass is in danger of breaking its stroke limit. A harmonic balance analysis concludes that these nonlinear control laws do not induce limit cycles. The performance of actuators with different parameters is also compared. A relation is presented that associates the modal frequency of the structure to these parameters. It is also found that large stroke with small mass offers the best performance with the nonlinear control in place. / Master of Science