A proof-mass actuator is an actuator for structural control that accelerates a proof-mass in linear motion imparting an equal and opposite force to the structure. A continuous motion of the proof-mass in one direction will eventually lead to a collision of the proof-mass with its stops, imparting shocks to the structure and possibly causing damage to the actuator. In this paper we propose nonlinear control laws for preventing collisions of the proof-mass with its stops. The linear control loop gains are sized such that the proof-mass will not hit its stops under commanded motions. Then a nonlinear control loop is added to increase the restoring force to the proof-mass when the proof-mass is close to its stops. It is shown that the nonlinear control law increases the operating region over an actuator with only linear control loops. It is also shown that the nonlinear control law allows the mass of the actuator to be decreased while maintaining the same performance.