To improve the modal damping of a structure, some types of active dampers such as an electro-dynamic or an electro-hydraulic servo damper were investigated and reported. However, they have a few defects such as the large size of force generator and their expensiveness. A simple servo damper which has a couple of electro-magnets in push-pull operation is introduced, and digital controller which utilize the proportional-derivative control algorithm is developed to increase the stiffness and damping of the structure. Electro-magnetic servo damper constructions are tested to increase damping ratio of the ram system such as a planer machine tool and with good results.
