Abstract: A piezoelectric sensor for monitoring the kinetic momentum of a movable member has a piezoelectric sensor assembly composed of a flexible plate member having at least one plane surface and a piezoelectric element fitted onto the plane surface of the flexible plate. A mass weight member rigidly attached to the piezoelectric sensor assembly for movement therewith, the mass weight member being so configurated and arranged as to exert oscillation energy thereof essentially around the center axis of the piezoelectric sensor assembly. Exertion of vibration or oscillation energy of the mass weight member essentially around the center axis of the sensor assembly may cause greater deformation in the sensor assembly for higher sensor output.