| Large civil engineering structures present a difficult set of test requirements in order to determine the dynamic characteristics. These structures are characterized by large mass, high damping, and low natural frequencies, which result in the need for large dynamic forces in that low frequency region. Conventional excitation systems used for dynamic analysis of civil structures are typically large hydraulic or rotating imbalance shakers. A novel excitation technique has been developed through the exploitation of the nonlinear dynamic response of a mechanical system with active damping. This work examines the characteristics of the nonlinear dynamic response and the resulting sub and super harmonic content that arises. The desired result is to amplify the sub harmonic response in a prototype excitation system for the purpose of low frequency structural excitation. Mechanical production of the sub harmonic excitation, known as force frequency shifting (FFS), has evolved from the application of an oscillatory force in a spatially varying fashion to designs with active components. Previous FFS applications used active damping components with an open loop or predetermined control signal. This work explores the use of feedback control of the active FFS component. Evaluation is performed by a combination of analytical analysis, simulation and experiments. Magnetorheological (MR) damping, a mature active technology, is used to generate the force frequency shifting response. Bang-bang and PID feedback controllers are analyzed and implemented with the objective of increasing the low frequency transmitted force and reducing harmonic distortion. The controllers were implemented on a fully instrumented laboratory scale prototype. The experimental test results showed that regardless of the feedback controller used, improvements in the sub harmonic transmitted force magnitude were not realized compared to open loop operation. However, nominal reductions of 2-4% in the harmonic distortion were demonstrated. The use of a purely dissipative actuator with a limited time response and limited variation in dissipative properties proved to be the factor preventing the desired performance objectives from being achieved. |
