بررسي تاثير ميرايي در كاهش حساسيت سيستم‌هاي كنترل فعال سازه‌يي به تاخير زماني

EFFECT OF DAMPING ON SENSITIVITY REDUCTION OF ACTIVE STRUCTURAL CONTROL TO TIME DELAY

يكي از مشكلات عمده در سيستم‌هاي كنترل فعال، وجود تاخير زماني است. تاخير زماني مي‌تواند تاثير قابل‌ملاحظه‌‌يي در عملكرد سازه در هنگام زلزله داشته باشد، به‌طوري‌ كه به ناپايداري كلي سازه نيز بيانجامد. در اين نوشتار، ابتدا سازه‌ي موردنظر نسبت به تاخير زماني تحليل حساسيت و سپس مقدار بحراني آن تعيين شد. بررسي‌ها نشان دادند تاخير زماني بحراني سازه به نوع زلزله‌‌ي ورودي حساس نيست و فقط به خصوصيات سازه و سيستم كنترلي ارتباط دارد. لذا براي مهار اين پديده و افزايش بازدهي سيستم كنترل، از ايده‌ي افزايش ظرفيت ميراكنندگي سازه استفاده شد. افزايش ميرايي به‌گونه‌يي انجام شد كه فقط ظرفيت جذب انرژي سازه تغيير كند. الگوريتم مورد استفاده، الگوريتم كنترل گسسته‌ي بهينه‌ي آني است. ارزيابي‌ها نشان داده‌اند افزايش ميرايي مي‌تواند تا اندازه‌ي زيادي حساسيت سازه و سيستم كنترل را نسبت به تاخير زماني محدود كند. به اين شيوه مي‌توان سازه‌هاي كنترلي را كه با گذشت زمان كارآيي خود را از دست داده‌اند، مجدداً احيا كرد.

Time delay is a major problem in active structural control, which may lead to an inactive control performance or even result in instability of the whole structure. Sensitivity of structures to time delay may refer to characteristics of the building, the control system, and/or external loadings. If a control system is designed based on the ideal parameters of the building and control mechanism, it may be inefficient, because it does not take note of the practical problems encountered in reality. But, if a control system is perfectly designed, it may suffer from some changes in structure and/or control parameters. Hence, time delay may always affect the performance of a controlled structure. In this paper, in order to evaluate the influence of this effect, sensitivity analysis of an eight-story model building, with respect to time delay, is carried out. The control system is an active mass damper/driver (AMD) installed on the top floor. The control algorithm is the discrete instantaneous optimal control (DIOC) method, previously presented by the second author. This algorithm is powered using stable matrices to overcome the time step problems that the instantaneous optimal control methods confront. First, a critical time delay is recognized, in which the controlled structure is unstable. By changing input excitation (strong ground motion), the effect of input seismic load on critical time delay is examined. Using different earthquake loads: El Centro, Parkfield, and San Fernando, results in the same critical time delay for the mentioned buildings. Then, the stiffness of the building stories are increased without changing the mass of the floors. It is recognized that there is a relation between critical time delay and the first period of the controlled building. The proposed coefficient is about 1/7 to 1/6 of the fundamental period of the controlled building. Second, by introducing passive dampers in such a manner that the capacity of the energy absorption of the building is increased without increasing its stiffness, critical time delay is evaluated, again. There is no change in critical time delay, but extensive results show that increasing damping to a large extent may alleviate the sensitivity of the controlled building to time delay. In other words, increasing the damping of the first mode of the controlled building may extensively increase the efficiency of the control system. Hence, this may be a good solution for older active control buildings, which suffer from time delay effects.