After development of passive devices such as base isolation and TMD. The next logical steps is to control the action of these devices in an optimal manner by an external energy source the resulting system is known as active control device system. Active control has been very widely used in aerospace structures. In recent years significant progress has been made on the analytical side of active control for civil engineering structures. Also a few models explains as shown that there is great promise in the technology and that one may expect to see in the foreseeable future several dynamic “Dynamic Intelligent Buildings” the term itself seems to have been joined by the Kajima Corporation in Japan. In one of their pamphlet the concept of Active control had been explained in every simple manner and it is worth quoting here.
People standing in swaying train or bus try to maintain balance by unintentionally bracing their legs or by relaying on the mussels of their spine and stomach. By providing a similar function to a building it can dampen immensely the vibrations when confronted with an earthquake. This is the concept of Dynamic Intelligent Building (DIB).
The philosophy of the past conventional a seismic structure is to respond passively to an earthquake. In contrast in the DIB which we propose the building itself functions actively against earthquakes and attempts to control the vibrations. The sensor distributed inside and outside of the building transmits information to the computer installed in the building which can make analyses and judgment, and as if the buildings possess intelligence pertaining to the earthquake amends its own structural characteristics minutes by minute.
Active Control System
The basic configuration of an active control system is schematically shown in figure. The system consists of three basic elements:
- Sensors to measure external excitation and/or structural response.
- Computer hardware and software to compute control forces on the basis of observed excitation and/or structural response.
- Actuators to provide the necessary control forces.
Thus in active system has to necessarily have an external energy input to drive the actuators. On the other hand passive systems do not required external energy and their efficiency depends on tunings of system to expected excitation and structural behavior. As a result, the passive systems are effective only for the modes of the vibrations for which these are tuned. Thus the advantage of an active system lies in its much wider range of applicability since the control forces are worked out on the basis of actual excitation and structural behavior. In the active system when only external excitation is measured system is said to be in open-looped. However when the structural response is used as input, the system is in closed loop control. In certain instances the excitation and response both are used and it is termed as open-closed loop control.
Control Force Devices
Many ways have been proposed to apply control forces to a structure. Some of these have been tested in laboratory on scaled down models. Some of the ideas have been put forward for applications of active forces are briefly described in the following:
Active-tuned Mass Dampers (TMD)
these are in passive mode have been used in a umber of structures as mentioned earlier. Hence active TMD is a natural extension. In this system 1% of the total building mass is directly excited by an actuator with no spring and dash pot. The system has been termed as Active Mass Driver (AMD). The experiments indicated that the building vibrations are reduced about 25% by the use of AMD.
Various analytical studies have been done using tendons for active control. At low excitations, even with the active control system off, the tendon will act in passive modes by resisting deformations in the structures though resulting tension in the tendon. At higher excitations one may switch over to Active mode where an actuator applies the required tension in tendons.
The liquid sloshing during earthquakes has assumed significance importance in view of over flow of petroleum products from storage tank in post earthquakes. One of the important consideration with sloshing is that is associated with a very low damping. The wave height was controlled through force applied to the side wall by a hydraulic actuator. The active control successfully reduced wave heights to the level of 6% of those without control, for harmonic excitations at sloshing frequency. For earthquake type excitation the wave heights were reduced to 19% level.
Conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity. But the new techniques like Energy Dissipation and Active Control Devices are a lot more efficient and better.