2.4.2 ACTIVE BRACING SYSTEM
  1. SEMI-ACTIVE PROTECTION SYSTEM
A semi-active control system typically requires a small external power source for operation (e.g. a battery) and utilizes the motion of the structure to develop the control forces, the magnitude of which can be adjusted by the external power source. Control forces are developed based on feedback from sensors that measure the excitation and/or the response of the structure. A semi-active control system generally originates from a passive control system which has been subsequently modified to allow for the adjustment of mechanical properties. For example, supplemental energy dissipation devices which dissipate energy through shearing of viscous fluid, movement of fluid through orifice, and sliding friction have been modified to behave in a semi-active manner. The mechanical properties of these systems may be adjusted based on feedback from the excitation and/or from the measured response. As in an active control system, a controller monitors the feedback measurements and generates an appropriate command signal for the semi-active devices. As in a passive control system, however, the control forces are developed as a result of the motion of the structure itself. The control forces are developed through appropriate (based on a pre-determined control algorithm) adjustment of the mechanical properties of the semi-active control system. Furthermore, the control forces in many semi-active control systems primarily act to oppose the motion of the structural system and therefore promote the global stability of the structure. Semi-active control systems generally require a small amount of external power for operation (on the order of tens of watts)[12].
Semi-active control systems are a class of active control systems for which the external energy requirements are orders of magnitude smaller than typical active control systems. Typically, semi-active control devices do not add mechanical energy to the structural system (including the structure and the control actuators), therefore bounded-input bounded-output stability is guaranteed. Semi-active control devices are often viewed as controllable passive devices.