Active Vibration Control & Smart Structures

Active vibration control is defined as a technique in which the vibration of a structure is reduced by applying counter force to the structure that is appropriately out of phase but equal in force and amplitude to the original vibration. As a result two opposing forces cancel each other, and structure essentially stops vibrating.

Techniques like use of springs, pads, dampers, etc have been used previously in order to control vibrations. These techniques are known as ‘Passive Vibration Control Techniques’. They have limitations of versatility and can control the frequencies only within a particular range of bandwidth. Hence there is a requirement for ‘Active Vibration Control’.

‘Active Vibration Control’ makes use of ‘Smart Structures’. This system requires sensors, actuators, a source of power and a compensator that performs well when vibration occurs. Smart Structures are used in bridges, trusses, buildings, mechanical systems, space vehicles, telescopes, and so on. The analysis of a basic structure can help improve the performance of the structures under poor working conditions involving vibrations.

Active Vibration Control & Controller Objective

According to the way in which controllers are designed, the control techniques have been classified such as Independent Modal Space Control (IMSC) and Coupled Control (CC).

Independent Modal Space Control (IMSC)
Space structures are characterized by a large number of degrees of freedom, so control of such systems requires a new approach. In IMSC method the control laws are designed in the modal space for each mode independently. The problem reduces to a set of independent second order systems so that control is possible.

IMSC requires an appreciable less amount of energy than the coupled control. It gives larger choice of control techniques including non linear control. In this method the Performance Index ‘J’ is independent of the actuator location.

IMSC requires that the number of actuators must be equal to the number of modes of vibration. With this constraint the possibility of actuator failure becomes critical and there is a requirement of a spare actuator. IMSC cannot be used with available actuators. Actuators incompatible with IMSC cannot provide the relative force distribution IMSC demands.

Modified Independent Modal Space Control (MIMSC)
One of the predominant difficulties in active control of flexible structures is that such structures have a number of vibration modes within or beyond the bandwidth of the controller. In active control of flexible structures, spill over can occur because only a few vibratory modes are dealt with by the controller. Although modal space based optimal control is known to avoid spill over, it requires a large number of sensors and actuators. MIMSC algorithm proposed by Baz-et-al is used to minimize the number of actuators. With it an artificial neural network is also used to identify the system characteristics and reduce the number of sensors. MIMSC control has an excellent closed loop structural damping.

Coupled Controllers
Coupled controllers are used when simultaneous control of multimodes is required. Two feed back laws are used for Coupled control design: state feedback and output feedback.

Output feedback allows us to design plant controllers of any desired structure. In output feedback method, the feedback gain k is chosen to minimize the quadratic cost or the performance index. Performance index PI is the functional relationship involving some system characteristics in such a manner that the optimum conditions as per the requirements may be determined from it.