Active Vibration Control - Mechanical Engineering PDF Download

In this lecture, we will discuss about different strategies for active vibration control.

Different Strategies for vibration control:

Strategies (ii), (partly) (iv) and (v) are related to Active Vibration Control.

Applications of Active Vibration Control includes:

a. vibration and shape control of flexible systems like optical mirror
b. gitter control of high precision instruments
c. active suspension system for ride comfort in advanced vehicles

Application 1: Vibration & Shape Control of Optical Mirror

Active Control of Optical Aperture

This type of mirrors are ideally suitable for light weight ultra-large space telescopes. A set of such flexible mirror segments could be assembled to form the actual mirror. The surface quality is < 30nm. Stroke requirements for such adjustments is <2µm. Usually PZT actuators are bonded behind deformable silicon mirror membranes for this purpose. An electric field applied perpendicular to the piezoelectric layer plane will induce lateral contraction and thereby cause large out of plane deformation of the membrane.

Application 2: LSS: A future Interferometric Mission

• Error in the optical path length: nanometre
• Pointing error of individual telescope: nanoradian
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Expanded view of the legs of a 6 DOF Isolator 

For a 6 DOF active Isolator:

• Piezo -actuators offer transmission of low frequency torque and suppression of high frequency vibration isolation
• Alternate to piezo -actuators are Terfenol - D rod, voice-coil etc.

Active Vibration Control as shown in this diagram involves design of suitable vibration sensors, processing of sensory data and then feeding back necessary signal to the actuators for vibration control.

In a more abstract way the same could be depicted by a block diagram as shown here.

Figure: Block diagram of AVC system

where, r – reference signal, e – error signal, s-variable in the frequency domain, H – Transfer function of the sensor, G – Transfer function of the vibrating system, d- disturbance, Gc - Transfer function of the controller and y – output/response of the system.

Occasionally, if the nature of the disturbance is known then a feed-forward technique is adopted as shown here.

Special cases: Shunting of mechanical energy to electrical energy

This involves transformation of vibrational energy by the piezoelectric material and designing a suitable electrical network to dissipate this energy.

The piezoelectric patch is represented as a capacitor and a voltage source in series

Implementation of Controller: Inertial Actuator (Electro-mechanical System)

With the advent of new technologies, actuators and sensors are getting miniaturized and integrated to the structure. Thus, there is a transition from traditional active control to smart structure based active control. A case study of helicopter rotor vibration control will illustrate this.

Electro-Hydraulic Actuator

Smart Structural Control

Traditional active control can essentially control a finite number of vibrating modes of a continuous system. This may create instability due to the unwanted excitation of the higher modes. Smart structural control, on the other hand, is distributed in nature. It consumes less energy, gurantees stability and could be integrated easily to the vibrating body.

Comparison between two methods of AVC

Important issues related to AVC

• Active Vibration Control (AVC) is important when there are stringent specifications on Performance and Weight Savings
• Passive solutions are in general cheaper than AVC.
• One should not consider that AVC will always give better performance and it can compensate for a bad design. It should be considered as the last resort.
• Feedback control can compensate external disturbance only in a limited band, outside the bandwidth the disturbance is actually amplified.
• Semi-active Vibration control is being considered as a trade-off solution.

Reference

• Moheimani and Fleming – Piezoelectric Translators for Vibration Control and Damping, Spiringer
• Moheimani , Halim and Fleming, Spatial control of vibration, Theory and Experiments, Series on Stability, Control and Vibration of Systems
• L. Meirovitch , Dynamics and Control of Structures
• A. Preumont , Vibration Control of Active Structures : An Introduction, Kluwer Academic
• D. J. Inman, Vibration with Control, Wile