Lecture 3: Active Vibration Control - · PDF file · 2017-08-04file:///D |/chitra/vibration ... Lecture 3: Active Vibration Control ... This type of mirrors are ideally suitable for

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file:///D|/chitra/vibration_upload/lecture3/3_1.htm[6/25/2012 12:20:06 PM]

Module 1: Overview of Vibration Control

Lecture 3: Active Vibration Control

The Lecture Contains:

Different strategies for vibration control

Comparison of feed forward and feedback control

Implementation of controller

Smart structural control

Comparison between two methods of Active Vibration Control (AVC)

Important issues related to AVC

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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Control

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.

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file:///D|/chitra/vibration_upload/lecture3/3_2a.htm[6/25/2012 12:20:06 PM]

Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol

Applications of Active Vibration Control includes:

a. vibration and shape control of flexible systems like optical mirrorb. gitter control of high precision instrumentsc. active suspension system for ride comfort in advanced vehicles

Application 1: Vibration & Shape Control of Optical Mirror

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Active Control of Optical Aperture

This type of mirrors are ideally suitable for light weight ultra-large space telescopes. A set of suchflexible 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 behinddeformable silicon mirror membranes for this purpose. An electric field applied perpendicular to thepiezoelectric layer plane will induce lateral contraction and thereby cause large out of plane deformationof the membrane.

Application 2: LSS: A future Interferometric Mission

Error in the optical path length: nanometre

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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 frequencyvibration isolationAlternate to piezo -actuators are Terfenol - D rod, voice-coil etc.

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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

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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.

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

Figure: Feed forward block diagram

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Module 1: Overview of Vibration Control Lecture 3: Introduction to Active Vibration Cpntrol

Special cases: Shunting of mechanical energy to electrical energy

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

Figure: Equivalent electrical circuit

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

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

Figure: Inertial actuation system

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

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Electro-Hydraulic Actuator

Figure: Traditional Active control with Hydraulic Actuator

Figure: The Hydraulic Actuator

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Smart Structural Control

Figure: Smart structural control

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

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Comparison between two methods of AVC

Important issues related to AVC

Active Vibration Control (AVC) is important when there are stringent specifications on Performance andWeight 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 baddesign. It should be considered as the last resort.Feedback control can compensate external disturbance only in a limited band, outside the bandwidththe disturbance is actually amplified.Semi-active Vibration control is being considered as a trade-off solution.

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Reference

Moheimani and Fleming – Piezoelectric Translators for Vibration Control and Damping, SpiringerMoheimani , Halim and Fleming, Spatial control of vibration, Theory and Experiments, Series onStability, Control and Vibration of SystemsL. Meirovitch , Dynamics and Control of StructuresA. Preumont , Vibration Control of Active Structures : An Introduction, Kluwer AcademicD. J. Inman, Vibration with Control, Wiley

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Lecture 3: Active Vibration Control - · PDF file · 2017-08-04file:///D |/chitra/vibration ... Lecture 3: Active Vibration Control ... This type of mirrors are ideally suitable for