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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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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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
Electro-Hydraulic Actuator
Figure: Traditional Active control with Hydraulic Actuator
Figure: The Hydraulic Actuator
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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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Module 1: Overview of Vibration Control Lecture 3: Active Vibration Cpntrol
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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