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Peyman Poozesh, Christopher NiezreckiWind Technology Testing Center (WTTC)

July 11, 2016

Multi-Camera 3D DIC

Structural Evaluation of a Utility-Scale Wind Turbine Blade

Using a Multi-Camera 3D DIC

Static Fatigue

Structural Dynamics and Acoustic Systems LaboratoryUniversity of Massachusetts Lowell

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Problem Statement

One hindrance to the rapid evolution of new wind turbine blade designs is wind

turbine testing and certification process

Static Test

Fatigue Test

A typical 50m utility-scale blade requires:

‒ ~200 strain gages costing $35k-$50k

‒ ~20 high sensitivity accelerometers costing $12K-$30k

‒ Takes 3 weeks to setup the blade for the

test

Transducers are able to measure at discrete locations.

Prior knowledge of high strain and maximum displacement

areas is needed.

Photo by Dennis Schroeder | NREL

(b)

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A single pair of DIC cameras may not be able to measure the

whole utility scale wind turbine blade.

Adjusting the distance between the cameras and object reduce the

accuracy.

The complex curvature of the test structure may produce visual

blind spots that cannot be covered by a conventional stereo

camera pair.

Blade monitoring system – Based on Stereo-photogrammetry:

Solution And Approach

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Solution And Approach

Multi-camera 3D Digital Image Correlation (3D DIC) system:

– Composed of several conventional stereo DIC systems.

– System considers any two cameras as a stereovision system.

– Each stereo vision system can measure a limited area of the object.

– The resulted displacement fields are stitched together to form global

point clouds.

Master Stereo DIC Slave Stereo DIC

CCD1

CCD2

CCD3

CCD4

Integrated DIC System

G

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Proposed Methodology – Multi-Camera System

Measurement - Multi-Camera System

Output-Only System Identification

The performance of

system identification

methods in the presence of

noise is unknown

Modal

Parameters

( )

3D Digital

Image

Correlation

3D Point

Tracking

Method

Full-field

Strain and

displacement

Phase I

Phase II

Output-only

system

identification

Attainment

of full-field

3D displacement,

strain and modal

parameters of

a utility scale

blade

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Phase II – Optically-Based Modal Test

A multi-camera measurement system is used to estimate modal

parameters of a utility scale wind turbine blade.

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Project Synopsis

Attainment of full-

field displacement,

strain, and modal

parameters on utility-scale blades

Accuracy of 3D DIC in

measuring large areas

may be too high

Different stitching

approaches induce

varying errors that are not

yet quantified

The physical constraints

associated with WTTC will be identified

The accuracy using a single

stereo-vision system will be determined

Conduct an experiment on a

large area to identify measurement error

Evaluate different dynamic

stitching methods (PCA,

SVD, and ICP) on a lab-

scale blade

The robustness of each of the techniques will be resolved

Analytically and experimentally

evaluate the sensitivity of system ID on modal parameter estimation

Accurate prediction of damping

Understanding the effectiveness of system ID considering noise

Scaled mode shapes

Improving the accuracy of estimated modal parameters

Use an initial displacement to scale

frequency response function and

select optimal excitation to excite all the modes

Measurement - Multi-Camera 3D DIC

Output-Only System Identification

Problem

Solution

Outcome

The performance of

system identification

methods in the presence of

noise is unknown

Mode shapes extracted

from OMA is un-scaled

and vibration modes may

not be well excited