3D MODEL GENERATION FOR DEFORMATION ANALYSIS USING LASER SCANNING DATA OF

A COOLING TOWER C. Ioannidis(a), A. Valani(a), A. Georgopoulos(a), E. Tsiligiris(b)

(a)Department of Rural and Surveying Engineering, National Technical University of Athens

Email: cioannid@survey.ntua.gr(b)Public Power Corporation S.A.

Greece

Introduction

The Hellenic Public Power Corporation S.A. required the 3D survey of the external and internal surfaces and the production of a 3D solid model of an old cooling tower in order to record its current state, decide for repairs if necessary and investigate the possibility of upgrading it. NTUA was assigned with the task and the details of the survey and the results are presented.

Equipment

• HDS2500 (FOV 40ox40o) • HDS3000 (FOV 360o horizontal and 270o vertical

angle)

spot size = 6mm

position accuracy = ±6mm (in 50m range)• Reflectorless total station

The tower in numbers

83 m

56 m

97 m80

diagonals

40pedestals

The tower in numbers

• 26 stations of the geodetic network that was established (3 stations inside the tower)

• 2,900 geodetically acquired check points • 22,000,000 points acquired by the laser scanners• 27 scanner set ups• 6 days of fieldwork

Sn: Geodetic station Ln: Scanner set up

Registration

• 27 scans to register – 20 scans for the interior of the tower

• 10 for the lower part • 10 for the upper part

– 7 scans for the exterior of the tower• 16 targets were measured and used for registering the

10* scans of lower part of the interior• 20 targets were measured and used for registering the

scans of the exterior

* 10 scans that cover the upper part of the interior were acquired with no targets

Registration with no targets

Object shape The top of the tower is a horizontal ring

Solution refinement The upper- and lower- part point clouds are compared via difference vectors that are calculated on a grid defined on the overlapping area and the relative ω and φ rotations and relative translation are thus eliminated

Solution approximationA plane is fitted on a selection of points that belong on the top horizontal ring and through the coefficients of the equation of the plane the ω and φ rotations that must be applied so that the plane be horizontal are calculated

Registration

•Cyclone was used for the registration•All of the scans registered in a common reference system•For all overlapping scans cloud constraints were created•There were no common targets nor overlapping scans between the interior and exterior

Interior: 53 constraints (30 cloud constrains)Mean Absolute Error= 5 mm

Exterior: 39 constraints (7 cloud constraints)Mean Absolute Error= 4 mm

3D Modeling for Finite Element Analysis

Finite Element Analysis (FEA): a computer-based numerical technique for calculating the strength and behavior of engineering structures

3D Models for FEA: – Ordinary CAD models are usually unsuitable– A mesh of a NURBS surface is normally required – Required formats: IGES, ACIS, STEP and STL

3D Model characteristics:– Simplified models– “Geared” for FEA

Data preparation for 3D modeling

• Noise removal• Creation of different point clouds for the parts

of the tower• Creation of 3D faces for parts that were

impossible to scan (e.g. pedestals, inside part of the shell extending from the lintel up until the doorstep)

3D Modeling of the Cooling-Tower

• Data: Laser scanner point clouds and 3D faces• S/W: Raindrop Geomagic Studio 7

SHAPE MODE

Polygonal mesh creation

Assembly of Tower parts

Corrections ofpolygonal mesh

Boundarydefinition

PHASE MODE

Patch definitionand corrections

Griddefinition

NURBS creationand corrections

Exportingto IGES

Shape ModePolygonal mesh

creation

Assembly of Tower parts

Corrections ofpolygonal mesh

Boundarydefinition

Shape ModePolygonal mesh

creation

Assembly of Tower parts

Corrections ofpolygonal mesh

Boundarydefinition

Important corrections

• Deletion of crossing triangles

• Deletion of floating triangles

• Hole filling

• Spike removal

• Relaxing

Shape ModePolygonal mesh

creation

Assembly of Tower parts

Corrections ofpolygonal mesh

Boundarydefinition

Shape ModePolygonal mesh

creation

Assembly of Tower parts

Corrections ofpolygonal mesh

Boundarydefinition

Phase ModePatch definitionand corrections

Griddefinition

NURBS creationand corrections

Exportingto IGES

Phase ModePatch definitionand corrections

Griddefinition

NURBS creationand corrections

Exportingto IGES

Phase ModePatch definitionand corrections

Griddefinition

NURBS creationand corrections

Exportingto IGES

Phase ModePatch definitionand corrections

Griddefinition

NURBS creationand corrections

Exportingto IGES

Accuracy evaluation

GEODETIC DATA• 1250 geodetically acquired points on the

external surface of the tower• Only 146 points deviate more than ± 3 cm from

the polygonal surface model (μ =-1 cm, σ = ±1.5 cm)

Accuracy evaluation

MATHEMATICAL SURFACE• A one-sheeted hyperboloid was fitted on

the data and using the equation 18.000 simulation points were calculated

• There are areas where deviations of ±20 cm are observed but the greatest part fits the mathematical model quite well (μ =-2.4 cm, σ = ±4cm)

Conclusions

• The use of a commercial laser scanner (±6mm at 50m) and the processing of the acquired data with Cyclone (registrations) and Geomagic (3D model generation) leads to results of adequate accuracy and satisfying quality for applications such as this

Thank you for your attention