Form Metrology
for AM
Primary Investigator:
Petros Stavroulakis
Form metrology
• Measurement of the external shape of
something
• Involves ‘large-scale’ measurements of metres
down to centimetres generally with an
accuracy of 100’s of micrometres
Why is form metrology important?
• Allows mass-assembly of mechanical products
Exact measurement of shape allows for predictable
performance when bringing parts together in an
assembly and is required to achieve a specific
performance (car engine, turbine blade)
• Keeping form controlled to tight tolerances
minimises material waste
In some cases this is the most important part of the
product cost (improves profits for manufacturing plants,
makes them more competitive)
Types of form metrology
Most used solutions are:
• Mechanical
– Coordinate measuring
machines (CMMs)
• Optical, most common:
– Laser scanning
– Fringe projection
Constraints introduced by AM
Constraint
type
Surface
texture
Form
geometry
Constraint
Diffuse reflectance (an
example of a flat surface
created via a selective laser
melting metal process is
shown in Figure 3)
Reason
Typical surfaces produced via layering and
powder based metal AM manufacturing
technology are not optically smooth to
visible wavelengths
Freeform, multiple
occlusions and shadows
exist
Little restriction in created geometry
permits for complicated shapes with large
number of discontinuities and line of sight
occlusions present
Material range Appreciable variation of
absorption/reflection
properties between
different materials
Inhibits the overlay of specific wavelengths,
more important for laser triangulation than
structured light
Current focus of AM metrology
activities in group
Three top priorities:
• Investigation of the UK’s requirements for AM
metrology
• Creating a technical review of existing form
measurement systems and their applicability
to AM
• Use of a priori knowledge (structured light
system models and object CAD data) to
improve the measurement process via IRM
(information-rich metrology)
Current focus of AM metrology
activities in group
• The UK’s requirements for AM metrology
• Completing the technical review of existing
form measurement systems
• Use of a priori knowledge (structured light
system models and object CAD data) to
improve the measurement process via IRM
(Information Rich Metrology)
Current status of UK AM metrology
needs review
• Investigation plan for UK AM metrology:
– Look at global AM landscape
– Create a realistic snapshot of the current UK AM
industry and supply chain
– Identify the greatest metrology challenges for AM
industry (currently in collection)
– Tie challenges in with fellowship project aims
Timeline of EPSRC effort to collect
AM data
•
•
•
The EPSRC AM CIM’s aim is to produce a UK-specific document regarding the
current roadblocks in AM and an actionable plan to present to government after
election and in time for the budget
Their timeline is very relevant and close to our project/fellowship needs
Metrology is placed in the top requirements for evidence collection and hence will
be a big part of this report
Metrology is mentioned among the most critical
barriers for the growth of the UK AM industry
Code title
# workshop
Post-Its
# references
from on-line Summary of common perceived barriers:
submissions
Materials
138
223
Design
99
208
Skills / education
68
125
Cost / investment /
financing
62
156
Standards /
regulation
61
105
Measurement /
inspection /
testing
23
127
IP / protection /
secrecy
17
14
Understanding properties in different processes/machines/applications, QA,
costs, availability (IP constraints, independent suppliers), use of mixed materials,
recyclability, biocompatibility.
Need for guides and education programmes on design for AM – better
understanding of design for AM constraints, availability of AM-skilled designers,
security of design data.
Lack of appropriate skills (design, production, materials, testing) preventing
adoption, up-skilling current workforce vs. training of next generation,
education of consumers, awareness in schools.
Funding to increase awareness and reduce risk of adoption (testing, scale-up,
machine purchase) – especially for SMEs, understanding of full costs (inc. postprocessing, testing), cost of materials.
Perceived or actual lack of standards – all sectors / sector specific (esp. aero /
health / motorsport), for processes / materials / software / products /
applications.
Need data libraries, standards for tests (general and sector specific),
materials/ in-process / final part, tests for higher volumes, nondestructive testing, QA through lock-in cf. open access to data.
Balancing need for openness to share knowledge with need for commercial
protection to capture value from investments, enforcement of IP rights.
Source: Update report 2 – UK AM National Strategy Effort http://www.amnationalstrategy.uk
First results of EPSRC effort: UK AM
Landscape
In total 143 organisations have provided responses out of which the
UK AM landscape per sector was as follows:
% of
Sector
contributions
Manufacturing
55
Materials
34
Machinery
19
Information
10
Aerospace
12
Defence
10
Creative
7
industries
Life sciences
5
Sector
Automotive
Motorsport
Nuclear
Electronics
Construction
Rail
% of
contributions
3
3
3
3
1
1
Marine
1
Oil and Gas
1
Current focus of AM metrology
activities in group
• The UK’s requirements for AM metrology
• Completing the technical review of existing
form measurement systems
• Use of a priori knowledge (structured light
system models and object CAD data) to
improve the measurement process via IRM
(Information Rich Metrology)
Technical review
The technical report created which has been sent to
Review of Scientific Instruments and is at peer review
stage describes the following:
• Presentation of overview of form measurement systems
available for 3D measurement
• Comparison of 3D form measurement systems and
selection of the ones most applicable for industrial AM
applications
• Calculation of fundamental accuracy and uncertainty limits
for metrological solutions available for AM (laser
triangulation, fringe projection)
• Presentation of practical considerations and additional
sources of uncertainty (beyond the fundamental ones) and
discussion of areas which need improvement
Summary of Technical Review: Measurement
tolerances required in industry
Figure from: E. Savio, L. De Chiffre, and R. Schmitt, CIRP Ann. - Manuf. Technol. 56, 810 (2007).
Other industrial requirements
Characteristic:
Value used:
Source:
Recommended by VDI/VDE
2634-1 standard on industrial
non-contact 3D scanning
Maximum volume of
measurement area
(distance):
Dimensional tolerance:
Up to:
(2000 × 2000 ×1500) mm
Resolution and
accuracy:
As high as possible, typically
ten times better than the
tolerance range in order to
provide adequate
measurement confidence
interval and appropriate
process control.
N/A
Measurement time:
As fast as possible, typical
requirement is to be faster
than the manufacturing
process itself.
N/A
Varies with industry, typically As per previous figure
≤ 100 μm (±50 μm)
Active vs. Passive systems
• There are two general families of 3D optical
measurement active and passive:
– By active systems we mean systems that use their own
light source to perform the measurement
– By passive systems we refer to systems that depend
on already available light to perform the
measurement
• Generally, for higher measurement speed and
accuracy people select active systems whereby
for cost effectiveness people prefer passive
List of optical systems available
Classification of active and passive 3D form measurement techniques (Source: G.
Sansoni, M. Trebeschi, and F. Docchio, Sensors 9, 568 (2009) )
3D form measurement
technique
Laser triangulation
Structured light
Stereo vision
Photogrammetry
Time of flight
Interferometry
Moiré fringe range contours
Shape from focusing
Shape from shadows
Texture gradients
Shape from shading
Shape from photometry
Passive
Active
X
X
X
X
X
X
X
X
X
X
X
X
X
Active form measurement systems
Source: S. Se and N. Pears, 3D Imaging, Analysis and Applications (2012)
• For our internal spec, a measurement volume of (700x700x700)
mm and an accuracy smaller or equal to 100 μm is required to
accommodate most AM. Laser triangulation and pattern projection
are the most appropriate techniques to measure AM parts.
Comparison of current popular non-contact
optical form technologies in industry
Laser Triangulation
Fringe Projection
Point-to-point or Line-to-line
Areal
Angle based
Intensity based
Direct measurement within field of view
Requires ambiguity removal:
-Wrapped phase
-Scaling factor
Less software involved
More software steps involved
Laser point illumination
Lamp illumination
Moving parts
No moving parts
Principle of operation of
laser triangulation
h = d1 - d2
Uncertainty Ishikawa diagram for
laser triangulation
Principle of operation of
fringe projection
Uncertainty Ishikawa diagram for
fringe projection
Where do we go from here?
A) Improve fringe projection and laser scanning
B) Develop other measurement technologies
which could be used for form measurement
Shear interferometry
Active shape from focus
Projected patterns used to texturize scene in order to assist with
reconstruction of featureless objects.
Projection Moiré
Holographic profilometry
Frequency modulated continuous
wave ranging
Close-range photogrammetry
Source: L.M. Galantucci, F. Lavecchia, and G. Percoco, J. Comput. Inf. Sci. Eng. 13, 044501 (2013)
Comparison chart of promising
techniques:
Technique:
Best dimensional
resolution reported in
height axis:
Measurement
Range:
Type of view:
Works on optically
rough surface:
Active focus
detection through
shear
interferometry
10 μm (on optically
smooth surfaces)
resolution reduced for
rough surfaces
300 mm
Point-to-point
YES
Time of flight
15 μm
500 mm
Full field
YES
Close-range
photogrammetry
~15 μm
30 mm - 100 ma
Full field
YES
Volume Holographic 15 μm – millimetres
Profilometry
20 cm – 2 m
Full field
YES
Holographic
Profilometry
10 μm
50 mm - 300 mm
depending on
object reflectance
Full field
YES
Moiré profilometry
typically ~25 μm depends
on grating frequency used
N/A
Full field
YES
Current focus of AM metrology
activities in group
• The UK’s requirements for AM metrology
• Completing the technical review of existing
form measurement systems
• Use of a priori knowledge (structured light
system models and object CAD data) to
improve the measurement process via IRM
Flow chart of IRM-enhanced structured
light system operation for AM
3D model of object to be
measured (AM CAD)
Send optimized views to
measurement rig to take
measurements
Simulation of
projections for specific
object model in
combination with the
instrument model.
Selection of minimum
amount of views
required to fully
describe object
Comparison of initial
model with points
acquired
Fill-in gaps found either
with new projections or
more accurate
secondary system
(triangulation)
Initial simulation results
Confirmed the ability to crate a setup which collects scattered light from the object to
correctly replicate the fringe pattern as seen by the image CCD.
Form Metrology lab at UoN
NUB3D SIDIO XR Structured light 3D scanning system:
Human Testing (with consent)
AM reconstruction of 3D scanned
point cloud
Future plan
Aim: ‘One-shot’ 3D reconstruction system with maximum available accuracy and resolution via
multi-view paradigm and inspection planning that will be customized to specific objects.
Thank you for your attention!