**Next message:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Previous message:**Spielman, Roger L: "RE: Question about laser for SLS"**In reply to:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Next in thread:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Reply:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Reply:**Bert van den Berg: "Re: little bit off topic (about inspection)"**Reply:**SiderWhite: "Re: little bit off topic (about inspection)"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

Hi

A very interesting discussion indeed. Here is my input coming from a Free

form modeling perspective.

So Why is it difficult to spec free form geometry?

By freeform I am assuming you mean parametric (and not parametric as in

associative) geometric curves and surfaces. Which in turn means mostly B

Spline and NURB - a variant of B Spline curves and surfaces. Even though the

parametric surfaces are mathematically represented in the solid(surface)

modeling kernels, the display, manufacture (STL file) etc always requires

them to be tessellated to a certain resolution. So imagine a curve being

represented by a chunk of st. lines. Now these lines may be short and within

"tolerance" but still are not accurate representation. Another problem is

that unlike algebraic curves and surfaces there is no "root" solving to see

if a point is numerically on the curve or surface. All techniques are

numerical like Newton's method to find how close a point is to the curve or

surface and usually expensive to perform.

Another problem is ... more theoretical but when designing algorithms we

have to consider all possibilities... A parametric curve/surface has the

play of domain and range. I.e. you pick a point in the U,V domain of the

surface and you can get a point on the surface. This mapping is unique i.e.

for every point in the domain there is a unique point in the range. However,

the inverse is not true. For example if a curve self intersects. The point

of intersection maps to two different points in the domain (although

individual mapping of each point in domain maps to unique point which is the

intersection point).

So how does it play into finding the nearest point problem. For something

like a Newton's method to work you must start with a initial good guess

otherwise the solution may not converge. And the guess is in the parameter

domain. So if you apply a techniques to map the point (scanned point) to the

domain of the surface and you start with a wrong inverse mapping you will

possible never converge to the solution.

So what is "good enough" solution. Approximate the free form surface with a

reasonable tessellation and then compare the scanned points to this

tessellation - so now you are comparing the scanned data to a close enough

approximation of the surface. Now is it good enough - that depends. On

tolerances used, how good the approximation of the original surface is,

noise in the scanned data etc etc. If you compared every scanned point to

the original NURB surface model you could take literally days to create the

color map.

Hope above is useful - more than u wanted to know but I wanted to convey

that even though Parametric surfaces are wonderful gifts to man kind they

have their own set of problems !!.

AR

---------------------------------

Dr. Anshuman Razdan

Technical Director PRISM

Email: razdan@asu.edu

http://prism.asu.edu/~razdan

MC 5106 Arizona State University

Tempe AZ 85287-5106

Phone: (480) 965 5368

Fax: (480) 965 2910

-> -----Original Message-----

-> From: owner-rp-ml@ltk.hut.fi

-> [mailto:owner-rp-ml@ltk.hut.fi]On Behalf Of

-> Tommy Tucker

-> Sent: Monday, March 20, 2000 10:44 AM

-> To: Steve Pitt; rp-ml@bart.lpt.fi

-> Subject: RE: little bit off topic (about inspection)

->

->

-> Steve,

->

-> I thought your question was very interesting and was

-> surprised not to see

-> more discussion. Scanners and three-dimensional measurement

-> equipment have

-> been discussed a lot on this list, but this is a subject

-> that rarely comes

-> up. Everyone thinks its great to inspect free-form shapes

-> but doesn't say a

-> whole lot about what they mean by it.

->

-> The main advantage in free-form surface inspection using

-> scanning/digitizing

-> technology has been the use of color mapping the errors from

-> measured points

-> to CAD surfaces. You raise an interesting question as to

-> whether this is

-> enough. Most of the other features you mentioned require a

-> tolerance.

-> Free-form surfaces should to, but how are these spec'd out?

-> Any input from

-> others on the list would be appreciated. My company is in a

-> position to

-> provide real innovation in this area based on input received.

->

-> One area I have seen a tolerance used for free-form surfaces

-> is turbine

-> blades. Generally, these are spec'd out by cross-sections

-> along the blade's

-> length. This has always bothered me because it takes a 3D

-> geometry and

-> simplifies it to 2D. With modern modeling systems, why can't a 3D

-> tolerancing scheme be imposed? In any event, you may want

-> to look into

-> turbine blade inspection and how inspection planning is

-> performed for these

-> products.

->

-> Tommy Tucker

-> (vc) 408-855-4372

-> (fx) 408-855-4360

-> tommy@paraform.com

-> http://www.paraform.com

->

-> > -----Original Message-----

-> > From: owner-rp-ml@bart.lpt.fi

-> [mailto:owner-rp-ml@bart.lpt.fi]On Behalf

-> > Of Steve Pitt

-> > Sent: Friday, March 03, 2000 3:23 AM

-> > To: rp-ml@bart.lpt.fi

-> > Subject: little bit off topic (about inspection)

-> >

-> >

-> > Hello List,

-> > I am Ph. D. student and my research topic is about

-> inspection planning.

-> > I have a question about inspection.

-> > For inspection of freeform surface, what should be inspected?

-> > There exist a lot of inspection features such as plane,

-> cylinder, etc.

-> > In that case, sampling several points is enough.

-> > But I think that freeform surfaces are different from the features.

-> > Just is it enough to see the difference between point data and

-> > the original

-> > surface?

-> > Or the surface which is reconstructed from point data must be

-> > compared with

-> > the original one?

-> > Which way is a CMM used for inspecting freeform surface?

-> > I respect the answer from anyone who has expriences for

-> freeform surface

-> > inspection.

-> > Tnank you in advance.

-> >

-> > Steve Pitt

-> >

-> > ______________________________________________________

-> > Get Your Private, Free Email at http://www.hotmail.com

-> >

-> >

-> > For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

-> >

->

->

-> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

**Next message:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Previous message:**Spielman, Roger L: "RE: Question about laser for SLS"**In reply to:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Next in thread:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Reply:**Tommy Tucker: "RE: little bit off topic (about inspection)"**Reply:**Bert van den Berg: "Re: little bit off topic (about inspection)"**Reply:**SiderWhite: "Re: little bit off topic (about inspection)"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

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