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-----Original Message-----

From: Anshuman Razdan [mailto:razdan@asu.edu]

Sent: Tuesday, March 21, 2000 8:39 PM

To: 'SiderWhite'

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

Glenn

Thanks. I am not familiar with the Lim Cheraghi method. Can u give me a

bibliographic reference or send me a copy of the paper if its handy. I agree

in general with you about changing the method. One of the reasons Least Sq

is liked because the scanned data is itself not free from noise. So, a local

approximation smoothes out this noise (at least theoretically). It is not

necessary to fit a surface model to scanned data though. Direct comparison

of points to the surface for planar and prismatic surfaces can be performed.

The problem is even though visually (and otherwise) you know that this is a

piece of cylinder it not apparent to the algorithms unless its specifically

mentioned as "feature" definition. Try creating a part of a sphere in a CAD

package and save it as an IGES NURBS file. Its a text file and look at it -

damn difficult to see that the surface network came from a sphere.

I would love to pursue the paper and perhaps talk intelligently about it.

Thanks for you many contributions to the RPML. I enjoy your comments - one

of more balanced heads :)

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: SiderWhite [mailto:SiderWhite@worldnet.att.net]

-> Sent: Tuesday, March 21, 2000 6:52 PM

-> To: razdan@asu.edu

-> Cc: Rapid Prototype Mailing List

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

->

->

-> Anshuman:

->

-> Nice piece on parametric geometry.

->

-> Regarding inspection of freeform geometry, all CMM software

-> that I know of

-> use the approximation method of Least Squares to "best fit"

-> a point cloud

-> back to a solid and/or surface definition model. But could

-> some better

-> methods be adapted for 3D geometry usage to gain exact

-> values such as the

-> minimum zone-convex hull method or the optimization

-> technique suggested by

-> Hossein Cheraghi and H.S. Lim (an LP method where the

-> objective function is

-> to minimize the tolerance zone)? These methods are used to measure

-> straightness and flatness of surface elements and surfaces

-> in the "XY"

-> direction but could a 3rd dimension be feasibly added to

-> these methods?

->

-> Regards,

->

-> Glenn Whiteside

->

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

-> From: Anshuman Razdan <razdan@asu.edu>

-> To: 'Tommy Tucker' <tommy@paraform.com>; 'Steve Pitt'

-> <steve2k73@hotmail.com>; <rp-ml@bart.lpt.fi>

-> Sent: Monday, March 20, 2000 12:59 PM

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

->

->

-> > 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/

*> -> >
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**Next message:**Anshuman Razdan: "FW: little bit off topic (about inspection)"**Previous message:**RapidPrototype@aol.com: "Sanders Design International, Inc. (Definitely Not SPI!)"**Maybe in reply to:**Steve Pitt: "little bit off topic (about inspection)"**Next in thread:**Anshuman Razdan: "FW: little bit off topic (about inspection)"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

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