**Next message:**Bert van den Berg: "Re: little bit off topic (about inspection)"**Previous message:**Tommy Tucker: "RE: little bit off topic (about inspection)"**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)"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

Tommy

Good point - good initial guess - that's where the key is. So how do u

intend to provide a good guess. What about points that happen to lie close

to patch boundaries? But imagine 3-5 iterations for million points on a

complicated part. I will let you do the floating point calculations.

AR

-> -----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 1:15 PM

-> To: razdan@asu.edu; 'Steve Pitt'; rp-ml@bart.lpt.fi

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

->

->

-> What if you use the result from the reasonable tessellation

-> to provide good

-> initial guesses to the Newton method? My experience is that

-> given a good

-> initial guess, Newton's method will converge in 3-5

-> iterations. This is a

-> small additional cost compared to searching the tessellation.

->

-> Tommy Tucker

-> (vc) 408-855-4372

-> (fx) 408-855-4360

-> tommy@paraform.com

-> http://www.paraform.com

->

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

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

-> > Sent: Monday, March 20, 2000 10:59 AM

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

-> > 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:**Bert van den Berg: "Re: little bit off topic (about inspection)"**Previous message:**Tommy Tucker: "RE: little bit off topic (about inspection)"**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)"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

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