Reporting "perimeter" of an "ellipse"

[Rockmondd]

Need help on reporting perimeter of an ellipse. I can get length and width NOT Perimeter!!

I am using v3.5 MR2

Thanks.

[craiger_ny]

If you have length and width give me your math and it can be done

I just checked the math on that and it can be coded I'm sure. If not with PCDMIS then for sure with a .bas. It is the math I'm not sure about. It looks like the stuff that would be the easiest to code is likely to have some error. If there is a formula that calculates the exact circumference of the ellipse then that is what I need to be educated on. This would require you to be able to measure these dimensions (a & b) of an ellipse:

http://www.efunda.com/math/areas/images/EllipseGen0.gif

This may or may not be the formula:

http://www.efunda.com/math/areas/images/EllipseGen2.gif

When you decide on a formula and how you are going to measure those two dimensions I'll help you out. I am posting this because I have never measured an ellipse (I know nothing about them) so perhaps this can all be done with PCDMIS. I have no idea, I'm not at a machine right now. If someone can measure one and dimension it go with their solution but if PCDMIS just can't get it done let me know. It can be coded. I just don't want to set you up with something that does math for you and hand you a bad formula. I know what that is like (it's embarrassing).

Craig

[Rockmondd]

Thanks for your reply.

It is a large (11.5" x 7") plastic part and out of shape badly (reason why customer want perimeter not major/minor diameter).

If I can get "5% max error" that should be good enough for now.

What if I scan the ellipse will it output the length (perimeter)?


Tons of thanks.
Rockmond

[Rockmondd]

Hello Graig,

Please help me to set up code using following formula. It claims only 0.4%error !!

I will make a "linear closed scan" then construct an ellipse by it.

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

The YNOT formula

This formula gives an approximation for the perimeter of an ellipse.



The perimeter of an ellipse with half-axes a and b is obtained as:

4 ( ay + by)1/y

with y = ln(2)/ln(pi/2) ~ 1.534928536...(y is called the YNOT-constant)

Rewritten as a function of a and the excentricity,e, this formula becomes:

4a ( 1 + (1-e²)y/2)1/y

This approximation formula, first proposed by Roger Maertens, makes an error of at most 0.4%.

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

Thanks.

[craiger_ny]

OK, I'll write something that passes a and b to a script. I'll let you take care of the math part. Give me a bit. It has been hectic here. If I have time I'll do it 2 ways and you can pick which one you want to use.

Craig

[John Kingston]

I googled "circumference of an ellipse" and came up with a plethora of sites. The wikepedia link seems to have numerous versions. Some more exact than others.

http://en.wikipedia.org/wiki/Ellipse

I'm not sure if PCDMIS variables can handle that math. Craig is good with VB and should be able to help more than I.

[cmmguy]

On a badly shaped elipse it seems the only way to do it is to scan the elipse and add the distances between the scan points together. Assuming X/Y plane
You will still have to contend with the tip compensation.

Dim Circumference
For i = 1 to number of points
if i <> lastpoint then
Circumference= Circumference+ SQR((point(i).X-point(i+1).X)^2 +(point(i).Y-point(i+1).Y)^2)
else
Circumference= Circumference+ SQR((point(lastpoint).X-point(1).X)^2 +(point(lastpoint).Y-point(1).Y)^2)
end if
next i

[craiger_ny]

Create a .bas with this in it (I named it DoSomeMath.bas and put it on my C:\ drive, you can do what you'd like):

[COLOR="RoyalBlue"]Sub[/COLOR] main(a [COLOR="royalblue"]As Double[/COLOR], b [COLOR="royalblue"]As Double[/COLOR])
[COLOR="royalblue"]Dim[/COLOR] dblTheResult [COLOR="royalblue"]As Double[/COLOR]
dblTheResult = [COLOR="SeaGreen"]'do your math here, this is where you perform math on a and b[/COLOR]
[COLOR="royalblue"]Dim[/COLOR] strTheFile [COLOR="royalblue"]As String[/COLOR]
strTheFile = "C:\Test.txt"
[COLOR="royalblue"]Open[/COLOR] strTheFile [COLOR="royalblue"]For Output As[/COLOR] #1
[COLOR="royalblue"]Write[/COLOR] #1, dblTheResult
[COLOR="royalblue"]Close[/COLOR] #1
End Sub

You may need to declare your constant y for doing the math. I'm not here to tell you what formula to use or how to employ it. I need to tread lightly in areas that output a derived result.

Here is your PCDMIS code:

            ASSIGN/A_VALUE = where ever your a value comes from goes here
            ASSIGN/B_VALUE = where ever your b value comes from goes here
CS1        =SCRIPT/FILENAME= C:\DoSomeMath.BAS
            FUNCTION/Main,SHOW=YES,A_VALUE,B_VALUE,,
            STARTSCRIPT/
            ENDSCRIPT/
FPTR       =FILE/OPEN,C:\TEST.TXT,READ
V1         =FILE/READLINE,FPTR,{THE_RESULT}
            FILE/CLOSE,FPTR

THE_RESULT is the result of the math performed on a and b. You can report that variable as any other (do what you'd like).

I thought maybe I'd write one that does the math and sets the PCDMIS variable through automation. I have never automated setting a variable in PCDMIS so knowing I could figure it out I am hoping instead someone here will post it and save me the BS (like I said it has been hectic here).

Craig

[Rockmondd]

Thanks EVERYBODY!!

I am working on it.

[Rockmondd]

Hi.............. Formula method giving me inconsistance readings probably due to out-of-shape. Probably I should stick with "Circumference"

Hello cmmguy........... would you help me a bit more to get it work.

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

SCN1 =FEAT/SCAN,LINEARCLOSE,SHOWHITS=NO,SHOWALLPARAMS=NO
MEAS/SCAN
BASICSCAN/LINE,SHOWHITS=NO,SHOWALLPARAMS=NO
ENDSCAN
ENDMEAS/
ELL1 =FEAT/ELLIPSE,RECT,IN
THEO/2.9233,0.0015,0.1666,0,0,1,11.5118,7.3303,1,0.0026 ,0
ACTL/2.868,0.0004,0.1656,0,0,1,11.5664,7.3852,1,0.001,0
CONSTR/ELLIPSE,BF,SCN1,,
DIM LOC1= LOCATION OF ELLIPSE ELL1 UNITS=IN ,$
GRAPH=OFF TEXT=OFF MULT=1.00 OUTPUT=BOTH
AX NOMINAL +TOL -TOL MEAS MAX MIN DEV OUTTOL
L 11.5118 0.0000 0.0000 11.5664 11.5664 11.5664 0.0546 0.0546 ------>
H 7.3303 0.0000 0.0000 7.3852 7.3852 7.3852 0.0550 0.0550 ------>
END OF DIMENSION LOC1

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

Thanks !!!

[cmmguy]

I have used the TP scan before but evaluated against CAD, there may be a "circumference" or scan length characteristic already there for scans. Maybe someone else would know.