Ellipse Perimeter Formula Help

[jsteinhauer]

Hi All,

I am trying to write a formula for finding the perimeter of an ellipse. I found a good formula to get a approximation, but I'm not able to get Revit to except it as a formula. The error message I receive is, 'Parentheses expected after function name'. Any help would be appreciated.

Thank you in advance,
Jeff S.

[jsteinhauer]

So if I make a number parameter for Pi and call it PIE, I can get the formula to work as expected. So I guess my next question is, can someone re-write the following formula to solve for b.

P = Pi[3(a+b)-sqrt((3a+b)*(a+3b))/2

Thanks,

Jeff S.

[contact.andrewk968454]

I've heard of an ellipse, but what is an eclipse (in math terms)?

[jsteinhauer]

I've heard of an ellipse, but what is an eclipse (in math terms)?

Sorry. I've got a newborn in the house. Sleep is in short supply.

[LP Design]

Wow, that is a nasty equation. Unfortunately I don't maths. (ahem) I mean, my understanding of complex algebra is insufficient to find a solution.

It may help, however to know what you are trying to accomplish. I was recently trying to make a complex family work by using some ridiculous formulas until it was brought to my attention that I didn't need any of them if I just changed which parameter was controlled by user input...

-LP

[contact.andrewk968454]

Are you sure you want to rewrite the equation to solve for the minor axis of the ellipse? I'd think you'd know both the major and minor axis and would want to just report the perimeter.

[jsteinhauer]

So I am creating a operable canopy family. I have a truss built and a partial ellipse. I know the perimeter of the partial ellipse when it is fully extended, and I want to have the same perimeter when it is collapsed. Knowing both 'P' & 'a' for both instances I would like it to solve for 'b'. This would drive the ellipse of the canopy down to make it look like the canvas material was drooping when not fully extended.

Thanks,
Jeff S.

[LP Design]

Wow, that's a pretty ambitious project. I suggest you change your approach slightly. #1, don't get stuck trying to make it mathematically perfect. In this case the visual result is most important, not the exact dimensions. When the project goes out for construction you should be getting shop drawings with the true construction details.

#2. determine Maximum Depth (front to back in your attached image) and Maximum Height parameters and set them equal. That would give you the maximum amount that the canvas would droop if it were fully closed. Add a third parameter for "open %" and 2 more for "actual depth" and "actual height".

Link each actual parameter to a reference plane as a function of the % parameter. With this setup, as "open %" approaches 100, "actual depth" approaches "max depth" and as it approaches 0, "actual height" approaches "max height".

Finally you just need to play with the length formulas and link them to the % parameter so that you get the elliptical droop rather than a semi-circle.

If you were to take a virtual tape measure along the edge in your model it would grow and shrink between 0% and 100% open but the visual effect should still be achieved, and you don't need to do calculus to achieve it.

-LP

[Alex Page]

So if I make a number parameter for Pi and call it PIE, I can get the formula to work as expected. So I guess my next question is, can someone re-write the following formula to solve for b........

Note that if you want to use the inbuilt value of "pi" - use "pi()" instead so your formula should read:
P = Pi()*[3(a+b)-...etc

[Revitaoist]

This would drive the ellipse of the canopy down to make it look like the canvas material was drooping when not fully extended.

Thanks,
Jeff S.

Instead of reinventing the wheel, just have nucleus calc the gravity for you.

http://labs.autodesk.com/utilities/nucleus/