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2013 Equation Curve issues- units, trig, and transforms

6 REPLIES 6
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Message 1 of 7
88nwilson
668 Views, 6 Replies

2013 Equation Curve issues- units, trig, and transforms

I'm doing a complex series of cycloids and trochoids with equation-driven curves to create clearances for moving parts.

 


First, it seems arbitrary that units for "t" switch depending on whether it's the radius or theta in polar coordinates. I'm quite familiar with how to correct the unit errors, but it's frankly irritating. I'm diving my arc-sines by 1 degree, I'm multiplying fractions by a centimeter... it would be easier if t were always unitless, or allowed a defined unit (e.g. always a degree, a radian, or a centimeter).

 

Second, is there a way to create, then constrain from, an origin point? I'm building these rather involved curves, then I need to transform a polar curve in cartesian directions. I can only constrain my two endpoints- very frustrating to have to build points on calculated coordinates, then constrain to those points, especially when "show transform" is an option, but "edit transform" is not. I just want my 0,0 to be visible and constrainable.

Inventor 2013
Win 7 Pro SP1 (x64)
6 REPLIES 6
Message 2 of 7
johnsonshiue
in reply to: 88nwilson

Hi! If I understood how Equation Curve works correctly, parameter 't' is unitless. The unit for angle is dictated by Document Settings (Tools -> Document Settings -> Units). It is either degree or radian.

For constraining to a point, you can copy and paste the Equation Curve and then the copied curve will be free to be constrained to any point.

Thanks!



Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer
Message 3 of 7
88nwilson
in reply to: johnsonshiue

Thanks for the response. I guess "t" is unitless... it just must be the other parameters that were acting in a less than elegant way. My arc-sine/ 1 deg was a surprise, for example, if logical.

 

The constraint issue is the opposite of the problem you seem to think I have. I want my equations to be MORE fully constrained upon creation, such that in virgin state, it will show my origin point, and have neutral rotation about the origin, so I can constrain my origin to the origin of my model and know exactly where my equation is, then constrain some reference (e.g. the first point) to a horizontal to know my angle is correct. Instead, I need to constrain my first and last points, making a transform difficult.

 

Suppose, for example, I have a semicircle as an equation, r(t) = 1 and theta(t) = t, t= [0,180]. I have to constrain my first point to a horizontal, constrain my last point to a horizontal, then constrain a distance from my first point to the origin and likewise to my last point (which becomes redundant, but prevents the ambiguous case). A more complex curve, I need to constrain my end points per my limits of theta(t) and r(t), with three total dimensions, all calculated.

 

What I would prefer, in the example of the semicircle, would be a reference "0,0" point in the equation which I can constrain to my origin. Further, a reference "r,0" line to constrain horizontal would facilitate a defined angle. It would make sense to also be able to contraol these from the "show transform" tab in the "edit equation" window, after creating the curve.

 

Are there options anywhere to allow these origin-based constraints to be applied?

Inventor 2013
Win 7 Pro SP1 (x64)
Message 4 of 7
johnsonshiue
in reply to: 88nwilson

Hi! If possible, could you send the file to me so I can take a closer look (johnson.shiue@autodesk.com)?

Many thanks!

 



Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer
Message 5 of 7
AdamEllison
in reply to: 88nwilson

I'm having this problem too. I'm drawing a cosine-modified circular profile (r=20+0.5*cos(50*a)) as part of a cam path profile and due to the problem described above am unable to fully constrain my sketch (the equation is plotted from 0 to 360, so the end points are coincident).

If the "show transform" values were editable, I could at least edit the values to make sure my equation origin lines up with the part origin.

 

This is otherwise a fantastic feature, significantly better than a number of other CAD packages. We just need to be able to edit and constrain the curve origin please (at least make it fixed by default)!

 

Thanks

 

Message 6 of 7
88nwilson
in reply to: AdamEllison

Bingo- "fixed" would be a great default setting.

 

SolidWorks has a more advanced equation-driven surface package, but I don't relish the thought of a full migration. I'd hate to have to import STEPs of anything this complex, though.

 

Adam, you could probably do a pair of tangent lines for your constraints at the calculated values of 0 and 180 degrees, then constrain your start/end point. That's what I've been doing (although only through 180 or less)- I draw my r(0) vector and constrain its endpoint to the curve origination, and its other endpoint to my origin, then draw a tangent line at r(90) or r(180), or on my case, I'm often able to do r(n), with n being my last value of t. A full [0,360] curve doesn't quite make that possible, though.

 

Also- I wish Inventor could handle singularities. Sin(2t)/Sin(t) from (0,90] shouldn't be that complicated. But I wind up having to do my inputs as Sin(2t)/Sin(t) dfrom [0.01, 90] instead.

Inventor 2013
Win 7 Pro SP1 (x64)
Message 7 of 7
johnsonshiue
in reply to: AdamEllison

Hi! I guess the default behavior of the equation curve is more focused on creating the curve shape. It allows the curve to be relocated with transformation captured. Like you have already found out, if you want the curve stays where it is, it is better to add "Fixed" constraint to the whole curve.

I will follow up with development team and see what the implication it might be if we change the default behavior to Fixed as opposed to Free.

Many thanks!

 



Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer

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