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How to create a logarithmic spiral 4

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Theophilus

Theophilus

Mechanical
Dec 4, 2002
3,407
Does anyone know of a way to create a logarithmic spiral? I need to generate a surface for a part to follow in an animation, and this appears to be the shape I need.

I have a hinge sort of detail. In normal SolidWorks I can accomplish what I need with a gear mate, but Animator doesn't do anything with the gear mate and lets everything flop around as if the mate didn't even exist (figures).

So here's the situation. I have two main parts (like doors), connected with an intermediate hinge piece. The intermediate piece supplies the axles for the doors. The doors rotate from folded atop one another to parallel with one another (180* of motion). However, the hinge piece rotates only 90* in this same course.

So the door rotates 180* about the other door, while the hinge piece rotates only 90*. I'd like to extrude a surface--which is starting to resemble a spiral of some sort--that I can use to control the rotation of my door when I rotate the hinge piece only 90* such that the door rotates 180* exactly. (This is quite a complicated hack for an Animator limitation--anyone know of anything simpler?)

So--is this what I'm looking for? If so, with the ratio of angle change of the door to the hinge piece being 2:1, what would the formula look like (remember, I'm an industrial designer, not an engineer--I didn't have that class [well, I probably did, but ditched the engineering career for ID at that point]). I found this, but don't know how to turn this into something useful within SolidWorks:

Thanks,



Jeff Mowry
Reason trumps all. And awe transcends reason.
 
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I'm having a bit of difficulty picturing your doors. Can you put up a screenshot?
 
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Jeff,

Could you use a cam to control the motion (with cam mates of course)? You can hide this cam part, but it would still be influencing the motion.

- - -Updraft
 
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Rob ... the SW spiral creates an Archimedian (even pitch) spiral. Theo' needs a logarithmic (increasing pitch) one.

Theo ... could you construct one using a spline and the rectangles as shown in It would only be an approximation, but may be close enough.

Like Handleman, I'm having trouble picturing the "door" arrangement. A picture would really help.



[cheers]
 
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I have a gear model where I approximated an involute with a spline. I graphically laid out reference points and then "connected the dots".

[bat]Honesty may be the best policy, but insanity is a better defense.[bat]
-SolidWorks API VB programming help
 
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Thanks for all the replies, folks! Sorry about no picture (yet)--gotta run and catch a plane. I don't think I'll be able to use a spline of any type unless it's perfect--since I'll try to run a point along this curve to guide it. (I'll see if I can upload a pic real quick without missing my flight. ..running late. ..as usual.)

Thanks again!



Jeff Mowry
Reason trumps all. And awe transcends reason.
 
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An image:

So the top door opens counter-clockwise. You can see an approximated spline drawn in, but this is far too sloppy to work by mating a point off the end of a line from the top door (maybe I can explain better later).

THANKS for your help--my mind's turned to drizzle with late nights!



Jeff Mowry
Reason trumps all. And awe transcends reason.
 
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At the risk of sounding stupid, as I've never used Animator, does a symmetric mate work in Animator? Could you set up a symmetric mate using some planes on the doors as the bread and a plane on the hinge as the peanut butter on the symmetric mate sandwich? Sort of like this file?
 
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I played around with the following and got it to work, well more or less. On one piece I created a helix coincident with the axis of rotation with 2 revolutions at a pitch of 1. On the other piece I created a helix coincident with the axis of rotation with one revolution at a pitch of 2. In the assembly I added a follower part that was constrained to a plane through the axis of rotation, and mated its origin coincident to both helixes. The pitch 1 helix has to rotate twice as far as the pitch 2 helix to allow the follower to travel the same vertical distance. This seemed to lock up in the assembly, but I was able to get it to move smoothly between two endpoints in animator. I had to temporarily suppress one of the two helix-follower mates in order to set the start and end points of the animation.

I think this technique could also be used to create a rack and pinion mate in earlier versions of SW. The pinion has a helix, and the rack has a sloped line.

Eric
 
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Eric,

I'd be interested in seeing your solution as an image or file (typing from airport). Sounds like something I might get to work for me.

Animator is strange--it doesn't treat mates, etc. as SolidWorks does--so I wonder what set of rules it's following and why the SolidWorks rules don't work. It's now very common for me to expect certain parts that move just fine in SolidWorks not to function at all in Animator--or to give me the dreaded red diamond if it does (which usually hoses the rest of the animation).

Certainly this sort of thing makes it difficult to make money when bidding on animations from SolidWorks files. (Oh well, I knew what I was getting into this time.)

Thanks again, everyone, for helping out. With all these ideas, I can probably hack together a solution when I get back in the office. (Speaking of the office, the Sangres had a fresh dusting of snow on them this morning--also saw some big-horn sheep on the way through the mountain pass--I love this place!)



Jeff Mowry
Reason trumps all. And awe transcends reason.
 
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I believe this has changed somewhat is SW 2008 Jeff. Animator, physical dynamics and CosmosMotion now all use the same interface and set up. Once you've set up a motion study you just decide which solver to use. I believe the same rules in SW now apply or can be transfered to the motion studies.

Rob Rodriguez CSWP
Eastern Region SWUGN Representative SW 2007 SP 2.0
 
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Eric,

That's a pretty impressive thought process, but seems incredibly complex for the function desired. Perhaps I'm wrong (I can't get your assembly to move at all on my machine), but it appears that I understand the desired effect - force one component to rotate at half the speed of the other. The assembly I posted achieves that with the symmetry mate. It works for infinite rotations and drags just fine whichever component you want to drag.
 
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I would have to say TheTicks method is the more sound from an engineers POV. I don't want to start a my MCAD program is better then yours but Pro/E 2001 has powerful Mathematical Functions ability. I've been asking SWx for that type of capability for a few years now....

Jeff, I would get out your calculus book and use those logarithmic spiral equations to define your path. You can use Excel to set this up then output the results to a CSV file then read it into SWx as 3D points. I've done this when designing gerotor pumping elements.

Heckler [americanflag]
Sr. Mechanical Engineer
SWx 2007 SP 4.0 & Pro/E 2001
o
_`\(,_
(_)/ (_)

This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
 
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handleman,
I agree that the paired helixes were binding up in SolidWorks, although I was able to get them to move in the animator (with some suppressing and resuming of mates). Your file with the symmetry mate works well and gave me another idea.

The following is a 5 bar linkage similar to what Jeff has already. There are 2 extra links which form it into a polygon, and the far vertex is constrained to lie on the midplane of the hinge link. It moves correctly in both SolidWorks and the animator. It’s a lot easier on the silicon brain cells than the helixes.


Eric
 
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Jeff,

What about using two rotary motors in Animator, one for the hinge and one for the door set to the same direction of rotation, but twice the speed of the hinge? This would eliminate all the extra parts/constraints and the subsequent load on the SWX solver and should give you the motion described.

- - -Updraft
 
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