Angle constraint 180 or 0

[Dropjeslover]

Hi all,

the typical problem with angular constraints is also hitting me/us here. I have seen many topics with similar problems, but no real solution. I'm starting to agree with one stating that this constraint is driven by an evil imp who just wants to annoy humans.

My (simplified) problem is shown in the attached picture below.

I have 3 risers/pedestals, each with a near identical part on top of it. I want the triangles to point to the same direction. With the angular constraint I mate the midplanes of each item to the midplane of the risers/pedestals. For simulation purposes, they have to rotate a little bit between (for example) +26.3 and -19.7 degrees, all points pointing towards the same spot.

However when I place the constraint and put the angle to 0º, the picture shows what Inventor (2014) thinks is 0º. I am having great difficulty (near impossible) with convincing Inventor that the middle triangle is rotated 180º... I want to get rid of entering the desired angle [180 +/- 26.3].

I use the directed angle constraint.

Any thoughts?

[jtylerbc]

First, are you sure that the pedestals are all oriented the same way?  From the picture, it appears that two out of the three aligned the same way, and the one in the middle is working backwards.  Since the pedestals are cylinders, there isn't any obvious visual cue that they are all oriented the same.

Second, if you are using the angle constraints with planes, it is common to have some goofy results.  Fortunately, you can fool the evil imp in many cases.  The angular constraint tends to behave much better if you use edges (or an axis) instead of faces or planes. 

[MikahB]

Also, when I run into issues with Angle Constraint I go straight for the Explicit Reference Vector option where you can define the "normal" - seems to help every time and make a much more stable constraint.

[Dropjeslover]

They are all orientated the same way. The actual item I'm working on are more of a crane type construction (Marine loading arms), like this:

[img]http://upload.wikimedia.org/wikipedia/commons/4/46/Marine_Loading_Arm_KLEa.jpg[/]

There is a pedestal with a bearing on top, and on top of that is a unit which holds the rest of the arm(s).

Same problem occurs with other sub assemblies of the unit. As it is mostly built out of piping and the end connection has almost 6 degrees of movement, it is rather hard to constrain otherwise...

[mcgyvr]

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@MikahB wrote:
Also, when I run into issues with Angle Constraint I go straight for the Explicit Reference Vector option where you can define the "normal" - seems to help every time and make a much more stable constraint.

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+1.. The explicit reference vector option was added only a few years back to solve this exact problem.. Seems many don't know about it and don't use it. 

[Dropjeslover]

so in my case, the normal would be the axis of the pedestal?

[rdyson]

It doesn't matter where the normal is defined. The normal only determines the direction of positive rotation. It follows the right hand thumb rule. (Is that still taught?)

[Dropjeslover]

I vaguely remember something like that yeah haha 😛
However, it does not define what the starting point is (the difference between 180 or 0 degrees).

[brendan.henderson]

Recently I raised a support case on this type of behaviour. The support rep pointed out the following :-

"This is about confusion in Angular constraint behaviour. From assembly constraint’s perspective, an angle between two objects can be defined in several ways as long as the selected geometry maintains the angle. So, the participating components can flip 180 degree and the constraint is still considered solved correctly. 

All of the Angular constraints in this assembly was created by Directed Angle constraint. It is better to use Explicit Reference Vector type instead. Make sure you select the face normal to the view direction since the components are supposed to rotate on the view plane. "

The support rep also supplied a DOC file from the developers explaining this a bit further. I have attached a section of that text for your reference :-

"Consider there is a pair of chopsticks. Each chopstick has a tip and a handle. If I say, I want the two chopsticks to be at an angle of 30 deg. How many solutions do you see? There will be actually 8 solutions.

1)      ChpA_Tip and ChpB_Tip form 30 degree and they are at the top

2)      ChpB_Tip and ChpA_Tip form 30 degree and they are at the top

3)      ChpA_Handle and ChpB_Handle form 30 degree and they are at the top

4)      ChpB_Handle and ChpA_Handle form 30 degree and they are at the top

5)      ChpA_Tip and ChpB_Handle form 30 degree and they are at the top

6)      ChpA_Handle and ChpB_Tip form 30 degree and they are at the top

7)      ChpB_Tip and ChpA_Handle form 30 degree and they are at the top

😎      ChpB_Handle and ChpA_Tip form 30 degree and they are at the top

Some of these solutions may seem redundant. But, if you play around with two chopsticks in the space, you will understand these solutions are all unique and valid. Each solution gives a unique position and orientation of this chopstick assembly in the space. This helps explain why Inventor has three different choices of Angular constraint. The latest addition to Angular constraint is “Explicit Reference Vector” type, now default. The main reason the third option is added is because Inventor used to have trouble distinguishing between solution#1 and #2 for a given set of geometry and angular value. With ERV, Inventor can easily tell that the requested angular value (+-) would result in different direction."

Also, just like it does with a Mate or Flush constraint, it matters which plane/surface/face is picked first. Do 2 tests, 1 selecting an Origin plane and the other a plane on the part to be rotated with a rotation value. Then do it again in reverse. See the picture for an explanation.

FWIW, I also gives a thums up to Excplicit Vector Reference and it being made the default Angular constraint.