Flexible Subassemblies - Another good idea implemented poorly

When FSA where first introduced, I was thrilled. I do a lot of
mechanism design with complex relative motions and FSA looked like they
would be a tremendous productivity boost. Unfortunately, they haven't
panned out. Sure it works OK when you have one very simple FSA, but
anything beyond that is extremely flaky. As an example, I have an
assembly with two FSAs in it. The FSAs are very simple, and as
mechanisms, they are identical. One is a linear actuator, and the other
is a linear slide. Each has has a limit mate to constrain its stroke,
and each has one DOF along its axis.

The slide is 'fixed' in the main assembly, but flexible. The actuator
is mated such that each of its two components is fixed relative to a
corresponding component in the slide. It is also mated such that the
limit mate on each is not violated.

A grand total of 2 mates, and SW chokes on it. Neither mate can be
solved. Also, the limit mate in each FSA fails as well. Open up the
FSA and rebuild and it corrects itself, but the main assy fails. If I
suppress the limit mate in one of the two FSAs, all is well, except that
the mechanism doesn't work properly. Without both limit mates, one FSA
can move beyond its internal limit.

Jim S.

 

When FSA where first introduced, I was thrilled. I do a lot of

I ran into a similar issue, and I think the problem is with the limit
mates and not necessarily the FSAs (although I agree that FSAs can be
flaky).

The issue is that once a limit mate is applied, no other mate can be
applied that defines the limit mate's exact position. Ex.: Create a
simple cylinder and use a limit mate to define its stroke limits (10-20
for instance). NOW, apply another mate that defines the exact stroke
length that does not violate the limit mate (15 for instance). This
will immediately cause errors, even though all conditions are/can be
met.

Thus, in my case, I wanted to crate a ramp that was actuated by a couple
of cylinders. My main assy would contain the ramp, base, and FSA
cylinders. The cylinders had limit mates applied defining their motion
limits. Then, in my main assy, I would apply an angular mate to define
the ramp. BOOM!! mate errors due to fully constraining the cylinder
length with my angular dim, even when the angular mate caused the
cylinders to be in a position that satisfied the limit mate.

I wish an error would only occur if the angular mate caused the limit
mate(s) to become unsatisfied.

 

Yes, that is part of it. However, since the limit mate is part of a
flexible subassembly, I'm not trying to define the exact position of the
limit mate, strictly speaking.

The silly thing in that my model moves correctly when dragged, obeying
either limit mate, despite the FM claiming that the mates cannot be solved.

Jim S.