Re: Multiple group membership for nodes

["David Meeker" <dmeeker@xxxxxxxx>]

--- In femm@xxxxxxxxxxxxxxx, "niallcaldwell" <niall.caldwell@xxxx> wrote:
> Firstly, many thanks to David for his advice over the slotted 
> armature problem I had. I was not aware that the "lamination" feature 
> essentially did this BH scaling that I was trying to do. The point 
> about radial gap is well taken- this is the case in most of my 
> designs. I presume that, if the material is working in the linear 
> portion of its BH curve, and the gap is small in comparison with the 
> major dimensions of the armature and the slot circumferential width, 
> then a good approximation to obtain the same gap reluctance is to 
> increase the radial gap by the same ratio as the "lamination factor" 
> at the outside of the armature? 

It might be reasonable to fudge the gap width in this circumstance. 
However, I'd suggest using Carter's Coefficient as a guide for picking
the effective air gap. I'll have to look up the formula--I don't
remember it off the top of my head. Anyone have a good web reference
for Carter's Coefficient.

> One other point which has come up. I often want to parameterise 
> models so that I can sweep through a design space to find an optimum 
> of two or more variables. I want to use Lua with nested "for" loops 
> to do this, for instance generating armature force for a range of 
> diameters and lengths of the armature. 
> 
> To do this from the starting point of existing geometry I need to 
> select nodes by group in Lua- all the nodes on the outside diameter 
> are in group 1, while all the nodes at the top end are in group 2. 
> The trouble is, there is always at least one node which wants to be a 
> member of both groups (ie the outer top corner). Is there any way to 
> assign multiple groups to entities?

Well, you can't assign multiple groups, but you could assign a
different group number for the elements that are supposed to be in
more than one group (e.g. group number 21 for elements that are
supposed to be in both groups 2 and 1 or something...). Then, you can do
selectgroup(1)
selectgroup(21)
to get group1 plus the shared section. When you do multiple
selectgroups, the previously selected group doesn't get unselected.

> The other option is to build up the entire problem in the lua script, 
> bypassing the GUI, creating keypoints with coordinates calculated by 
> functions of defined parameters. That way individual nodes 
> coordinates can be a function of many parameters. This is the most 
> powerful and general way of parameterising, and the way I used to 
> work with Ansys becuase its GUI is such a dog compared to femm's, but 
> really.... life is too short! 
> 
> BTW this same nested loop technique can be used to generate lookup 
> tables of force and inductance at a range of currents and armature 
> positions, for export to dynamic simulation packages (AmeSim, Easy5, 
> Simulink) This allows the complete dynamic simulation of a solenoid 
> taking into account damping, spring, inertia, colomb friction, 
> electric circuit switching... I have heard that transient phenomena 
> eg. eddy currents can be taken into account by running a frequency 
> analysis in FEMM and inserting parasitic lumped parameters into the 
> dynamic model.
> 
> Niall