Reply To: Negative stiffness in x-position between PM and steel stator

[Bill Bulat]

I'm not sure this is exactly what you're asking for, but I found this old MAPDL input file that uses two spring elements (COMBIN14 and COMBIN39) to posit nonlinear negative stiffness between two nodes (to mimic net magnetic force versus separation):

fini

/cle

 

/sys,del test_neg_K_05*.png

/fil,test_neg_K_05

 

/pnu,node,1

/pnu,type,1

/tri,lbot

/esha,1

/ics,1,5

/pbc,u,1

 

 

C***********************************

C*** MODEL

C***********************************

/prep7

 

n,1,,1 ! NODES

n,2

n,3,,2

 

et,1,39,0,0,2 ! NONLINEAR SPRING (COMBIN39) ATTRIBUTES

r,1, -0.5,10-1.25, -0.4,5-1.25, -0.3,3-1.25

rmore, -0.2,2-1.25, -0.1,1.5-1.25, 0,1.25-1.25

rmore,  0.1,1.10-1.25, 0.2,1.05-1.25, 0.3,1.03-1.25

rmore,  0.4,1.02-1.25, 0.5,1.015-1.25

 

et,2,14,,,2 ! LINEAR SPRING (COMBIN14) ATTRIBUTES

r,2,1.25/50

 

type,1 ! SINGLE NONLINEAR COMBIN39 ELEMENT

real,1

e,2,1

 

type,2 ! SINGLE LINEAR COMBIN14 ELEMENT

real,2

e,3,2

 

d,1,uy ! BOUNDARY CONDITIONS

d,3,uy,50

 

 

fini

 

/solu ! SOLVE

auto,off

outr,all,all

 

nsub,1

d,2,uy,-0.5

solv

 

nsub,10

d,2,uy,0.5

solv

fini

 

eplo

/sho,png $eplo $/sho,close $/wait,2

 

/post26 ! VERIFY FORCE-DEFLECTION

nsol,2,2,uy

xvar,2

rfor,3,2,fy

/axl,x,Node 2 UY Displacement

/axl,y,Reaction Force @ Node 2

!/yra,0,10

plva,3

/sho,png $plva,3 $/sho,close $/wait,2

Note that as node 2 at the bottom of the two element model is moved upward, the reaction force becomes increasingly negative (the springs are pulling upward on node 2 with increasing force as node 2 gets closer to node 3).

 

In Mechanical, you would have to include commands like these in a command object, maybe written in such a way that the springs are attached to remote point nodes scoped to surfaces.

 

--Bill