why does 2 link manipulator with fixed and joint support has the same results in modal analysis?

- November 4, 2021 at 4:16 am
  
  Amirmkr
  Subscriber
  
  I have defined a 2 link manipulator using mpc184 between the 2 beams. In one problem I have defined fixed support in the following code:
  
  /PREP7
  ET,1,BEAM188
  ET,2,MPC184
  KEYOPT,2,1,6
  KEYOPT,2,4,1
  MPTEMP,1,0
  MPDATA,DENS,1,,7830
  
  SECTYPE,1,BEAM,RECT,Beam01,0
  SECDATA,0.01,0.01,2,2,0,0,0,0,0,0,0,0
  
  MPDATA,EX,1,,2.068e11
  MPDATA,PRXY,1,,0.3
  
  LOCAL,11,0,1,0,0,0,0,0,1,1,
  SECTYPE,2,JOINT,REVO,Joint1
  SECJOINT,,11
  
  CSYS,0
  
  N,2,0,0
  N,3,0.5,0
  N,4,1,0
  N,5,1,0
  N,6,1.5,0
  N,7,2,0
  
  TYPE,1
  SECNUM,1
  
  E,2,3
  E,3,4
  E,5,6
  E,6,7
  
  TYPE,2
  SECNUM,2
  E,4,5
  
  D,2,All,0
  
  ANTYPE,2
  MODOPT,LANB,10
  EQSLV,SPAR
  MXPAND,10, , ,0
  LUMPM,0
  PSTRES,0
  MODOPT,LANB,10,0,0, ,OFF
  FINISH
  /POST1
  SET,LIST
  
  In the other problem I have defined a joint support in the following code:
  
  /PREP7
  ET,1,BEAM188
  ET,2,MPC184
  KEYOPT,2,1,6
  KEYOPT,2,4,1
  MPTEMP,1,0
  MPDATA,DENS,1,,7830
  
  SECTYPE,1,BEAM,RECT,Beam01,0
  SECDATA,0.01,0.01,2,2,0,0,0,0,0,0,0,0
  
  MPDATA,EX,1,,2.068e11
  MPDATA,PRXY,1,,0.3
  
  LOCAL,11,0,0,0,0,0,0,0,1,1,
  SECTYPE,2,JOINT,REVO,Joint1
  SECJOINT,,11
  
  LOCAL,12,0,1,0,0,0,0,0,1,1,
  SECTYPE,3,JOINT,REVO,Joint2
  SECJOINT,,12
  
  CSYS,0
  
  N,1,0,0
  N,2,0,0
  N,3,0.5,0
  N,4,1,0
  N,5,1,0
  N,6,1.5,0
  N,7,2,0
  
  TYPE,1
  SECNUM,1
  
  E,2,3
  E,3,4
  E,5,6
  E,6,7
  
  TYPE,2
  SECNUM,2
  E,1,2
  
  TYPE,2
  SECNUM,3
  E,4,5
  
  D,1,All,0
  
  ANTYPE,2
  MODOPT,LANB,10
  EQSLV,SPAR
  MXPAND,10, , ,0
  LUMPM,0
  PSTRES,0
  MODOPT,LANB,10,0,0, ,OFF
  FINISH
  /POST1
  SET,LIST
  
  Why do both problems give the same natural frequencies?
- November 4, 2021 at 4:24 am
  
  Amirmkr
  Subscriber
  
  Even if you put
  
  D,2,ux,0
  D,2,uy,0
  D,2,uz,0
  D,2,rotx,0
  D,2,roty,0
  
  to the first case you will get the same results.