direction of servo walls

Submitted by zamir on Fri, 11/01/2013 - 07:30

Hello all,

I cannot get the servo wall to move in the -Z direction, even if I use a negative target_val. This is how I issue the command:

variable lid_force equal -1400
fix lid0 all mesh/surface/stress/servo file LIGGGHTS_files/STL_files/in/p_lid.stl type 2 move 0 0 ${lid_height} com 0. 0. ${lid_height} ctrlPV force dim z target_val ${lid_force} vel_max 0.01

When I print the z coordinates of the wall, f_lid0[9], they are strictly increasing over time no matter what values I input for target_val.

I really want my servo controlled walls to move in the negative z direction. Please help.

FYI, I am using liggghts v2.3.8.

Thanks,
Zamir

PS: I edited the command a bit, my first post was inaccurate.

Forums:

zamir | Sat, 11/02/2013 - 06:55

I've got this solved, but the developers of fix_mesh_surface_stress_servo should take a read. I have learned something very interesting about this command:

When I issue the fix mesh/surface/stress/servo with a target_val as either -1400 or v_lid_force, it works fine. However, when I use the variable reference ${lid_force}, it does not work as expected.

I placed a few cout << commands in the source code and I find that when I use the variable reference ${lid_force}, the set_point_ variable in fix_mesh_surface_stress_servo.cpp always equals 1, no matter what I set lid_force to, whereas if I use v_lid_force for target_val, set_point_ is correctly printed as -1*v_lid_force.

Perhaps we can include a line in the documentation that indicates that the user should use the v_variable flavor of equal-style variables for target_val in order to get expected behaviour.

Thanks,
Zamir

aaigner's picture

aaigner | Mon, 11/18/2013 - 20:07

Hi Zamir,

Thanks for your command. I will check this...

Cheers
Andi

aaigner's picture

aaigner | Tue, 11/19/2013 - 11:08

Hi Zamir,

I wrote a very small test case and the servo wall works with all versions (${ctrlForce}, v_ctrlForce). Can you send me your case or better a minimal example?

Bests,
Andi

zamir | Thu, 11/21/2013 - 05:04

Even I cannot reproduce the problem anymore. I will certainly send you a script file when I can reproduce the behaviour.