Issues with mesh/surface/stress/servo command

Submitted by mschramm on Tue, 04/16/2019 - 00:38

Hello,
I am attempting to use the servo command to place a "constant" force onto a bed of particles in the z direction.

I am using the following commands to do so

variable drum_diameter equal 0.250
variable plung_start_height equal 0.5
variable target_force equal 1000.0*${drum_diameter}*${drum_diameter}*PI
fix Plung all mesh/surface/stress/servo file STL_Files/Plung_225.STL type 2 scale 0.001 &
move 0.0 0.0 ${plung_start_height} com 0.0 0.0 ${plung_start_height} ctrlPV force &
axis 0.0 0.0 -1.0 target_val ${target_force} vel_max 2.0 kp 1.0

I am then accessing the plunger forces by
variable plung_fz equal abs(f_Plung[3])

I use abs because I have seen a few cases where the force has been reported as negative.

Using these commands yields some interesting results.
I should be seeing about 200 Newtons as an average force but this is not the case.
I am seeing two groups, one averaging around 340 and the other around 68.

Is this due to my values for vel_max and kp? Should I try smaller values?

Any help is greatly appreciated.

Thank you