Shear Cell Test

Hi I want to perform numerical experiment of a Shear cell test by applying the periodic boundary conditions (as shown in Figure, on a box of 0.0015*0.0015*0.0015m). The aim is to find the friction properties of the particles. The particles are inserted using a factory for 4 seconds (0-4 sec) and then a force of (0.225N) is applied through a top plate for one second (4-5sec). For the next 20 seconds (5-25sec), the force on the top plate is changed to (0.9N) an the shearing action is planned to be induced due to motion of the top plate with a velocity of (1mm/s) in the negative x-direction. I have some questions and issues that I need help in and I would appreciate any help.

The force I am applying to the top plate is through (servo command), where I am adding the force applied plus the weight of the plate (by ctrlPV force target_value) applied at the location given as the center of plate (com 0.0 0.0 0.0044). My plan was to define a plate at the top of the domain at 0.0015m and then it should travel a certain distance coming down based on the height of the particles and then I should see the plate touching the particles and apply the force. However, using the servo command I can't see any motion of the plate. However, I could see the particles getting pushed down which shows a force being applied. I checked the stress on the plate and it gives the value equal to the force I am applying which gives hint I am doing something right. Could any body who has done the same problem of compression test or shear test help me, how to be sure that we are applying the right force at the right position?

Also, For shearing action, I can't move the top plate on which I have already applied a compression force, because LIGGGHTS gives error to remove the previous fix. So, I apply the shear action by moving the bottom plate? Will it give the same result as moving the top plate. Which I think so it should.

I will appreciate any help in this. I am attaching my LIGGGHTS script with this post.