Compute Force acting on mesh

Yes, see for example:
https://www.cfdem.com/forums/tutorial-3-dem-flow
Have a look at fix messflaeche all mesh/surface/stress file STL/messflaeche.stl type 2 scale 0.001 stress on and

variable time equal step*dt
variable fx equal f_messflaeche[1]
variable fy equal f_messflaeche[2]
variable fz equal f_messflaeche[3]
variable Mx equal f_messflaeche[4]
variable My equal f_messflaeche[5]
variable Mz equal f_messflaeche[6]
dump forces all mesh/vtk 1000 post/dumpI*.vtk output interpolate id stress stresscomponents
fix forceslog all print 100 "${time},${fx},${fy},${fz},${Mx},${My},${Mz}" file post/forces.csv title "t,Fx,Fy,Fz,Mx,My,Mz" screen no


Hi all,
I am a beginner here. My simulation currently employs one servo wall and three non-servo walls. Two of the non servo walls are moved at a constant velocity in this assembly. All the walls are stls and I am needed to track the forces and positions on all four meshes. How do I do this?
My code for tracking the forces on three walls given below and as you can see there are 9 variables and I get only 7 outputs, which confuses me since the numbers doesnt seem right.

variable fx_botcyl equal f_cad2[1]
variable fy_botcyl equal f_cad2[2]
variable fz_botcyl equal f_cad2[3]
variable fx_pl equal f_cad3[1]
variable fy_pl equal f_cad3[2]
variable fz_pl equal f_cad3[3]
variable fz_top equal f_cad4[3]
variable xpos_cyl equal f_cad2[7]
variable xpos_pl equal f_cad3[7]

fix forceslog all print 1000 "${fx_botcyl},${fx_pl},${fy_pl},${fz_pl},${fz_top},${xpos_cyl},${xpos_pl}" file post/forces.txt title "fx_botcyl,fy_botcyl,fz_botcyl,fx_botpl,fy_botpl,fz_botpl,fz_toppl,xpos_botcyl,xpos_botpl" screen no

Please help me with this.

Thank you!!