Hi vicpopo,
why did you make your simulator so small? Because lack of space? Maybe this video could help you:
https://www.youtube.com/watch?v=zNUBZfrOXUc
The g-center in this video is much lower, so they can achieve biggest angles.
The problem of small 6dof simulator is that there is very small x,y,z (surge, sway, heave) moves. The biggest benefit of 6dof platform is that you can combine 6 moves, where surge, sway and heave is very important (professional platforms often use only world orientation for pitch and roll, yaw slip for yaw and g forces for xyz).
When you have small simulator like yours, you have practically "4dof" not 6dof.
In my video I combined axes like this: SURGE(x-sim sway)-longitudinal force, SWAY(x-sim(surge)-lateral force, HEAVE-vertical force, PITCH-pitch (world orientation)+longitudinal force, ROLL-roll (world orientation)+lateral force, YAW-yaw slip angle
https://www.youtube.com/watch?v=aIng_hVEuoc
The platform-base ratio is also very important. Platform = Base => biggest xyz moves. Platform < Base => biggest xyz angles.
Try to experiment on paper/in excel/... See example:
https://www.youtube.com/watch?v=9zb-xDrxjb8
I´m looking forward to see your progress,
Richard