Hello Fu-Kwun Hwang

At last I found some time to check your applet.
Unfortunately there are still a few bugs.

If the mass reaches the bottom of the loop for the first time, the arrow of the centripetal force and normal force become suddenly twice the size they had one step before.

After the loop the normal force shrinks suddently

After the first 90 degrees the normal force suddenly shrinks almost to nothing.
From here 180 degrees later the opposite happens the normal force grow suddenly to the size of the centripetal force.