SOLVED!
For those who still don't understand, this topic is about an error
I found in the ms excel sheet "Robot Arm Calculator"
about inverse kinematic
http://www.societyofrobots.com/robot_arm_tutorial.shtmlIf you put an input (-8,0) the end effector points in the wrong position.
After doing some calculation, I realize the error is in the formula:
To solve the joint angle, you need to add 2 more variable which are:
K1: (C74) =D12+D13*C64
K2: (C75) =D13*C65
and for the theta and psi, rewrite
theta:(C66) =ATAN2(C62,C63)-ATAN2(C74,C75)
psi: (C67) =ATAN2(C64,C65)
The graph should be O.K. now.
Regards