On Which Principle Robotic Arm Works?

There can be many ways of making of an robotic arms, various techniques can be applied for various application, definition may change with technological advancements,For me a tweezer is also a manually operated robotic arm that is directly operated with human hand,Bulldozer is an another kind of robot with an arm,

Perhaps you may find some more bureaucratic definitive answer to your question with more members reapplying you, or you may Google it.

In the principle of closed loops.

Read about: servos, feedback, resolvers, encoders, motion control, linear actuators and all related data.

Yahlasit

On the mechanical side, two relevant principles would be torque and leverage.

I'm afraid the OP asked only for "the" principle. You will have points taken off for supplying two principles.

I think we'd need to specify "movement" as being the principal. One could argue for "force" also: in the real world, it is difficult to achieve movement without force... and there are some applications in which force is the main concern (clamping, for example).

Although I am sure that there is something in our answers that will help the OP, I wonder if he was thinking in more fundamental terms. Perhaps desire is the fundamental principle of robotics. For a robot to do something, someone must desire that the robot do something.

I also wonder if the OP is a fan of robotic ethics:

1. A robot may not injure a human being or, through inaction, allow a human being to come to harm.