what is the meaning for the orientation of pr2 arm in cartesion controller?

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I try to use the keyboard control the pr2 simulator, but Ĩ can not get a clear idea about the mean setting of orientation.

cmd.pose.orientation.x=-0.00244781865415;
cmd.pose.orientation.y=-0.548220284495;
cmd.pose.orientation.z=0.00145617884538;
cmd.pose.orientation.w=0.836329126239;

That is in the demo source code. How can I set this value to make the gripper is vertical face the table so as to grasp the object on the table.