lot to every one helped me...

you need to determine how fast you want it to move - how much weight to
lift, holding reqmts, etc - hydraulics are frequently used, but if
motion doesn't have to be too snapy a piezo motor provides high holding
torque and is light/small

Thank you for your time.
i want the motion to be smooth, and the motor must endure weight 1 KG
as a maximum.
the main problem is in the connection between the motor and the
joint.

so, try reading about them - start here
http://www.sensorsmag.com/sensors/acoustic-ultrasound/an-introduction-piezoelectric-motors-1076