HAL, which weighs 23kg, is comprised of robotic 'limbs', and a backpack containing the suit's battery and computer system. It is strapped to the body and controlled by thought. When a person attempts to move, nerve signals are sent from the brain to the muscles, and very weak traces of these signals can be detected on the surface of the skin. The HAL suit identifies these signals using a sensor attached the skin of the wearer, and a signal is sent to the suit's power unit telling the suit to move in unison with the wearer's own limbs.