The world's largest retailer will add shelf-scanning robots to 650 more U.S. stores by the end of the summer, bringing its fleet to 1,000. The six-foot-tall Bossa Nova devices, equipped with 15 cameras each, roam aisles and send alerts to store employees'  handheld devices when items are out of stock, helping to solve a vexing problem that costs retailers nearly a trillion dollars annually, according to researcher IHL Group. The new robots, designed by San Francisco-based Bossa Nova Robotics Inc., join the ranks of Walmart's increasingly automated workforce which also includes devices to scrub floors, unload trucks and gather online-grocery orders.