# A Master-Slave Separate Parallel Intelligent Mobile Robot Used for Autonomous Pallet Transportation
## 1 Introduction  
Logistics is an important part of the manufacturing enterprise [1; 2], and the pallet is one of the logistics carriers in the factory [3]. The automation of pallet transportation is an important factor which affects the smart factory logistics [4]. Pallet handling equipment mainly refers to manually operated hydraulic trucks and electric or fuel forklifts, both of which require a large working space for operation. When the goods are delivered, it is necessary for the operator to handle the loading and unloading of the goods. A large amount of labor is required, which does not meet the requirements of factory automation. In recent years, with the development of factory intelligence, flexible manufacturing systems and automated warehousing, the autonomous navigation forklift or forklift AGV (Automated Guided Vehicle) has gradually become an important device for solving the internal logistics of pallet transportation in the factory [5; 6]. Compared with the traditional forklift, the autonomous navigation forklift AGV is improved with intelligent components such as external sensors, navigation and positioning modules, software scheduling, and power management system. Forklift AGV can transport the pallets placed on the ground to meet the unmanned logistics requirements [7; 8; 9]. As early as 2015, the forklift company, for example, Dematic or Jungheinrich, launched a forklift AGV based on the traditional forklift. For AGV products based on forklift, a single steering wheel or a differential fixed double steering wheel with a front integrated steering motor was generally used as the drivingwheel, and the two-forked casters with two auxiliary wheels. Forklift AGV could move freely by controlling a single steering wheel or a differential double steering wheel. Forklift AGV used the traditional forklift structure and had the function of stacking high pallets. However, due to its own structural characteristics, the overall size and its self-weight are both large. And the arrangement of the driving wheels and the attached wheels determined that this type of AGV cannot achieve zero-radius rotation. It requires a rather large working space [10]. In order to adapt to small working space of factory logistics, another type of pallet transportation robot was designed and used. The pallet transportation robot moved underneath the pallet rack to lift and delivered the pallet to a destination, which was placed on a pallet rack with a certain height in advance. In 2007, Professor [PERSON] of the University of Stuttgart, Germany, proposed and designed the double-pronged pallet handling robot, the Doppelkufen system [11], to minimize the required transportation space. The Doppelkufen system used a visual recognition ribbon to guide between different production lines. In order to realize autonomous navigation, the two separate double forks connected with each other physically, and peripheral sensors were assembled to develop a compact forklift robot capable of zero-radius rotation and omnidirectional motion, for example, Agumos G130 [12] of Melkus Mechatronic GmbH and Nipper [13] of Dutch F3 Design. Combining with the advantages of both the traditional forklift AGV and the compact forklift AGV, INTEREST Services GmbH of Germany introduced the Agilox product [14]. Agilox could perform palletizing automatically. It had a heavy self-weight and high height and cannot move underneath the pallet directly like a forklift. However the pallet needed to be placed on a custom-height pallet rack in advance. In order to solve the problems, this paper mainly introduces a master-slave separate parallel intelligent mobile robot for autonomous pallet transportation in the smart factory logistics. The separate parallel intelligent mobile robot consists of two independent sub robots, which is similar to two forks of the forklift, but these sub robots do not have any physical connection with each other to achieve the pallet transportation. It can move underneath the narrow free entry of the pallet and has a load capacity of up to 1 ton.