The application research of mobile robots and wireless sensor networks in auto guide vehicle

The application research of mobile robots and wireless sensor network in laserautomatic guided vehiclesGuoqing ZhangNational University of Defense Technology, Changsha,Hunan, China 410073Keanuzhang@Zhongming PanNational University of Defense Technology, Changsha,Hunan, China 410073Chunmingpan@Abstract—Wireless communication is applied frequently in laser automatic guided vehicle (AGV) communication system, but at present the system usually adopts the wireless static central control mode in which vehicles of AGV system are controlled and managed by a center control unit and cannot communicate with each other. An AGV wireless control system based on wireless sensor network and mobile robots control is proposed to improve the intelligence and efficiency of AGV system, with this communication system all the vehicles are connected by wireless sensor network to implement inter-vehicle communication and distributed control, this enhances the degree of autonomy and flexibility of vehicles, and improves the efficiency of AGV system.Key Word: Wireless sensor network, Mobile Robot, Automatic guided vehiclesI.INTRODUCTIONWith the development of the logistic technology, laser automatic guided vehicles (AGVs) are applied widely in many fields for its features of auto guidance, accurate positioning, optimal route and safety. In modern automatic warehouses, each laser AGV runs as a mobile robot that receives the destination address and instructions from the control center, and walks along the specific route to the destined spot to finish the assigned operations. Laser AGV are usually work under the scheduling of the upper computer system, and integrated into the complex production monitoring and management with automatic logistic software.Wireless communication technology is applied in the AGV system in the result of the mobility of AGVs. However, the most present used wireless communication systems run in a center control mode, in other words, all AGV communicate only with the upper control center, not with each other. All the data are stored and exchanged through the center computer system. This sets an even higher demand of the center control system, and limits the individual autonomy, scale and flexibility of AGVs.This paper proposes the application research of mobile robots and wireless sensor network in laser automatic guided vehicles. Wireless sensor network is the wireless communication network characterized by large scale, self-organization, and dynamic topology. Each node in the network has the ability of data processing, storing and communication. In the AGV system that establishes on wireless sensor network, each vehicle work as a node which can communicate through self-organized network, and process the data more efficiently. In turn, the intelligence of AGVs as mobile robots is enhanced.II.FUNCTIONS OF AGV NODEEach vehicle of logistic AGV system within WSN works in a wide range like a mobile and autonomous robot performing many functions. These functions are classified as below:1)Vehicle Locating Function. In some automatic indoor warehouses, the navigation mode of AGVs is free route mode, thus the routes of vehicles are not fixed, and they can be adjusted according to the real situation. This mode makes a demand that the position of each vehicle should be acquired instantaneously. At present the applied positioning method is laser reflection method. A laser emitter and placed on the top of vehicles, and Cubic Corner Reflection (CCR) route signs are distributed in the area where vehicles run. The laser emitter keeps rotating by the driving of a motor while the vehicle is moving, in each circle the photo counting circuit collects the angular information of route signs through the reflection. The software that consists of the matching, locating and optimal algorithms can determine the actual position of each vehicle. This method has the advantage of high accuracy, good flexibility and reliability.2)Vehicle Control Function. This function includes vehicle driving, speed control, break control and safety control etc, and is mostly performed by the control of motors in vehicles. When a vehicle gets an instruction, a calculated route is established, and then the vehicle should be driven properly with optimal speed to the destined spot. Additionally, the safety control such as collision avoidance and buffering is necessary, so infrared or ultrasonic detector should be equipped in vehicle nodes.3)Vehicle Communication Function. As shown in Fig. 1, the vehicles in AGV system communicate in the wireless sensor network. Each vehicle is equivalent to a node in the network, and the vehicle nodes can establish the self-organization network. The information such as the position, speed and destination is transmitted among the vehicles.4)Vehicle Sensing Function. Each vehicle collects the data from the sensors such as load cell, temperature meter or ultra-sonic sensor etc. These data are used to aid the warehouse monitoring or AGV system control. The vehicles in the system work as mobile sensors to monitor the circumstance of working area.Figure 1. The AGV communication systemA distributed control is realized on the base of the wireless sensor network. Each vehicle node can make the decision of route selecting or task assigning by its own. Then the intelligence of vehicle node of mobile robot is improved, and the stress on the control center can be relieved significantly.III. THE FRAME OF AGV SYSTEM CONTROL In order to perform all the functions mentioned above, vehicles are taken as mobile robots in the wireless sensor network. Robots are a compelling instance of those artifacts that comprise and deliver the ambient space. Modern multi-robot applications have moved away from the historical view of monolithic control systems. They are already perceived to belong to a larger, open distributed network made up of different sensors and effectors, and they extends this view by explicitly addressing the need for interoperability between robots and wireless sensor networks(WSNs).Under the circumstance of wireless sensor network, mobile robots have some new features. First, mobile robots can sense the space distributed information of the environment through WSN, so the full sensing information can be acquired. Second, the information resources are abundant, and they are distributed in each node, hence the information could overlap and be redundant. Third, while mobile robots enter the network, they are not only transmitting data through the network, but also providing the data that they sense. Fourth, mobile robots can sense the information of their own through the wireless sensor network. Fifth, nodes in WSN are scattered in performances, dynamic in distribution and related to circumstance, so the information in WSN is of space character.Therefore, vehicles in wireless sensor network, like mobile robots, have much higher intelligence and decision-making ability. They can utilize the network to exchange and share the information distributed in the entire network while they are moving in the network. However, the dynamic uncertainty of each vehicle is increasedcorrespondingly.The AGV system control frame is shown in Fig. 2. In this frame, AGV control system is the key part that connects the route sign location unit, vehicle motion control & sense unit and wireless communication system.Figure 2. The Frame of AGV Control SystemThe Route sign location unit is in charge of location of vehicle in the warehouse by the photoelectric CCR route sign measuring system. As shown in Fig. 3, several CCR route signs are fixed in the work area of AGV , and the laser sensor is placed on the top of AGV . The coordination values of CCR route signs are known, the laser sensor scans them around to acquire the azimuth angles αi . With these data, the location unit can calculate the position of AGV in the work area.Figure 3. The AGV communication systemThe vehicle motion control & sense unit is in charge of acquisition of different sensor data and control of motor. This unit is the Sensing and executing part of vehicles, it collects the data from different sensors such as sonic sensor and infrared sensor, and provides them to the upper control system; meanwhile it receives the instructions from the upper control system, and drives the motors to control the activity of vehicles.Route Sign LocationSonic SensorVehicle Motion Control & SenseAGV Control SystemOther SensorsInfrared SensorAdj. Speed MotorAGV Wireless Communication SystemControl CenterLocal SinkWireless sensor networkCCR2CCR1CCR3CCR4CCRnAGV Initial Directionαn α3 α2α1AGV wireless communication system unit is in charge of organization of wireless sensor network, routing message and transmit data. The network formed by the AGV wireless sensor node is an Ad Hoc network, which is an infrastructureless mobile network. There are no fixed routers in the network, Instead the mobile vehicle nodes themselves function as routers which discover and maintain communication connections. The flexibility of the network increases the complexity of communication protocol. Taking routing protocol as example, there are two major categories: on-demand such as AODV and DSR, and proactive such DSDV and CBRP etc.With this frame, the ability of data processing and control of vehicle nodes is enhanced and meet the requirements of flexibility, intelligence, and compactness.IV. THE EMBEDDED DESIGN OF VEHICLE NODE The Vehicles in AGV system are endowed with intelligent behavior, such as location, avoiding collision and self-organizing network, which makes the vehicle node design more complex, so an embedded design is adopted. As shown in Fig. 3, the hardware platform is built mainly upon the Philips ARM chip LPC2214 and the Jennic wireless communication chip Jn5131x. The Former is an embedded chip that based on a 16/32 bit ARM7TDMI-S CPU with real-time emulation and trace support, together with 128/25kB of embedded high speed flash memory, and the latter is one of low power, low cost wireless microcontrollers suitable for IEEE802.15.4 or ZigBee applications, the chip integrates a 32-bit RISC processor, with a fully compliant 2.4GHz IEEE802.15.4 transceiver, 192kB of ROM, a selection of RAM sizes from 8kB to 96kB, and a rich mixture of analogue and digital peripherals.Figure 4. The Embedded System Design of Vehicle NodesThe LPC2214 is in charge of several functions. First, it reads the data from the photoelectric decoder circuit which acquires the position information of the laser-reflection locating route sign. Second, it reads the data from the different sensors or controls the motors by the A/D interface. Third, it connects the wireless sensor network circuit made of Jennic5131 through SPI. The Jennic5131integrates 802.15.4 wireless communication protocol and a 32-bit RISC processor, so it is in charge of organization of WSN and transmission and reception of message.The application of vehicle node relies on three main modules as shown in Fig.4. The multi-functions and complexity of mobile communication presents a challenge to develop the software integrating locating, sensing and communication.The laser guide locating module acquires the angel data of route signs real time and perform the route sign matching algorithm, provides the position data to LPC2214 core control module. The wireless communication module is programmed based on the Jennic5131 internal integrated protocol stack, it mainly perform the network organization and management. The data transmission between the wireless communication module and LPC2214 core control module is performed by the Intelligent Peripheral (IP) in Jennic5131 and SPI in LPC2214. The IP interface is provided for systems that are more complex, where there is a MCU that requires a specific wireless peripheral. The wireless peripheral may be controlled via one of the UARTs but the IP interface is intended to provide a high-speed, low-processor-overhead interface.Figure 5. The Software Architecture of Vehicle NodeV. CONCLUSIONSThis paper presents the research on the application of wireless sensor network and mobile robots in laser automatic guided vehicle. The intelligence and flexibilityof AGVs as mobile robots is enhanced because ofLPC2214Jennic5131Photoelectric DecoderA/D to Sense and ControlFlashMotorsUltrasonic Sensor SensorInfrared SensorSPII/OM C UIEEE 802.15.4 PHY/MAC NWK LAYER Protocol Jennie APIA T-Jennie APILPC2214 Core Control ModuleIPLaser Guide Locating ModuleSPIGPIOutilization of wireless sensor network communication technology. The vehicles in the network can organize the network autonomously and share the information such as position, speed, direction, load status etc. Compared with traditional center control way, this way can take full advantage of distributed processing ability of mobile vehicle, and alleviate the load of the center server. Due to the complexity of the mobility of vehicles, the further work is to develop more optimized network control protocol and application software.R EFERENCES[1]Jun Luo,Jean-Pierre Hubaux. A Survey of Inter-VehicleCommunication. Technical Report, Switzerland: EPFL (Ecole Polytechnique Federale de Lausanne), IC (Informatique & Communications), 2004.[2]Asher H. J., Galler B.A. Collision Warning using NeighbouringVehicle Information. Intelligent Transportation: Realizing the benefit, Houston, Texas, USA, 1996.[3]Lars P. A framework for integration of processes in autonomoussystems. 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