During the operation of a factory, the flow of materials determines the production efficiency of the factory. Recently, production lines have gradually added automation equipment, but the supply or handling of materials to and from the production line still relies on manual handling operations. This often results in unsmooth logistics and interrupted production flow. To avoid interruptions in supply, and reduce storage and production space, Automated Guided Vehicle (AGV) technology offers an unmanned management solution.
Introduction of AGV
An Automated Guided Vehicle, AGV, or Robotic Forklift is a special application for automated material handling. It is an independent handling system that provides material storage and transportation during the manufacturing process. It is suitable for handling diverse materials between various loading and unloading points. To increase production line flexibility and reduce costs, the power is usually supplied by batteries. The path is usually composed of wires buried under the floor or reflective paint on the floor surface. A sensor on the vehicle guides the vehicle along the wire or paint.
An Automated Guided Vehicle is unmanned and can travel according to a predetermined path or program, saving labor and improving efficiency. The core value of an AGV is its ability to facilitate logistics planning, production experience, control logic, and mass production implementation. After years of research and development and field verification, AGV technology has been developed to perform logistics handling operations by connecting with MRP and MES production control systems to achieve Industry 4.0 efficiency.
What Kinds of AGV Control Systems Are There?
Three main technologies used in the AGV control system (AGV system, or AGVS) are: Navigation, Layout Designing, and Guidance. AGV control systems are divided into the following three parts:
Ground Control System (Stationary System):
The AGV upper control system is the core of the AGV system. Its main function is to perform task distribution, vehicle management, traffic management, communication management, etc. for multiple AGV machines in the AGV system. Task management, like the process management of a computer operating system, provides an explanation and execution environment for the AGV ground control program, scheduling operations according to task priority and start time, and various operations on tasks such as start, stop, cancel, etc. Vehicle management is the core module of AGV management. According to the request of material handling tasks, it allocates and dispatches the AGV to perform tasks. Shortest AGV walking times and shortest walking path of AGV are calculated, and the walking process of the AGV is controlled and directed to timely issue loading and unloading. According to the AGV's physical size, operating status, and path conditions, traffic management is provided for AGVs to automatically avoid each other, and at the same time avoid deadlock of vehicles waiting for each other. Communication management provides communication between the AGV ground control system, the AGV stand-alone ground monitoring system, ground IO equipment, vehicle simulation system, and the host computer. The AGV uses radio communications, and a wireless network needs to be established. Each AGV communicates with the ground system, but there is no communication between the AGVs. The ground control system uses TCP/IP, polling mode and multiple AGV communications with the ground monitoring system.
An Onboard System is an AGV stand-alone control system. After receiving the instructions from the upper system, it is responsible for the navigation, guidance, route selection, vehicle driving, loading, and unloading operations of the AGV.
- Navigation: A single AGV uses its own navigation device to measure and calculate its position and heading in global coordinates.
- Guidance: The single AGV calculates the speed value and steering angle value of the next cycle according to the current position, heading, and preset theoretical trajectory. This is the command value of the AGV movement.
- Searching: According to the instructions of the upper system, the single AGV pre-selects the path to be run through calculations, and reports the results to the upper control system. Whether it can run or not is determined by the upper system according to the location of other AGVs. The path of a single AGV is determined by the actual working conditions and is composed of many segments. Each segment indicates the starting point and ending point of the segment, as well as the driving speed and steering information of the AGV in the segment.
- Driving: A single AGV controls the operation of the vehicle through the servo device according to the calculation result of the guidance and route selection information.
Navigation/Guide Mode and Features:
The Main navigation/guidance technologies that have been used to help AGV realize unmanned driving, navigation, and guidance include the following:
- Cartesian Guidance: Uses a positioning block to divide the AGV's driving area into several small coordinate areas. The guidance is realized by counting the small areas. Generally the AGV follows photoelectric or electromagnetic tapes. The advantage is that the path can be modified, the guidance is reliable, and the environment has no special requirements. The disadvantage is that the ground measurement installation is complex, the workload is large, the guidance accuracy and positioning accuracy are low, and it cannot meet the requirements of complex paths.
- Wire Guidance: A metal wire is embedded into the driving path of the AGV, and the guiding frequency of the metal wire controls the guidance of the AGV. Its main advantage is that the lead is concealed so is not easy to be contaminated or damaged. The guiding principle is simple and reliable, is easy to control and communicate, has no interference from sound and light, and has low manufacturing cost. The disadvantage is that the path is difficult to change and extend, and it has great limitations on complex paths.
- Magnetic Tape Guidance: Like electromagnetic guidance, it uses tape on the path instead of burying metal wires under the ground. Guidance is achieved through magnetic induction signals. Flexibility is better, and it is easier to change or expand the path. Tape laying is simple and easy, but this guidance mode is susceptible to interference from metal materials around the loop, and the tape is susceptible to mechanical damage, so the reliability of the guidance is greatly affected by the outside world.
- Optical Guidance: Paint or stick ribbons on the driving path of the AGV realize the guidance by simply processing the ribbon image signals taken by a camera on the vehicle. Flexibility is better, and the ground route setting is simple and easy, but the system is very sensitive to contamination and mechanical abrasion. Environment requirements are high, guidance reliability is poor, and accuracy is low.
- Laser Navigation: This system uses a precise laser reflector around the driving path of the AGV. The AGV emits a laser beam through a laser scanner and collects the laser beam reflected by the reflector to determine its current position and heading. The trigonometric calculations are used to realize the guidance of the AGV. The biggest advantage of this technology is the precise positioning of the AGV; the ground does not require other positioning facilities, the driving path can be flexible and changeable, and it is suitable for a variety of on-site environments. It is the advanced guidance mode preferred by many foreign AGV manufacturers, but the disadvantage is the high cost of manufacturing. Due to its limitations in relatively harsh environments, it is not suitable for outdoor use.
- Inertial Navigation: By installing a gyroscope on the AGV, and installing a positioning block on the ground in the driving area, the AGV can determine its own position by calculating the deviation signal (angular rate) from the gyroscope and data signals from the ground positioning block. This technology is widely used in the military field, and its main advantages are it has advanced technology compared with wired guidance, and it has a lower ground processing workload, and strong path flexibility. The disadvantage is that the manufacturing cost is high, and the accuracy and reliability of the guidance are closely related to the manufacturing accuracy of the gyroscope and its subsequent signal processing.
- Visual Navigation: Image recognition of the environment in the AGV driving area to realize intelligent driving is a kind of guidance technology with great potential. At present, there is no practical AGV that adopts this kind of technology. But it it predicted that the combination of image recognition technology and laser guidance technology will make AGV accurate and reliability for guidance, driving safety, and intelligent memory recognition.
- Global Position System (GPS) Navigation: GPS uses satellites to track and guide controlled objects in a non-fixed road system. This technology is still developing. It is usually used for outdoor long-distance tracking and guidance. Its accuracy and ability to control depends on the fixed accuracy and number of satellites in the system and other factors such as the surrounding environment of the subject. The result is iGPS and dGPS, which have much higher accuracy than civilian GPS, but the manufacturing cost of ground facilities cannot be afforded by ordinary users.