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Introduction
This article gives comprehensive information about automated guided vehicles (AGVs).
Read further to learn:
What are Automated Guided Vehicles?
Types and Applications for AGVs
Overview of an AGV Navigation System
How AGV Locomotion Works
And much more…
Chapter 1: What are Automated Guided Vehicles?
Automated guided vehicles (AGVs), or mobile robots, are types of guided robotic systems with a wide range of motion and accessibility. They are self-contained and can move along a line, surface, or space. AGVs differ from immobile robotic arms attached to a base with links and joints. However, the function of robotic arms is sometimes enhanced when they are placed on an AGV.
Combining AGVs and robotic arms creates a mobile platform that performs versatile functions such as remote handling, telemanipulation, scanning, and probing. AGVs are used in various applications, including manufacturing, warehousing, inspection, exploration, transportation, and military.
AGV systems are a branch of automation with complex controls and sophisticated guidance systems that allow them to travel long distances and complete multiple tasks. They have various navigation systems, including perception, localization, path planning, and motion control, that can be controlled by an onboard computer, central computer system, or dispatcher.
AGV paths are carefully selected to remove any potential hindrances, blockages, or obstacles that may interfere with the system's smooth operation. Additionally, AGV systems must have a smooth, even surface upon which to operate since they are not designed to move over holes, bumps, or cracks.
During the pandemic of 2020, AGVs became popular due to the social distancing rules and the growth of e-commerce. The increase in demand led to the development of more reliable and efficient AGV systems. The factors of the pandemic also impacted factories that were required to have social distancing. As with e-commerce, factory owners turned to AGVs to move products, tools, equipment, and resources.
With the rapid growth of AGVs has come the growth of wireless connectivity, which is the foundation upon which AGVs operate. AGVs are constantly on the move and depend on a reliable connectivity platform for seamless and efficient operation.
Chapter 2: Types and Industrial Applications of AGVs
AGVs are generally used in logistics. Other functions such as exploration, inspection, and service robotics comprise only a fraction of the industry. Thus, automated guided vehicles are categorized according to their load and method of transport.
Forklift AGV
Forklift automated guided vehicles are simply automated guided vehicle navigation systems integrated into a forklift. They are suitable for floor-level pallet pick-up and can stack pallets at various heights. Forklift AGVs are widely used in automatic storage and sorting systems, particularly automatic warehouse racking. The navigation system can be overridden to allow manual control.
Underride AGV
Underride automated guided vehicles, or automated guided carts (AGCs), are a type of AGV that lifts a load by driving underneath a basket or cart and lifting it slightly. Underride AGVs can orient and drop the load at the destination without intervention. They are mostly used in hospitals for delivering food, linens, and medical supplies.
Towing AGV
Towing or tugger automated guided vehicles pull undriven carriers or trailers. Since their load-carrying does not involve lifting, they can handle multiple loads, in contrast with forklifts and underride AGVs. However, they are solely for transport and cannot position the loads to their location.
Unit Load AGV
Unit load automated guided vehicles are designed to transport unitized or palletized goods. This type does not lift the load off from the floor, which requires other lifting equipment such as conveyors, cranes, or forklifts for loading and unloading.
Assembly AGV
Assembly automated guided vehicles, or tunneling AGVs, are used for transporting goods to an assembly process. Since assembly processes are controlled environments, assembly AGV navigation is much simpler than other types with lower driving speeds. They are highly maneuverable, allowing them to fit and orient into assembly stations.
Tunneling AGVs pull a companion frame to a drop-off location. When it arrives at the location, it may pass over an RFID puck that triggers the AGV’s next stop. Assembly AGVs are more efficient and cost-effective than forklift AGVs because they only require a companion frame to deliver assembly parts or components.
Heavy Load AGV
Heavy load automated guided vehicles are widely used in paper and steel mills, where rolls of finished products are transported for storage or distribution. They have more robust construction than the other types and are equipped with more safety equipment.
Mini AGV (Small Load Carrier)
Mini automated guided vehicles, also known as small load carriers, are designed to transport small parts or objects and usually work in swarms or fleets. Their locomotion system is usually composed of a three-wheel drive system for stability and minimal turning radius, and they can move at high speeds with great flexibility. They are commonly used in high-selectivity racking systems.
AGV Scissor Lift
AGV scissor lifts are an ergonomic method for positioning large products during assembly. As a product arrives at a workstation, it is raised or lowered such that a worker does not have to reach or bend to add their portion of the assembly process. AGV scissor lifts can lift one or two tons as high as 50” for convenient and easy access.
When an assembly process is scattered through various stations in a facility, an AGV scissor lift can be programmed to travel between stations regardless of their position. Additionally, as with all AGVs, scissor lifts can be programmed to travel between rooms or deliver completed assemblies for storage or shipping. Such programming improves efficiency and reduces assembly time. The movement and lifting capacity of AGV scissor lifts saves workers and protects them from injuries.
Truck Loading AGV
Truck loading AGVs are a unique type of AGV that do not require guidance systems to unload or load a truck. Referred to as automated trailer loading (ATL) AGVs, they can load and unload trucks without any modifications to the truck's trailer or the dock. Truck loading and unloading AGVs can load pallets or unit loads in any load pattern, including mixed orientations. They can drive over dock plates and other uneven surfaces, carrying loads of up to four pallets.
Lasers and natural targeting navigation systems allow the AGV to see the interior of the trailer without the need for modifications to the dock, trailer, or dock area.
Cobot
Cobot is an inclusive term for a collaborative robot that can safely work with humans. They have sensitivity sensors that control their motions. If they sense an interruption in their motion, they stop and go into safety mode, unlike normal robots that will continue to operate and possibly injure people.
The combination of AGVs and cobots can perform a wide array of tasks. With the lifting strength of a collaborative robot and the mobility of an AGV, these specialized machines are capable of decreasing the strain of repetitive movements and alleviating worker stress.
The advantages of AGVs and cobots include:
Reduced Downtime – In the past, cobots needed to be shut down to be repositioned. When combined with an AGV, they can easily be moved.
Fast and Easy Programming – Instead of programming the AGV and cobot separately, they can be programmed together.
Smaller Footprint – Cobots are valued for their small footprint. This advantage is enhanced by combining them with an AGV.
Healthier and Safer Working Conditions – AGV cobots have a wide assortment of cameras and sensors, making it possible for them to work safely with people.
Quality Work – Since AGV cobots consistently work at the same speed and force, their repetitive work is always of the highest quality.
Flexible Applications – AGV cobots can have different tools attached, such as robot arms, pick-up shelves, racks, pallet lifts, and conveyors.
Improved Productivity – As in all robotic devices, adding an AGV cobot dramatically improves efficiency and productivity.
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Chapter 3: The AGV Navigation System
Navigation is the ability of the guided vehicle or mobile robot to determine its location and autonomously know where it should proceed while avoiding collisions and unsafe conditions. Navigation can be divided into four fundamental components: perception, localization, path planning, and motion control.
Perception of Surroundings
The concept of acquiring data for mobile robot navigation is broader and more complex than it is for robotic arms. The perception of mobile robots, similar to robotic arms, is achieved using sensors. Its higher sophistication arises from its ability to measure and relate its position globally or over a wide range.
Sensor Classification
Sensors are classified according to two functional axes: proprioceptive/exteroceptive sensors and passive/active sensors. The two sets of sensors are classified by what they do and observe; proprioceptive/exteroceptive sensors monitor the AGV, while passive/active sensors send energy into the work environment.
Proprioceptive sensors measure internal values like battery levels, wheel position, motor speed, load, temperature, and current. They can be encoders, potentiometers, gyroscopes, and compasses.
Exteroceptive sensors observe aspects of the environment, such as distances, electromagnetic wave intensity, and acoustic amplitude. Examples of exteroceptive sensors are sonar, IR sensitive, and ultrasonic distance sensors.
Passive sensors use temperature probes, microphones, and cameras to examine the environment and absorb energy such as loads, electromagnetic waves, or vibrations.
Active sensors send energy into the environment and measure the reaction. An example of an active sensor is a sonar that transmits an acoustic wave, the echo of which is analyzed and measured.
Types of Sensors
Tactile Sensors
These include contact switches and proximity sensors. They can measure mechanically, through physical contact (limit switches), or through other physical phenomena such as magnetism (reed and hall effect switches) and electric induction (inductive switches).
Heading Sensors
These include compasses and gyroscopes. Heading sensors are used to determine the orientation of the robot relative to a fixed, external reference point or frame.
Wheel and Motor Sensors
Wheel and motor sensors measure a motor or wheel’s angular position, speed, and acceleration. An example is an encoder in a servo motor where the feedback signal is used to control the motor drive.
Motion and Speed Sensors
These sensors measure the robot's speed relative to a fixed or moving object. These sensors are exteroceptive, in contrast with proprioceptive wheel and motor sensors.
Acceleration Sensors
These are sensors used to determine the robot’s acceleration. Most of the time, acceleration is a less important value. Position can be indirectly determined from acceleration, initial position, and orientation through dead reckoning. A combination of acceleration and heading sensors is commonly referred to as the Inertial Measurement Unit (IMU).
Beacon-Based Sensors
These sensors use a known fixed reference point or frame to determine a robot’s position and orientation. An example is the global navigation satellite system (GNSS), where robots use an electronic receiver that receives orbital data and compares it with the time-of-flight measured by three or more satellites to calculate its position and orientation.
Active Ranging Sensors
Active ranging sensors are sensors capable of transmitting and receiving signals. A signal is radiated towards an object or reference point that reflects part of the signal, which is then measured and analyzed using concepts of reflectivity, time-of-flight, and triangulation. Examples of active ranging sensors are lidar, radar, and sonar.
Visual Sensors
Vision is a high-level feature that allows the robot to analyze captured images to determine their localization. They can also perform additional functions such as obstacle avoidance and object recognition.
Localization and Orientation
After gathering information from the environment or a fixed reference frame, the robot evaluates the data to determine its position and orientation relative to the environment through a process known as localization. A robot’s position and orientation can be determined through odometry (dead reckoning) or triangulation from fixed reference frames. In most cases, this is not enough, especially when high accuracy is required.
The environment often has unknown obstacles and restrictions that are constantly changing. In addition, the sensors and effectors have issues regarding accuracy and precision. To achieve full autonomy and proceed with the succeeding steps of navigation, mapping creates a model of the environment that the robot can use to determine its location, orientation, and goal. Mapping helps it update its information in real time, a process known as Simultaneous Localization and Mapping (SLAM).
Path Planning
Path planning is the process of determining the sequence of actions needed to be performed by the robot to reach its destination. This is the robot’s cognition process, where it analyzes the map of the environment and creates output in the form of a program or instructions. If some attributes in the map change, the robot must be able to measure these changes and adjust its actions accordingly. Moreover, the robot not only determines how to get to its target location but also optimizes its path by reducing the path length while avoiding obstacles.
In path planning, four different concepts need to be described. These are the robot geometry, the robot effectors’ degrees of freedom, the map of the environment, and the initial and target configurations. To solve the robot’s path planning, these four concepts must be translated into what is known as the configuration space. In the configuration space, the possible configurations of the robot and the space occupied by the obstacles are represented. The robot in the configuration space can be represented as a point defined by coordinate vectors instead of a rigid body. By reducing the robot to a point, the obstacles are somehow inflated by the size of the robot to compensate. Knowing the possible configurations of all objects on the map, a robot’s trajectory can be determined, corresponding to a continuous curve or path.
Motion Control
Motion control is the ability of the robot to execute its planned or programmed sequence of actions by feeding input signals to its drivers, actuators, and effectors. For mobile robots, the control system is typically a closed loop. The most common closed-loop control used in robotics is Proportional-Integral-Derivative (PID) control, a form of feedback control. Feedback control allows the robot to correct any disturbances or errors to its trajectory by continuously measuring parameters internally and externally. Using a PID controller mathematically represents the error signal and the proportional, integral, and derivative gains. These three factors allow the controller to quickly eliminate the error while maintaining a stable signal and avoiding overshoot.
Zone Blocking
Zone blocking is controlled by a central AGV system controller where only one AGV is allowed into any given zone. It is used in sections of the guide path where there are intersections, stations, and turns. The process allows multiple vehicles to be released into high-traffic intersections and prevents interference between vehicles.
Accumulative Blocking
With accumulative blocking, AGVs are not controlled by a central system but by remote object detection sensors. It is a process used for long, straight sections of guide paths where AGVs sense the approach of a slower AGV or one that has stopped along the guide path. With accumulative blocking, AGVs tend to accumulate or collect behind each other but move up to the next intersection. The process is faster and more efficient than zone blocking.
Chapter 4: Types of AGV Navigation Systems
The whole navigation system is created by integrating the processes of perception, localization, path planning, and motion control. In addition, different types of navigation systems can be created with the combinations of sensors, controllers, programs, and algorithms. Enumerated below are the most commonly used navigation systems for automated guided vehicles.
Physical Guides
Physical guides include guide tracks, tapes, and wires detected either actively or passively. This navigation system uses fixed reference points or environment landmarks, which are measured and evaluated by sensors and controllers. Since the automated guided vehicle will rely on predetermined paths for navigation, the process of path planning can be preprogrammed into its system.
An example is an inductive guide track, or wire guidance system, which is made of a current-carrying conductor embedded into the ground or floor. To sectionalize the tracks, it is divided into segments that can be turned on or off. An alternating current flows through the wire, which generates electromagnetic waves that can be detected by the mounted sensors at the bottom of the automated guided vehicle. The sensor is composed of two coils. Currents are induced through these coils, which become analog signals fed to the feedback controller.
Other examples of physical guides are magnetic, metallic, and optical guide strips. These strips are mounted on the floor’s surface and detected by magnetic, inductive, or optical proximity sensors at the bottom of the AGV. Magnetic proximity sensors operate through a physical phenomenon called the Hall effect, which allows them to detect magnetic materials. Inductive proximity sensors, on the other hand, are also active sensors that utilize the principle of electromagnetic induction to detect metallic materials. Optical sensors determine the path by detecting the recognizable features (color) of the tape. Physical guides are less expensive than wire guidance systems and can easily be reconfigured. However, they are not suitable in dirty and high-traffic areas.
Anchoring Points
Anchoring points are also physical guides, but they allow free navigation. Instead of installing a predetermined path using wires and tapes, a grid of permanent magnets (magnetic bars) or transponders is set on the floor to determine the location and bearing of the automated guided vehicle. Like the magnetic strip guide, magnetic proximity sensors are also mounted on the bottom of the AGV. The path followed by the robot is based on preprogramming or path planning.
Laser Navigation
Laser navigation is a free navigation system that uses active-ranging light sensors for localization. Markers such as reflective foils or tapes are mounted on walls or objects. These markers can be easily detected by the laser sensor. A minimum of three markers are needed to enable triangulation. Since the automated guided vehicle performs localization and path planning, the trajectory is highly flexible. The optimum path can also be computed.
Global Positioning System (GPS)
GPS navigation is used outdoors where artificial markers are impractical to install. The GPS satellite acts as a beacon that sends data to the AGV, which is then used to triangulate its position. The downside of solely relying on GPS is its low accuracy, especially indoors. To maintain a reliable signal, there must be a clear line of sight between the satellite and the automated guided vehicle.
Wireless Connectivity for AGV Navigation
The rising popularity of AGV systems has increased the need for reliable connectivity systems to ensure fast and efficient operation. There are certain factors that need to be considered when designing and planning a wireless connectivity system to ensure optimum reliability.
Reliability
In any manufacturing operation, electrical inference such as ground loops and conveyor belts can interrupt the performance of an AGV. Additionally, the vibrations from the operation of an AGV can also interfere with its operation. Wireless navigation systems must be reinforced with radio frequency (RF) and power isolation to protect against electrostatic discharge damage and inrush from motor currents.
The various protective features of an AGV keep wireless connections stable in the harshest and most electrically active environments. Aside from ensuring reliable service, ambient electrical protection increases the working life of an AGV.
Continuous Operation
The roaming technology of AGVs makes them constantly search for and switch to different access points with stronger signals. The working environment for AGVs should provide optimal and smooth roaming transitions as well as seamless connectivity.
Wi-Fi coverage determines the rate at which an AGV can find an access point. This necessitates wireless devices with multiple input and multiple output (MIMO) capabilities, which removes the need to install more access points.
An operation's type of wireless communication system is essential for smoothly integrating an AGV system. The first and most important step is identifying the wireless local area network (WLAN) settings. In addition, external antennas on the AGV will allow it to have broader Wi-Fi coverage.
Regardless of all the proper precautions, there may still be interference in the environment from obstacles, such as walls, pillars, or large equipment, which can prevent finding access points and lead to collisions. Installing a request to send and clear to send (RTS/CTS) mechanism will prevent such occurrences and help avoid collisions.
Security Software
Protection of the wireless network using various security protocols is necessary since unauthorized access can shut down the system. All wireless communication devices must be secured so they can only be accessed by approved personnel. This can be ensured using management software that allows it to see the network environment and control access to all connections.
Chapter 5: AGV Locomotion
Locomotion is the ability of automated guided vehicles to propel themselves from one point to another. Wheels are a common mode of locomotion due to their simplicity and minimal friction loss. However, there are certain limitations for wheeled automated guided vehicles in environments with irregular and uneven surfaces. This is where legged robots are usually preferred.
Wheeled AGVs
The wheel is an artificial technology used due to its high efficiency and simple mechanical implementation. They can have a wide degree of freedom, high stability, and excellent maneuverability. In robotics, wheels can be classified according to their kinematics. The different wheel types used in automated guided vehicles are enumerated below.
Standard Wheel
A standard wheel offers two degrees of freedom: rotation around the wheel axle and around the contact point. The axis of rotation around the contact point passes through the center of the wheel, usually along the plane of the wheel. This design allows steering without imparting additional forces to the robot chassis, provided that the wheels are aligned with no camber and toe.
Caster Wheel
As with the standard wheel, a caster wheel offers two degrees of freedom. One is rotation around the wheel axis, while the other is around an offset from the center of the wheel. Caster wheels are generally used to provide support for the chassis. It is rarely used for maneuvering and delivering motion since steering using caster wheels exerts forces on the chassis. The main advantage of using caster wheels is their automatic alignment when moving forward after turning.
Mecanum Wheel
Sometimes referred to as Swedish wheels, a Mecanum wheel has three degrees of freedom: rotation around the wheel axis, rotation around the rollers, and rotation around the contact point. A Mecanum wheel has rollers along the circumference of the main wheel arranged at 45° angles. Another configuration of this type is an omnidirectional wheel with rollers oriented at 90° angles. To provide omnidirectional movement, three or more Mecanum wheels are installed on the chassis and rotated in clockwise and counterclockwise rotations.
Ball or Spherical Wheel
A spherical wheel offers three degrees of freedom or rotation around all three axes. True spherical wheels are mostly conceptual since implementing them into a large chassis brings issues such as opposing rotation between drivers, unpowered stability, and difficulties of power transmission with the drive system.
Different wheels can be combined and configured to produce the necessary stability and maneuverability. A statically stable robot requires a minimum of two wheels. Two-wheel stability can be achieved by lowering the robot’s center of mass below the wheel axle. For three or more wheels, stability is achieved by containing the center of mass within the polygon formed by the points of contact between the wheel and the ground. In terms of maneuverability, the usual configuration is a two-wheel drive with one or two undriven wheels for steering. A differential is used for maneuvering with fixed standard wheels or wheels with only one degree of freedom. Four-wheel drives also exist, which use two pairs of driven and steered standard wheels or four individually driven and steered Mecanum wheels.
Legged AGVs
A legged AGV is another terrestrial automated guided vehicle developed to take advantage of its high maneuverability in irregular terrain. It can cross gaps or holes as long as the reach of its legs exceeds the width of the gap. However, this type is less often used in industrial applications due to limitations in load-carrying capacity and the mechanical complexity of the leg assembly. A leg is composed of several links and joints that typically require independent drivers. Multiplying this with the number of legs significantly increases the robot’s weight, decreasing its load-carrying capacity. Moreover, additional actuators require more power and control. Effort in developing legged AGVs is mostly consumed by designing the kinetics and controls of the legs instead of focusing on other aspects such as navigation and localization.
Aerial AGVs
Aerial AGVs use different principles of flight. To summarize, aerial automated guided vehicles can be divided into two: Lighter Than Air (LTA) and Heavier Than Air (HTA). LTAs are balloons and blimps. Balloons are limited to elevation control, while blimps have propellers to move laterally. HTA, on the other hand, are gliders, planes, and rotorcrafts. Gliders and planes rely on wings and airfoils and their dynamic reaction with air. Rotorcrafts generate lift and lateral movement using rotary blades or propellers. Among the aerial AGVs, rotorcraft is the most promising for practical use. They can be made very light, compact, and easy to control. Moreover, they can take off and land vertically. Unmanned aerial vehicles (UAV) or drones in the form of rotorcraft are now available for consumer use in applications such as photography, inspection, navigation, and agriculture.
Submersible/Aquatic AGVs
The principles used in this type are similar to aerial AGVs. Submersible automated guided vehicles, also known as autonomous underwater vehicles (AUV), can be likened to blimps that float using buoyant force and propel laterally using rotor blades. They are used in scientific and industrial applications such as seafloor mapping, environmental monitoring, and pipeline and cable inspections.
Roll Handling AGVs
Roll handling AGVs carry large rolls of paper, plastic, or steel coils. They are designed to retrieve and deposit rolls to specified locations. Their forks point towards the rear of the vehicle and go under the roll to raise it two to four inches from the floor. Once the roll is raised, the vehicle proceeds to its assigned destination. Loads for roll handling vehicles typically weigh up to four tons with diameters from 60 to 110 inches (152.4 to 279.4 cm).
Conclusion
Automated guided vehicles (AGV), or mobile robots, are types of guided robotic systems that are not bounded by a fixed range of motion. They are generally used in logistics.
Navigation is the ability of the guided vehicle or mobile robot to determine its location and autonomously know where proceed while avoiding collisions and unsafe conditions.
The whole navigation system is created by integrating the processes of perception, localization, path planning, and motion control. Common navigation systems are physical guides, anchoring points, laser guides, and GPS.
Locomotion is the ability of AGVs to propel themselves from one point to another. The most common mode of locomotion is through rolling components or wheels, which are widely used due to their simplicity and minimal friction loss.
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