The Automated Guided Vehicle Engineering Essay

CHAPTER 2

An automated guided vehicle system (AGVS) is a material handling system that uses an independently operated, self-propelled vehicle that is guided along defined rails in the floor. The automobile is powered by means of an AC Servo motor. The definition of pathways is generally achieved by using wires embedded in the ground, painted stripes or by using rails to steer the vehicle.

types of agv

There will vary types of AGV's each which works predicated on a particular principle. The types can be classified as follows

Driverless trains

This type includes a towing vehicle (which is an AGV) that pulls a number of trailers to form a train. It had been the first type of AGVS to be introduced and continues to be popular. It is used in applications where heavy payloads must be moved in large distances in warehouses or factories with intermediate pickup and drop-off point along the route.

Fig 2. 1 Driverless Trains

AGV pallet trucks:

Automated guided pallet trucks are used for palletized loads along determined routes. On this typical application the automobile is backed into the loaded pallets by human workers. The driver drives the pallet truck to the guide path, programs its destination and the automobile proceeds automatically to the estination for unloading. The capability of the AGVS pallet truck ranges up to6000 lb, some trucks are capable of handling two pallets rather than one. A recently available introduction related to pallet trucks

AGV is the Forklift AGV. This vehicle can perform significant vertical movement of its forks to attain loads on shelves.

Fig 2. 2 AGV Pallet trucks

AGVS Unit Load Carrier:

This kind of AGVS is utilized to go unit loads from one station to another station. They are often equipped for automatic loading and unloading through powered rollers, moving belts, mechanized lift platforms, or other devices. Variations of the machine load carriers include light load AGV's. The light load AGV is a relatively small vehicle with a corresponding light load capacity. It can no require the same large aisle width as the traditional AGV. Light-load guided vehicles are made to carry a partially completed subassembly through a sequence of assembly workstations to build the product.

AGVS technology is definately not mature, and the industry is continually working to develop new systems in response to new application requirements. A good example of a fresh and evolving AGVS design involves the placement of a robotic manipulator on an automated guided vehicle to provide a mobile robot for performing complex handling tasks at various locations of the plant. These robot vehicles are seen to be useful in clean rooms in the semi conductor industry.

Fig 2. 3 AGV Pallet trucks

applications of agv:

Automated guided vehicle systems are being used in a growing number and in a variety of applications. The application trend is parallel to the vehicle types described above. We group the applications in to the following five categories

Driverless train operations:

These applications involve the movement of large quantities of material over relatively large distances. For example, the moves are within a big warehouse or factory building, or between buildings in a huge storage depot. For the movement of trains consisting of 5 to 10 trailers, this becomes a competent handling method.

Storage/distribution systems:

Unit load carriers and pallet trucks are usually used in these applications. These storage and distribution procedures involve the movement of materials in unit loads from or even to other specific locations. The application always involves the interface of the AGV with various other automated handling or storage system, such as automated storage and retrieval system (AS/RS) in a distribution center. The AGVS gives the incoming loads or unit loads from the getting dock to the AS/RS; the AS/RS retrieves the pallet loads or items from the storage and transfers them to vehicles for delivery to the shipping dock. When the rates for the incoming loads and outgoing loads are in balance, this mode of operation permits loads to be carried in both directions by the AGVS vehicle, thereby increasing the handling the machine efficiency.

This kind of storage/distribution operation can also be applied in light manufacturing and assembly procedures in which the work-in-progress is stored in a central storage space and distribution to individual workstations for assembly or processing. Electronics assembly is an example of these types of applications. Components are "knitted" at the storage section and are delivered in tote pans or trays by the guided vehicle to the assembly workstations in the plant. Light load AGV systems are being used in these applications.

Assembly line operations:

AGV systems are being found in a growing number of assembly line applications, based on the trend that started out in Europe. In these applications the production rate is relatively low and there are a variety of different models made on the production line. Between your workstations, components are knitted and placed on the automobile for the assembly procedures that are to be performed on the partially completed product at another station. The workstations are usually to be arranged in parallel configuration ton add to the versatility of th1e line. Unit load carriers and light load vehicles are the types of AGVS used in these assembly lines.

Flexible manufacturing systems:

Another growing application of AGVS technology is Flexible Manufacturing Systems (FMS). With this application, the guided vehicles are the material handling systems in FMS. The vehicles deliver work from the staging area to the average person workstation, the automobile also moves work between stations in the manufacturing system. At a workstation, the work is transferred form the automobile platform into the workshop of the station for processing. On the completion of processing by that station a car comes to grab the task and transport it to another area. AGVS systems give a versatile material handling system to complement the versatility of the FMS operation.

Miscellaneous applications:

Other applications of Automated Guided Vehicle system includes non-manufacturing and non-warehousing application, such as mail delivery in the office buildings and hospitals material handling operations. Hospital guided vehicle transport, meal trays, linen, medical and laboratory supplies and other materials between various departments in the building. These applications typically require movement of the vehicle between different floors of a healthcare facility. AGV systems are capable to summon and use elevators for this function.

VEHICLE GUIDANCE AND ROUTING

There are several functions that must definitely be performed to use any automated guided vehicle system successfully. These functions are

Vehicle guidance and routing

Traffic control and safety

System management

The term guidance system identifies the methods where the AGVS pathways are defined and the vehicle control system that follows the pathways, as indicated above, there are two principle methods currently used to define the pathways along the ground: Embedded guide wires and paint stripes. Of both types, the guide wire system is more common in warehouse and factory applications.

In the guide wire method, the wires are usually embedded in a small channel cut into the surface of the ground. Following the guide wires are installed, the channel slot is filled so as to eliminate the discontinuity in the wire. The signal is of relatively low voltage, low current so that as the frequency is at the number of 1-15 KHz. These signal levels creates a magnetic field along the pathway that is accompanied by sensors on-board each vehicle. Two sensors are attached to the vehicle on either side of the guide wire. When the automobile is moving over the course such that the guide wire is directly between your two coils, the intensity of the magnetic field measured by each coil will be equal. If the automobile strays to 1 side or the other, or if the guide wire path curves, the magnetic field intensity at the two sensors will me different. This difference can be used to control the steering of the motor, which make the mandatory changes in the automobile direction to equalize the two sensor signals, thereby tracking the defined pathway.

When paint stripes are being used to define the vehicle pathways, the vehicle possesses an optical sensor system that is capable of tracking the paint. The paint can be taped sprayed, or painted on the floor. One system uses a 1-in-wide paint strip containing fluorescent particles that reflect an ultraviolet (UV) light source on the automobile. An up to speed sensor detects the reflected light in the strip containing fluorescent particles that reflects an UV source of light on the automobile. An onboard sensor detects the reflected light in the strip and controls the steering mechanism to follow it. The paint guidance system id useful in environments where electrical noise would render the guide wire system unreliable or when the installation of guide wires in the ground surface wouldn't normally be appropriate. One problem with the paint-strip guidance system is that the paint strip must be maintained.

A safety feature used in the operation of most guidance systems is automatic stopping of the vehicle in the event that is accidentally strays lots of inches on the guide path. The automated stopping feature prevents the vehicle from moving off of the guide path. It is capable of locking to the guide wires or paint strip. If moved within the same few inches from it. The distance is referred to as vehicle acquisition distance.

The use of micro process or controls up to speed the vehicles has resulted in the development of feature called dead-reckoning. This term identifies the ability of the automobile to travel along the way that will not follow the defined pathway in the floor. The microprocessor computes the number of wheel rotations and the procedure of the steering motor necessary to cross a steel plate in the factory floor, or to depart from the guide path for positioning at a load/unload station. In the completion of a dead-reckoning maneuver, the automobile is programmed to return to within the acquisition distance of the guide path to resume normal guidance control.

Routing can be an AGVS system which can be involved with the condition of selecting among alternative pathways open to the automobile in its happen to be a precise destination point in the system. An average guided vehicle layout, the one which exploits the capacities of modern AGVS technology, contains features such as multiple loops, branches, side-tracks and spurs, as well as the required pickup and drop-off stations. Vehicles in the system must decide which way to take to reach a defined destination point.

When a vehicle approaches a branching point when a guide path splits into 2 directions, a choice must be produced as to which path the vehicle should take. This is sometimes referred to as a conclusion point for the automobile. A couple of two methods used in commercial AGV systems allowing the vehicle to decide which path to take

Frequency select method

Path switch select method

In the frequency select method, the guide wires leading into the two separate paths have different frequencies. As the vehicle enters the decision point, it reads an identification code on to the floor to identify its location. Based on its programmed destination, the vehicle selects one of the guide paths by deciding which frequency to track this method takes a separate frequency generator for each and every frequency that is used in the guide path construct. this results in that several generators are needed in the system. Additional channels must often be cut in to the floor with the frequency select solution to provide for by pass channel where only the key channel must be powered for vehicle tracking.

The path switch selects by using a single frequency through the guide path layout. To be able to control of a vehicle at a choice point, the energy is powered down in any way branches except the main one on which the automobile is to visit. To perform a routing by the road select switch method. The guide path layout must be split into blocks that may be independently fired up and off by means of control installed on the floor-mounted switching device connected to the control unit for the relevant block. The control unit activates the desired guide paths and turns from the alternatives branches or branch.

In the rail guide method, the AGV travels on rails. The rails are fitted with micro-switches at the respective positions wherever necessary and the automobile in guided by the rails to the respective storage section, the micro-switches positioned on the rails are used to detect the reach of position by the automobile these micro-switches are linked to the PLC and trigger the reset of the pulse train output being produced by the PLC. The micro-switches also determine the length of return travel the AGV has to go to be able to reach the foundation or the finished product collecting section. The routing and branching method adopted is similar to that of the embedded wire or paint strips guidance method as explained above.

traffic control and safety:

The reason for traffic control for the AGV is to prevent collision between your vehicles traveling along the same guide path in the layout. This purpose is usually accomplished by method of a control system called the blocking system the term "blocking" claim that a vehicle ahead of it. There are many means used in the commercial AGV system to accomplish the blocking. These are

on board vehicle sensing

zone blocking

On-board vehicle sensing involves the use of some type of sensor system to find the occurrence of vehicle and carts ahead of the same guide wire. The sensor used on commercial guided vehicle includes optical sensors and ultrasonic systems. Once the on-board sensor detects an obstacle before it, the automobile stops. Once the obstacle is removed, the automobile is avoided and the traffic is controlled. Unfortunately, the potency of forward sensing is bound by the features of the sensor system to find vehicles before its guide path. Since the sensors themselves are far better in sensing obstacles directly prior to the vehicle, these systems are most appropriate on layouts that contain long stretches of straight pathways. These are less effective at turns and convergence points where forward vehicles might not be directly before the sensor.

The idea of zone control is simple. The AGV layout is divided into separate zones, and the operating rule is the fact that no vehicle is permitted to enter a zone if that zone is already occupied by another vehicle. The length of any zone is enough to carry one vehicle plus an allowance safety and other considerations. These other considerations are the amount of vehicle in the system, the size and complexity of the layout, and the objective of minimizing the number of separate zone controls. When one vehicle occupies confirmed zone, any trailing vehicle is not allowed into that one zone. The key vehicle must proceed into the next zone prior to the trailing vehicle can occupy the given zone. By controlling the forward movement of the vehicles in the separate zones, collisions are prevented and the traffic in the overall system is controlled. The idea is its simplest form. More difficult zone control schemes separate any two vehicles with a blocked zone.

One method of implementing zone control is to use separate control units for each and every zone. These controls are mounted over the guided path are actuated by the automobile in the zone. Whenever a vehicle enters confirmed zone, it activates the block in the previous zone to block any trailing vehicle from continue and colliding with the present vehicle. As the present vehicle moves into the next zone, in place, zones are fired up and off to regulate vehicle movement by the blocking system.

In addition to avoid collisions between vehicles, a related objective is the safety of human beings who might be located along the way of the vehicle traveling in the system. There are several devices that are usually included on an automated guided vehicle to do this safety objective. One of the safety devices is an obstacle-detection sensor located at the front of every vehicle. This is often the same on-board sensor as which used in the blocking system to find not only other vehicles, but also people and obstacles in the road of the vehicle. These obstacle-detection systems are usually predicated on optical, infrared, or ultrasonic sensors. The vehicles are programmed either to avoid when an obstacle is sensed ahead of it, or to slow down. The reason behind slowing down would be that the sensed object may be located off aside of the vehicle part, or directly prior to the vehicle beyond a turn in the guide path. In either of the cases, the vehicle should be permitted to proceed at a slower speed until it has passed the thing or rounded the turn.

Another safety device included on virtually all commercial AGV vehicles can be an emergency bumper. The bumper surrounds leading of the automobile and protrudes ahead of it by the distance which may be a foot or even more. If the bumper makes connection with an object, the automobile is programmed to brake immediately. Depending on the speed of the automobile, its load, and other conditions, the braking distance will vary from several inches to many feet. Most vehicles are programmed to require manual restarting after an obstacle face with emergency bumper.

Other safety devices on the automobile include warning lights and/or warning bells. The unit alert individuals who the vehicle is present.

Finally, another safety feature that prevents runaway vehicles is the inherent operating characteristic of the guidance system: if the system strays by more than a few inches from the defined path, the automobile is programmed to avoid.

system management:

Managing the operation of your AGVS deals principally with the problem of dispatching vehicles to the points in the system where they may be needed inn a timely and efficient manner. The function is determined by reliable procedure of other system functions discussed above. There are a variety of methods generally found in combination to increase responsiveness and effectiveness of the overall system.

The dispatch methods include

On-board control panel

remote call stations

central computer control

Each guided vehicle has some type of control panel for the purpose of manual vehicle control, vehicle programming, and other functions. Most commercial vehicles have the capability to be dispatched through this control panel to confirmed station in the AGV's layout. Dispatching with an on-board control panel represents the cheapest level of sophistication on the list of possible methods. Its advantages are that it offers the AGV's with the flexibility and responsiveness to changing demands on the handling system. Its advantage is that it needs manual attention.

The use of remote call stations is another method which allows the AGVS to react to changing demand patterns in the machine. The simplest form of call station is a press button mounted close to the load/unload station. This provides a sign to any passing vehicle to avoid at the station in order to accomplish a load transfer operation. The vehicle might then be dispatched to the desired location by means of the on-board control panel.

More advanced call stations includes control panels mounted close to the various stations along the layout. This technique permits the vehicle to be stopped at confirmed station, and its own next destination to be programmed form the handy remote control panel. This represents a more automated method of the dispatching function and pays to in AGV systems that are capable of automatic loading and unloading operations.

Both of the cal station methods described here involve a human interface with the AGVS at the loading/unloading station. One of these is an automated production workstation that receives recycleables and sends completed parts through the AGVS. The workstation is interfaced with the AGVS to demand vehicles as had a need to perform the loading and unloading procedures.

In large factories and ware houses systems involving a higher degree of automation, the AGVS servicing the factory or warehouse must also be highly automated to accomplish efficient operation of the complete production- storage-handling system. Central computer control can be used to accomplish computerized dispatching of vehicles according to a preplanned schedule of pickups and deliveries in the layout and/or in response to calls from various loads/unload stations in the machine. Within the dispatching method, the central computer issues commands to the vehicles in the machine concerning their destination and procedure to perform. To perform the dispatching function, the central computer must possess real-time information about the location of every vehicle in the system such that it can make appropriate decisions concerning with vehicles to dispatch from what locations. Hence, the vehicle must continually communicate their whereabouts to the central controller.

There are difference in the manner these central computer dispatching system operate. Among the variations involves the distribution of the decision responsibilities between the central controller and the average person vehicles. At one extreme the central computer makes practically all the decisions about the routing of vehicles and other functions. The central computer plans out the routes for every vehicle and controls the operation of guide path zones and other functions. At the contrary extreme each individual vehicle possesses substantial decision-making capabilities to make its routing selection also to control its operations. The central computer is still needed to control the entire scheduling and determine which vehicles is going to the various demand points in the machine. However the vehicles themselves decide which route and control their own load transfer operations. Vehicles in this second category tend to be known as "smart" vehicles.

To accomplish the system management function, it is effective to monitor the monitor the overall functions of the AGVS through some form of graphics display. Despite having the central computer control it is still desirable for human managers to have the ability to start to see the overall system operations, to be able to monitor its general status also to spot problems. A CRT colour graphics display is often used for these purposes in modern guided vehicle systems.

Another useful tool in undertaking the systems management function is something performance report for each shift of AGVS operation. These periodic reports of system performance provide summary information about proportion uptime, downtime, and range of transactions made throughout a shift, and more descriptive data about each station and each vehicle in the machine. Hard-copy reports containing this kind of information let the system managers to compare operations from shift to shift and every month to maintain a higher level of overall system performance.

2. 7 agv construction:

The AGV is rectangular structure. The outer frame is constructed using square pipe of size 20mm, length 40cm, width 30cm and height 20cm. the base of the AGV is mounted on four PPC wheels on all four corners through and iron bush type rod which is bolted combined with the frame. A horizontal shaft of 30cm in length is inserted between the two wheels attached to the bush.

At either ends of the horizontal shaft the two PPC wheels are mounted and are bolted at the ends. The horizontal shaft rod is of 12mm in diameter and an aluminum pulley of diameter 25mm is fixed in the heart of the horizontal shaft. The other pulley is mounted on the servo motor shaft, which is also of diameter 25mm. Two 30 cm square pipe are taken welded in the bottom of the frame of the AGV, to be able to provide support to the vehicle.

Slots are made in both welded square pipes to be able to seat the servo motor perfectly. A groove shaped setup is made with a steel plate according to the dimensions of the servo motor such that it can be seated perfectly.

With the assistance of a flat belt of thickness 15mm and length 50mm the two aluminium pulleys, one attached to the shaft of the servo motor and the other to the shaft in between the wheels are coupled. The belt has minimum slippage and the power transmission from the motor to the wheels is maximum with minimum percentage of power loss.

On top of the outer frame a plywood plank is put and screwed to underneath which acts as the bottom and a 50mm overall height is maintained to support the object, so that it will not fall off while shifting the automobile.

A sensor is placed on the AGV on the plywood plank such that it can sense the objects falling into the AGV; it can help to allow the servo motor according to the logic fed in to the PLC.

AGV Design Specifications:

AGV wheel

Diameter = 100mm

Square pipe = 18 x 18mm

Length = 400mm

Width = 300mm

Height = 246mm (without wheel)

Total height = 300mm

Wheel connecting rod = 10 x 10mm

Diameter = 15mm

Coupling diameter = 50mm

Motor support rod = 20 x 20mm

Pulley support

Height = 140mm

Length = 50mm

Width = 10mm

Fig 2. 4 AGV Graphical Representation

2. 8 AGV Working:

The Automated Guided Vehicle adopts the Barcode Technology to go to the respective storage sections. An AC servo motor is utilized to drive the automobile; the servo motor has a drive that controls the motor positioning and movement by making use of a built in encoder.

When ever an object travels on the conveyor the barcode label pasted on the finished product is sensed by the barcode scanner, a sensor present in the AGV also senses the existence of the object once the object comes sliding down the conveyor. The bar code scanner produces an ASCII output. This is given as an input the PLC. The PLC in turn generates a pulse train output (PTO) based on the sort of product. This PTO output subsequently is fed to the servo drive. The servo drive drives the servo motor predicated on the pulses received.

If product A exists on the AGV the PLC generates a specific amount of pulses that will do to drive the automobile to the respective storage section. The PLC similarly generates different kinds of pulse outputs for different kinds of products so that all product can check out its particular storage section.

The AGV moves combined with the servo motor on rails to the respective station. The rails have micro switches attached to them at respective positions in front of each storage section, this helps in keeping track of the movement of the AGV. It also acts an input to the PLC to generate the pulses in the reverse direction for the AGV to come back to the home or origin position to be able to acquire other finished products dispatched by the conveyor.

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