AGV Control Systems

OverviewNavigationSteering ControlTraffic ControlBattery TechnologyCostsSupporting TechnologyVendors

NavigationFixed-pathThe paths are continuousThe paths are fixed, but can be changedThe paths are well marked on the floorExamples: Wired, Guide tape, opticalOpen pathVehicles are offered more variation if not an infinite number of ways to navigate the open space between two points.

NavigationWiredThe wired sensor is placed on the bottom of the robot facing the ground. Then a slot is cut in the ground and a wire is placed an inch below the ground. The sensor on the AGV detects the radio frequency being transmitted from the wire and follows it.Pros/Cons: The wire is embedded into the ground and so can not be easily damaged. However, installing the wire and maintenance or replacement can be more difficult (costly).

NavigationGuide TapeTape can be magnetic or colored, and is laid out on the floor along the desired path. The AGV is fitted with a sensor to follow the path of the tape. Tape can also be used to tell the AGV to speed up, slow down, or stop.Pros/Cons: Tape does not require cutting a path into the factory floor for the entire travel route. On the other hand, it lacks the advantage of being embedded into the floor in high-traffic areas where tape can become damaged or dirty. Of course, tape can also be easily replaced or relocated if the course needs to change. Also, the tape does not need to be energized like in a wired system.

NavigationOptical Guidance NavigationThe guidepath is a 1" wide ultra-violet painted stripe on the floor. Each AGV has a sensor that contains an ultra-violet light that illuminates the path. Optical guidance is primarily used for non-industrial systems. It can be applied to a variety of different floor surfaces including: concrete, tile, wood, carpet, etc.

NavigationLaser Target NavigationThe wireless navigation is done by mounting retroreflective tape on walls, poles or machines. The AGV carrys a laser transmitter and receiver on a rotating turret. The laser is sent off and received again, and the angle and distance can be calculated and stored in the AGVs memory. The AGV has reflector map stored in memory and can correct its position based on errors between the expected and received measurements.It can then navigate to a destination target using the constantly updating positionPros/Cons: Tape is not in danger of wearing down or breaking in a high-traffic area, and the AGV path can be changed by reprogramming the AGV rather than moving tape. Many laser guided AGVs have rather sophisticated technology that make changing paths very quick and easy. Costly.

NavigationInertial (or gyroscopic) NavigationA computer control system directs and assigns tasks to the vehicles. Transponders are embedded in the floor of the work place. The AGV uses these transponders to verify that the vehicle is on course. A gyroscope is able to detect the slightest change in the direction of the vehicle and corrects it in order to keep the AGV on its path. The margin of error for the inertial is 1 inch.Pros/Cons: Inertial can operate in nearly any environment including tight aisles or extreme temperatures and has a longer lifespan than other guidance options.

NavigationCartesian NavigationLocation precision is accomplished by way of an optical or magnetic grid pattern that covers the entire floor area. Using underside sensors, when the vehicle detects the gridlines, it has a precise location reference relative to its assigned path and can correct its heading and speed. A grid based navigation system also utilizes odometer, distance and angle measurements by the vehicle. Pros/Cons: Possible travel paths can be infinite. Magnets or markings must be set but do not need to be removed (just replaced in case of wear or breaking)

Steering ControlDifferential speed controlDifferential speed control is the most common. Two sets of wheels are driven. The sets can be driven at the same speed to go straight, or one can be driven faster than the other to turn in that direction. With this method it is easy to maneuver in small spaces. This setup for the wheels is not used in towing applications because the AGV would cause the trailer to jackknife when it turned.Steered wheel controlThis type of steering is similar to a cars steering. It is more precise in following a path than the differential speed controlled method. This type of AGV has smoother turning but cannot make sharp turns in tight spots. Steered wheel control AGV can be used in all applications; unlike the differential controlled. Steered wheel control is used for towing and can also at times have an operator control it.Video: http://www.egeminusa.com/swf/Fork-Over_AGVS.html

Traffic Control Zone controlThe favorite in most environments because it is simple to install and easy to expand. Each AGV has a wireless transmitter to transmit a signal and a sensing device to receive this signal and transmit back to the transmitter. If an area is clear any AGV can enter and pass through the area. When an AGV is in the area all AGV attempting to enter the area must stop and wait.

Pros/Cons: Zone control is a cost efficient way to control the AGV in an area. A problem with this method is if one zone goes down all the AGVs are at risk to collide with any other AGV.

http://www.mhia.org/media/elessons/agvs1/understandingagvssect1_.htm

Traffic Control Technical PaperDeadlock prediction and avoidance for zone-control AGVs; Authors: Yeh M.-S.,Yeh W.-C.Source: International Journal of Production Research, Volume 36,Number 10, 1 October 1998 , pp. 2879-2889(11); Publisher: Taylor and Francis LtdDeadlock problems of zone-control uni-directional automated guided vehicle systems (AGVS) are discussed in this paper. The motivation of this research was to propose a dynamic approach to enable prediction in real time and to avoid deadlocks that are caused by sharing guidepath zones for a class of AGVS so that both utilization of resource and overall throughput can be improved. The proposed algorithmic procedure, in accordance with the states gained and generated from the obtained model, can serve as a functional module for the operation of a zone-control AGVS without the need to revise the original vehicle control extensively.

Traffic Control Forward sensing controlUses collision avoidance sensors to avoid collisions with other AGVs, people, or other objects.

Sonic: works like radar, sends a high-frequency chirp out and then waits for a reply, from which it can determine if an object is ahead of it and take action to avoid collision

Optical: similar to sonic, uses an infrared transmitter/receiver, sends a signal which gets reflected back

Bumper: physical contact sensor, most have them as fail-safes

Limitations:The problems with these are they can only protect the AGV from so many sides. They can be relatively hard to install and work with as well.

Battery ChargingBattery SwapRequires an operator to manually remove the discharged battery from the AGV and place a fully charged battery in its place after approximately 8 - 12 hours (about one shift) of AGVs operation. 5 - 10 minutes is required to perform this with each AGV in the fleet.

Battery ChargingAutomatic / Opportunity Charging"Automatic and opportunity battery charging allows for continuous operation. On average an AGV charges for 12 minutes every hour for automatic charging and no manual intervention is required. If opportunity is being utilized the AGV will receive a charge whenever the opportunity arises. When a battery pack gets to a predetermined level the AGV will finish the current job that it has been assigned before it goes to the charging station.

Battery ChargingAutomatic Battery Swap"Automatic battery swap" is an alternative to manual battery swap. It requires an additional piece of automation machinery, an automatic battery changer, to the overall AGV system. AGVs will pull up to the battery swap station and have their batteries automatically replaced with fully charged batteries. The automatic battery changer then places the removed batteries into a charging slot for automatic recharging. The automatic battery changer keeps track of the batteries in the system and pulls them only when they are fully charged

Who, what for, when usedAircraft, automotive, any large-scale production factoryHospitalsTransportation industry

CostsRule of Thumb System Pricing Ranges (costs shown are per vehicle)

Examples of complexity as used in the 4 tables:Level 1: Simple Manual Vehicle Dispatch, Load/Unload, No Central Controller, No Host Interface.Level 2: Medium Automatic Vehicle Dispatch, Load/Unload, Central Controller, Product Tracking, Multiple Path Options.Level 3: More Automatic Vehicle Dispatch, Load/Unload, automatic coupling/uncoupling (applies to tuggers only), Central Controller, Complex Host Interface, Ethernet Link, Product Tracking, Multiple Path Options Multiple Transfer Heights, etc.

Supporting TechnologyBatteries / Chargers / MotorsComputer Hardware and/or Software Controls and Controlling Devices (e.g.: Sensors)Radio Frequency/Data Communications Equipment

Use in Industry

Rules/LimitationsSpeed limitationsLevel of complexityCostSafety

Primary vendors of technologyComplete AGV systemsEgemin Automation Inc. Frog AGV Systems Inc.HK Systems, Inc. JBT Corporation (formerly FMC Technologies)Jervis B. Webb Company Transbotics Corporation

Components and/or supporting productsDanaher Motion Sr software and hardware equipment SICK, Inc. sensors, safety systems and automatic identification

StandardsMaterial Handling Industry of America

American Society of Safety Engineers

ANSI/ASME B56.5-2004Safety Standard for Guided Industrial Vehicles and Automated Functions of Manned Industrial Vehicles

Safety and perceived safety is main issue

Class applicationAn AGV system has an average travel distance per delivery LD=200m, and an average empty travel distance LE=150m. Load and unload times are each TL=TU=24s and the speed of the AGV is vC=vE=1m/s. The available time is AT=51.3min/hr/vehicle. How many vehicles are needed (nC) to satisfy a delivery requirement of RF=30 dels/hr?

Reminder:

TC = TL + TU + LD / vC + LE / vE Delivery cycle time (min)WL = RF*TC work by handling system per hr (min/hr)nC = WL/AT num of required vehicles

Class applicationTC= [24s +24s +200m+150m/(1m/s)]/60= 6.633min/delWL = 30del/hr * 6.633min/del= 199min/hrnC= (199min/hr)/(51.3min/hr/vehicle)= 3.88vehiclesnC= 4 vehicles required

SummaryEfficientCost-effectiveMaintainableHighly developed in Japan and EuropeGrowing in the US

Referenceshttp://www.mhia.orgwww.egeminusa.comwikipedia.comGroover text