AGVs - WAREHOUSINGON THE RIGHT TRACKAGV's or conveyors? Conveyors orAG V's? Gordon SkimmingsofSM Consultants describes a recent AGV installation and weighsthe pros and cons.

The AGV installation is in a warehousein the UK which is owned and operatedby a leading third party warehousingand distribution company. Operating

24 hours a day, seven days a week, it receives,stores and distributes canned products. Palletloads are received in mixed batches of 18 palletson curtain-sided trailers fitted with conveyor beds.Pallets are stored within the warehouse in doubledeep racking. There are 44 rows of this rackingwith 22 aisles. At each aisle there is a gravity rollerinput station and a separate output station, eachwith the capacity to hold two pallets. Pallets are putaway and retrieved by narrow aisle trucks fittedwith double reach forks.

When a trailer arrives at the warehouse,electrical power is plugged into the trailer' bedconveyors and the load is then powered off thetrailer onto a matching bed of powered rollersmounted on the dock. There is a manual off-toadfacility for trailers not fitted with roller beds, or inthe event of breakdown of the off-load system.Automatic off-loading of an 18 pallet load takesless than two minutes.

The pallets are then indexed forward on thewarehouse conveyor, through a gauging station totwo AGV auto-loading stations. The presence ofpallets on the stations is detected by sensors andAGVs are called forward. There are nine AGVs in

the system at present and this can be expanded to15 if required. An AGV takes on a pallet andtransports it past an automatic bar code reader -which reads the bar code label attached to everypallet - linked to the warehouse computer. Thecomputer allocates pallets to racking locations in amanner which spreads the product across theaisles and matches products in the racking, whileat the same time ensuring that the number ofpallets destined to an aisle does not exceed thecapacity of the input station.

Within the aisles, pallets are zoned according tothe popularity of the product. Man-rider narrowaisle trucks pick up pallets from the aisle inputstations and deposit them in the racking, inaccordance with instructions transmitted to thetruck-mounted radio terminals by the computer.On completion of an input, the terminal displays anoutput instruction. The accuracy of deposit andwithdrawal by the narrow aisle trucks is verified bymeans of a hand-held bar code reader mounted oneach truck. The driver uses the reader to read thepallet label and a label on the beam of eachlocation. The bar code reader is linked to thecomputer which signals acceptance or error to the •driver.

Transport of pallets to and from the aisles iscarried out by the AGVs. The AGV control systemis linked to the computer from which it receives

AGVs provide automatictransfer to marshalling

lanes

aisle destinations for incoming pallets anddispatch marshalling destinations for outgoingpallets. The AGV addressing points are positionedsufficiently far away from the loading station toallow the computer time to select the destinationwhile the AGV is travelling. There is a facility formanual input of AGV destinations in the event offailure to read the bar codes or breakdown of thesystem.

Outgoing loads are automatically marshalled ongravity roller lanes in the marshalling areas. TheAGVs collect outgoing pallets from the aisles anddeposit them on the marshalling lanes accordingto instructions signalled to the AGV control systemby the computer. The AGVs are fitted with solid topshuttles, powered from the AGV's battery, whichrun out from the AGV and which lift or lower thepallet at the pick up and deposit.stations. Fastbattery charging is facilitated by roller beds underthe battery and matching changing trollies.Opportunity charging stations are installed at twoof the three AGV home stations.

Why AGVs and not conveyors?The operational requirements of the installation

were studied and closely specified by SMConsultants. Four suppliers were invited to tenderfor a conveyor solution with the option ofAGVs. Two suppliers were shortlisted:one offered a single 'carousel' conveyor withpowered input/output stations, and the otheroffered AGVs with gravity input/output stations.The decision to opt for AGVs was taken followingthe discussion of the following factors:

The initial capital cost was virtually the same forboth solutions.

While both provided an automatic link betweenincoming trailers and the aisle input/outputstations, only the AGVs provided automatictransfer to the marshalling lanes. This link couldhave been provided by conveyors but at additionalcost and complexity.

AGVs left the floor open which allowedunobstructed access to all parts of the warehouse;this was particularly relevant in the marshallingarea.

AGVs eliminated all forktruck handling otherthan dispatch vehicle loading at the front of thedocks.

The AGV solution dramatically reduced thenumber of sensors in the system, and sensors arestill the main cause of system stoppage. Whilesensors can normally be repaired quickly, thefrequency with which they fail means that constantmaintenance cover is needed.

Other than those on the AGVs themselves, theAGV solution eliminated all drive motors andgearboxes, lift and lower tables, divertors andturntables. This significantly extends the meantime between failures and avoids critical conveyorcomponent failures which could stop the flow ofboth inputs and outputs.

When an AGV fails, it can be taken out of thesystem while the remaining AGVs are able to

MANUFACTURING ENGINEER APRIL 1990

operate normally, which means that immediatemaintenance work is not necessary. Failure of aconveyor system, on the other hand, requiresimmediate attention and therefore constantmaintenance cover. This is a major on-going costin a warehouse which otherwise does not requiresuch attendance.

In the event of catastrophic failure of the AGVsystem, forktrucks can be drafted in to transportpallets to and from the aisle input/output stations.

Drawbacks with AGVsDespite the benefits the AGV system has over a

conveyor system, it is vulnerable in three mainareas. These are if there is a loss in thecommunication link with the warehouse computer;where there is failure of the AGV communicationloop; or when the AGV controller fails.

If the link with the main computer goes down, itcan be overcome, with some loss of capacity, bymanually entering data into the AGV controller intothe warehouse computer via a VDU terminal.However, with the AGV controller and the maincomputer being located in the same building, lossof communications between them should beinfrequent; it should also be noted that such a lossin communications would affect a conveyor systemequally.

There is no standby provided if the AGVcommunications loop fails, but as these arehistorically reliable a standby is not necessary.

Failure of the AGV controller is most likely to becaused by a circuit board failure so plug-in sparesare held on site and diagnostics are built into thesystem. Occurrences of this sort should be rareafter the first few weeks and will only involve anemergency call out if personnel on site cannotcope. A spare control unit is held on site in case ofa major failure.

The cost of adding more AGVs into the systemas its throughput increases is offset by the fact thatconstant maintenance is not required, fewerforktrucks are required because of lower powercosts.

It should be noted that the conveyor solutionoffered an inherent capacity beyond that providedby the maximum of 15 AGVs. This comes about

Q Despite the benefits ofAGV's over conveyors, they are

vulnerable in three main

areas

because of the response time to pallets on the aisleoutput stations provided by a conveyor solution.The time taken for an AGV to respond will dependon where the AGV is relative to the aisle fromwhich the output is waiting. While this has beenpartially offset by the provision of AGV homestations along the length of the AGV route,response times of between two and four minuteswill be normal for an installation which has over 50AGV destinations. Although conveyors willconsistently clear output stations within 30seconds, this was not considered to be sufficient tooutweigh the advantages of the AGVs.

The weight given to each factor when decidingwhether to go for an AGV or a conveyor solutionwill vary between installations. For example, if theinstallation is 'in a factory where existingmaintenance personnel are on call, the differencein maintenance costs between the two solutionswill probably carry little weight. This stresses theimportance of an informed and detailed study ofthe requirement and conditions before a choice ismade.

Problems and solutionsThe floor: At an early stage, but after the

contract had been placed, the AGV supplierexpressed doubts on the long term durability of thefloor surface in the main AGV tracking aisle. Theproblem is similar to that experienced with narrowaisle trucks in that both follow a constant tracking

The heart of the machine- getting into the guts of

the AGV

path. If the floor surface is not constantly flat, rutswill occur in the tracks and the surface will breakup. AGVs and narrow aisle trucks also require aneven surface. The floor specification depends onthe weight imposed on the surface and the stabilityof the load. In the case of narrow aisle trucks, theaisle's width relative to the truck and load width isimportant. Also, the possibility of the load settlingout of its original profile while it is stored on theracks must not be overlooked.

The AGVs: These presented few problems otherthan component and connection failures in the firstweeks of operation and this can be expected fromall types of automation equipment. There weresome minor floor cutting changes and controlsoftware modifications during commissioning -again to be expected.

Automatic off-loading: problems wereencountered during commissioning in the align-ment of the trailer to match the receivingconveyors. These problems are inherent inautomatic off-loading systems which involve two ormore docks at different sites.

It is essential to achieve a constant heightinterface between the trailer and the dock conveyoras the load is shifted between the two. This is mostoften achieved by welding a downward projectingsteel frame at the rear of the trailer and installingmatching mounting shoes at the docks. Lateralalignment depends on the tolerance allowedbetween the side guides of the mounting shoes.

A common alternative is a kingpin at the rear ofthe trailer which locates in a fifth wheel coupling atthe dock. This is often supplemented by a lateralspring loaded steel plate on the ground under therear wheels of the trailer. With both options, it isessential that the trailer is docked in a straight lineand is locked to prevent it moving away from thedock edge as the load is transferred. If thedownward projecting steel frame solution ischosen, the design must be such that the weight ofthe trailer is taken off its suspension as it mountsthe shoes. It is also important before committing toan automatic off-loading system that, exhaustivetests are carried out with everyday pallets andloads across the full range of products as fewcompanies will be able to guarantee that only goodpallets and loads will be in the system.

Gravity input/output stations: These presentedthe biggest problem and the solution is still beingmonitored. The problem centres around the degreeof slope and braking necessary to allow heavy andlight loads with varying pallet quality to roll downthe gravity rollers and stop within the requiredtolerances without disturbing the load.

In addition, the original support of the gravitychannels was not sufficiently strong to eliminatedeflections under load. This had the effect ofdecreasing the number of rollers in contact withthe pallet, thus increasing the load imposed on thebearings of those rollers which are in contact.Deflections also caused the pallet bearers to strikethe crown of the rollers which caused the pallets tostop before reaching the end of the track. M3

MANUFACTURING ENGINEER APRIL 1990