SubscribeThe continuing expansion of the baggage handling system in Heathrow Terminal 1 by ALSTEC involves extensive computer modelling.
This is conducted by the company i) to confirm that the expanded system will produce the predicted increase in capacity, ii) to investigate the impact on operations as the various installation phases are implemented, and iii) to study the performance of the layout under a variety of fault conditions. Principal Engineer Rob Paterson outlines the process.
Computer Models of Complex Baggage Handling Systems
There is a limit on the extent to which the performance of a complex baggage handling system can be predicted from engineering drawings and accumulated experience. The computer modelling of a proposed installation, however, allows its draft mechanical layout to be fully tested, to ensure this meets the requirements of the specification. Moreover, a simulation enables design options to be assessed, and may lead to the identification (and subsequent resolution) of bottlenecks within the layout.
Designed and installed by ALSTEC during the 1990s, the baggage handling system in Heathrow Terminal 1 has been operated and maintained by the company in partnership with BAA's Baggage Team since its completion. It has three distinct areas: the fully automated early bag store, the transfer baggage facility (handling bags transferred to or from T2, T3 or T4), and the main sortation hall, containing two original and two new tilt tray sorters linked to 198 flight make-up chutes.
In 2001, BAA ordered a major expansion of the system. This was due to the decision by British Airways and British Midland to increase the number of long haul flights leaving from T1, as well as the need to accommodate the anticipated growth of passengers in the years up to 2008 - when T5 is scheduled to become operational. Since BAA wanted to confirm that its business objectives would be fulfilled by ALSTEC's recommendations, the generation of a system simulation was included in the contract.
Heathrow's Baggage Handling System Expansion
The expansion project currently under way has four stages. The first and largest of these, encompassing the installation of the new sorters and 26 dedicated premium check-in facilities, was completed in October 2003. An ongoing requirement is that all modifications must be implemented with the minimum of disruption to operations at Heathrow. The company's project team works closely with the BAA Baggage Team to achieve this goal. Ultimately, the peak baggage throughput will be increased to over 14000 bags per hour.
Many benefits have resulted from running scenarios on the T1 model, variants of which correspond to each stage of the project. To give a specific instance, at the outset it indicated that once the new sorters were functioning, the read rates of some of the older scanners in the main hall would be insufficient. Consequently, these devices were replaced. (This was not predicted from static topological flow studies of steady state flow rates.)
Baggage Handling Systems Simulation
The simulation is used to determine the best way to operate the installation under standard conditions. This is far from obvious, given the layout's complexity and routing possibilities. Particular areas studied include the allocation of chutes in order to optimise bag transit times, and the means of responding to a high demand for early bag storage.
Non-standard conditions - both intended and unintended - are also investigated as a matter of course. For example, some of the nine discrete build phases comprising stage 2 require the extensive diversion of bags from check-in to avoid conveyor routes that are temporarily out of commission as new equipment is added on. Scenarios are devised to overcome the failure of a sorter or a key piece of equipment. As a result, personnel have a list of solutions to implement in the event of a problem being experienced in reality.
In addition, as the expansion continues, the model is employed to confirm that the system will continue to meet (or exceed) the performance targets set by BAA. For example, 85% of the bags checked in must proceed to a chute in 10 minutes, while 97% of the incoming transfer bags must do this in 12 minutes.
Finally, the simulation is used to give demonstrations to airlines, to ensure that they remain happy with the support services ALSTEC provides.
Production
The accuracy of any simulation is determined by the accuracy of the information from which it is created. In this case, statistics were available regarding the actual installation, as it existed in 2001. These were augmented by figures concerning T1's flight arrivals, transfers and departures (and thus baggage movements) - together with their projected increases in every year up to 2007. A site survey was conducted to unearth additional details, such as the frequency of various types of line failure, and the time it took to resolve these situations.
On the basis of the accumulated data, the system was modelled using AutoMod(tm), a software package developed by Brooks Automation. It produces true-to-scale 3D animations that can be understood at a glance. In summary, virtual conveyors and resources - representing machines or individuals conducting particular operations - were positioned on a grid configured to the dimensions of T1. (AutoMod provides a set of templates for this purpose.)
For each item of equipment or person depicted, a value was entered for the processing time, and for any other pertinent details. Next, the crucial control logic was written. This dictates i) when bags enter the simulation, ii) the route they will take thereafter, and iii) their final destinations. The intention was that each model variant would have logic directly applicable to the real layout as the expansion project progressed.
Once some routine tests were completed, assessments were conducted to confirm that the initial simulation mirrored the system (as it was in 2001). The outputs of the model were compared to those of the actual installation. BAA then confirmed that it was an accurate representation. It was felt necessary to ensure that this was the case prior to any experimentation, in the light of the complexity of the T1 system.
Operation
The simulation takes 40 minutes to recreate 24 hours of operation at T1. The baggage input data consists of four profiles: bags arriving at the check-ins, incoming transfer bags for outbound flights, outgoing transfer bags for T4, and early bags received from there for temporary storage. (Bags are quickly moved between T1 and T4 in carts via a 1.5 km. tunnel with two destination coded vehicle (DCV) lines.) These profiles fix the number of bags entering the model during every fifteen minute period of the day. Information is broken down by flight, and sometimes further sub-divided by class and on-carriage details.
Many other parameters may be set. For example, the discharge of bags from the check-ins can be adjusted, to prevent - or possibly produce - bias, and the direction of travel of the collector conveyors can be reversed. Scanner read rates and the clearance times of flight make-up chutes can also be varied. In order to highlight patterns of behaviour, bags can be coloured to indicate their handler, type or destination. Their colour may also denote that a particular condition has been met.
A random behaviour setting ensures that certain events occur at specific locations in accordance with their actual probability per 100 bags in transit. (They include occasional conveyor stoppages, induction failures, and the arrival of out-of-gauge bags.) Unless this setting is adjusted, however, running a particular model multiple times will always reproduce the same events. This is important for analytical purposes.
All input parameters form an integral part of the results of every run. Output tables and charts provide a wealth of statistical and graphical information relating to the performance of the overall system and its component areas. Each run has a unique reference number (which automatically appears on any print-outs).
The broader picture
The development of a baggage handling system simulation is a process in which airport managers should be closely involved. This is especially true at the outset, where agreement must be reached on the extent of the model, its anticipated inputs and outputs, the level of detail it will possess, and the desired performance requirements - in terms of factors such as times, and queuing limits. It is also necessary to define the set of variables governing the simulation. This determines the scope of the 'what if' scenarios which can be explored.
For the airport, the critical factor in deciding whether to proceed with a simulation is usually the time available, related to the deadlines associated with the project in question. There is a balance to be struck here, however, with regard to the total life span of the relevant installation. Time allocated at the outset to test the proposed system - and how it responds to projected increases in throughput - may radically reduce time expended two or three years down the line making (costly) modifications to the actual equipment.
For further information, contact ALSTEC Ltd., Cambridge Road, Whetstone, Leicester LE8 6LH, UK. Tel: +44 (0) 116 201 5057. Fax: +44 (0) 116 284 3851. Web site: www.alstec.com Email: airport.sales@alstec.com
