UTO LASH April 15, 2002 Published by Brooks Automation, Planning & Logistics Solutions Vol 15 No 4
Reserving Induction Space to Evenly Allocate Limited Conveyor Capacity By Carlo Minervino and Jacques Natchia Brooks Automation, Planning & Logistics Solutions - Europe INTRODUCTION When several lanes of conveyor transfer onto the same destination section, it is frequently necessary to manage the destination section’s limited capacity. Otherwise, upstream lanes transfer with little or no delay, which creates lengthy delays for downstream transfers. This article presents a method of managing limited conveyor capacity by reserving induction space to evenly distribute loads based on the order of their arrival at conveyor transfers. (In actual systems, modern PLC technology allows such strategies for controlling transfers when conveyor capacity must be shared by several induction lanes.)
cation times, makes conveyor flow organization and optimization a crucial consideration in these systems. The example model demonstrates how to reserve induction space to manage the transfer of bags from check-in counter conveyors to an associated collector. After providing a brief description of the system, the article discusses the algorithm that controls space reservation. A copy of this article, AF0402.PDF, and a zip file containing the accompanying example model, AM0402.ZIP, are available on the Brooks Automation FTP site in autoflash/2002/apr02 (for information on accessing the FTP site, see “FTP Information” on page 8). 8).
To illustrate a system in which the allocation of conveyor space is important, this article includes an accompanying model of an airport baggage handling system (BHS). Baggage handling systems have been greatly impacted by the increased use of air transportation. The high volume of bags in a system, combined with the need to meet gate allo-
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EXAMPLE MODEL OVERVIEW The example model simulates a baggage handling system at a small, imaginary airport. As illustrated in figure 1, the BHS consists of two collectors, each fed by several checkin conveyors. The system also includes two X-ray scanners, two screening operators, one tomograph, and two final carousels from which bags are loaded by air side operators onto dolly trucks.
The data file that drives the example model consists of each flight’s departure time, the aircraft type, the flight number, and the number of passengers checking in. Furthermore, the passengers do not arrive at the check-in counters all at once, but every 10 minutes according to the following arrival curve.
loading carousels
collectors
check-in conveyors
X-ray scanners
Figure 2 – Flight data file and passenger arrival curve
screening operators
Description of the check-in system tomograph
Figure 1 – Example model layout
Passenger arrivals at the check-in counters are generated from a departure flight’s input data. This solution is preferable to generating the arrivals according to a statistical distribution because it allows the simulation to exploit historical input data and to more accurately evaluate different scenarios.
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An airport check-in system is usually comprised of several check-in counters arranged in one or more groups. Checkin counter operators constitute the interface between the passenger terminal and the BHS. The operators allow only those bags whose weight and dimensions are within the defined limits to enter the system. All the bags that are processed by a particular check-in counter group are conveyed to an associated collector. The two collectors represent input points for the bags carried by passengers entering the airport. Each collector is a single, straight conveyor section that collects all bags coming from the same check-in counter array. Because each collector is a continuous section, as illustrated in figure 3, the system cannot stop some bags on the collector in order to allow other waiting bags to transfer. A collector runs in no-stop mode, which means only those bags on the checkin conveyors that find available space are able to transfer to the collector.
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bags wait on the check-in conveyors conveyors until they are able to transfer to the collector
check-in conveyors
Figure 3 – Transfers from the check-in conveyors to a collector
If the flow rate of bags transferring to a collector is significantly smaller than the collector capacity, the probability of a bag finding a window of space for induction onto the collector is reasonably high. In this case, you do not need to develop custom methods of allocating conveyor space. Problems can arise, however, when the flow rate of incoming bags is comparable to the collector capacity. Especially during peak flow periods, upstream bags are able to transfer easily onto the collector, while downstream bags experience lengthy delays waiting for a sufficient amount of induction space. To prevent this imbalance, you must treat conveyor space as a scarce resource and implement reservation strategies to equally distribute the collector capacity among the waiting bags. Tip:
To determine the amount of space that must be allocated to each bag that is transferring to the collector, you can use the following two parameters: 1. The colle collect ctor or veloci velocity ty.. 2. The maxim maximum um flow flow rate rate the colle collector ctor is expected to manage. The following formula provides a simple relationship to evaluate the required moving space on the collector: section velocity (m/s) × 3600 moving space = --------------------------------------------------------------------section capacity (bags/hr)
For a collector velocity of 0.5 m/s and a maximum flow rate of 900 bags/hr, this formula returns a space window of 2 m. This result represents the minimum amount of space that must be present between the leading edges of two successive bags to prevent bags from exceeding the maximum collector flow rate. This result also represents the induction space that must be available for a bag to transfer to the collector.
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RESERVING INDUCTION SPACE ON A COLLECTOR To prevent bags from transferring to the collector without a reservation, a continuous sequence of control loads travel down the collectors throughout the simulation. A control load represents a window of space that can be reserved by a waiting bag. Each control load is defined with a length equal to the required moving space for bags on the collector. The control loads are listed in one of two arrayed load lists (one for each collector). Bags on the check-in conveyors can search these lists when attempting to reserve a space on the collector. Each control load has an attribute that indicates whether or not it has been reserved. When a bag claims a space on the conveyor, it changes the control load’s attribute to indicate the location where the bag is waiting to transfer. When the control load arrives at the bag’s location, the control load is sent to die and the bag replaces it on the collector. This process is discussed in greater detail in the following sections.
Procedure executed by bags waiting to get on a collector Each bag that is created in the simulation has two attributes that are initialized to the number of its current check-in counter:
AI_ AI_Che CheckInCou Counter
•
AI_I
The second attribute, AI_I, is an index value that the bag uses when searching for a control load to reserve on the collector. By decrementing the value of the AI_I attribute, the load is able to search progressively backward on the collector to find a control load that has not yet passed the bag’s check-in conveyor. An array of two load lists ( VOL_Window) lists the loads that are traveling on each collector, respectively. When a bag is ready to transfer to a collector, it searches the collector’s list of control loads to find a window of space that has not yet been reserved. Each control load sets the value of its AI_CheckInCounter attribute to the number of the check-in conveyor to which it is currently traveling. By comparing the value of the bag’s AI_I attribute to the control load’s AI_CheckInCounter attribute, the bags can determine if the listed control load is currently traveling to the load’s location.
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When a bag finds a control load that is traveling to its location on the collector, it checks to see if the load has already been reserved. If a bag has not yet been reserved, its load attribute AI_OrderLocation is equal to zero. If the control load has not been reserved, the bag reserves it by changing the control load’s AI_OrderLocation attribute from zero to the number of the check-in counter where the bag is waiting. If the bag does not find an available window, it decrements the value of its AI_I attribute by one and repeats the search, thereby searching those control loads on the collector that are traveling to the preceding check-in conveyor. If every window prior to the bag’s check-in counter has already been reserved (that is, AI_I is equal to “1” and AI_CycleEnd is equal to “0”), the bag adds itself to the VOLNoWindow load list and then waits to be ordered on the OL_WaitWindow order list. The procedure that bags execute to find an available window on the collector is shown below: begin P_CheckIn arriving procedure . . . travel to conv:Out_CheckIn(AI_CollectorNum, AI_CheckInCounter) set AI_I to AI_CheckInCounter set AI_CycleEnd to 0 /* While there are unsearched windows on the collector */ while AI_CycleEnd=0 and AI_I>0 do begin for each VWindow(AI_CollectorNum) in VOLWindow(AI_CollectorNum) do begin /* If window is traveling to bag’s counter and has not yet been reserved */ if VWindow(AI_CollectorNum) AI_CheckInCounter=AI_I and VWindow(AI_CollectorNum) AI_OrderLocation=0 then begin /* Reserve window */ set VWindow (AI_CollectorNum) AI_OrderLocation to AI_CheckInCounter set AI_CycleEnd to 1 break end end
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/* if no window is found */ if AI_I=1 and AI_CycleEnd=0 then begin insert this into VOLNoWindow(AI_CollectorNum) at end break end else /* Search previous window */ dec AI_I by 1 end wait to be ordered on OL_WaitWindow(AI_CollectorNum) send to P_HBS end
Procedure executed by control loads traveling on a collector Each control load is listed in the VOL_Window load list that corresponds to the collector on which it is traveling. The control loads travel from station to station, until they are reserved and replaced by a bag or until they reach the end of the collector and are sent to die. Each time a control load arrives at a station on the collector, the load increments the value of its AI_CheckInCounter attribute, which indicates the next station to which the load is traveling on the collector. The load list and load attribute allow bags on the check-in conveyors to search for a window of space on the conveyor that has not yet passed the bag’s check-in location. Before starting to travel on the collector, each control load executes a procedure to see if any bags were unable to reserve a window and are waiting on the VOLNoWindow load list. If the load list contains one or more bags, the control load reserves itself for the first waiting bag and removes the bag from the load list. To reserve itself for a bag, the control load sets its AI_CheckInCounter attribute to the bag’s check-in counter number and then sets the value of its AI_CodePart attribute to 2. The AI_CodePart attribute causes the control load to execute the second part of the P_MovingWindow arriving procedure. This part of the procedure causes the load to travel directly to the bag’s check-in counter, order the waiting bag to continue, and then send itself to die so the bag can claim the control load’s place on the col lector. If, on the other hand, the VOLNoWindow load list is empty, the control load inserts itself on the VOL_Window load list and travels to the end of the collector. At each station on the collector, the control load checks the value of its AI_OrderLocation attribute (which could have been changed by a waiting bag) to see if it matches the check-in counter the control load is currently passing.
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else inc AI_CheckInCounter by 1
The procedure that creates control loads and controls their movement is shown below: begin P_MovingWindow arriving procedure . . . move into conv:CollectorInput(AI_CollectorNum) travel to conv:CollectorGener(AI_CollectorNum) /* Clone the next window on the collector */ clone 1 load to P_MovingWindow /* Determine if any waiting bags could not claim a window */ if VOLNoWindow(AI_CollectorNum) size>0 then begin for each VNoWindow(AI_CollectorNum) in VOLNoWindow(AI_CollectorNum) do begin set AI_CodePart to 2 set AI_CheckInCounter to VNoWindow(AI_CollectorNum) AI_CheckInCounter remove VNoWindow(AI_CollectorNum) from VOLNoWindow(AI_CollectorNum) break end end if AI_CodePart=1 then begin /* Add itself to list of windows and travel to the end of the collector */ insert this into VOLWindow(AI_CollectorNum) at end while AI_CheckInCounter <= KI_DeskNum(AI_CollectorNum) do begin travel to conv:Coll_Check (AI_CollectorNum, AI_CheckInCounter) /* If window has been reserved at current station */ if AI_OrderLocation = AI_CheckInCounter then begin set VI_I to AI_OrderLocation order 1 load satisfying AI_CheckInCounter = VI_I from OL_WaitWindow (AI_CollectorNum) to continue break end
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end /* Remove from window list and die */ remove this from VOLWindow(AI_CollectorNum) end else if AI_CodePart=2 then begin travel to conv:Coll_Check (AI_CollectorNum, AI_CheckInCounter) set VI_I to AI_CheckInCounter order 1 load satisfying AI_CheckInCounter=VI_I from OL_WaitWindow (AI_CollectorNum) to continue end send to die end
This reservation process evenly allocates the collector’s capacity among the waiting bags by reserving space for each bag in the order that they finish processing and are ready to transfer to the collector.
CONCLUSION When multiple conveyor lanes transfer to the same continuous conveyor and the destination section’s capacity is comparable to the total injecting flow rate, PLC devices are usually needed to control baggage injection and minimize transfer delays. This article presents a method to simulate the reservation of induction space on a conveyor and evenly distribute the conveyor’s capacity among transferring loads. The approach described in this article represents one of the many possible methods PLC technicians and system experts may adopt to allocate conveyor capacity. In addition to accurately simulating a system for scenario analysis, the described algorithm can be used in a simulation model to demonstrate to subject matter experts the gains that can result from implementing a space window reservation rule.
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A UTO UTOFLASH Publisher Brooks Automation, Inc. Planning & Logistics Solutions 5245 Yeager Road Salt Lake City, UT 84116 (801) 736-3201 FAX (801) 736-3443
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