Project Proposal:
Simulation of Cheshire Direct Conveyor System
By:
Kosta Karachristos
Ralph Minaya
For:
Ernesto Gutierrez-Miravete
Professor of:
Simulation Modeling and Analysis – DSES-6620H01
ACKNOWLEDGEMENTS
We would like to give special thanks to the Cheshire Direct Management Team
for their involvement in our project for Course DSES-6906 Simulation & Modeling
Analysis at Rensselaer Hartford.
2
TABLE OF CONTENTS
Abstract
4
1.0 Introduction
4
2.0 Objective
5
3.0 Background
5
4.0 Scope
6
4.1 Receiving
6
4.2 Automatic Conveyor
6
4.3 Stock Put-away
7
5.0 Requirements
7
5.1 Receiving
7
5.2 Automatic Conveyor
7
5.3 Stock Put-away
8
6.0 Data Collection
8
6.1 Receiving
8
6.2 Automatic Conveyor
8
6.3 Stock Put-away
9
7.0 Model Definition
9
8.0 Current and Proposed System Behavior
11
8.1 Current System Results
11
8.2 Proposed System Results
11
9.0 Model Validation and Verification
12
10.0 Conclusion and Recommendations
13
11.0 References
13
Appendix A: Flow Diagram of System
14
Appendix B: Pictures of System Components
16
Appendix C: Data Collection Tables
20
Receiving, Conveyor, & Stock Put Away
Appendix D: Fitted Distributions
23
Receiving & Stock Put-away
Appendix E: Current and Proposed Models Output
28
3
ABSTRACT
This report determined the daily schedule between the receiving work centers and
the put-away work centers to minimize bottlenecks on the automatic conveyor at
Cheshire Direct. With observations of the system, it was concluded that imbalances
between the rate bin boxes were loaded vs. the time required to unload the bin boxes
caused bottlenecks.
Defining the speed and loading/unloading rates of work centers assisted in
simulating the existing conveyor system. With the assistance of the simulation software,
it was evident that the rate that receiving inputs merchandise far exceeded the rate that
the put away personnel can stock into location. The existing system loads merchandise
onto the conveyor at a rate of 0.55 boxes/min, creating bottlenecks that would leave 200
bin boxes still on the conveyor after an 8hr shift.
Receiving should load 0.7 boxes/min in order for the automatic conveyor system
to run without major bottlenecks. Running at 0.7 boxes/min enables the conveyor to run
with constant flow and minimal bottlenecks of only leaving 5 bin boxes in system after
an 8hr shift.
1.0 INTRODUCTION
Cheshire Direct is a distribution center that supports catalog operations for
Bloomingdale’s by Mail (BBM), Macy’s By Mail (MBM), and Macys.com.
These
catalogs and web sites offer products in home-furnishings, men’s and women’s designer
clothing, jewelry, lingerie, cosmetics and fragrances. Consumers can place orders by
phone, mail in forms, or electronically through the web site.
Once customers place their orders, they must be packed and shipped within 24hrs,
creating a tremendous demand on the work centers. The centers consist of receiving,
4
automatic conveyors, and stock put-away areas. In the receiving area, merchandise is
processed in two different locations, and released to the subsequent work center. Items
move from one work center to the next through the automated conveyor system, which
diverts the products to the proper stock area via barcode scanners. In the put-away area
goods are stored in the appropriate shelving locations.
Adhering to an hourly schedule at the receiving end, all processed merchandise is
presented to the automated conveyor regardless of its existing utilization. Despite the
current conveyor capacity, the product is released, causing bottlenecks when the system
is busy. As a result, many man-hours are spent clearing bottlenecks in the system.
2.0 OBJECTIVE
The goal was to coordinate the workload between the receiving and stock putaway work centers, based on system capacity, to minimize bottlenecks in the downstream
process.
3.0 BACKGROUND
This section describes the process flow before merchandise can be shipped to
customers. The process consists of receiving, automatic conveyors, and stock put-away
areas. See Appendix A for a simplified schematic of the real world system applied to our
project. Also, Appendix B shows pictures of each process area.
3.1 Receiving
This aspect of the process allots to the receiving of merchandise from various
product manufacturers. Merchandise is prepared for put-away in the receiving prep
mezzanine, and at the pallet receiving areas. Receiving packs the merchandise into bin
boxes, which are labeled with bar codes and proper storage location information.
3.2 Automatic Conveyors
5
Once the boxes are on the conveyor system, the barcode system directs them to
the put-away location.
3.3 Stock Put-away
Stock put-away personnel (3 persons) remove boxes from the conveyor and
transport them to the proper storage locations.
4.0 SCOPE
Our methodology will consist of modeling the existing conveyor system with
specific assumptions, and based on the results, adjustments will be made to achieve the
desired results. Assumptions, necessary data, and performance metrics are described in
the sections following:
4.1 Receiving
Assumptions:
 Merchandise is readily available, no transport time required to conveyor
 There is no box in the system initially
 No operators are included in this activity
 Receiving prep mezzanine, and pallet receiving areas considered as one
processing area
 Boxes are pulled via FIFO
 Never experience failure
Data Collection:
 Arrival time based on existing production data
-Boxes/Min
4.2 Conveyor
Assumptions:
 Conveyor will serve as input queue
 Entities (boxes) will move at constant rate (ft/min), & no distance between
each other
 Never mechanical experiences failures, except bottlenecks
Data Collection:
 Conveyor speed (ft/min) – defines entity speed
 Conveyor length (ft) – defines queue length
 Conveyor capacity – defines max number of boxes that fit into queue length
Performance Metrics:
 Bottlenecks/Day – when entity number in conveyor is > than conveyor
capacity
 Conveyor downtime – waiting time in queue/conveyor
6
4.3 Stock Put-away
Assumptions:
 Server is initially idle
 3 persons considered as resources
 Storage location is readily available, no walking distance for resource
Data Collection:
 Put-away rate
-Boxes/Min
Performance Metrics:
 Daily % Utilization
5.0 REQUIREMENTS
Putting this work together required understanding the system through the different
components, that is Receiving, the Conveyor system, and Stock. A general overview of
the system was provided by the manager of Stock and Industrial Engineering, the
supervisor of the Receiving Department at the first visit.
5.1 Receiving
Different visits were made to the different departments so we could learn the
processes. In Receiving we observed the way the bin boxes were inducted in the
conveyor system.
We also observed the speed the bin boxes were coming to the
conveyor. The most difficult part of this project in receiving was the data collection
process. All the input data for the model was gathered from the daily production report,
which took a considerable amount of time.
5.2 Automated Conveyor System
The behavior of the conveyor system was analyzed. Being a fifteen year old
conveyor, the flow is not only affected because of the way the bin boxes are being
currently released from Receiving, but also from failures due to mechanical malfunctions.
We had to measure physically the conveyor because no blue print was available at the
moment. The same thing was done with the conveyor speed. Samples of bin boxes had
7
to be tracked in the system to determine the average speed of the system. We had to
watch the conveyor system working for more than eight hours during morning and
afternoon hours. We also noticed that releases were more frequent during afternoon
hours.
5.3 Stock
Several employees in Stock were interviewed along with supervisory personnel to
identify what were the real causes of the bottlenecks. None of the staff knew accurately
how many hours they put on clearing the bottlenecks on a daily basis, but we knew that
the amount of time they were spending on that was more than two hours per shift. Once
again, the input data for the model was gathered from daily production reports.
Once the current model was validated and verified, it was run. The results were
discussed with the Manager of Stock to check if the model was reflecting the reality of
the system.
6.0 DATA COLLECTION
This section describes how the data collected for receiving, automatic conveyors,
and stock-put away areas was utilized to determine the loading rates of receiving, and
unloading rates of stock-put away areas. Also, the conveyor parameters were calculated.
The data collection can be seen in Appendix C for receiving, conveyors, and put-away.
6.1 Receiving
Receiving data was gathered form production records generated in the receiving
Detailed and Pallet work centers. Daily volumes were converted into bin per minutes.
The number of weeks considered was 18 weeks.
6.2 Automatic Conveyor
The list below summarizes the conveyor parameters calculated:


Conveyor Length = 714 ft
Conveyor Speed = 122 ft/min
8

Conveyor Capacity = 327 boxes
The overall length of the system was derived from manually measuring the
conveyor used between the receiving and stock put-away centers. The conveyor speed
was calculated from a time study conducted at different sections of the conveyor system.
Ten boxes were timed for each section of the conveyor and an average time was
calculated. The table in Appendix C lists the length of the different sections and the
average time it took to travel along that distance and the overall average time for a box to
travel through the system = 122 ft/min.
6.3 Stock Put-away
Put away data was gathered from the warehouse management system reports
(Base Plus 105) for 18 weeks. Total weekly volumes consisted of three different items,
1) Active cartons, 2) Reserve cartons, and 3) Truck put away cartons. Volumes were
then added to get the total put away volumes handled by the 3 employees. The total put
away weekly hours for these respective volumes was calculated from recorded data.
Weekly volumes and their corresponding put away hours appear in Appendix C. The
average put away time per bin box = 1.02 minutes.
7.0 MODEL DEFINITION
The bin boxes arrive at the receiving end and then they are loaded onto the
conveyor. The conveyor transports them down to the scanner, and then they are diverted
to the putaway area. A detailed description of the model components follows.
Locations
The model was built taking in consideration the locations below:
9
Automatic Conveyor System
The conveyor has a total length of 714 feet, and a width of 2.87 feet. The conveyor
capacity is infinite, and runs at a constant speed of 122 fpm. It does not experience any
down time. The rules that govern the conveyor system is FIFO (first bin box to enter
onto the conveyor will be the first to be put away).
Putaway Personnel
There are 3 putaway persons who take the bin boxes off the conveyor and store them.
These persons are Putaway_Persons, Putaway_Persons2, and Putway_Persons3. They
putaway bin boxes as they come down the conveyor following the oldest bin box. The
putaway rate is 1 bin box per minute. There is not downtime associated with any of the
putaway persons, and they can handle one bin box at a time.
Entity
The entity used in the model is the bin box. The bin box is 2.00 wide by 2.17 long. It
moves through the conveyor system at a speed of 122 fpm. There is not distance between
the bin boxes moving through the conveyor system.
Processing
Bin boxes arrive at an initial constant rate of 0.55 bin per minute. They move through the
conveyor system at a constant speed of 122 fpm, and then they are putaway to the
specific locations at a rate of 1 bin box per minute following a Weibull distribution.
Arrivals
Bin boxes arrive to the conveyor system following a normal distribution that varied from
0.55 to 1.0 bin boxes per minute and standard deviation of 20% in every case.
10
8.0 CURRENT AND PROPOSED SYSTEM BEHAVIORS
Once the distributions were determined for each area, the current modeled system
was simulated for verification and determining bottlenecks/loses in the system. Through
much iteration, we discovered the correct arrival rate for bin boxes to be introduced into
the system. For the system altering the conveyor speed and the number of personnel in
the put away area could not be considered. This was so because the real system conveyor
has many convergence’s and we do not want to hire or eliminate a job at the facility.
8.1 Current System
The fitted distribution for the production data recorded was the Weibull distribution. The
average utilization for the 3 resources was around 98 %, with 871 total entries into the
system. The average minutes of a bin box in the system were 56.36 minutes. A total of
197 bin boxes remained in the system for the amount of time the simulation was ran.
These results can be seen on Appendix E.
8.2 Proposed System
Different experiments were run to come up with the proposed systems. We considered
an average daily production of 11,000 units and assumed a normal distribution for the bin
box arrivals. We altered the arrival rate of the existing model from .55 to 1 min per bin
box and assumed a 20% variation. Table 1 shows the iterations of the proposed model
versus the existing model. The % Utilization of each put away person, and bins left over
on the conveyor system were the driving factors to determine the appropriate arrival rate.
Comparing the utilization and bins left over for the different runs the optimum Model
was the one with a mean of 0 .7 bins per minute and standard deviation of 0.14. This
distribution allows 686 bin boxes to be processed, and a utilization of 98.11% of the three
11
Table 1: Current & Proposed Results
Model
Current (6 replications)
Proposed (6 replications)
Current and Proposed Results
Average
Distribution
Total Entries
Minutes
Weibull (24.2, 0.563)
871
56.48
N (0.55, 0.11)
874
57.31
N (0.60, 0.12)
799
40.23
N (0.70, 0.14)
686
8.05
N (0.80, 0.16)
597
5.82
N (0.90, 0.18)
536
5.79
N (1.0, 0.20)
480
5.79
Left Over
Bins
197
199
125
15
7
7
6
Person 1
98.67
98.69
98.66
98.27
86.82
80.69
78.63
Utilization
Person 2 Person 3
98.55
98.43
98.57
98.47
98.53
98.41
98.12
97.98
86.16
85.56
78.19
73.41
73.97
55.45
put away persons, with only 15 boxes left over on conveyor, therefore bin boxes
minimized. See Appendix E for details of iteration outputs.
9.0 MODEL VERIFICATION & VALIDATION
The simulation model correctly reflected the conceptual model. The conveyor is
an accumulation conveyor with previously stated length and speed. Since the conveyor is
a location, the routing of the bin box (the entity) was assigned to the conveyor from
receiving and from the conveyor to the put away area. The time in queue was defined as
the time a bin box takes to get from the beginning of the conveyor to the put away area.
Since the length of the bin box is 2 ft long and the speed of the conveyor is 122 ft/min,
the average time for the box to travel the length of the conveyor is 5.5 min. Studying the
time it takes boxes to get to the stock put away area was about 55 min on the existing
system. This value is very close to the existing model distribution (56.48min) for average
minutes in queue as seen on Appendix E. Furthermore, the put away rate of each person
is 1 bin box/min, and assuming a normal distribution of 1 min should give us an average
time in queue of 5 min and small number of bins left over. The simulation was run with
the assumed normal distribution and the average time in queue from the existing model
output was 5.79min.
12
Therefore checking with actual studied activities of the conveyor and comparing
them to the model out put we were confident to conclude that the model correctly
describes the conveyor system at Cheshire Direct.
10.0 CONCLUSIONS & RECOMENDATIONS
When we gradually increased the amount of bin boxes sent from the Receiving
area to the conveyor system from 0.55 to 0.7 bin boxes per minute the Stock’s putaway
personnel utilization went up to 98%, and the amount of bin boxes that remained on the
conveyor went down to 17 units. Bringing the arrival rate closer to the putaway rate
based on the system capacity minimizes conveyor bottlenecks and increases putaway
utilization. Below are listed the recommendations for the proposed system based on the
observations from 10 replications of the new model.
1. Bin boxes must be released at a rate of 0.7 units per minute from the receiving
end following a pre-set daily schedule.
2.
Receiving personnel must communicate with Stock personnel before every
release to make sure they are available for putaway.
3. Arrangements must be made with stock in the event the arrival rate should
increase due to production volumes in receiving.
Different bin box releases from the Receiving Area must be done at a rate close to the
putaway rate in then Stock Putaway Area, 1.0 bin per minute, so the flow through the
conveyor system be smooth.
11.0 REFRENCES
1. C. R Harrel, Ghosh & R. Bowden. Simulation Using ProModel, 3rd ed. McGraw Hill,
Boston, 2000.
2. A.M. Law & W.D. Kelton. Simulation Modeling and Analyiss, 3rd ed. McGraw Hill,
New York, 2000.
13
APPENDIX A: FLOW DIAGRAM OF SYSTEM
14
Appendix A: Flow Diagram Of System
Receiving
Area
Bin Boxes
CONVEYOR SYSTEM
Storage Locations
Put-Away
15
APPENDIX B: PICTURES OF SYSTEM COMPONENT
16
APPENDIX B: Receiving
PREP AREA
CONVEYOR
17
APPENDIX B: Automatic Conveyor
Conveyor
Bin Box
Bar Scanner
18
APPENDIX B: Stock Put Away
Stock Locations
Bin Box
19
APPENDIX C: DATA COLLECTION TABLES
20
APPENDIX C: DATA COLLECTION TABLES
RECEIVING AREA
PALLET & DETAIL
DATE
8/28
8/29
8/30
8/31
9/1
9/5
9/6
9/7
9/8
9/11
9/12
9/13
9/14
9/15
9/18
9/19
PALLET
0
0
0
0
1016
643
0
0
382
726
499
214
1198
188
437
742
DETAIL
382
1049
1053
1207
449
857
449
768
816
538
568
1387
0
0
0
0
BIN SUM
382
1049
1053
1207
1465
1500
449
768
1198
1264
1067
1601
1198
188
437
742
Units/Min
3056
8392
8424
9656
11721
11998
3592
6144
9581
10109
8538
12805
9587
1502
3494
5932
Min
5659
14986
12573
18569
22115
20336
6414
11592
19162
17735
14721
23282
17120
2590
6239
10785
Bins/Min
0.54
0.56
0.67
0.52
0.53
0.59
0.56
0.53
0.5
0.57
0.58
0.55
0.56
0.58
0.56
0.55
DATE
9/20
9/22
9/25
9/26
9/27
9/28
9/29
10/2
10/3
10/4
10/5
10/9
10/10
10/11
10/12
10/16
PALLET
488
1031
934
790
166
693
905
41
1182
821
229
261
434
478
844
474
DETAIL
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
BIN SUM
488
1031
934
790
166
693
905
41
1182
821
229
261
434
478
844
474
Units/Min
3904
8251
7474
6321
1329
5540
7237
325
9458
6565
1829
2088
3470
3822
6751
3789
Min
7508
14226
13346
11493
2556
9552
14474
625
16307
11936
3153
3600
6088
7644
12502
7149
Bins/Min
0.52
0.58
0.56
0.55
0.52
0.58
0.5
0.52
0.58
0.55
0.58
0.58
0.57
0.5
0.54
0.53
10/18
10/19
10/20
10/24
10/25
10/26
10/27
10/30
10/31
11/3
11/6
11/7
11/8
11/9
11/10
11/13
PALLET
456
392
291
473
321
349
544
157
483
1188
558
543
580
1249
645
527
DETAIL
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
DATE
BIN SUM
456
392
291
473
321
349
544
157
483
1188
558
543
580
1249
645
527
Units/Min
3649
3133
2331
3786
2566
2793
4355
1252
3865
9500
4462
4347
4642
9989
5160
4217
Min
6635
6266
3951
6884
4349
4900
8375
2159
7027
16379
7828
8694
8596
18847
9382
8434
Bins/Min
0.55
0.5
0.59
0.55
0.59
0.57
0.52
0.58
0.55
0.58
0.57
0.5
0.54
0.53
0.55
0.5
Average Bin/min:
0.56
Appendix C (Cont.)
CONVEYOR SPEED CALCULATION
Length (ft)
Time (min) Speed (ft/min)
357
3.18
112
244
2.04
119
357
2.97
120
714
5.26
135
122
Total Average
PUTAWAY DATA COLLECTION TABLE
6/03
6/10
7/1
7/8
7/15
7/22
7/29
8/5/00
8/12
8/19
8/26
9/2
9/9
9/16
9/30
Flat Active Cartons
978
1038
1832
913
2523
1867
1977
951
1813
2261
1295
1509
1839
1512
3495
Reserve Carton
728
829
1096
622
1642
1417
1854
796
1574
2030
1342
1028
1188
827
1283
Truck B Carton
1011
1421
664
414
1675
681
1330
974
1040
1091
1618
1813
892
1123
3144
HRS/WK
44.44
53.62
59.31
32.14
95.94
65.52
84.91
44.54
72.92
88.79
69.57
70.99
64.57
56.76
129.41
Total Packed
2717
3288
3592
1949
5840
3965
5161
2721
4427
5382
4255
4350
3919
3462
7922
Bins/Min
1.02
1.02
1.01
1.01
1.01
1.01
1.01
1.02
1.01
1.01
1.02
1.02
1.01
1.02
1.02
2
3
APPENDIX D: FITTED DISTRIBUTIONS
APPENDIX E: CURRENT & PROPOSED MODEL OUTPUTS
2