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Capstone Project Final Report Small Lot Delivery System
IE 4800/4880
Fall 2008/Winter 2009
Team Members: David Naujokas
(Project Leader) (734) 216-0901
[email protected] Nashwan Fatteh
(Team Member) (313) 492-0678
Instructor: Dean Pichette [email protected]
Alper Murat, Ph. D [email protected] Office: 313-577-3872 Cell: 313-443-4429
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Acknowledgments
We would like to give a special thank you to our Wayne State University Senior
Project advisors, Dr. Alper Murat and Dr. Dean Pichette for their help and support in this
project. Also Dr. Darin Ellis who helped the group in the ergonomics part of the project.
Executive Summary
Currently, Chrysler is using excel spreadsheets to do their efficiency and
ergonomic calculations. The problem that was given was to produce AMPS, Automatic
Manufacturing Planning System, screenshots that will allow Chrysler to do their
calculations within AMPS instead of the excel spreadsheets and manual methods that
they are using now. Before the screens could be produced, the efficiency spreadsheet had
to be analyzed to determine where the numbers were coming from and to make sure that
the way it was being done was correct. For the ergonomic assessment, the equations had
to be found and the appropriate data needed to be in the spreadsheet. The equations that
were used were the NIOSH 81 multi-lift equation and NIOSH 91 multi-task equation.
With the ergonomic advancements, the IE will not have to leave their desk
in order to do this assessment. Before the OR model (Operations Research), the
tugger driver would complain that their list gave them problems then the IE would
take that list and run the ergo assessment by picking the parts that were on the
list in AMPS and run the assessment. With the OR model, the ergonomic
assessment is in place while calculating the routes and hours for the day.
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Table of Contents
Acknowledgements .............................................................................................. 2
Executive Summary ............................................................................................. 2
Background.......................................................................................................... 4
Previous Problem................................................................................................. 4
Problem Statement ............................................................................................... 5
Project Objectives ................................................................................................ 7
Methods & Approach ........................................................................................... 8
Constraints for Model........................................................................................... 8
NISOH 91 Lifting Equation ................................................................................. 9
Objective Function............................................................................................. 12
Model Creation .................................................................................................. 12
Results and Expected Impact.............................................................................. 14
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Background AMPS is currently used as a user-friendly planning tool for direct-labor control
and man-power assignments. Indirect labor (material handling) assignments for bulk
materials are in the process of being integrated into AMPS. Small lot deliveries constitute
a significant portion of the line feeding operations in Chrysler assembly plants. Further,
its importance is expected to grow in accordance with inventory reduction efforts. Work
assignments (Tugger operators, routes, delivery schedules) for small lot deliveries are
currently performed via spreadsheets. Whenever there is a need for change in these
assignments (i.e., efficiency, ergonomic considerations), these spreadsheets need to be
manually updated.
Previous Problem
The challenge that Chrysler faced is that the indirect labor is not being
implemented in the Automated Manufacturing Planning System (AMPS). The Small Lot
Delivery System which is the indirect labor consist of Tugger routes is only being
recorded and updated on an excel spreadsheet. The spreadsheet is a good process to
monitor the routes, but employees manually updating the spreadsheet will cause data to
be incorrect. Every time the spreadsheet gets updated there is no standard process that the
employees follow. The problem with efficiency was it had to be validated by the group
and make sure that the way efficiency was being determined was taking into account all
the fixed variables. The ergonomic problems that Chrysler is having is the time that it
takes time for the employees to go and study the routes of each individual tugger driver.
They have to follow the driver and get the appropriate measurements in order to perform
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the assessment. The specific problem to this is that tugger driver has a different route
every day, so Chrysler is only getting the day’s ergonomic assessment, and not the
overall assessment of that route. So the problem is that Chrysler would like to implement
these spreadsheets into their AMPS program.
Problem Statement
The challenge that Chrysler is currently facing is Routes are determined solely on
hourly usage rather then a complete shift. The IE’s would rather have a list for the
tuggers on a daily basis then an hourly basis. This option will save time and money.
Ergonomics is done after routes and efficiency is calculated. This is a problem because
the tugger would complain of heavy lifting or too many parts. The IE would then have to
collect data and measurements to check for any ergonomic concerns. All the data for the
Tugger routes are in excel spreadsheet. This group wants to design an OR model that
will help save the company time and money. The OR model will not only plan routes but
will also include ergonomic feasibility checks.
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Figure 1. Tugger Driver Process
Tugger Picks up Pick List
Tugger Picks up
Parts in CMA
(First Lift)
Tugger Drives to Drop Location
Is there still parts on tugger
Tugger Drives to
empty box location
drop point
Tugger Drops off
empty boxes and
Sorts them by size
Tugger drops off part at location
Tugger picks up
empty container and places it on
tugger.
Tugger route is complete
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Project Objectives Chrysler has asked WSU for help in fulfilling their need to put in place an OR
model to plan routes. The OR model will also include ergonomic checks within the
model. The project objectives were to analyze, understand and design an excel
spreadsheet based work assignment for the Small Lot Delivery System which includes a
segment of an assembly line.
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Methods & Approach In the beginning stages of the project, the problem Chrysler is having with
efficiency and ergonomics needed to be defined. The steps used for this project were:
first, go through the Tote Simulation spreadsheet that was provided by Chrysler to clearly
understand what is involved in creating spreadsheet; second, to find any mistakes that
were made; third, determine what the necessary constraints are for a model; and fourth, to
determine how to incorporate the NIOSH 91 equation into the model. The data was
collected by the previous group that completed their capstone in 2007/2008. Due to the
time it was very unfortunate that this group didn’t have the chance to take a tour to the
plant to getter a better understanding of how the tugger replenishes the lines.
Constraints for Model
There are three main constraints for this model. First, the inventory levels must
be kept at a level sufficient to prevent any line stoppages from lack of parts. This
requires a minimum constraint for each station greater than zero. Second, the inventory
levels cannot exceed the actual capacity at each station. This provides a maximum
inventory constraint for each station. Since actual maximum inventory values were
unknown to the team, they were estimated based on part consumption rates.
The second main constraint is that delivery amounts can only be integer values.
Inventory only arrives in full containers and therefore, the tugger operators cannot move
partial part containers to the assembly line.
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The third main constraint is that relating to the ergonomics. The ergonomic
equation used for this project is the NIOSH 91 lifting equation. The implementation of
this equation is explained in the next section.
NIOSH 91 Lifting Equation
The NIOSH 91 lifting equation is as follows:
RWL = LC x HM x VM x DM x AM x FM x CM
RWL = Recommended Weight Limit
LC = 51 lbs
HM = Horizontal Multiplier - HM = 10/H, where H is the distance from the hands to the
midpoint between the ankles. It can be estimated using the following two equations:
When V is greater than or equal to 10, H = 8 + W/2 where W is the width of the
container.
When V is less than 10, H = 10 + W/2
If H is less than 10, the multiplier equals 1.0.
VM = Vertical Multiplier – VM = 1 – ( 0.003 | V-30| ), where V is the vertical distance
between the hands and the floor.
DM = Distance Multiplier – DM = 0.82 + ( 1.8/D ), where D is the vertical travel distance
of the object. If D is less than 10 inches, DM equals 1.0.
AM = Asymmetry Multiplier – AM = 1 – ( 0.0032 A ) The asymmetry angle, A, is
measured from the mid-sagittal line of the worker. For the purpose of this project, the
AM is assumed to equal 1.0 since all of the lifts are either from a general storage location
or directly off of the tugger meaning that the container is directly in front of the worker
when it is being lifted.
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FM = Frequency Multiplier – F is the frequency of lifts in lifts/min. It can very from 0.2
lifts/ min up to 15 or greater. There is no equation for this multiplier and must be pulled
from a table. This table is shown in figure 2.
CM = Coupling Multiplier – CM is a classification modifier with three options, Good,
Fair, and Poor. It is also based off of a table shown in figure 3. For this project, CM is
assumed to be 1.0 since all of the containers have good handles and the components are
unlikely to shift during the lift.
Figure 2. Frequency Multiplier Table
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Figure 3. Frequency Multiplier Table
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Objective Function
It was determined that there can be several objective functions for this model. It
can be a. to minimize the total work of the tugger operator, b. to minimize inventory
levels of parts on the line, or c. to minimize the cost both underutilizing the tugger
operator and the cost of inventory levels. The equations for calculating total work were
directly based on the efficiency calculations for the previous year’s project.
Model Creation
The left side of the model is based directly upon the previous year’s project.
Figure 4 captures a screenshot of this section.
Figure 5. Left and Middle Sections of Model
Since the only variables for the NIOSH equation that are different for each part
number are HM, VM, and L, these are the only multipliers displayed on the model. FM
is also listed but is based on the hourly frequencies. In order to simplify the model, HM
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and VM are chosen to be the smallest multiplier for which containers are chosen for each
hour. In other words, the model chooses the smallest multiplier for which parts are
actually being moved on the route. L is also chosen in the same way except that the
largest one is chosen. This inherently adds a factor of safety for each route because the
RWL will always be smaller than what it would be if a multi-lift equation was used. In
order for the model to be expanded to include the full number of parts that are used in
SHAP, the multi-lift equation could not be used due to its much higher complexity.
The middle section of the model is where the solution and the RWL are displayed.
The solution is broken into hour routes where the numbers represent the number of
containers to be delivered to each station for that hour’s route. Just below the solution
are the NIOSH equation calculations. It shows the HM, VM, and FM used for each
hour’s equation. It then calculates a RWL for that route and checks it against the
maximum L for that hour.
The following screenshot shows the inventory calculations for each part.
Figure 6. Right Side Section of Model
The Max All Inventory column is the maximum allowable inventory for each part
number. For the purpose of this project they are estimated based on the hourly
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consumption of parts. The next column, Max Invent, is the actual maximum inventory
level for each part. The Initial Supply column is where initial inventory levels before
hour 1 are entered.
Results and Expected Impact The model created for this project successfully finds solutions that meet the
constraints established. However, the objective function of minimizing inventory failed to
produce any results. This may be a limitation with the solver function in excel. On the
other hand, however, setting the objective function to minimize the cost of both
underutilization and cost of inventory successfully solved the model. This is almost the
same as minimizing inventory except has the added benefit of maximizing the work of
the tugger operator.
It should also be noted that the model cannot find solutions when the initial
inventory values are equal to zero. What this means is that there must be some inventory
at the stations in order for this model to be applicable to the plant floor. The reason that it
cannot find a solution is because the tugger operator would have to deliver parts to every
single station if the initial inventory was zero and this would greatly violate ergonomic
feasibilities.
The impact of this project should be that Chrysler will be able to create routes for
their tugger operators that will both be the most efficient as possible as well as keeping
those routes ergonomically feasible without ever having to do an ergonomic assessment
at the floor level. This will both save lots of time for both the industrial engineers and the
tugger operators. They may also be able to further reduce costs due to lowering overall
inventory levels.