Vensim software work required for 3 students 3 copies
Lisa09
06.pdf
Dr. Daniel Xing Email: [email protected]
EBUS-504
Operations Modelling and Simulation
Lecture 5
Use of variables and attributes
University of Liverpool
Management School,
UK
Key learning outcomes
1. What are variables and attributes?
2. Use variables and attributes under different scenarios
3. Use variables and attributes in Witness to improve your
model
Recall the definitions
Variables
Variables provide an abstraction for features of the model whose values typically change as the
model evolves over the course of the observation interval
Attributes • Characteristic of all entities: describe, differentiate
• All entities have same attribute “slots” but different values for different entities, for example:
• Time of arrival
• Due date
• Priority
• Color
• Attribute value tied to a specific entity
• Like “local” (to entities) variables
• Some automatic in Arena, some you define
Recall the definitions
Variables Attributes
Independent Attached to one group of entities
Their values change as model evolves Each entity can have a collection of attributes
Information under one attribute remain unchanged
e.g. e.g.
Number of outputs the colour of your parts
Time to despatch a delivery the number of components for an assembly
Number of serviced customers/day the age of someone in 2020
A simple scenario
A simple scenario
, B3, B4
, B2, B4
, B2, B4
A simple scenario
A simple scenario
M1 Logic
A simple scenario
M2,3,4 Logic
A simple scenario
A simple scenario
Free set of questions
• Q1: How many parts have arrived from each kind?
• Q2: For how long they are in the system?
• Q3: Which buffer holds the most of the parts?
• Q4: How long does each part wait in every buffer or conveyor?
• Q5: In average, how long does it take to generate one A5?
A simple scenario
A simple scenario
In the following example, an attribute will be created in order to
capture the arrival time from Part A2, A3, A4 to B2, B3 and B4.
Therefore, we will calculate how long a part a A2, A3, and A4 stay at
each buffer.
In order to calculate this time, three variables are going to be
created, which will capture this time.
In Witness, we use the internal variable TIME to know the exact time
of the simulation.
A simple scenario
Attr_A2_Time_in: attribute that captures the arrival time for A2 to
B2. In this case, at B2 Actions on input the following expression is
considered: Attr_A2_Time_in = Time
Remark: it will be the same for A3 and A4.
A simple scenario
A2_waiting, A3_waiting and A4_waiting are variables
representing the total waiting time for the most recent outputted A2,
A3, and A4 in their corresponding buffers.
At Actions on output in each buffer, we have:
A2_waiting = TIME - Attr_A2_Time_in
A3_waiting = TIME - Attr_A3_Time_in
A4_waiting = TIME - Attr_A4_Time_in
Why not attributes here?
A simple scenario
Run simulation to 1000 minutes
Proposed Exercise
1. Numbers of parts A1, A2, A3, A4, and A5 generated;
2. Time it takes to generate a part A5
3. How long does it take for the 100th A5 to be produced?
New scenarios
Company ABC purchases a group of white balls to produce different
final products. The purchasing will be done every 10 minute with
batch size 5 and there is 2 balls in initial stock. M1 can paint all the
white balls (one at a time and it costs 3 minutes) into three different
colours (blue, red and green). Then based on the colour of a ball, a
blue ball will be further processed by M2-M3 and red ball will be
further processed by M3-M4 and a green ball will be further
processed by M2-M4. All M2, M3, M4 are single machines with cycle
time 5 minutes.
How can we use variables and attributes to better simulate above
system?
Dr. Daniel Xing Email: [email protected]
EBUS-504
Operations Modelling and Simulation
Lecture 5
Use of variables and attributes
University of Liverpool
Management School,
UK
