Scheduling Flashcards
Describe push scheduling
The orders are planned and issued centrally, the longest time process sees the new order first
Same order given to all processes at the same time
Describe pull scheduling
Processes are triggered by a replenishment signal, upon withdrawal of material from inventory the preceding process is authorised to start processing, the final process sees the new order first
What are the objectives of scheduling
Minimise completion time, lateness, tardiness, number of tardy jobs
How to minimise average completion time of 4 jobs with the processing times: A - 11 B - 3 C - 4 D - 2
Shortest processing times earlier:
D_ 2 B_5 C_9 A_20
average completion time = (2+5+9+20)/4
How to minimise maximum lateness of 4 jobs with the processing times and due date: A - 3 - 9 B - 3 - 8 C - 4 - 16 D - 5 - 9
Put jobs in order of earliest due date:
B_3 A_6 D_11 C_15
maximum lateness (job D) = 11-9 = 2
How to calculate lateness
completion date - due date
How to minimise average tardiness
Use a Modified Due Date (MDD), put jobs in order of increasing MDD
MDD = max(due date, time + processing time)
Calculate MDD for all after noting what t is (add all the processing times of jobs in the list already), then add the lowest MDD to the end of the list
How to minimise the number of tardy jobs using Moore’s Algorithm
1) Schedule jobs by EDD (lowest due dates)
2) If no job is tardy, go to step 6
3) Find first tardy job, this is k
4) From jobs 1 to k, remove job with longest processing time
5) Return to step 1 with one fewer job to consider
6) Schedule is the EDD schedule, plus removed jobs (any order)
How to find a tardy job
When the completion date is larger than the due date
What is makespan for a 2-machine flowshop
Time between first job starting and last job finishing
How to minimise makespan in a 2-machine flowshop, using Johnson’s Rule
1) From jobs not yet assigned, find the job with shortest processing time on either m/c
2a) If the shortest processing time occurs on m/c 1, assign job to next free slot
2b) If the shortest processing time occurs on m/c 2, assign job to last free slot
3) Go back to step 1 until all are assigned
What is an assembly line
Consists of a sequence of operations
What is takt time
Takt time = available time per period / demand per period
How to calculate minimum number of stations required
Find cycle time (minutes per item)
Work content = sum of all processing times
number of stations = work content / cycle time
How to calculate balancing loss
balancing loss = 1 - (total processing time) / (stations x cycle time)
What is a fixed-position layout
A layout that requires the product to remain at one location
What is a process layout
Layout that groups together machines, equipment or people with similar functions
What is a product layout
A layout that dedicates equipment and workers to specific products on a linear route
What are the advantages and disadvantages of a process layout
Advantages: resources are general purpose, flexibility is high, capacity utilisation is high
Disadvantages: frequent changeovers, high inventory levels necessary, throughput time very long, control and planning required
What are the advantages and disadvantages to product layouts
Advantages: faster processing rates, lower inventories, fewer changeovers
Disadvantages: low flexibility, low utilisation, high risk of layout redesign
What are hybrid layouts
Layouts that attempt to combine advantages of process and product layouts by grouping disparate machines into cells to work on similar products
How to select a layout type
Low volume and high variety: fixed-position
More volume and less variety: process
Even more volume and even less variety: hybrid
High volume and low variety: product
