Week 8 Flashcards
What are the 2 types of short-term schedulnig?
- Forward scheduling
Scheduling is started as soon as the requirements are known. It may cause buildup of WIP - Backward scheduling
The due date is thought of first, and then the time to start is found. Schedule jobs in reverse order
What are the goals of short-term scheduling?
Minimize:
1. WIP inventory - number of jobs in system is used to identify
2. Waiting time for customers - average number of lates
3. completion time - average completion time
maximize
utilisation - percent facility is utilized
What are the 3 approaches in determining how to load jobs in work centers?
- Input/output control or ConWIP- constant WIP
- Gantt Charts
- Assignment method
What is input/output control or CONWIP?
input/output control is about keeping WIP constant. It wants to produce x items only when x items have been released/finished. It manages facility work flows
What are CONWIP cards?
ConWIP is a technique to keep WIP constant. It controls amount of work. ConWIP card travels with a job through the process and when the job is done, it returns to the initial workstation and authorizes entry. limits work amount
What are gantt load charts?
Gantt load charts show how tasks are divided and assigned to on which days. Disadvantage is it does not account for unexpected disturbances and needs to be frequently updated. It can also be used to track job progress
What is the assignment method?
Assignment method is based on binary LP. It uses constaints of 1 job per resource and 1 resource per job.
What is the hungarian method of assignment method?
- minus lowest number in row for each cell in row. minus lowest number in column for each cell in column
- draw minimum lines necessary to cover maximum zeros as possible
- If lines equal the amount of columns/rows, stop. If less, then find the smallest number in uncovered cell and minus it from other uncovered cell. add that to the intersections of lines
- do lines again. look at doc
what is sequencing and the different sequencing rules?
Sequencing is the task of choosing which tasks should be done first.
FCFS - first come first serve. orders placed first are done
EDD - earliest due date. orders with the earliest due date are done
SPT - shortest process time. tasks that can be done fastest
LPT - longest process time
CR - critical ratio. the time remaining till due date divided by time required for order
What are the definitions of CR?
CR = 1 means you have exactly enough time left to complete task
CR < 1 means you dont have enough time left to complete your job. required time for task is larger than time remining till due date
CR > 1 means job is ahead of schedule and has some slack.
Place order from smallest CR to largest
What are the priority rules/formulas?
Average completion time = sum of flow time / number of jobs
Utilisation = sum of process time / sum of flow time
Average number of job = sum of flow timre / sum of process
average job lateness = sum of late days / number of jobs
What is processing time
The task time, how long it takes for task to be doneW
What is flowtime?
the total time it takes to do task when previous tasks are taken into account. So the time between order placed and order finished. order may have been placed at same time but has to wait for previous orders to be finsiehd. the time between the release of a job to a work center until the job is finished.
What are the general rules of the sequence rules
EDD has the smallest average job lateness
SPT has the best completion times but disadvantage is long duration jobs wait long
FCFS is considered fair in customer pov
What is the theory of constraint?
TOC is all about the constraints that limits the objective or firm’s ability to achieve a goal.
