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MODULE 3 > 6 - Scheduling > Flashcards

6 - Scheduling Flashcards

1
Q

Which are the Inputs/outputs of the Production scheduling table?

A 
Input= Production plan.
Output= Plan of jobs, Plan of people, Plan of resources.
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2
Q

Mention some Critical issues in Production scheduling.

A

*Huge amount of data:

  • technical
  • managerial
  • Uncertainty of working data -variability (e.g product variants, etc)
  • unpredictability (eg. failures,etc))
  • Uncertainity (eg. rowking times, etc)
  • Difficulties in the objective definition.
  • efficiency (e.g machine saturation)
  • or effectiveness (delay minimization, low completion times)
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3
Q

In Production Scheduling, define Due Date and mention others plans related with Due Date jobs.

A

Due Date (DD): It is a date in which a job should be completed (forced by higher planning levels or by final client)

  • EDD = Earliest Due Date: The earliest planned date an order may be completed
  • LDD = Latest Due Date: The latest planned date an order may be completed
  • ESD = Earliest Start Date: The earliest planned date at which it is possible to start working the order
  • LSD = Latest Start Date: The latest planned date at which it is possible to start working the order without missing its due date
  • Slack = DD – Processing time – ESD (static, before releasing job)
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4
Q

In Production Scheduling, which is the meaning of JOB?

A
  • It is a working unit composed by physical units that are all of the same kind.
    It could be a single piece or a box or a batch.
    It could happen that there is no correspondence between job and client order.
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5
Q

In Production Scheduling, which is the meaning of ORDER?

A

The product or service required by a customer.

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6
Q

In Production Scheduling, which is the meaning of ROUTING?

A

-The sequence of stages of production that an order has to go through. The routing specifies many details including standard time, tooling required, etc.

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7
Q

In Production Scheduling, which is the meaning of PREEMPTION?

A

-Stopping of the processing of one job on a machine in order to process another one that is more urgent.

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8
Q

In Production Scheduling, which is the meaning of PASSING?

A
  • Overtaking between jobs waiting to be processes, for urgency reasons.
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9
Q

Which are the Classifications profiles in Production Scheduling? (be schematic in the 3-axis diagram)

A

X - Specific objectives.

Y - Production system typology

  • plant configuration
  • type of constraints considered

Z - Used resolution technique.

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10
Q

Mention some of the “possible production system configurations” in Production Scheduling.

A
  • Single machine
  • Parallel machine
  • Job Shop
  • Flow Shop
  • Open Shop
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11
Q

About the basic scheduling principles.. What does the FORWARD SCHEDULING consist?

A

-It starts from the ESD (static, before realising job) of each job and plan ahead the individual operations of the routing of the job, calculating, with the help of the lead time of each operation, the ESD and EDD of each operation.

The processing is done for all jobs, considering operations already allocated and, thus, also the EDD (Earliest Due Date) of the plan is calculated (the latest of the EDD’s).

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12
Q

About the basic scheduling principles.. What does the BACKWARD SCHEDULING consist?

A

-It starts from the DD (Due Date) of each job (equal to the LDD (Latest Due Date) ) and individual operations of the job are planned BACKWARDS calculating their LDD and LSD (Latest Start Date), based on their routing and related lead times of each operation. The Schedule is done for all jobs and, thus, also the LSD of the plan is obtained (the smallest of the LSD)

The operations are allocated on the various resources of the system defined by the technological routing, also considering the operations allocated by other jobs.

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13
Q

*Mention the main objectives in Basic Scheduling

A
  • Minimize average Lateness.
  • Minimizing average tardiness.
  • Minimizing average Flowtime.
  • Minimizing number of late jobs (tardiness different from zero)
  • Minimizing “Makespan”
  • Maximizing average rate of saturation of the system (working time / available time).
  • Composed Weighted Criteria of the previous ones.
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14
Q

How can we define the Priority Index?

A

The job to be loaded on the machine is chosen among the ones available based on a priority index (also referred as a “Figure of worthiness”).
-The priority index is computed for each job based on one or more of the following variables:

  • Processing time
  • Set-up time
  • Due date
  • Job state
  • Machine/plant state
  • Delay costs
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15
Q

How can we define the Dispatching Rules?

A
  • Often, due to the COMPLEXITY and the DYNAMIC NATURE of the SYSTEM and ENVIRONMENT analyzed, an operative plan, that pre-defines allocations and operations sequences, is not formulated: if it’s formulated, it happens to be interrupted and often cancelled, so not generating good results
  • Instead, RULES to take decisions each moment are provided
  • This approach is generally (with some rare exceptions) HEURISTIC but flexible and relatively easy to use
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16
Q

Provide the scheme of the Dispatching Rules (3-axis diagram)

A

X - Information Update
- Static:
The reciprocal priority of two jobs in queue at the same machine does not change over time
The priority index of each job can be computed once for all when the job enters the queue at the machine

  • Dynamic:
    The reciprocal priority of two jobs in queue at the same machine can change over time
    The priority index of each job must be re-computed any time a decision is taken

Y - Variables used for making decision

Z - Information Extent
- local: The priority index for a job on a machine is calculated only with information regarding the operations on that (i.e. only the next) machine

  • Global: The priority index for a job on a machine is calculated with information regarding also the operations on other (i.e. all the remaining) machines
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17
Q

Provide examples of Dispatching Rule

A

*Processing time or Set-up time
-SPT (Shortest Processing Time) [see later]
-LPT (Longest Processing Time)
-TSPT (Truncated Shortest Processing Time): as SPT, but with a waiting time threshold; when overcome, the FIFO [see later] is applied
-LWKR (Least Work Remaining): min. residual total processing time
-TWORK (Total Work): min. total processing time
-MSUT (Minimun Set-up Time): min. set-up time on the machine
…
*Due date
-EDD (Earliest Due Date) [see later]
-OPNDD (Operation Due Date) [see later]

  • System status
  • NINQ (Number In Next Queue): min. number of jobs already queued for the next operation
  • WINQ (Work In Next Queue): min. total processing time (i.e., sum of the processing time of jobs) already queued for the next operation

*Job status
FIFO (First In First Out)
LIFO (Last In First Out)
FISFS (First In the System First Served): the first job that entered the system for the first time
FROP (Fewest Remaining Operations): min. number of residual operations
MROP (Most Remaining Operations): max. number of residual operations

*Economic factors
COVERT: max. ratio = delay cost / residual time

*Weighted rules
SPT/LWKR: min  PT + (1- ) RT (PT = processing time; RT = residual total processing time)

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18
Q

Mention the Benefits of Dispatching Rules

A
  • Simple
  • Limited number of information needed
  • Flexibility of use
  • Easy to transfer and to reconfigure
    • They are not strictly linked to production system characteristics
  • Decision taken at the latest
    • A plan defined a priori becomes obsolete after few hours
  • Good practical results
  • Few valid alternatives for a job shop
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19
Q

In which constist The Shortest Processing Time. [SPT]

A
  • Variable: processing time
  • Static
  • Local
  • First the job with the shortest processing time on the considered machine
  • In case of single machine, it minimizes the average flowtime
  • In general, it provides very good results in terms of average flowtime
  • Risk of large delays of the longest jobs
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20
Q

In which consist The Earliest Due Date [EDD]

A
  • Variable: Due date
  • Static
  • Local
  • First the job with the earliest due date
  • It provides very good results in terms of number of tardy jobs
  • It neglects the processing time therefore it penalizes the longest jobs
21
Q

In which consist Slack per Operation [S/OPN]]

A
  • Variables: Due date, processing time, present time (now)
  • Dynamic
  • Global
  • Compute for each job the SLACK = due date – present time – residual total processing time
  • Compute the rate S/OPN = SLACK / number of residual operations
  • First the job with lowest S/OPN
  • It provides very good results in terms of punctuality (especially if the due date have been “reasonably” defined)
  • It needs to replicate a computation every time the rule is applied
22
Q

Define a Local Index of Information Extent

A

-Priority Index for a job on a machine is calculated only with information regarding the operations on that machine. (i.e. only the next)

23
Q

Define a Global Index of Information Extent

A

The priority index for a job on a machine is calculated with information regarding also the operations on other machines (i.e. all the remaining machines).

24
Q

Define in the Information Update the Static Index.

A
  • The reciprocal priority of two jobs in queue at the same machine does not change over time.
  • The priority index of each job can be computed once for all when the job enters the queue at the machine.
25
Q

Define in the Information Update the Dynamic Index.

A
  • The reciprocal priority of two jobs in queue at the same machine can change over time.
  • The priority index of each job must be recomputed any time a decision is taken.
26
Q

Describe the approach of Operative Planning-based approach of the Production scheduling

A

[i.e. scheduling in the strictest meaning of the word] This approach tries to solve a priori and completely the scheduling problems (when, where) and the detailed production plans are made once and for all.

27
Q

Describe the approach of Production Control-based approach of the Production scheduling

A

[i.e. Dispatching rules] This approach aims at making production decision in the short-term as they are requested, moving in the direction of the objective chosen. It adopts almost always heuristic solving techniques.

28
Q

Mention the benefits of Dispatchng rules?

A
  • Simple
  • limited number of information needed.
  • Flexibility of use.
  • Easy to transfer and to reconfigure (they are not strictly linked to production system characteristics).
  • Decision taken at the latest. ( a plan defined a priori becomes obsolete after dew hours.
  • Good practical results.
  • Few valid alternatives for a job shop
29
Q

On what does the Production Scheduling consists?

A

Operational Planning (or scheduling): the activity that actually launches production orders

Determination of:

  • what
  • how much
  • When
  • **Where

To produce:
*At the level of single batch of each item and at machine level
*On a short term horizon (normally weekly)
*With temporal detail level the day or the hour
–
Data:
*Production orders generated in the previous planning phases
*Resource availability
Actual state of production systems and inventory level.

30
Q

Of what is the Operational planning (or scheduling) is made?

A
  • Off-line scheduling

* On-line Scheduling

31
Q

about Off-line scheduline?

A

the programming tasks (performed off-line) that allows to plan on a short horizon (usually weekly) the workload coming from the medium-term planning, taking into account the constraints and objectives of the typical production system to schedule. This type of planning can be done manually with support of computer (usually a PC) for shop floor scheduling

32
Q

about On-line scheduline?

A

on-line scheduling (which can be performed for human operation or for automated systems) is composed by:

*Job loading:
it carries out the placing of the job in the production system with its workload, after this the job is considered as eligible for the subsequent control mechanisms of production management (i.e. dispatching);

*Sequencing (or Dispatching):
it is the process of activation of individual steps (operations) which are part of the processing of a job; in general, dispatching is in charge of the selection of the job to be activated from the different jobs in the queue in front of individual resources of the system.

33
Q

Which is the requested data on Production Scheduling?

A
  • Production orders:
  • what
  • how much
  • when
  • Bill of materials
  • For assembly
  • Raw material / semi-finished product
  • Production or assembly -cycles
  • Machines
  • Tools / Fixture
  • Processing times
  • Yield
  • Set-up
  • Calendar of availability of resources
  • Machines
  • Workforce
  • Tools / Fixture
  • System status
  • In-progress works
  • Resources status
  • Inventory levels
34
Q

Production Scheduling- Hypotheses?

A
  • It is not possible to call the decisions taken at higher planning level into question.
  • Quantity and delivery date are fixed
  • Available resources (workforce, machines, tools, …) are completely known and it is not possible to change them
  • Usually it is supposed to have only one critical resource (generally the machines)

-In this case, the other resources (e.g., workers) are supposed to be always available

  • Jobs are completely defined from a technical point of view (cycles, processing, set-up times, machines, … that can be used for every operation)

-No alternative routings are allowed

  • Transportation time between two following phases are negligible
  • A job is available for an operation as soon as it has finished to be processed by the previous one
  • -
  • Presence and influence of decoupling point is not taken into account
  • -
  • A machine can process only one job at any given time
  • A job can only be processed by one machine at any given time
  • It can not be processed in parallel by two identical machines
  • It can not begin to be processed on a new machine until it has not completed to be processed on the previous one

*Stock keeping costs are neglected
Due to the short planning horizon

35
Q

Possible production system configurations - SINGLE MACHINE?

A

there is only one machine and all jobs must be processed on it. Once a job has been processed by the machine, it is completed

36
Q

Possible production system configurations - PARALLEL MACHINE?

A

each of them can perform the only operation required for all the jobs

  • Identical
  • Not identical (processing time dependent on the machine)
37
Q

Possible production system configurations - JOB SHOPS?

A

each job must be processes on some or all the machines (or shops), but each job may have a different routing

38
Q

Possible production system configurations - FLOW SHOP?

A

consists of many machines or shops, each of them can perform a different operation (e.g., drilling, turning, …), but all the jobs have the same pre-defined routing

39
Q

Possible production system configurations - OPEN SHOP?

A

consists of many machines or shops, each of them can perform a different operation (e.g., drilling, turning, …), but jobs routing has not been fixed a priori on technological basis (no precedencies).

NOTE; Used essentially for checking available production capacity, disregarding the constraints raised by fixed job routing (precedencies)

40
Q

Which are the possible constraints (limitations) of Production Scheduling?

A
  • Setup times
  • Indipendent or dependent on the sequence
  • In case, dependent on machine state (past history)
  • Preemption allowed or not allowed
  • Passing allowed or not allowed
  • Processing time deterministic or stochastic
41
Q

Is the Production Scheduling phase more Deterministic than the other ones belonging to the overall PP&C process?

A

YES
Therefore, dta on jobs and resources can in fact be assumed as known.

For this reason, further assumption can be considered as verified:

  • A machine (or workcenter) can work one workpiece at a time,
  • The effect of the interoperational buffers is negligible,
  • Transportation times are negligible,
  • The time of any single operation can be considered deterministic.
42
Q

what does the Production schedule mean in terms of foreseeing?

A

Schedule generation means to foresee when the start and end of each operation of the job will occur, taking into account the availability of resources required (from the technological cycle).

43
Q

About Basic Scheduling pronciples (Forward & Backward). In which cases is better to use the former and latter?

A

FORWARD (—>) it is better to plan the activities with the Forwards approach for the jobs in the closest time horizon, immediatley after Tnow (time now).

BACKWARD (

44
Q

Why is it better to plan activities for jobs in the closest time horizon by means of FORWARD approach?

A

The reason is because in the near time horizon it is better to move forward without loosing a time that will never be possible to recover,

while in the far away horizon it is better to plan at the latest with the purpose of not inefficiently anticipate the production activity.

Note: Of course the two approaches must then be properly “reconciled” in between.

45
Q

Which are the two approaches used as Scheduling solving techniques?

A
  • Operative Planning-based approach (i.e. scheduling; in the strictest meaning of the word):
    This approach tries to solve a priori and completely the scheduling problems (when, where, ..) and the detailed production plans are made once and for all
  • Production Control-based approach (i.e. dispatching rules).
    This approach aims at making production decisions in the short term as they are requested, moving in the direction of the objective chosen. It adopts almost always heuristic solving techniques.
46
Q

What does Loading mean?

A

Loading defines the job that is to be released to a production system

-Job loading is the process of releasing a job order as a new workload to a production system: when released a job is made available to the processing of sequencing algorithms.

47
Q

What does Sequencing mean?

A

The sequence in which orders are processed at a work centre has a significant effect on performance (especially under overload conditions)
Note: In practice: companies use “rule-of-thumb” algorithms & basic dispatching rules

48
Q

On what does the OPNDD – Operation Due Date consist?

A
  • Variables: Due date, processing time, ongoing operation
  • Static
  • Global
  • It calculates for each operation an OPNDD by dividing the interval between the moment when the job enters the system and the due date among all the operations
  • In equal parts, or
  • In parts proportional to processing times
  • First the job with the earliest OPNDD for the ongoing operation
49
Q

Name the possible production system configurations

A
  • SINGLE MACHINE
  • PARALLEL MACHINES
  • JOB SHOP
  • FLOW SHOP
  • OPEN SHOP

MODULE 3 (6 decks)

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