sequencing theory

Sequencing


Q1 how do scheduling activities differ for projects, mass production and process industries?
ANS
In the process industry continuous and batch production systems can be distinguished.There exists also semi-batch production which combines features from both. Plants producing only a limited number of products each in relatively high volume typically use special purpose equipment allowing a continuous flow of materials in long campaigns,i.e.,there is a continuous stream of input and output
products with no clearly defined start or end time.Alternatively,small quantities
of a large number of products are preferably produced using multi-purpose equipment which are operated in batch mode,i.e.,there is a well-defined start-up,e.g., filling in some products,well-defined follow-up steps defined by specific recipes, e.g.,heating the product,adding other products and let them react,and a clearly definedend,e.g.,extracting the finished product. Batch production involves an integer number of batches where a batch is the smallest quantity to be produced,e.g.,500 kg. Several batches of the same product following each other immediately establish a campaign. Production may be subject to certain constraints,e.g.,campaignsare built up by a discrete number of batches,or a minimal campaign length (or production quantity) has to be observed.Within a fixed planning horizon,a certain product can be produced in several campaigns; this implies that campaigns have to be modelled as individual entities.
Another special feature in the refinery or petrochemical industry or process
industry in general is the pooling problem ,an almost classical problem in nonlinear optimization. It is also known as the fuel mixture problem

Q2why is scheduling a job shop so difficult?

Ans:-Scheduling has become a critical factor in many job shops in order to determine their capacity for more work and be able to schedule their work more efficiently. Job shop scheduling becomes more and more difficult when you deal with assemblies and/or multiple components which need to be made in an efficient manner.
Job Shop Scheduling is an optimization problem in which ideal jobs are assigned to resources at particular times. The most basic version is as follows: We are given n jobs J1, J2, …Jn of varying sizes, which need to be scheduled on m identical machines, while trying to minimize the total length of the schedule (that is, when all the jobs have finished processing). Scheduling job shops is much more complex then the basic version above because there are many more constraints involved. In the above method the machines are identical which is not going to happen in an actual job shop. Online problem vs offline problem In an online scheduling system the scheduling application looks at a single piece of information at a time and does not know whats coming down the pipeline. In this situation the entire input is not known from the start. Because it does not know the whole input, an online algorithm is forced to schedule the job without knowledge of the entire input available at the start. The online algorithm needs to make a decision about that job before the next job is presented. In an offline scheduling system the scheduling application the system is given the whole problem data from the beginning and is required to answer scheduling problem given in an optimal way. Because an online scheduling system does not know the whole input, an online algorithm is forced to make decisions that may later turn out not to be optimal, and the study of online algorithms has focused on the quality of decision-making that is possible in this setting.
Q.no 3-
What the function are typically performed by a production control department ?

Ans –
Production control department responsibilities consist of
1. Loading—checking the availability of material, machines, and labour. The MRP system
plans for material availability. CRP converts the material plan into machine and
labour requirements, and projects resource overloads and under loads. Production
control assigns work to individual workers or machines, and then attempts to
smooth out the load to make the MRP schedule "doable." Smoothing the load is
called load levelling.
2. Sequencing—releasing work orders to the shop and issuing dispatch lists for individual
machines. MRP recommends when orders should be released (hence the name,
planned order releases). After verifying their feasibility, production control actually
Releases the orders. When several orders are released to one machine center, they must
be prioritized so that the worker will know which ones to do first. The dispatch list
contains the sequence in which jobs should be processed. This sequence is often based
on certain sequencing rules.
3. Monitoring—maintaining progress reports on each job until it is completed. This is
important because items may need to be rescheduled as changes occur in the system.
In addition to timely data collection, it involves the use of Gantt charts and input/output
control charts.
Q.no 4 –
Explain Johnsons rule and its Procedure .
Ans-Since few factories consist of just one process, we might wonder if techniques exist that will
produce an optimal sequence for any number of jobs processed through more than one
machine or process. Johnson's rule finds the fastest way to process a series of jobs through a
two-step system in which every job follows the same sequence through two processes. Based
on a variation of the SPT rule, it requires that the sequence be "mapped out" to determine
the final completion time, or makespan, for the set of jobs. The procedure is as follows:
1. List the time required to complete each job at each process. Set up a one-dimensional
matrix to represent the desired sequence with the number of slots equal to the number
of jobs.
2. Select the smallest processing time at either process. If that time occurs at process 1,
put the associated job as near to the beginning of the sequence as possible.
3. If the smallest time occurs at process 2, put the associated job as near to the end of the
sequence as possible.
4. Remove the job from the list.
5. Repeat steps 2-4 until all slots in the matrix have been filled or all jobs have been
sequenced.
Question 5

What is PRIORITY SEQUENCING

Choice of right sequencing rule based on one criteria criteria
becomes quite critical as no singular rule is ideal for all situations
some of the criteria used are :
  Set-up cost or change over cost
  Work in progress inventory cost
  Idle time
  No. of jobs late
  No. of jobs early
  Average no. of jobs
  Average flow time
  Average time to complete a job


SINGLE CRITERIA PRIORITY SEQUENCING RULES

1. First come first serve (FCFS)

2. Shortest processing time(SPT) this rule minimize the WIP
inventory cost and at same time the throughput time

3. Longest processing time (LPT) jobs having longest processing
time are scheduled first

4. Least slack rule Highest priority is given to the job which had
least slack
Slack= available time-processing time

5. Earliest due date(EDD) the jobs having their earliest due date
given highest priority

6. Preferred Customer order

7. Random selection

8. (TSPT) this rule sequence the jobs acc. To the SPT except the
jobs have been waiting for a time period longer than a specified
truncated time are given higher priority than other jobs

9. COVERT it uses the ratio of expected delay cost to the
processing time
COVERT=expected delay cost/processing time

10. Least change over cost priority is given by analyzing the
total cost of the making machine change over cost b/w the jobs



Question 6
What are sequenceing rules ? How are they evaluated?
EVALUATION OF SEQUENCING RULES:


1. Average job flow time is define as the total time spent by job
in system
Average job flow time =job waiting time +job processing
time

2. Average no. of jobs flowing in the system (waiting or being
processed from the beginning of sequencing when the last
job is finished

3. Average job lateness is defined as the diff. b/w the actual
completion time of job and its due date.

4. Average earliness of job if a job is completed before its due
date the lateness value is –ve and its magnitude is referred as
job earliness of the job

5. No.of tardy jobs
No. of jobs which are completed after the due date





7. What three functions are typically performed by a production control department?


Process Planning (Routing)

The determination of where each operation on a component part, subassembly, or assembly is to
be performed results in a route for the movement of a manufacturing lot through the factory.
Prior determination of these routes is the job of the manufacturing engineering function.

Loading

Once the route has been established, the work required can be loaded against the selected
machine or workstation. The total time required to perform the operation is computed by
multiplying the unit operation times given on the standard process sheet by the number of parts
to be processed. This total time is then added to the work already planned for the workstation.
This is the function of loading, and it results in a tabulated list or chart showing the planned
utilization of the machines or workstations in the plant.

Scheduling

Scheduling is the last of the planning functions. It determines when an operation is to be
Performed, or when work is to be completed; the difference lies in the detail of the scheduling
procedure.



8.Give the type of operations (manufacturing or service) and suggest which scheduling objectives might be appropriate for each ?


In a manufacturing content, intermittent systems are traditionally referred to as Job-Shops. As the work order arrive , the work load on the facility increases.Some work centres may be idle at the same time that orders are severely overloaded. A work centre may experience a large build-up of orders awaiting processing.
The sequencing in which the waiting jobs are processed is critical to the efficiency and the effectiveness of the intermittent system. Sequencing affects how many jobs are completed on time versus late , costs incurred for setup and change-over , delivery load times , inventory costs , and the degree of congestion on the facility.

SERVICES

Intermittent systems may be used in both the product and service organizations. In the restaurants and the automobile repair shops for example conversion systems are similar to concept to those in manufacturing job-shops.


Q 10- What information is provided by the critical sequencing rule? How does it differ from Slack?
Ans-   Variations on the DDATE rule include minimum slack and the smallest critical ratio. Slack considers the work remaining to be performed on a job as well as the time remaining (until due date) to perform that work. Jobs are processed first that have the least difference between the two as follows:
SLACK = (due date-today’s date)-(remaining processing time)
The critical ratio uses the same time as slack but arranges it in ratio form so that scheduling performance can be easily assessed. Mathematically, the CR is calculated as follows:
CR= time remaining      =   due date-today’s date
        Work remaining          remaining processing time
If the work remaining is greater than the time remaining, the critical ratio will be less than1. If the time remaining is greater than the work remaining, the critical ratio will be greater than 1. If the time remaining equals work remaining, the critical ratio exactly equals 1. The critical ratio allows us to make the following statements about our schedule:
If CR>1, then the job is ahead of schedule
If CR<1, then the job is behind schedule
If CR=1, then the job is exactly on schedule

Q12 What are gantt charts & why are they used so often ?
Ans-- A Gantt chart is a type of bar chart that illustrates a project schedule. Gantt charts illustrate the start and finish dates of the terminal elements and summary elements of a project.Terminal elements and summary elements comprise the work breakdown structure of the project.Gantt charts also show the dependency (i.e., precedence network) relationships between activities. Gantt charts can be used to show current schedule status using percent-complete shadings.
Gantt charts have become a common technique for representing the phases and activities of a project work breakdown structure (WBS), so they can be understood by a wide audience all over the world.

Q.NO.13 EXPLAIN THE CONCEPT BEHIND INPUT/OUTPUT CONTROL. DESCRIBE HOW GATEWAYWORK CENTERS, DOWNSTREAM WORK CENTERS,AND BACKLOG EFFECT SHOP PERFORMANCE?
ANS:   Input/output is a technique that allows operation to manage facility work  flow. It is used to control the size of the queues in front of work centres, thereby helping to control manufacturing lead times. Refer to as "push system" of linking work centres. When   batch of items is completed at one work centre, it is pushed to the next work centre, where it waits in a queue until it is selected to be worked at that work centre.
                Input-output Control is important because it is a form of queue control, and a great
        portion of the time that a job spends in a plant is spent waiting in queues. In many job shops
        Input-output Control is used to monitor and control the amount if work in a queue at a work
        centre so that it stays within reasonable bounds. Queue times, therefore, are more consistent
        and predictable. The actual output, in standard hours of works, that flows from a work centre
        is the demonstrated actual capacity of the centre.
                If the capacity differs from the planned amount of work over a few scheduling periods,
        the problem needs to be investigated. If the work that flows to work centre is greater than the
        actual output, the queue in front of the centre will grow. On the other words, If the works is
        arriving faster than it is being processed, we are overloading the facility and a backlog
        develop.Overloading causes crowding in the facility, leading to inefficiencies and quality    
        problem. If the works is arriving at a slower rate than it is being performed, we are
        underloading the facility and the work centre may run out of work. Underloading the facility
        results in idle capacity and wasted resources.


Q.no.14 EXPLAIN THE DIFFERENCE BETWEEN FINITE AND INFINITE SCHEDULING??
ANS: Finite Scheduling
Scheduling type within capacity planning that takes account of the capacity loads which already exist.  Finite scheduling calculates the start and finish dates for operations in the order.  It is a detailed scheduling strategy with which you schedule orders and operations, taking into account the existing resource load. A resource overload cannot occur.
Infinite Scheduling
A detailed scheduling strategy with which you schedule orders and operations, without taking into account the existing resource load. It is therefore possible for resource overloads to occur.
R/3 does planning without consideration for capacity situations. So if MRP says you need 500 parts on 3/1/04, it schedules them all to be built at the same time, even though you can only do 100 at a time. Assume you have a fixed lot size of 100, you'll get 5 planned orders for 100 to start on the same day. This is "Infinite Planning". APO would recognize that constraint, and instead schedule out the 5 orders over time. The important part of that is that it also will schedule out the deliveries of the components for 5 different days. This is "Finite Planning".
Now, assume old fashion MRP. It schedules all 5 orders for the same day, and the buyers go out an get all of the components for the same day. Then the planner realizes he can't do all 5, and manually changes the schedule, and manually spreads out the 5 orders. The buyers will recieve rescheduling notifications, but not until the scheduler does the manual rescheduling.
You could call this "Infinite Scheduling", but that only means the same thing as Infinite Planning.
15 How does theory of constraints differ from traditional scheduling?
Sol-THE THEORY OF CONSTRAINTS
The theory of constraints is a philosophy that has a lot in common with Just-in-Time but also has some critical differences. There are two basic differences. The first is that the theory of constraints accepts the existence of a constraint, at least temporarily, and focuses the improvement effort on the constraint and related workstations. The second is that the theory of constraints uses overlapped production (transfer batch not equal to the process batch) to schedule work through a batch production environment, while Just-in-Time provides no scheduling mechanism for a batch environment. Thus, the theory of constraints scheduling approach has wider applicability than Just-in-Time (although Just-in-Time's continuous improvement philosophy and quality emphasis clearly is applicable to batch production environments).
There are five steps to the theory of constraints: identify the constraint, exploit it, subordinate everything else to it, elevate the constraint, and avoid inertia when the constraint shifts. In exploiting the constraint, the drum-buffer-rope scheduling technique and buffer management are used. In finding ways to elevate the constraint, the techniques of effect-cause-effect and the cloud diagram often are useful.

Traditional scheduling

Long before there was absolute priority, Unix systems were scheduling the CPU using this system. When Posix came in like the Romans and imposed absolute priorities to accommodate the needs of realtime processing, it left the indigenous Absolute Priority Zero processes to govern themselves by their own familiar scheduling policy.
Indeed, absolute priorities higher than zero are not available on many systems today and are not typically used when they are, being intended mainly for computers that do realtime processing. So this section describes the only scheduling many programmers need to be concerned about.
But just to be clear about the scope of this scheduling: Any time a process with a absolute priority of 0 and a process with an absolute priority higher than 0 are ready to run at the same time, the one with absolute priority 0 does not run. If it's already running when the higher priority ready-to-run process comes into existence, it stops immediately.
In addition to its absolute priority of zero, every process has another priority, which we will refer to as "dynamic priority" because it changes over time. The dynamic priority is meaningless for processes with an absolute priority higher than zero.
The dynamic priority sometimes determines who gets the next turn on the CPU. Sometimes it determines how long turns last. Sometimes it determines whether a process can kick another off the CPU.
In Linux, the value is a combination of these things, but mostly it is just determines the length of the time slice. The higher a process' dynamic priority, the longer a shot it gets on the CPU when it gets one. If it doesn't use up its time slice before giving up the CPU to do something like wait for I/O, it is favored for getting the CPU back when it's ready for it, to finish out its time slice. Other than that, selection of processes for new time slices is basically round robin. But the scheduler does throw a bone to the low priority processes: A process' dynamic priority rises every time it is snubbed in the scheduling process. In Linux, even the fat kid gets to play.
The fluctuation of a process' dynamic priority is regulated by another value: The “nice” value. The nice value is an integer, usually in the range -20 to 20, and represents an upper limit on a process' dynamic priority. The higher the nice number, the lower that limit.
On a typical Linux system, for example, a process with a nice value of 20 can get only 10 milliseconds on the CPU at a time, whereas a process with a nice value of -20 can achieve a high enough priority to get 400 milliseconds.
The idea of the nice value is deferential courtesy. In the beginning, in the Unix garden of Eden, all processes shared equally in the bounty of the computer system. But not all processes really need the same share of CPU time, so the nice value gave a courteous process the ability to refuse its equal share of CPU time that others might prosper. Hence, the higher a process' nice value, the nicer the process is. (Then a snake came along and offered some process a negative nice value and the system became the crass resource allocation system we know today).
Dynamic priorities tend upward and downward with an objective of smoothing out allocation of CPU time and giving quick response time to infrequent requests. But they never exceed their nice limits, so on a heavily loaded CPU, the nice value effectively determines how fast a process runs.
In keeping with the socialistic heritage of Unix process priority, a process begins life with the same nice value as its parent process and can raise it at will. A process can also raise the nice value of any other process owned by the same user (or effective user). But only a privileged process can lower its nice value. A privileged process can also raise or lower another process' nice value.
16   what are some typical issues involved in employee scheduling?
Sol-  Top 5 Challenges Caused by Traditional Employee Scheduling

Manual Staff Scheduling has Low ROI
Creating an employee schedule by hand is a daunting task for managers and supervisors. Matching worker availability to staffing requirements and controlling labor costs at the same time is complicated. When managers produce a schedule manually, they often end up causing inflated payroll in some areas while leaving other departments chronically understaffed.
Worse yet, achieving this sub-optimal result usually takes over 8 hours per week just to create a schedule for 70-100 workers. A manager who takes employee scheduling seriously may spend much more time on this task – up to 14 hours per week. Still, the outcome can be disappointing simply because this method is inefficient.

Employees Aren’t Kept Up to Speed
In today’s dynamic production environments, last minute changes are common. This means you have probably noticed the following problem with manually created employee schedules: workers are frequently unaware of last minute changes. That’s because they have no visibility in their schedules. This causes unintended time & attendance violations that managers and HR staff members have to investigate and address.
When an employee doesn’t show up to cover his/her shift, other workers must stay longer. This increases your overtime costs. Or, you have to cope with the inevitable problems caused by running a shift with a skeleton crew.
In addition to being confused by the lack of a transparent staff scheduling process, employees may also become resentful. It isn’t unusual for workers to feel that covert decisions regarding time off requests and shift scheduling are being made out of favouritism.

Payroll Errors Eat Up Valuable Resources
When it comes to getting timesheets turned in to Payroll, the problems with manual employee scheduling become even more evident. Each timesheet has to be tediously compared with logged hours. Managers must review each sheet after the fact, correct it, and send the edits to Payroll. This is not a productive use of time for your department heads. Of course, if a busy manager makes an error in editing, payroll employees have to edit (by hand) again.

Unbudgeted Expenses Cause Conflict
A decentralized, manual scheduling process makes it tough to keep staffing expenses and unapproved overtime under control. This is a common cause of unbudgeted departmental expenditures. Without real-time access to staffing data, current schedules, and budgeting parameters, managers have difficulty finding the most suitable, cost efficient employee to fill a gap. They have no choice but to make an uninformed decision and cope afterward with the consequences of incurring unbudgeted expenses.

Labor Law Compliance Risks
Inconsistency and poor record keeping are the most common offenses that trigger fines for labor law and worker pay violations. Without visibility in employee scheduling (and complete, accurate records), HR may be unaware of a problem until it is too late. This not only puts your company at risk for being penalized by government agencies, it also opens you up to litigation from employees.


Q:17
What quantitative techniques are available to help employee schedule & advanced planning and scheduling system ?
ANS:
Employee scheduling has lots of option because labor is a very flexible resource .The assignment method of linear programming is designed for employee scheduling problem of assign worker with difference performance rating available.Large scale linear programming is currently used by Mc Donald to schedule its large contingents.
Most employee scheduling problems are solved by heuristics (rules of thumb):
Employee scheduling heuristics
1. N= no. of worker
D = demand of workers
X = Day working
O = Day off
2. Assign the first N- D1 worker day1 off, assign next N-D2  worker 2 off and similar to next .
3. If no. of worker is less than 5 , assign the remaining workers so that consecutive days off are possible
4. Assign any remaining work to part time employee .
5. If consecutive dayoff desire consider switching scheduling among days with same demand requirements .
Q: 18
Look for advanced planning and scheduling software on the internet .write the summary of the techniques presents .
ANS:
Advanced planning and scheduling (also referred to as APS )a refers to a manufacturing management process by which raw materials and production capacity are optimally allocated to meet demand. APS is especially well-suited to environments where simpler planning methods cannot adequately address complex trade-offs between competing priorities . APS simultaneously plans and schedules production based on available materials, labor and plant capacity.
APS has commonly been applied where one or more of the following conditions are present:
1. Make-To-Order (as distinct from make-to-stock) manufacturing
2. capital-intensive production processes, where plant capacity is constrained
3. products 'competing' for plant capacity: where many different products are produced in each facility
4. products that require a large number of components or manufacturing tasks .
Advanced Planning & Scheduling software enables manufacturing scheduling and advanced scheduling optimization within these environments.
Advantages:
•Includes advanced scheduling features like overlap, storage constraints, and custom constraints.
•Gives the look-and-feel of Microsoft products for easy usage.
•Manages all constraints by taking into account materials, machines, labor, and tooling.
•Infinitely customizes activity labels on our powerful Resource Gantt and Job Gantt.
•Shows resource utilization and availability by looking at an organized Capacity Plan graph and grid.
•Highlights problems, like bottlenecks, in red so you’ll never miss a scheduling conflict again