Planning & control activities, Loading, Sequencing, Johnson’s Rule of Sequencing, Scheduling, Forward & Backward Scheduling, Gantt Charts

Planning & control activities

Planning & control requires the reconciliation of supply & demand in terms of volumes, timing & quality

Loading

The amount of work that is allocated to a work center

Finite loading

Approach which only allocates work to a work center (person, machine etc.) up to a set limit. 

Particularly relevant for operations where

- It is possible to limit the load
- It is necessary to limit the load
- The cost of limiting the load is not prohibitive

Infinite loading

Approach to loading work which does not limit accepting work, but instead tries to cope with it. 

Particularly relevant for operations where

- It is not possible to limit the load
- It is not necessary to limit the load
- The cost of limiting the load is prohibitive

Sequencing

Decisions must be taken on the order in which the work will be tackled.

The priorities are often determined by some predefined set of rules, some are

• Physical constraints

- Priority of work is determined by physical nature of the materials

• Customer priority

• Due date (DD)

- Work is sequenced according to when it is ‘due’ for delivery, irrespective of the size of each job or the importance of each customer

• Last-in first-out (LIFO) (Unloading an elevator)

•  First-in first-out (FIFO) sometimes called ‘first come, first served’ (FCFS)

•  Longest operation time (LOT)

- Sequence the longest jobs first in the system

• Shortest operation time first (SOT)

- Sequence the shortest jobs first in the system

Johnson’s Rule of Sequencing

Technique for minimizing makespan for a group of jobs to be processed on two machines or at two work centers. (Makespan is the total time needed to complete a group of jobs)

• - Minimizes total idle time

Conditions must be satisfied

• Job time must be known & constant for each job
• Job times must be independent of the job sequence
• All jobs must follow the same two-step work sequence
• A job must be completed at the first work center before the job moves on to the second work center

Steps

• Select the job with the shortest time. If the shortest time is at the first work center, schedule that job first; if the time is at the second work center, schedule the job last. Break ties arbitrarily

• Eliminate the job & its time from further

• Repeat steps 1 & 2, toward center & complete all jobs

Judging sequencing rules

The following performance objectives are often used judge the effectiveness of sequencing rules

• Meeting ‘due date’ promised to customer (dependability)
• Minimizing the time the job spends in the process, also known as ‘flow time’ (speed)
• Minimizing work-in-progress inventory (an element of cost)
• Minimizing idle time of work centers (another element of cost)

Scheduling

Establishing the timing of the use of equipment, facilities, & human activities in an organization

• Effective scheduling can yield cost savings, increases in productivity, & other benefits
• The most complex tasks in operations management

Forward scheduling

Forward scheduling involves starting work as soon as it arrives

- Scheduling ahead from a point in time

Backward scheduling

Backward scheduling involves starting jobs at the last possible moment to prevent them from being late

- Scheduling backward from a due date

The choice of backward or forward scheduling depends largely upon the circumstance

Gantt charts

Chart used as visual aid for loading & scheduling purposes

•  A simple device which represents time as a bar, or channel, on a chart
• Purpose of Gantt charts is to organize & visually display the actual or intended use of resources in a time framework

A time scale is represented horizontally & resources are represented vertically

Scheduling work patterns

The schedule of work times effectively determines the capacity of the operation.

- Make sure that sufficient numbers of people are working at any point in time to provide a capacity appropriate for the level of demand at that point in time, is often called staff Rostering.

• Capacity matches demand
• The length of each shift is neither excessively long nor too short
• Working at unsocial hours is minimized
• Days off match agreed staff conditions (for example – staff prefer two consecutive days off every week)
• Vacation & other ‘time-off’ blocks are accommodated
• Sufficient flexibility is built into the schedule to cover for unexpected changes in supply & demand

Scheduling staff times is one of the most complex of scheduling problems

Monitoring & controlling the operation

Having created a plan for the operation through loading, sequencing & scheduling, each part has to be monitored to ensure that planned activities are indeed happening

Push & pull control system

Drum, buffer, rope

The drum, buffer, rope concept comes from the theory of constraints (TOC)

Drum: The bottleneck in the process should be the control point of the whole process, because it sets the ‘beat’ for the rest of the process to follow

Buffer: Potentially constraining resources outside of the bottleneck, the role is to keep a small amount of inventory ahead of the bottleneck operation to minimize the risk of having it be idle

Rope: The synchronizing of the sequence of operations to ensure effective use of the bottleneck operations, goal is to avoid costly & time-consuming multiple setups, particularly of capacity-constrained resources, so they do not become bottlenecks too

Five-step procedure

• Determine what is constraining the operation

• Exploit the constraint (constraint resource is used to its maximum)

• Subordinate everything to the constraint (focus on the constraint)

• Determine how to overcome (eliminate) the constraint

• Repeat the process for the next highest constraint

The degree of difficulty in controlling operations