3.1 What is shop floor control
(SFC)?
3.1.1 -
Definition
Shop Floor Control is defined as a system
for utilizing data from the shop floor to maintain and communicate status
information on shop/manufacturing orders and work centers. (Higgins, Leroy and Tierney 1996). It forms
the foundation of a production planning and control system and therefore plays
a crucial role in the overall design of a manufacturing system. However since
manufacturing systems are of such a large variety, different SFC designs exist
and these are typically customizations that fit the specific needs of a given
shop floor.
Scherer points out that the topic of Shop Floor
Control is not well understood owing to a theory and practice gap between the
situation in industry and in academia. In industry operator experience,
motivation and qualifications form the basis of Shop Floor Control while
academia concentrates on the problem of scheduling and its solution. In describing the situation in industry, he
identifies the shop floor as a provider of physical goods He further states
that it is faced with the challenge of becoming an agile entity within an
enterprise and within a network of enterprises forming a virtual organization.
He states that this challenge is posed by the current production environment,
which is constantly faced by changes and dominating customer demand. An example
of a study that tries to reconcile this gap is by Kenneth Mackay and John
Buzacott whose paper entitled “The application of computerized production
control systems in job shop environments” analyzes how the computer helps the
scheduler to do the task of scheduling in a job shop environment. In his paper,
he points out that analytical and alogorithmic aids have limited benefits to a
typical job shop. He suggests that the appropriate use of computer technology
can address information overload, cue filtering and assist the scheduler in
problem solving.
3.1.2 Objectives of Shop Floor Control
Spearman and
Hopp point out that Shop Floor Control plays an integral role in production and
when properly implemented it satisfies 4 objectives:
i.
It creates the ideal production system. In the various
literature surveyed, the ideal case was described as a pull system (to be
defined later in the chapter).
ii.
It provides an enabling environment for the workers
that makes the entire production system easy to understand. As a result the
system becomes easy to use.
iii.
It integrates easily with other planning functions. In
the case of MRP II, this would mean an ability to execute the plans generated
in long range planning and intermediate planning as well as providing feedback
to refine these functions.
iv.
It is has the flexibility to accommodate new ideas and
changes. This objective is aimed at creating an agile system that can meet the
challenges currently faced in industry. (Spearman and Hopp 1996,424)
3.1.3
Functions in Shop floor Control
Spearman and Hopp identify four
general functions that are carried out in Shop Floor Control
-
It co-ordinates the manufacturing resources
(material, knowledge, humans and information) on the shop floor. Material flow
control, which is a fundamental activity in most systems, falls under this
category. This function provides a mechanism that decides which job to release
to the factory, which job to work on at the individual workstations and what
material to move between workstations.
-
It provides real time control. Real time
simulations can be created based on the behavior of a plant which is determined
by analyzing three sets of data:
-
Standard WIP which refers to the quantity and location
of material between different manufacturing processes.
-
Status monitoring which involves the surveillance of
manufacturing resources other than material such as staffing levels and machine
status.
-
Throughput tracking which involves measuring the output
from a line or plant against an established production quota or customer due
date. This can then be used to forecast the need for overtime or staffing
shifts.
-
It carries out capacity feedback, which involves
the collection of data to update capacity estimates so as to ensure consistency
between high level planning modules and low level execution ones.
-
It enables quality control by giving the
operator of a downstream workstation the authority to refuse parts from an
upstream workstation on the basis of inadequate quality. (Spearman and Hopp
1996, 425)
1.2 What are the characteristics of a good SFC
design?
Scherer
describes shop floor control from a systems perspective. He notes that in order
to achieve control within the shop floor, the designer’s goal should be that of
developing a dynamic and flexible organization as opposed to finding an optimal
design. He gives a further breakdown of the SFC system using two different
perspectives:
-
Using cybernetic systems theory, the shop floor is part
of a larger cybernetic system that is highly complex and has chaotic behavior.
In such a system, the behavior is predictable only for a short time because of
the interactions, feedback and coupling between the different aspects of the
manufacturing system. The dynamics of behavior of the formal logic system and
its state variables as encountered in the real world can subsequently be used
to describe shop floor control.
-
Using sociotechnical systems theory, emphasis is laid
on the role of humans in production as they interact with machines on the shop
floor. By using the patterns of social and human behavior, it is possible to
describe and understand the action and logic of organizational development of
informal systems. (Scherer 1998, 453).
With these two
definitions in mind, Scherer proposes that the two important parameters to
consider in designing a control system (hence the SFC module) are the structure
of the system and the individual work tasks.
In terms of
structure, Spearman identifies three important considerations to bear in mind
when designing the SFC.
1.
Gross capacity control – Gross Capacity Control ensures
that the lines on the plant floor are close to optimally loaded when running.
This creates a stable environment for the production system. Gross capacity
control can be achieved by varying shifts, staffing levels, days per week and
hours per day or by using outside vendors.
2.
Bottleneck planning – Bottle neck refers to the slowest
process in a production system. Stable bottle neck provide the most ideal
situation because they are easier to maintain than moving ones. It is worth
noting however that bottlenecks can be designed by adding capacity to some
stations so that throughput is never constrained.
3.
Span of control – Span of control refers to the number
of employees under the direct supervision of a manager as well as range of
products and/or processes to be supervised. An ideal system will provide the
manager with information about what is needed further downstream as well as
information about the materials that will be arriving at different stations.
This information enables him to plan effectively.
According to
Scherer, a design that takes into account the individual work tasks should be
able to instill a capacity for self-design and lasting adaptability in the shop
floor control module. A system with this capacity gives the human an
opportunity to achieve three things:
-
Learn based on his qualifications and motivation
-
Gain experience through errors
-
Apply knowledge by carrying out independent actions.
In this way the
human can contribute to the increased flexibility and adaptability of the
entire production system without being driven to do so by people higher in the
hierarchical framework. Ultimately,
this enables the SFC module to meet objectives (i) and (iv) described
above.
3.3 SFC in Push systems and Pull systems
In general, SFC
systems are classified into two categories, Push and Pull, based on four
different criteria. These are described below under separate headings. Benton
and Shin provide the first three classifications while the fourth is proposed
by Professor Cochran of the MIT Production System Design laboratory.
1.
Nature of the order release (De Toni et al,
1988; Karmakar, 1989; Ding and Yuen 1991) -In pull systems, the order
release by which the flow of materials or components is initiated gets
triggered by the removal of an end item or a fixed lot of end items. In push
systems, production or material flow is initiated in anticipation of future
demand.
2.
The structure of information flow (Olhager and
Ostlund 1990; Hodgson & Wang (1991 a,b)) – In pull systems, local
demand from the next server triggers the physical flow of materials. The local
demand refers to orders while the server refers to a workstation. Such a system
is a decentralized control strategy where the ultimate goal of meeting orders
is disregarded in local workstations. Push systems use global and centralized
information in the form of customer orders and demand forecasts which are
released and processed to control all the levels of the production cycle.
3.
Practical approach associated with WIP level on the
shop floor (Spearman and Zazanis 1992) – In pull systems, a closed
queuing network is characterized by a bounded Work In Process (WIP). This
places a cap on the maximum amount of WIP that can be found within a cell or
between workstations on the shop floor. Push systems are characterized by an
open queuing network with infinite queuing space.
4.
Type of control system based on the classical
control model (David S. Cochran 1994) – A pull system provides
feedback each time a unit is produced. It uses a decoupler to detect the
difference between the desired quantity and the actual quantity produced. The
resulting error is converted into a signal that initiates production of the
machines upstream of the decoupler. A push system is an open loop control
system whereby the feedback in the output quantity is not used to effectively
control the manufacturing system. Any disturbance occurring to the system
causes a change in the output which is however not detected until the following
planning cycle. This change is caused by the time delay in information.
Uday Karmakar
summarizes the advantages of the two systems as follows:
Pull systems
- are cheaper because they don’t need
computerization (software and hardware); leave control and responsibility at
the local level; and offer attractive incentives for lead time management.
Push systems
– are good at material planning and
co-ordination; provide a hub for inter-functional communication and data
management due to their centralized
control; and are good at computing
quantities for work releases by interpreting forecasts into discrete product
orders but not so much for timing .The inability to meet the timing is caused
by the lack of dependable feedback based on the output of the system.
By combining
these complementary set of strengths, hybrid systems end up solving the
weaknesses found in MRP II. Based on the above classifications and advantages,
MRP II can be classified as a push system.