17-09-2014, 11:03 AM
Advances in Agile Manufacturing
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Abstract
An agile workcell has been developed
for light mechanical assembly in
collaboration with industrial sponsors.
The work cell includes multiple Adept
robots,a Bosch conveyor system,
multiple flexible parts feeders at each
robot’s workstation, CCD cameras for
parts feeding and hardware registration,
and a dual VMEbus control system. Our
flexible parts feeder design uses multiple
conveyors to singulate the parts
and machine vision to locate them.
Specialized hardware is encapsulated on
modular grippers and modular
worktables which can be quickly
interchanged for assembly of different
products. Object-oriented software
(C++) running in the real-time operating
system, VxWorks, is used for work cell
control. An agile software architecture
was developed for rapid introduction of
new assemblies through code re-use. A
simulation of the workcell was
developed so that controller software
could be written and tested off-line,
enabling the rapid introduction of new
products
Introduction
The agile-manufacturing project at Case
Western Reserve University was
publicly unveiled in 1996 at
this same conference1,2. This
multidisciplinary project
combines efforts of students and faculty
from threeas well as active engineering
collaboration from local
industry. The purpose is to construct a
system for light mechanical assembly
that is capable of rapid product
changeover as well as rapid introduction
of new product designs [1]. Our
objectives are to develop
specific techniques, to identify and
present relevant lessons, and to establish
general principles and philosophies for
the design of agile manufacturing
systems3. This knowledge is being
gained and experimentally validated
through the construction and
operation of a sophisticated, yet
industrially-relevant,
Mechanical Design Strategies for Agile Manufacturing
As introduced in [1], our system includes several
critical elements which enable agility.
Chief among the physical aspects are:
vision-based flexible parts-feeder
systems; the introduction of “spurs”
within the conveyor system; the use of
modular worktables
supporting specialty fixturing and
tooling; and multipurpose,
modular end-of-arm tooling (
Figure 2). Further progress has been
made in each of
these areas
Worktable Changeout Sequence
Carlisle et al. developed a lightweight
gripper that permits parts to rotate
passively under the force of gravity
without re-grasping9. However, some
parts were found to be too light to
overcome friction and had to be
brushed over a fixed lip to rotate them.
We solved this problem by designing a
novel, lightweight pneumatic rotary-jaw
gripper which actively rotates parts
without re-grasping. Such capability
provides the benefits of an extra wrist
axis at lower cost and complexity than a
servoed degree of freedom. A single
pneumatic cylinder is used to drive the
finger pads through a four bar
mechanism. The opposing jaws of the
gripper are mechanically constrained to
prevent relative rotational motion, which
helps assure dependable grasp during
rotation
Design for Agile Manufacturability
Within the last year, we have introduced
design for manufacturability
considerations in simultaneous
product/process design. Keeping the
capabilities of an agile workcell in mind
while designing new products
can improve system reliability and
decrease cycle-time. This concurrent
engineering approach follows the work
of Boothroyd and Dewhurst’s Design for
Manufacturing and Assembly
(DFMA)11, which espouses guidelines
such as aggressively minimizing the
number of components in an assembly.
Design for manufacturability teaches
that the interaction of the components in
a product is critical to a successful
automated assembly12. For example,
minimizing the forces required to
assemble a product simplifies the needed
hardware.Similarly, designing
mating parts with generous tolerances
and chamfers, whenever possible,
permits them to be self-aligning and
less sensitive to positioning inaccuracies.
A few, often simple, changes to a
product in the early stages of design can
have a marked impact during final
production13. In the context of agile
manufacturing
Computing Considerations for Agile Manufacturing
Integration and overall control of the agile
manufacturing workcell is embodied in computer
software, which is more complex than
typical machine control software. The
workcell is designed so that, if
possible, product changes are
accomplished mainly by modifying
software; hardware changes are
minimized
to reduce cost and delay. The workcell
software must be adaptable to new
products without becoming
unreliable or difficult to maintain.
Software engineering methodology
provides useful means to address these
issues. The techniques of object-oriented
design and programming are especially
pertinent because they call for
encapsulating potentially changeable
design features within software modules
called (object) classes. This
encapsulation, or hiding,
Controller Design and Software Environment
Our Adept 550 robots are controlled
from an Adept MV controller which
consists of proprietary processors
and I/O boards residing on a VMEbus.
Programming requires use of Adept’s
V+ language and operating system. For
most industrial applications, this
programming environment is sufficient,
but it is inadequate for an advanced
agile-manufacturing system. As a
language, V+ is relatively primitive, as
an operating system, it is similarly
restrictive, offering little or no support
for operations such as file manipulation
and task scheduling. At present, there are
no proven commercial alternatives to the
MV controller for controlling Adept
robots14. To overcome this obstacle, an
innovative open-architecture system has
been developed. Under this scheme, the
Conclusions
Our research into agile manufacturing
has been motivated by industrial
applications and has been grounded in
physical implementation. Through this
empirically-driven approach, we have
realized a variety of specific techniques
and general principles for the design of
agile manufacturing systems. From the
mechanical design perspective, the
critical issue of achieving agile parts
feeding can beoperation and reducing cycle time. For
agile manufacturing, there are some
additional considerations. Parts should
be analyzed with respect to their
mechanical behavior within a flexible
parts feeder, as well as with respect to
their ease of recognition by a machine
vision system. Rapid prototyping can be
exploited to test these properties
physically, before design specifications
are committed. Software considerations
are central to successful agile
manufacturing. An object-oriented
perspective is valuable for achieving
generality, maintainability, and
promoting ease of reprogrammability for
new applications.