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Developing a Hybrid Programmable Logic
Controller Platform for
a Flexible
Manufacturing System
Abstract:
In this
article, we present the design and implementation
of a flexible
manufacturing system
(FMS) control platform based on a programmable
logic controller
(PLC)
and
a
personal
computer
(PC)-based
visual
man-machine
interface
(MMI)
and
data
acquisition
(DAS)
unit.
The
key
aspect
of
an
FMS
is
its
flexibility
to
adapt
to
changes
in a demanding process operation. The PLC provides
feasible solutions to FMS
applications,
using
PC-based
MMI/DAS,
whereby
PLCs
are
optimized
for
executing
rapid
sequential
control
strategies.
PCs
running
MMI/DAS
front-ends
make
intuitive
operation
interfaces, full of powerful graphics and
reporting tools. Information from the
PC c
an be distributed
through a company’s local area network or web
using client
-server
technologies. Currently, with the
convergence of underlying microprocessor
technology
and
software
program-ming
techniques,
many
users
find
that
PLCs
provide
a
cost-effective
solution
to
real-
time
control
in
small-
to
medium-
sized
process
plants,
especially when combined with
supervisory PCs using hybrid systems. The major
work
of
this
article
demonstrates
that
PLCs
are
responsive
to
rapid
and
repetitious
control
tasks,
using
PCs
that
present
the
flow
of
information
automation
and
accept
operator
instructions,
thereby providing the user a tool to modify and
monitor the process as the
requirements
change.
Key Words:
< br>PLC
、
FMS
、
PC.
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1
．
Introduction
In
a
variety
of
product
manufacturing
industries,
the
most
automated
form
of
production is a Flexible manufacturing
system(FMS),first introduced in 1970s. Since the
FMSs can providea high potential for
productivity improvement in batch manufacturing,
the
number
of
FMSs
is
growing
substantially
(Groover
and
Zimmers,
1984).
The
acceleration
throughout
the
world
is
due
to
increased
global
competition,
reduced
manufacturing cycle
times, and cuts in production costs.
Generally,
an
FMS
consists
of
a
group
of
machines
or
other
automated
work
stations,
which
form
into
modular
subsystems,
such
as
CNC
machines,
robots,
vision
systems, and a process station. These
are interconnected by a materials handling system
and
usually
driven
by
a
computer(Maleki,1991).Each
modular
system
requires
an
individual
modular
control
system,
with
different
components
being
controlled
by
individual controller units. All of the
modular subsystems are controlled by computers as
usual. These controllers perform their
intended tasks under supervision of a higher level
controller.
To
the
system,
both
the
control
devices
as
well
as
the
flow
of
information
need to be automated. The key aspect of
an FMS is its ability to adapt to changes in the
control tasks. This flexibility
includes the quantities and varieties of part
types which it
can produce, the order
in which operations may be performed, and its
ability to reroute
parts back into flow
paths. In the end, the control platform should
have the capability to
automate the
flow of information.
Typically, there
are three types of control platforms used in FMSs:
minicomputers,
microcomputers,
and
PLCs
(Maleki,
1991).
The
minicomputers
are
best
suited
for
complex large-scale, continuous
,regulatory control applications . The PLCs are
used for
rapid
and
repetitious
logic
control.
Personal
computers
(PCs)
are
suited
for
operator
interface
functions. Primarily, PLCs are designed to replace
hardwiring relays, to operate
in an
industrial environment, to be easily
modified by plant engineers and
maintenance
personnel, and to be
maintained by plant electricians. Currently, with
the convergence of
underlying
microprocessor technology and software
programming, many users find that
PLCs
provide
a
cost-
effective
solution
to
real-time
control
in
small-to
medium-sized
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process plants,
especially when combined with supervisory PCs
using hybrid systems.
The
purpose
of
this
article
is
to
address
the
state-of-the-art
technology
of
FMSs.
The
design
and
construction
of
an
FMS
using
PLC-controlled
and
PC-based
visual
man-machine
interface(MMI)
and
data
acquisition
system(DAS)
are
presented.
It
is
organized as follows.
Section 2 begins with the description of the FMS
on the factory
floor
of
the
Center
for
Manufacturing
System
sat
the
NewJersey
Institute
of
Technology(NJIT).Section 3 shows the
operational description of the FMS. Sections 4
and 5 present the applications of PLC-
controlled and PC-based MMI/DAS for the FMS
at
NJIT.
Section
6
contains
a
summary
of
the
advantages
of
this
PLC-controlled
and
PC-based MMI/DAS for FMS
application.
2. Description of the FMS
SI handling conveyor system
This
consists
of
four
carts,
A,
B,
C,
and
D,
with
fixtures
mounted
on
each,
two
transfer
tables,TT1
and
TT2
,
and
dual
conveyors
which
transport
materials
to
each
workstation.
Figure 1
. Flexible
manufacturing system.
NASA II CNC
milling machine
The
milling
machine
accepts
rectangular
solid
blanks
and
machines
each
part
of
different
types according to its computer controller.
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GE P50 robot
A shared robot is
used to
load and unload the material between
the CNC milling
machine
and
the
conveyor
system,
and
between
the
parts
presentation
station
and
conveyor
system.
It
contains
five
computer
programs
assignable
by
the
PLC.
The
computer
programs direct the robot to load the material
between the parts presentation
station
and the carts and between the CNC machine and the
carts. The last two programs
place the
completed parts in the accept or reject area.
Parts presentation station
This station includes a gravity-chute,
which supplies rectangular solid blanks as raw
materials.
This
station
also
contains
two
bin
types,
one
each
for
accepted
parts
and
rejected parts.
Computer
vision system
The vision system
provides for the visual automated inspection of
the parts. It is a
menu-driven,
64-level gray scale, edge detection system.
Drilling machine
An
IBM7535
industrial
robot
with
an
automated
drill
as
an
end-
effector
drills
various
holes in the parts as directed.
In
summary, the FMS has two robots, one CNC mill, a
material transfer convey or
system
including transportation carts and positioning
limit switches, and a vision system,
which
are
supervised
by
a
GE-Series
Six
PLC
and
monitored
by
a
PC-based
visual
MMI/DAS.
3. Operational
description
The working cycle for this
FMS proceeds in the following manner:
lly, all four carts on the conveyor
system are empty and available for the raw
materials to be loaded onto them from
the parts presentation station.
GE
robot loads four parts, one by one, on to the four
carts on the convey or
system. The
carts move clock wise as they are being loaded.
3.
Figure
2
shows
the
positions
acquired
by
the
four
carts
once
the
four
parts
of
different types have been loaded.
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4.
The
IBM
robot
drills
various
holes
on
each
blank
part
as
the
cart
stops
at
the
drilling
machine.
5. The GE robot moves to the
conveyor, removes the part from the cart at
position
X1,and loads it into the
fixture located on the CNC machine table.
6. Once the part is loaded on the CNC
milling machine, the robot retracts, and the
milling machine mills the rectangular
part as required.
7.
After
the
milling
operation,
the
robot
arm
moves
to
the
milling
machine
to
remove
the part that was machined from the holding
fixture
.
Figure
2
. Loading state of the conveyor
system.
8. The robot returns the
finished part to the same cart on the conveyor.
9. A signal is sent to the vision
camera to inspect the part.
10. The
vision system analyzes the part and outputs a
signal that directs the robot to
accept
or reject the part.
11. The robot runs
either an accept program to place the part in the
accept bin or
runs a reject program to
place the part in the reject bin.
12.
The GE robot goes to the parts presentation
station and loads a new blank part
into
the cart.
13. The cart is released to
the system and the next cycle is started.
4. Control of an FMS with a PLC
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The significant
features of the FMS control system are as follows:
system
is
easy
to
configure
and
to
modify
to
accommodate
changes
and
updates, because of the ladder logic
capability of the system.
a similar
manner, the system is easy to program and
document.
system can be easily
maintained, and troubleshooting is
decreased because
on-line
diagnostics are provided to pinpoint problems and
decrease maintenance.
lly, the system
is readily interfaced with the computer.
The PLC is a general purpose industrial
computer which is widely used in industrial
process control. It is capable of
storing instructions to implement control
functions such
as
sequencing,
timing,
counting,
arithmetic,
data
manipulation,
and
communication
to
control
industrial
machines
and
processes.
The
PLC
is
chosen
to
perform
an
FMS
control
task based on the following features:
1) good reliability;
2) less
space required and operates in an industrial
environment;
3) easier to maintain by
plant engineer or technician;
4) can be
reprogrammed if control requirements change;
5) can communicate and network with
other computers.
In this application, a
GE-Series Six PLC is equipped with a memory bank,
and the
I/O
racks
are
loaded
with
the
following
input
and
output
interfaces:
120
VAC
input
modules
with
8
ports/module,
24
VDC
input
modules
with
8
ports/module,
and
120
VAC output modules with
8 ports/module.
5. PC-based visual
operator interface unit
With the
convergence of microprocessor technology and
software techniques,
the
PC
has become very useful in operator interface
applications. PCs running MMI/DAS
front-ends
make
powerful,
intuitive
operation
interfaces,
full
of
useful
graphics
and
reporting
tools.
Information
from
these
PCs
can
be
distributed
through
a
company’s
local area
network(LAN) or web using client-server
technologies.
A PC-based visual MMI/DAS
was developed to monitor the process and log data.
The functions of the MMI are twofold.
First, it opens a window between the
operator
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