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Manufacturing Automation Safety:
Is your company really prepared to operate safely and efficiently?
By Rob Dombek
In the manufacturing world, although safety
is always stressed as the most important part of the manufacturing
process, situations arise whether due to downtime, production
stress, poor maintenance procedures or poor engineering design
that tend to raise the safety issue.
Many a times I have seen and been a part
of situations where a manufacturing crisis arises due to a
machine malfunction that causes a slowdown in production or
an unscheduled downtime. More often than not, those individuals
that are paid to fix the problem (maintenance technicians
and plant engineers) feel the pressure from management to
get the machine back up and running ASAP.
This circumstance causes the greatest
likelihood of personnel letting down their guard concerning
safety, and the safe operation of machinery can get overlooked.
Sometimes quick fixes are applied to meet production requirements,
but these quick fixes oftentimes end up causing more problems
in the future if the time is not dedicated to thoughtfully
plan a solution and implement it correctly and thoroughly.
These circumstances are where I have seen
emergency stops jumpered, fuses shorted, safety interlocks
(limit switches, proximity sensors, light curtains) forced
to “fix” the problem. Although the problem can be
temporarily “quieted” so production can continue,
a “quieted” problem ends up becoming a very loud
problem at a later time.
So how do we overcome the issue of safety
vs. productivity? As an engineer, I know that having experienced
problems like this, engineers are compelled to try to “prevent”
it from happening again, either by over-designing the machine,
or by putting in additional (and often superfluous) safety
interlocks or procedures. Unfortunately, this often results
in an engineering “catch 22.” For example, when
an interlock is designed to be safe and also idiot-proof,
(that is, make it nearly impossible to override or force),
it usually makes it much harder for maintenance personnel
(who tend to like the simple designs) to access a device to
do general or preventative maintenance.
This is exactly where and why training and
feedback need to be a continual process in the manufacturing
environment. As personnel and responsibilities change, the
on-hand maintenance crew often does not have the specific
expertise for each specific machine. Continual training is
vital to keep the existing workforce efficient, and to also
train new personnel.
The alternative is that outside (expensive)
contractors may be needed to solve problems. I have often
heard statements like “well I know
how this particular machine operates in and out, but don’t
have a clue about how the packing machine works.”
If machine designs were consistent (wiring
methods, safety interlocks, etc.), then it would be advantageous
to have a “jack-of-all-trades” maintenance crew
maintaining the machines. However, as newer safety technologies
or methods are introduced, training must be an essential part
of the operation. With properly trained personnel, old machines
can be kept running safely and efficiently, while new machines
(which are designed safer due to newer safety regulations)
are more thoroughly understood and operated correctly; meaning,
the whole system can function, old and new, effectively and
safely.
Standards
There are application standards that exist
for specifics such as how to calculate safe mounting distances
for machine guarding using light curtains. For example, ANSI/RIA
R15.06 discusses the use of light curtains for robot guarding.
Although there are construction standards
for design, construction and testing of presence-sensing devices
(which are typically the devices used for automated machinery
interlocking safeties), there are currently no standards in
North America. IEC61496, an IEC standard entitled “Safety
of Machinery— Electrosensitive Protective Equipment”
is accepted as the default standard, and the Underwriters
Laboratories, Inc. has adopted the IEC61496 into their standards.
UL’s version contains specific examples
concerning such things as number of outputs required, the
need for key-operated switches, transformer construction,
and failure conditions.
OSHA and other organizations such as ANSI
provide information on proper machine guarding, construction,
and the use of automated machinery (machine tools).
The ANSI B11 Machine Tool Safety Standards
are probably one of the best sources of machine tool guarding
information for North America. The specific parts of B11 related
to automated machinery include the following:
• -B11:20: Manufacturing Systems/Cells;
--• -B11:21: Machine Tools Using Lasers
for Processing Material;
• -B11:22: Turning Centers and Automatic,
Numerically Controlled Turning Machines;
• -B11:23: Machining Centers and Automatic,
Numerically Controlled Milling, Drilling and Boring Machines;
and
• -B11:24: Transfer Machines.
Safety Methodologies
When an automated machine is being newly
designed or if designs are being implemented to upgrade older
machinery, there are three main safety methodologies and practices
that should be considered:
• -Design with maintenance in mind;
• -Interlocking principles and devices;
and
• -Safety controls and programmable
logic controllers (PLCs).
Although the main issues are divided into
different categories, often their details overlap one another.
Safety devices must be designed to account for future maintenance,
to use interlocking principles and elements to prevent the
operator from damaging the machine or personnel (or the machine
from damaging itself), and with new approved control methods
that can considerably save hardware and cabling costs and
lead to reduced troubleshooting time with increased diagnostic
capabilities.
Interlocking Principles and Devices
In discrete automated manufacturing, where
there tends to be a significant amount of moving machinery,
protective measures must be implemented to protect the worker
and machine from the moving devices. For instance, when a
machine guard (that will prevent access to a dangerous area)
is being designed, the security and type of interlocking principles
and devices should be thoroughly discussed between the facility’s
safety, maintenance and engineering personnel. While, for
instance, it may prove to be a simple design (and easily maintainable)
to use a limit switch to detect whether a machine guard door
is open, as opposed to using a specifically designed electronic
safety switch, or a mechanically actuated switch, thought
must be put into the design so that the interlock cannot be
“cheated.” By cheated, I’m suggesting that
someone can actuate the limit switch, with one hand, while
still able to open the guard door, but without shutting down
the machine.
Safety Controls and PLCs
New safety control devices marketed as Safety
PLCs are not really new. According to industry experts, the
Safety PLC concept traces its history to the late 1970s. Traditionally,
standard PLCs were used in pairs in process industries which
would allow a safe and orderly equipment shutdown by the redundant
PLC if the primary PLC fails. It is an expensive method requiring
a great deal of engineering, hardware, wiring and custom software
to implement the safety portion.
Newer Safety PLCs build the redundancy into
a single PLC chassis by incorporating multiple processors
that perform the same logic, checking each other, only writing
the outputs upon agreement. The dual processor Safety PLC
is priced about 25 to 30 percent higher over a comparable
standard PLC.
Several PLC manufacturers’ Safety PLC
lines incorporate two identical central processing units and
include a variety of built-in self-monitoring hardware systems
and diagnostics, while other manufacturer’s units go
even further and use tripling processors as well as triply
redundant input and output (I/O) systems.
The manufacturing facility’s product,
whether it is a continuous process or discrete product manufacturing,
will tend to dictate the type of Safety PLC to implement.
Fault tolerant systems are typically better for process industries,
while fail-safe systems tend to be better for discrete manufacturing.
For discrete manufacturing, you want to stop the motion immediately
so you do not hurt someone; for the process industry you want
to keep it running so you do not have a dangerous work environment
(overtemperature or pressure situations, leakage of flammable
or toxic materials, etc.) or cause economic damage in lost
product or equipment.
Although each Safety PLC method adds costs
to a machine’s design, when you consider the all inclusive
costs (design, commissioning, installation, etc.), the hardware
costs for a Safety PLC controlled system is maybe about 10
percent of the whole project.
The bottom line is when considering the
cost of implementing manufacturing safety; one should keep
in mind the results of a recent study by the American Society
of Safety Engineers which puts the ratio of indirect to direct
costs of an industrial accident as high as 8:1. If, through
a safely designed machine, and adequately trained personnel,
one accident is prevented (along with the investigations,
legal costs, worker compensation costs, insurance costs, etc.),
the costs for training personnel and purchasing the extra
safety equipment will pay for itself many times over—and
increase overall productivity.
Rob Dombek is a program manager for National
Technology Transfer, Inc. and has worked in the field of automation
technology for over 16 years with extensive experience as
an Electrical Controls Engineer
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