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The (r)evolution
in ergonomic two hand control devices
The need to
protect workers who routinely are exposed to industrial
hazards is basic. It requires well designed, properly
installed equipment and a trained work force that can
work safely. For these reasons, two-hand-control
technology will continue to develop as the need grows
for effective safety equipment.
The original
intent of two hand control systems is to protect the
operators' hands when loading and unloading parts into
machines, such as presses, metal forming machines or
guillotines.
Today,
two-hand control systems protect operators by requiring:
- A specific,
deliberate operator action to start the machine
cycle.
- Continuous
monitoring of the operator's hands during the
dangerous machine motion.
The First
Two-Hand Control Systems
With one early hand-protection method, the machine
operator wore leather cuffs tethered with cords and
pulleys to the machine-drive mechanism. This cuff system
allowed the operator to load or unload parts while the
machine was idle. When the machine started, the drive
mechanism pulled the cord, pulling the operator's hands
away from the machine's hazard point. While bothersome
at best, the system was effective - - if the operator
wore the cuffs.
Later entered
a more efficient method: a dual-push-button circuit
(with a simple wiring of the buttons in series) that
required the operator to push and hold both buttons
throughout the machine cycle. Operators soon discovered,
however, that they could "tie down" one push
button and use the other button to re-initiate a
machine cycle.
The unauthorized modification typically was an attempt
to be more productive, especially if the operator's pay
would be incentivised by the number of parts he would
produce.
While the "tie down" may have speeded the
load-unload process, it also exposed the operator to
serious hand injury - - or worse.
Improvements to the Early Systems
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In addition to
the push-and-hold procedure, the next generation of
two-hand control required both buttons to be released
before another machine cycle could begin. This
re-initiation circuit is used today.
In addition,
specifications for placing, orienting, and protecting
the push buttons helped to prevent operators from using
both push buttons with just one hand or a nearby tool.
Push button improvements led to palm buttons, which
required less force to activate.
These advances helped to create a safer workplace, but
left room for improvement in the areas of control
circuit reliability and ergonomic design.
The First Redundant, Self-Checking Control Modules
Early on, machine control designs consisted of relay
logic elements that connected power to the machine drive
motors or solenoid-controlled valves while the push
buttons were pressed. These conventional control
circuits usually could not detect failures that might
cause an unintended machine cycle or that might allow
the machine to continue operating after a hand was
removed from a button. This key element - the ability to
detect and reliably respond to such a failure - is the
essence of a safe system.
In the late 1970s, Europe introduced a new, two-hand
control module. It incorporated force-guided relay
circuits that were redundant and self-checking and that
required the input signals from both push buttons to
occur within 0.5 seconds. The force-guided relays were
critical: They could detect an internal welded contact
failure and respond by preventing another machine cycle
from occurring. The module's redundant relay
configuration ensured that if one relay failed, the
other would back it up, preserving the safety function.
Not only could the circuit detect certain faults, its
input simultaneity requirement ensured that the machine
would not cycle without a deliberate operator action. It
was only years later that North America adopted this
safer, more reliable control idea.
Ergonomic Considerations
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Industrial
ergonomic design became an issue in the early 1980s. The
benefits associated with reduced physical stress and
improved employee well-being were compelling factors for
health agencies, insurance companies, unions, and
manufacturers.
Push buttons in use at that time required significant
hand force to start and maintain a machine cycle. That,
combined with the repetitive nature of the two-hand
operation, caused a prevalent industrial problem:
repetitive stress injury. The need for a better
hand-activated device was evident.
Easier-to-activate electronic buttons entered the
picture in the early 1990s. They required no hand force;
an operator simply touched the sensing zone for the
electronically controlled outputs to turn on. These
touch buttons typically incorporated conventional,
general-purpose circuitry and used photoelectric or
capacitive sensing elements. Most manufacturers made no
claim that their electronic buttons could detect
safety-critical failures.
As the
actuators become more ergonomic, they also become easier
to defeat. So it is now more important for these
easy-to-operate actuators to also incorporate
self-checking technology, to maintain a high level of
safety.
Self-Checking Actuating Devices
An innovative self-checking process has been devised for
one new two-hand-control actuating device: Banner's STB.
The "Safety Touch Button" replaces traditional
push buttons, where the operator only needs to put his
finger or hand in the touch zone of the button. An
infrared beam that "looks" across this zone
detects the finger or hand and activates the output
circuit.
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How is the
self-checking achieved? After the devices power-up and
initial system self-checking routines are conducted, the
microcontrollers alternately activate the primary
emitter (E1) to create a light pulse (see Figures
2 and 3). The light pulse travels across the button's
touch zone (see Figure 1). The primary receiver (R1)
sees the light pulse if no obstruction blocks the beam
and generates a signal in response.
E2 and R2 form
a secondary emitter/receiver pair, which is used to
"test" the E1-R1 circuit. R2 looks for the
beam from E1, while E2 sends a second beam to R1 a short
time after E1 emits its beam. Because this secondary
pair is an internally protected path it is used as the
secondary safety circuit. If either the E1-to-R2 or the
E2-to-R1 beam is not detected, the device's internal
microcontrollers interpret this as a fault. This
self-checking capability, together with the safety
module, create a control-reliable system.
Fail-Safe Control Reliability
The European Union has adapted since many years safety
standards that describe in detail the concept and design
features, usage and installation of safety components.
The EN574 standard describes the functional aspects and
principles for design, and the application of such
devices on machines. It even defines its own set of two
hand control device types. As such, if a particular
machine has been identified as a risk level Category 4
(for example a press), it should be equipped with a two
hand control device meeting Type IIIC.
Banner
Engineering's DUO-TOUCH SG kits are unique ergonomic
style two hand control units that are certified by a
notified body meeting the requirements of the Type IIIC
as per EN574.
Need for Additional Guarding
A fail-safe control reliable two-hand-control system can
provide safe machine cycle control for operators, if
installed correctly. However, the system cannot protect
others who may be near the hazard point.
This is a very
real problem, and especially troublesome when a trainer,
supervisor, or coworker is directing or assisting an
operator at the point of operation.
A single two-hand-control system can protect only one
pair of hands. Additional safeguarding devices, such as
safety light screens or a second two-hand-control
system, can be used to protect additional personnel.
This is a good practice to follow whenever more than one
person might be exposed to the hazard point.
The need to protect workers who routinely are exposed to
industrial hazards is basic. It requires well designed,
properly installed equipment and a trained work force
that can work safely. For these reasons,
two-hand-control technology will continue to develop as
the need grows for effective safety equipment.
SourceTurck
by :
Peter
Mertens is the Sales Director Europe for Banner
Engineering Corp,
Koning Albert 1 laan 50 in Wemmel 1780, Belgium.
Christine Larsen is a Technical Writer with
Banner Engineering Corp.,
9714 10th Avenue North, P.O. Box 9414, Minneapolis,
Minnesota, USA.
Phone : ++32 2 456 0780,
Fax : ++32 2 456 0789
Email : mail@bannerengineering.be
Web : www.bannerengineering.com
Article Reference : 002070
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