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Recently, I had a chance to read a few chapters in a
McGraw-Hill's Manufacturing Engineering
Handbook. In a chapter entitled "Industrial Automation Technologies",
Andreas Somogyi of Rockwell Automation summarizes some advances in machine
safeguarding since the 1960s. As a student of the history of technology, I was impressed
with his synopsis and wanted to share some of it with you here. I've also incorporated
some materials I've written for GlobalSpec, CR4's parent company, about
specific safety products.
The 1960s: Hard
Guarding
During the 1960s, Somogyi begins, "fixed machine guards"
became "the primary method of protecting personnel from hazardous machinery".
Known as "hard guarding", this technique uses cages and other physical barriers
to prevent employee access to dangerous machinery. Preventing accidental
contact is an effective means of protection, of course, but hard guarding has
its limitations. As Somogyi explains, hard guarding "is not a feasible solution
when the application requires routine access by an operator or maintenance
personnel."
The 1970s: Guard
Locking
"By the 1970s", the author continues, "movable guards with interlocking systems
became the most prominent solution for applications requiring access to the
machine." Equipped with safety interlock switches, hinged or sliding guard
doors enabled operators to access equipment for maintenance; however, these
safety devices shut-off the machine's power if the safeguarding device was left
open. Some interlocking safety systems even included special products with guard
locking, a safety strategy that locks the machine guard in place until the
equipment is in "a safe condition".
"In terms of the marriage between safety and productivity",
Somogyi explains, the safety interlocks of the 1970s provided "the most reliable
and cost-effective solution". That's still the case today, but some
manufacturing processes require more frequent access to machinery. For potentially-hazardous
equipment that operates at thousands of cycles per day, the seconds needed to
open and close machine guards can add up to minutes or even hours of cycle
time.
The 1980s: Pressure Sensing
During the 1980s, factories began using light curtains and pressure-sensitive
devices to improve plant safety. Light curtains are photoelectric transmitters
that project an array of synchronized, parallel infrared (IR) beams to a photoelectric
receiver. When an opaque object interrupts one or more beams, a light curtain
controller sends a stop signal to the guarded machine. Pressure-sensitive
products for machine safeguarding include safety bumpers, safety edges, safety
mats, and safety sensors. When a moving part that includes such a safety device
contacts an operator, conductive plates touch and the output relays in the
controller are de-energized. In turn, a stop signal is sent to the machine.
According to Andreas Somogyi, safety sensors represented a
significant breakthrough not only because they "provide protection without
mechanical guards," but also because they "are less susceptible than interlock
switches to tampering by machine operators". In addition, pressure-sensitive safety
sensors provide "a degree of diagnostics" that is not available with
conventional relays and electromechanical switches.
The 1990s: Dedicated
Controllers and Integrity Monitoring
"Fifty years' worth of safety advances culminated in the safety control
domain of the 1990s," Somogyi concludes. The crowning achievement was the
"integration of hard guarding, safety interlocks, and presence sensing devices
into a safety system monitored and controlled by a dedicated safety controller
and integrity monitoring." Now, during the first decade of the twenty-first
century, providers of industrial machine safeguarding systems are offering
what Somogyi termed "distributed safety implementation" via embedded
intelligence and advanced networking capabilities.
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