Machine Safeguarding: Fifty Years of Industrial Safety

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.