Prioritizing system safety doesn't necessarily compromise productivity
While it’s often said that a company’s most valuable asset is its people, the human body is one of the most delicate things you’ll find in a manufacturing environment. Because of this, manufacturers put a high priority on accident-proofing factories, even if it requires extra operator steps or “false alarm” shutdowns. However, such compromises may soon be a thing of the past. Recent advances in operator interfaces and machine monitoring systems keep life and limb safer than ever and, in some cases, actually enhance productivity.
According to the U.S. Occupational Safety and Health Administration, most robot accidents do not occur during normal operation, rather they happen during maintenance when troubleshooting personnel have to enter a device’s working envelope. Because the average engineer does not have eyes on the back of his head, seamless systems that quickly and reliably communicate status and automatically react to approaching danger are a must.
Interface on the go
Fine for start-stop commands, stationary panel-mount HMI, have several inherent safety and productivity shortcomings when it comes to maintenance and troubleshooting.
For example, a diagnostics engineer may have to repeatedly walk hundreds of feet away to wiggle a limit switch and then trek back to check the control system. Or, two people may be involved, one at the panel, the other out by the physical problem source. Any miscommunication under these circumstances can lead to unexpected movement of large powerful machinery, putting people at risk.
New portable HMI devices, like MobileView terminals from Allen-Bradley, Rockwell Automation, Milwaukee, offer a safer and more efficient option. Of the three available versions, two are cabled, and one is wireless. One of the tethered pendants is designed specifically for safety applications and features an onboard E-stop. With this protection, operators can access applications in gated areas and one person can accomplish what used to take two. The cable tethers have a range of 5 to 20 meters, but they can be connected at different points on a production line. So, on large assembly lines, the E-stop is always handy, it does not need to be searched out in an emergency.
Today’s portable versions are close to panel-mount HMI in functionality, depending on the architecture, says MobileView product marketing specialist Keith Kersten. Using a Windows CE platform, they can act as thin client computers to remotely interface with machine and plant computers and run applications. This allows the full power of a server-grade computer in a harsh EMI environment.
The portable tools are made possible by advances in wireless communications, better batteries, stronger, more lightweight materials, and more compact electronics. Wireless versions make use of military wireless technology, employing multiple frequencies to block out noise and interference resistance.
Kersten says to expect a lot more wireless technology in industrial environments, particularly for troubleshooting, and, eventually, a wireless E-stop.
Many worker injuries don’t result from horrific, instantaneous accidents. Instead they develop over time as a result of repetitive operator motion. Proper ergonomics can prevent many of these problems. Buttons and switches that do not require forcible actuation are one example. By switching when the light beam is broken by simple insertion of a finger, STB buttons from Banner Engineering Corp., Minneapolis, eliminate hand, wrist, and arm stress associated with repeat switch operation. No physical pressure whatsoever is required. Microcontrollers in the button perform a continuous self-check and immediately detect any internal failure. The buttons are immune to ambient light, EMI, and RFI interference.
Light curtains protect by signaling when any of a series of beams of light is intercepted by someone or something that does not belong. A new design from Omron Electronics LLC, Schaumburg, Ill., raises the level of safety. First, all features are contained within the light curtain, rather than an external control box. For easier programming, the F3SN-A uses the industry’s first handheld programmer. All programming can be done through a copy-and-paste function. This is also the first light curtain to keep track of the order in which beams are broken for higher safety levels in a floating condition. Up to three curtains can be connected serially to protect multiple access points. The response time with safety relay outputs is 25 msec, so the device can be mounted close to the hazard.
Making the switch
The Boster from French company BTI is a standalone, contactless, coded machine safety switch. It performs both position control and magnetic latching and is rated for harsh environments. Like its industry-first predecessor introduced last summer, the switch features internal automatic failure monitoring.
Relaying the importance of safety
The first all-electronic safety relay series, PNOZ elog from Pilz Automation Safety LP, Canton, Mich., has it all over traditional mechanical devices: improved performance, longer life, and easier maintenance. With electronic relays, the semiconductor technology allows universal integration of input devices that use contacts, such as emergency stop relays and light curtains. The diagnostic output provides details on I/O so wiring faults, shorts across the input contacts, and incorrect input types can be identified.
But, electromechanical devices with certain safety features still have their place. Electromechanical safety relays are suitable for switching large loads, while electronic relays are more appropriate for medium and small loads and frequent switching cycles.
The difference between a standard and safety electromechanical relay lies in the contacts. A standard version uses a coil of wire and physical motion of metal contacts to switch power to loads. After repeated cycling, the metal contacts may weld together. When this happens, the machine will continue to run after an operator has initiated an Estop. Because of the resulting unsafe conditions, most regulations forbid use on hazardous machines.
For safety electromechanical relays, Pilz uses several relays in a unit, all with positive-guided contacts. The relays are wired so that if any one welds shut, the others will still be able to open the circuit. After a contact welds, the device will not restart until the relay is replaced.