MACHINE SAFETY, now more than ever, is the focus of companies that own, operate, or build machines. Safety requirements now are defined more clearly through a number of government, industry and trade-group standards. Through the early 1980s, most machines were shipped with little consideration for potential operator error or interference that might create an unsafe environment. As the number of injuries increased at an alarming rate, most notably to operators of manually fed punch presses, in jumped the lawyers. Several thousand lawsuits later, machine safety now is held in the highest regard by machine builders and their customers.
As these evolving standards become clearer, the design methodology in machine safety takes on some semblance of standard thinking among machine builders. Today, standards are more focused on the type of machine, with different machines having different standards. These standards allow machine builders to incorporate specific safety equipment on specific machines. From a suppliers perspective, the standards drive them towards more universal, less-dedicated safety control to provide more consistency from one machine type to another.
You easily could make the argument that manufacturers with legacy machinery not conforming to current standards tend to evaluate machine safety for human and machine protection, while increasing productivity, efficiency, and uptime. As these considerations become more clearly developed for various types of machines, control suppliers now are responding accordingly with many offering a new generation of solutions. For the most part, many of these new solutions, once installed, have met the majority of the manufacturers criteria, and have been proven to be far more reliable than hardwired systems.
To meet safety, productivity and environmental objectives many North American manufacturers take a task-oriented approach. Accessing many machine variables and controlling their limitsin essence creating safety zones during machine operationshas become the primary approach to safety by many manufacturers. In the past ten years smarter safety control systems that provide redundancy, monitoring, diagnostics and safety networking have become much more effective than absolute safety equipment. The different functionality levels of the various safety controls that are available today provide a multitude of solutions depending on the machine, interlocking devices and access requirements.
Towards that end, the strategies, tactics, and actions revolve around safety relays and safety controllers. This article will focus on the relays.
Machine Safety in North America
The Cascade Enviropac plant in Brampton, Ont., has a complex honeycomb cardboard-laminating line application. We needed a solution that allowed us to access certain machine sections without shutting down the entire machine, recalls Paul Laroche, plant engineer for Cascade. If the machine stops for too long, the glue station starts to dry, forcing substantial delays in restarting. Laroche says adjustments on the machine are required periodically, so temporary access is essential. Stopping the entire machine for these frequent adjustments would result in far too much lost product. However, throughout the eight safety zones on the machine, there are emergency-stop interlocks, says Laroche.
Bretco Electric, a system integrator in Milton, Ont., had the Cascade project. We had to independently control safety in five different zones on two different continuous production lines, says Brett Patterson, president of Bretco Electric. If an e-stop is pushed, we shut down the entire line, but if access is required in any given zone, we just shut down that zone. In both cases, Sick safety relays interface to barrier-mounted machine guards, and then send a contact interface on status directly back to an Omron PLC used for line control (See Figure 1 below). The continuous material flow culminates at a cut-to-length zone at the end of the line. Here another relay disables the cut-to-length servo directly should access be required.
FIGURE 1: SELECTIVE, EFFECTIVE SAFETY
Safety relays interface to barrier-mounted machine guards and then send a contact interface on status directly back to a PLC used for line control. (Click image to open an enlarged image.)
Regardless of the safety requirements, at the center is some form of safety intelligencea controller, module, monitor, and/or relay. The definition of the type of intelligence seems to depend on supplier definition and device functionality. Table I below sorts out the devices and compares features.
Custom machinery builder Centerline Tool, in Windsor, Ont., put safety relays on a welding cell. Its analysis of options led it to Pilz Automation Safety because of needed relay functionality and space-savings. Our objective was to reduce the number of relays while allowing complete safety monitoring, says Dan Dinuzio, controls manager at Centerline. The relays let us integrate all safety devices via DeviceNet, which provided a high level of safety diagnostics.
Pilz centers its approach on integrating lean manufacturing techniques with machine safety. We implement safety as early as possible to assure that lean principles are met and safety is integrated seamlessly, states Roberta Nelson Shea, general manager, Pilz USA. Pilz safety solution centers on its PNOZ safety relays. The latest in this series have different functionalities to meet various application and interlock requirements.
Pepperl+Fuchs has developed a series of safety products compatible with the Actuator Sensor Interface (AS-i) as a two-wire safety network solution. Helge Hornis, intelligent systems manager for P+F in North America, says AS-i isnt intended to replace fieldbus systems, but rather assist them to optimize their bandwidth and hence usefulness.
Central to Pepperl+Fuchs AS-i approach are its line of safety monitors, which are somewhat like configurable safety relays, but have less flexibility than a fully programmable safety controller. P+F prefers the term monitor in place of relay and controller because they do incorporate software for configuring the monitoring sequence. Also, these monitors are considered fail-safe, and provide machine shut-down outputs should anything in the sequence detect a disturbance.
Omron Scientific Technologies, Inc. (OSTI), Fremont, Calif., offers a host of safety relays designed for dedicated functionality. Relays range in function from stop motion detection to two-hand palm button control to dual-channel input units with time-delayed outputs that provide conformity to ANSI B11.19, Section 5.1.
A typical description for this type of safety relay would include the ability to accept standard or dual N/C inputs, or dual solid-state inputs from a light curtain; provide seven N/O outputs to route power to external device power contactors; and two N/C auxiliary outputs and four solid-state outputs for other purposes. External Device Monitoring is also provided, and it can be monitored in manual or automatic/manual reset modes
Standards Committees Can Define User Concerns
Mike Carlson, safety products marketing manager for Banner Engineering, has been involved in developing several safety standards recognized by the American National Standards Institute (ANSI), such as ANSI/RIS R15.06-1999 Industrial Robots and Robot Systems, ANSI B11.1-2001 Mechanical Power Presses, ANSI b11.3-2002 Power Press Brakes, ANSI B11.19-2003 Performance Criteria for Safeguarding, ANSI B11.20-2004 Integrated Manufacturing Systems, and now is chairman of ANSI B11.19 Performance Criteria for Safeguarding Committee.
First-hand involvement with standards groups has given us the ability to anticipate and more quickly respond to new safety requirements and trends in the industry, Carlson says. Participation also provides insight into the problems and issues that industry is grappling with when it comes to safety.
Carlson believes that the term relay is confusing to many end-users, because they envision a relay as being of the traditional electro-mechanical variety. So Banner uses the term safety modules in its Pico-Guard solutions when applied to intelligent safety control and monitoring, says Carlson. Moreover, Banner focuses on dedicated, machine mounted solutions and is less focused on networks connecting safety devices to safety-oriented PLCs.
What Can We Expect?
For the most part, safety installations meeting all the latest standards still are a developing market, especially considering all the legacy machines operating today. However, some significant results are in for those manufacturers that have placed machine safety at the forefront of their corporate culture. Consumer product manufacturer, Johnson & Johnsons long-term management goal is to create an injury-free workplace, while at the same time sustain world-class production.
J&Js safety strategy includes Zero Access and now embraces ANSI B155.1-2006 standards. The long-term results of have been impressive. According to a report presented at a recent Packaging Machine and Manufacturing Institute (PMMI) conference, J&J reported 23 worker amputations worldwide in 1982. As of September, 2006, J&J has enjoyed more than 14 months of amputation-free machine operation.
Thoroughness a Safety Essential
Brian Sherman, senior applications engineer, for Tri-Phase Automation, Hartland, Wis., has installed a number of safety solutions. One application was on a metal processing machine at a consumer product manufacturer that is a strong advocate of the AS-Interface bus. Tri-Phase implemented a thorough safety survey at the site before making their recommendations. Sherman says safety surveys typically must include the following data to develop a safety solution specification:
- Machine dimensions
- Measurements for the zones to be protected
- Access frequency and where access is necessary within the machine environment
- Mounting restrictions for interlock mechanisms
- Type of shutdown allowed should there be a safety breech
- Location and type of main machine controls and I/O
- Wire routing requirements
- Input requirements from safety monitors to main control
- E-Stop locations
- Location and packaging requirements for each remote safety monitor(s)
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