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.