1 February 2002
Control technologies for safety and productivity
Integrating safety control saves money, increases productivity.
By Frank Watkins and Kelly Flowers
Today, more than ever, manufacturers must increase productivity and efficiency while maintaining a work environment that is safe for their employees. Manufacturers understand, further, that taking care of their greatest investment, their employees, requires more than merely meeting safety regulations: It also involves integrating safety controls directly into a system so a worker is protected and productive in the workplace. Installing safety-specific products helps manufacturers constantly monitor the entire system to ensure that it is functioning properly and that man and machine do not meet hazardously.
Safety has long been a manufacturer requirement. Early safety products included emergency-stop push buttons in the early 1900s and control relays with interlocked contacts to provide electromechanical diagnostics in the 1940s. In the 1960s, engineers developed a safety system with relays and palm buttons acting as safety circuits for mechanical stamping presses.
In the 1970s and '80s, industrial equipment users began moving from relays to solid-state devices because operators could defeat relay control systems with electromechanical devices—but not with solid-state programmable devices. The Allen-Bradley 1774 programmable logic controller (PLC), for example, featured a protected memory module that ensured unalterable core memory storage; that system was used in many safety-critical applications, including those outside the manufacturing industry, such as amusement park rides and ski lifts. In the 1990s, solid-state safety control became even more popular.
Today, safety PLCs, pressure-sensitive safety mats, and light curtains are among the newest safety-related innovations, offering manufacturers the capability to safely and efficiently manage the safe start-up, continuous operation, and shutdown of automated systems. These state-of-the-art safety products, systems, and services satisfy global regulations and protect operators and tooling, all while increasing productivity.
SAFETY MARKET DRIVERS
As global distribution of products becomes the norm, machinery manufacturers and end users increasingly consider global machinery safety requirements when designing equipment. One of the most significant forces behind this international safety effort is the 1997 European law requiring all machinery built for use in the European Union and European Economic Area to comply with the Machinery Directive on safety. The law mandates that machine builders indicate compliance by placing CE markings on their machinery.
Though European in origin, this safety-related directive affects OEMs, end users, and multinational corporations everywhere. With components supplied from around the world, the final destination and use of a product often remains unknown to its manufacturer—hence, the rising need for a universal standard.
Safety in automation involves those control elements that prevent a hazardous condition from occurring. These controls usually reside in a separate, dedicated system but are sometimes a part of the primary system's overall control architecture.
These safety controls may involve simple systems, such as a guard door interlock switch and emergency stop switches or programmable safety controllers (essentially PLCs) designed specifically to monitor and regulate the safe operation of an entire system in unison with existing PLCs.
Safety controllers address a wide range of applications where safe and efficient start-ups and shutdowns are paramount to ensuring the well-being of operators and the protection of machinery. These applications include robot control, mechanical stamping press control, material handling systems, and packaging machinery.
SAFETY AND PRODUCTIVITY MERGE
Traditionally, safety and productivity in manufacturing plants pulled in opposite directions. The safer a system was, the less productive it was. Now, systems can provide enhanced productivity while meeting safety requirements. Companies using such safety systems increase throughput as well as showing a responsibility, giving workers confidence the company is taking their well-being into account.
As machine industry safety standards become statutory requirements, manufacturers must bear added costs in order to meet the regulatory requirements. The decision to upgrade an organization's equipment involves more than purchasing and installing safety products to meet specified codes; it also involves an investment in engineering and installation time. Many view these as sunk costs and lost production time. Others view it as an investment with a positive return.
Many applications benefit from investments in safety technology with increased productivity and efficiency, however. Consider, for example, an automotive operation using lockout/tag-out procedures for safety: System operators may have to use five or six locks to safely shut down the system, including the electronics, the pneumatics, and the robot. Shutting down the machine is time consuming and inefficient. If a safety control process were installed, operators could shorten the lockout/tag-out process, quickly troubleshoot the system at the point of error, and get the system running without stopping the production line.
Another example of how advanced safety products can improve productivity involves light curtains, or infrared beams that detect operator presence in dangerous areas. Typically, designers use a safety interlock gate to prevent operators from entering these areas. A gate takes about 10 seconds to open, 10 seconds to close. If operators do this 100 times a day, for example, they are losing as much as half an hour or more of productivity daily.
If traditional gates were replaced with light curtains, operators would simply break the infra red beam when entering dangerous areas, and the operation would come to a safe stop. Over time, the light curtain investment increases productivity.
Merging control and safety systems allows facility engineers to do routine maintenance or troubleshooting on a specific part of the machine while production continues on the rest of the machine, safely reducing work stoppages and increasing flow rates. A robot weld cell with a perimeter guard around it, for example, will in most of today's plants shut down the entire operation if an operator walks into the cell and breaks the safety chain. By using safety controllers and more intelligent safety equipment, manufacturers can structure the line so that when a person walks into a cell, only that section of the cell shuts down, and the rest of the cell maintains production.
When safety control systems integrate with control networks, the benefits are exponential. Over the life cycle of a machine, manufacturers gain enhanced diagnostics, communications, safe machine operation, and decreased operator downtime. This integration involves more than just controlling that machine under the normal, automatic modes of operation. It also includes controlling the machine under maintenance and repair modes so that services can be performed in a safe and controlled environment without having to stop an entire process. Companies are beginning to understand the relationship between the investment costs to get an operation running and, if integrated properly, the benefits associated with an integrated control and safety system that work hand in hand to keep the entire operation running efficiently and safely at the same time.
Safety and control integration also permits users to run one network to standard control and safety devices, reducing system wiring and minimizing start-up costs. Manufacturers traditionally use hard wiring to each device, which means they have a complex and often redundant system of wires running to each component. An integrated solution consolidates the control and safety wires on a single bus, or network, significantly saving the amount of wiring and associated start-up and maintenance costs.
In addition to wiring and maintenance savings, integrating safety and control permits immediate access to diagnostic data. This helps protect operators, but it also protects expensive tooling as well. The same diagnostic information that protects operators can protect against catastrophic failures of tooling, saving valuable downtime and increasing productivity.
Networked safety systems also provide valuable plant-floor information for manufacturing execution systems. If a machine goes down, manufacturers need to route production to another line or build another part that does not need that particular machine. This process helps keep the production time per part down and therefore increases overall productivity. Networked safety controls improve both the factory information system and the automation system. IT
Behind the byline
Frank Watkins is program manager, Safety Controls business, Rockwell Automation. Kelly Flowers is business manager, Safety Controls business, Rockwell Automation.