Safety control systems are now intimately tied to plant operations, Gary Milburn* writes.
DESPITE their importance, industrial safety systems can sometimes be seen as a necessary evil. Often tacked on as an afterthought to meet regulations, traditional designs generally provide immediate shutdown of all hazardous electrical/mechanical equipment upon demand of the safety system.
While such action is certainly required in the case of potential worker injury, there are many occasions–such as jams and spills–where it can take a great deal of time to get the plant back into full production.
The good news is that modern industrial safety systems represent a completely ‘new age’ of safety, courtesy of advanced technology, and new functional safety standards. These promote upfront safety system design as part of an holistic view of the plant and its processes.
This ‘functional safety’ not only focuses on risk assessment and appropriate safety measures, but also assists the design process. It is now easier to design safety systems to work with the plant production objectives, rather than against them.
In fact, deploying a modern safety system–one that is fully integrated with the standard plant automation system–can lead to improved productivity.
These new safety philosophies have been made possible by a fundamental shift in two related areas.
The first is significant developments in safeguarding and control technologies–notably the advent of new microprocessor-based technologies in lieu of electromechanical or hardwired control. The second is the evolution of global safety standards that allow new technologies to be incorporated into industrial safety systems.
The result is an entirely new breed of safeguarding products and safety control platforms. Safety technology can now be integrated into standard automation products–such as programmable automation controllers, programmable safety relays, plus AC and servo drives. In addition, high-integrity safety communications networks have also been developed to allow safety and non-safety messages and devices to exist on common media.
The synergy achieved by integrated safety and plant automation systems facilitates the improvements in productivity. Modern safety technology can help minimise downtime in a number of ways, including: improved troubleshooting and diagnostics; prompt return of equipment to production; and reduced service time by permitting some processes to safely continue operation.
Engineering safety into the process
Rapid diagnosis is critical when a fault is causing downtime and lost production.
However, traditional hardwired systems are difficult to troubleshoot, a problem exacerbated by electro-mechanical relays which are inherently less reliable than solid state technology. As a result, the precise cause of a fault such as an E-stop activation can require lengthy investigation.
If, on the other hand, each E-stop is wired into a safety I/O module that is connected via a safety network to the integrated standard/safety programmable automation system, the diagnostic information is easily provided to the PLC and HMI. The cause of the event is promptly diagnosed with minimal delay.
E-stop events can cause a deal more trouble than being difficult to diagnose. Frequently they occur when a machine is in full production, leading to machine downtime, alignment issues and product wastage.
To avoid this scenario, a safety system fully integrated with the control system can reduce unnecessary E-stop activation by initiating an ‘end-of-cycle stop’ which will inhibit machine access unless correctly invoked. When this ‘end-of-cycle stop’ is activated, the machine completes critical processes and is brought to a graceful halt. Work-in-progress is cycled out of the work cell, safe for operator or maintenance entry and ready for restart.
* Gary Milburn is Rockwell Automation Area Manager, 03 9896 0300, www.rockwellautomation.com.au.