"It's important to have a risk assessment to start with,"Pettersson explains. "After that, you must work through function descriptions and operating instructions. For us, it took a lot of time to find solutions for our new safety functions so they would be safe and productive. Once the instructions are finished, it's much easier to write the safety program. This is important because it can take lot of time to do a new RA and change safety functions and programs during commissioning if the initial solutions don't work out. It's also important to separate safety and the non-safety programs in the controls. This will make it easier to commission and test the safety functions, and these systems will be much easier to maintain because a clear, readable program will minimize the risk of any misunderstandings.”
Sandvik also adopted ABB's safety PLC with Profisafe network architecture via Profinet communications protocol to independently control the six safety zones. The new dual-processor safety CPU module and the safety I/O channels are integrated alongside the mill's existing PLC, which controls six dc and ac drives and motors. The dc motors drive the main steel belt and rollers, while the ac motors adjust roller position to achieve desired pressure during strip-steel processing.
This new arrangement with the safety PLC allows parts of the mill to remain operational while an operator gains safe access to some other zone, where safety is assured by disabling power. About 50 safety PLC I/O channels also link to gate switches, light curtains and E-stop buttons. The I/O points also monitor pressure switches to sense that hydraulic power is disabled for maintenance.
"Establishing safe zones inside machines allows power to be brought to a safe level without shutting down and having to resynchronize the entire machine," says Gary Thrall, senior product support engineer and TÜV-certified functional safety engineer at Bosch Rexroth (www.boschrexroth-us.com). "Similarly, safe-direction functions can be set up in safety zones, so all power won't have to be removed when operators are loading or unloading materials. This can save 10% on many production cycle times.”
Simpler Standards,Proactive Mindsets
To conduct thorough, uniform RAs and achieve the greatest practical safety at the design stage, there are a variety of domestic, regional and international machine-safety standards that builders can use. Most significantly, the ISO 13849 standard pushes machine builders and users to move from complying with traditional safety categories to instead calculating and achieving performance levels.
Grantek's Winter adds that, "Risk assessments are like resumes. Everyone has one, but they all look a little different. The important part is they all achieve the same basic purpose and contain the same basic information. For that reason, it's important to use an RA and risk reduction process that works for your organization. The first and most important step is to base your process on nationally recognized, consensus standards, such as ISO 12100, ANSI B11.0 or other industry-specific standards. The second and most challenging step is to develop a procedure and rules to support the process to ensure you have consistent results from one RA to another."
Winter reports that deciding which safety standards to follow begins with the location of the end user's manufacturing facility, and then OEMs, integrators, contractors and everyone else follows suit. "In the U.S., you start with federal and state OSHA regulations, and then use standards it has incorporated through reference, which are primarily American National Standards Institute and National Fire Protection Assn."Winter says that because recently revised ANSI standards are starting to harmonize with International Organization for Standardization (www.iso.org) and International Electrotechnical Commission standards, it's important to be aware of their contents too. In Europe, this burden is on machine designers and builders.
"Because U.S. and Canadian machine builders and system integrators are at the mercy of clients' requests, they'll follow national or internal standards most of the time,"Winter continues. "However, it's not uncommon for them to also embrace unique, company-specific, homemade requirements. Europe has a much more restrictive approach, and to meet the ‘presumption of conformity' of the EHS requirements in the Machinery Directive 2006/42/EC, you must follow one or more of the hundreds of harmonized standards listed in the European Union's official journal. In lieu of customer specifications, it's highly advisable to have an internal specification of minimum safety requirements and to list the standards chosen to demonstrate compliance.”
To help machine builders learn and perform ISO 13849-1's calculations, the free "Safety Integrity Software Tool for the Evaluation of Machine Applications" (SISTEMA) is offered by the German Social Accident Insurance organization's Institute for Occupational Safety and Health. The tool provides comprehensive support in evaluating safety in the context of ISO 13849-1.
Safe Motion, New Tools
Besides setting up safe operating zones, the most important benefit of integrating safety PLCs into machine designs is establishing safe direction, speed and other motion that won't allow operators to be injured.
Back at Sandvik's cold-rolling mill, the safety PLC provides safe speed control, which ensures that hands or fingers can't be trapped between the mill's belt and rollers. The safety PLC also allows Sandvik's engineers to use floating-point numbers that simplify safety programming required for tasks on the mill, such as calculating speed. Programming was further simplified by ABB's integrated PLC development tool, which supports safety PLC programming in its CoDeSys-based integrated development environment and supports the PLCopen Safety Library. ABB also provided its own safety code analyzer tool, which verifies safety programming rules.