The regulatory impetus for safety in stamping applications dates back to the OSHA general duty clause in the original OSH Act of 1970: “Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.” Hazards are determined through the process of performing a risk assessment, assessing hazards associated with the tasks performed in the workplace.
ANSI B11.0 calls out the requirements to take for risk assessments, and stipulates what is essentially a double-barreled approach: one, eliminate hazards; and two, reduce risks to a safe and acceptable level. The former involves changing tasks, functions, locations, and so on, and substituting materials; the latter is more complex, entailing engineering controls, warnings (e.g., signage, placards), safe operating procedures, training, and use of personal protective equipment. Two critical steps often forgotten in the risk assessment process are validation and verification:
- Validation is defined as “the process of confirming that a system design performs to a pre-defined confidence level.” The supplier, user or a third party may perform validation.
- Verification is defined as “the act of verifying system performance within the validation process.”
Presses and press-related automation can benefit greatly from performing a valid risk assessment and following the guidelines of ANSI Z244.1, Hazardous Energy Control; B11.0, Safety of Machinery; and B11.1 and B11.2, Safety Standards for Mechanical Power Presses
The Fatal Five
The leading causes of lockout accidents in press applications are often called “the fatal five”:
- Failure to stop equipment
- Failure to disconnect from the power source
- Failure to dissipate residual energy
- Unexpected start up of equipment
- Failure to clear work areas before reactivation
These causes are easily preventable if hazardous energy control procedures are followed and effective safety systems implemented.
The OSHA definition for a safety system focuses on single failure and single operating error. According to OSHA 1910.211, “safety system” means the integrated total system. This includes the pertinent elements of the press, the controls, the safeguarding and any required supplemental safeguarding, and their interfaces with the operator, and the environment—designed, constructed, and arranged to operate together as a unit, such that a single failure or single operating error will not cause injury to personnel due to point of operation hazards.
Improving Safety and System Complexity
Providing safe solutions to new, automated metal stamping presses and automation equipment creates challenges at many levels, but providing those solutions to a retrofit application intensifies those challenges due to time constraints during both install and startup. The specific challenge for retrofitting safety solutions in press controls is the need to simplify and improve the approach to press control by removing hardware and single-channel hardwired circuits.
Current safety technologies involving Safety PLCs (PROFIsafe) and ASI-Safe provide solutions that assist in faster installation times over conventional safety relay integration methods; they also allow for faster debugging time. Safety solutions have become simpler and more effective as these technologies have emerged.
Safety solutions for press controls have evolved along these general lines:
- First came traditional E-stop circuits. As presses became automated, some circuits became single string E-stop controls. These were single contact controls: the output passes through one contact. Their weakness was that contacts may weld closed, and then switches may fail.
- Next came traditional safety relay circuits that emerged when NFPA 79 was updated to allow the use of PLCs in safety-related functions. They are typically run in series, and safety relays switch dependently. There is little or no flexibility. Extra wiring to PLCs was required to monitor individual component status, which both increased fault sources and complicated device mapping. It takes multiple safety relays to effectively design the system safely with this approach, and the systems can be very large and difficult to de-bug.
- Today’s press safety upgrades are comprised of three basic steps:
- Replace the hardware safety circuit with safety PLCs (SPLC) or ASI-Safe.
- Upgrade the C/B control.
- Replace all safety devices.
The schematic below shows an upgrade that replaced safety relays with safety PLCs:
This configuration minimizes wiring and installation, speeds startup, improves diagnostics, and simplifies maintenance when compared to the traditional safety relay-based system. A particular advantage is the opportunity it affords to harmonize three control systems into one integrated control:
An alternative approach to using safety PLCs is ASI-Safe. AS-Interface is an open industrial communications network for data exchange between electro-mechanical input/output devices and automation controllers (i.e., PLCs). Among its capabilities:
- Data and power (24V & 30V DC) travel on a single cable.
- Offer standard/extended 31/62, 5/10 ms response time.
- Integrates safety-oriented signals up to CAT 4/ SIL3.
- Protects up to IP 65/67, special modules up to IP69K.
- Goes 100 m in any topology (line, star, tree, etc.), with extension plug up to 200 m, and with repeater up to 600 m.
ASI-Safe is compelling in press control safety applications for a host of reasons, including:
- Master-slave principle. AS-Interface’s one control module (master) system queries data from all networked data stations (slaves) at precisely determined intervals.
- Network construction. Offers exceptionally simple installation and limitless networking topology.
- Data integrity. Enables check and recheck of every message for possible transmission errors, and automatically corrects the problem.
- Cable and design. The system is easily assembled with its modular “click-and-go” design and familiar yellow cable that carries both power and data through a reverse-polarity protected line.
- Safety networking. With ASI-Safe, it is easy to add safety devices such as light curtains, E-stops, etc., to the AS-Interface network, with instant connectivity to any control platform.
Efficiency Based on Proven Standards
For fail-safe communication, Siemens Safety Integrated uses both the tried-and-tested field bus systems AS-Interface and PROFIBUS, as well as the innovative Industrial Ethernet standard PROFINET that allows for new approaches to safe and efficient machines and systems, such as wireless fail-safe communication via IWLAN.
With all communication solutions, the safety-relevant data are transmitted via the already available standard bus to allow for significant savings in terms of installation and engineering. Safe I/O modules can be combined with standard modules, and safe data can be re-used for diagnostic purposes on the standard level.
Several key benefits emerged from the actual upgrade cited in the Automation Summit presentation on the topic:
- Reduced start-up time
- Safety system can be verified for function in one to three shifts, depending on size and complexity of the system.
- Single press safety startup can be managed in less than a shift to return press to production quickly.
- Cost reductions
- Demonstrated quick, low-cost engineering changes to implement expansion or modifications.
- Maintenance recovery time reduced costs while improving diagnostics over standard safety relay systems.
- Further benefits in cost reductions were realized from Web browser-based diagnostics, reducing the cost of additional hardware related to diagnostics for the systems.
Article authored by Ted Sberna, White Horse Safety. For more details on services provided by White Horse Safety: http://www.whitehorsesafety.com.Have an Inquiry for Siemens about this article? Click Here >>