Simplified Safety: Rethink the Cost of Entry Using Safety PLCs

The bulk of safety projects industrial companies initiate today involve upgrading safety and control systems in existing production equipment.

03/24/2011


Advanced Engineering develops control systems using equipment from many equipment providers, including Siemens and Allen Bradley. Picking the right hardware and software plays a huge role in a company’s machine safety programs. We provide our value added through knowing which issues are important, using unbiased information about safety hardware and software, and doing system integration.

Over the past few years, safety PLCs have become much more common, and hence more often included in customers’ integration programs. The benefits of a safety PLC-based architecture extend well beyond simplified wiring and the ability to alter safety to accommodate changing demands.

Most customers have machines that are already being used in production and they want to add safety to them. Typically, they’re adding safety to two or three machines per year. These machines will already have safety devices, such as e-stops and interlock switches, which the machine builder incorporated into the original design. The value we add is to develop a solution to bring the machine up to the safety level required by a risk assessment. More than likely, these upgrades will include a safety PLC and fail-safe drives.

Developing a safety solution

The first step required for the upgrade is for customers to perform a risk assessment. Risk assessment is a systematic means for quantifying risk levels in order to determine the scope of required safety systems needed to protect personnel and machines from possible injury and damage. The outcome of the risk assessment is that the customer learns the level of safety that is required to be implemented on their system to comply to the North American machine-safety requirements.

Based on the risk assessment and the level of safety required on the machine, Advanced Engineering puts together a safety solution for the customer that includes a safety controller, such as a safety PLC, and necessary fail-safe devices, such as e-stops and light curtains.

The first consideration is what automation platform is currently being used on the machine. The next step is to analyze the most beneficial safety solution for the customer. The solution should be easy to integrate with their existing platform, and provide the most cost-effective implementation. The prime goal is to avoid causing a hindrance to machine operators, while being compliant. We also wish to increase productivity of the system as a whole.

We analyze different manufacturers’ safety solutions. For example, Allen-Bradley products use a standard processor and a safety-partner PLC in combination in the same rack. No safety inputs and outputs are allowed in that rack. Instead, safety signals, such as from interlocks, go to a remote panel, where they are formatted and sent over the network to the safety-partner PLC in the central rack.

Siemens, on the other hand, does both standard and safety processing in one controller. This allows implementing standard control logic for normal machine operation, and safety logic for the safety system in one module with standard and non-standard I/O mixed, either in the main rack or as distributed I/O stations.

Several different network offerings are available depending on the chosen safety solution. These include either Ethernet (PROFINET from Siemens) or PROFIBUS, then, Ethernet (EtherNet/IP from ODVA) , or DeviceNet (ODVA). Customers have flexibility to do everything over the network, which might get them to switch to Safety PLCs.

Similar criteria hold true while selecting variable speed drives. Rockwell Automation offers safe-torque-off options on Allen-Bradley drives, but still uses a hard-wired configuration, with a wire from the PLC to the drive. Whereas, Siemens’ fail-safe drives have a network offering, so there is one network cable to the drive, and no hard-wired safety connection.

Another critical factor is the time spent getting the hardware up and running. Installation can be a huge factor. There may be a lot of machines running with safety relays and e-stop strings, and often there are people who haven’t even heard of safety PLCs. Installation and setup time may even trump the always-critical financial factor. Buyers should examine tradeoffs and ensure that they understand safety, ease-of-use, and monetary considerations. Cost may not make a difference, for example, if one option costs more, but takes less time to install. The cost of hardware may not be as great as the income lost when production lines are down.

Some of the first things Advanced Engineering looks at are the amount of time the system will be down, and general ease of use. If switching from a standard PLC to a safety PLC, how much wire will have to be pulled? When you’ve got people in there troubleshooting at 1:00 am, you want the equipment to be easy to use. That’s often one of the first things people compare after cost. Those cost considerations must also account for the problems that can arise if protective technologies aren’t installed or maintained properly. If equipment or products are damaged, it’s expensive, but if humans are injured, expenses soar and the possibility of legal action rises.

After developing the initial safety solution, we then review it with the customer, making changes per customer requests – such as changing from light curtains, say, to safety mats, and vice versa.

Multiple revisions are done based on customer functionality and requirements, then we ensure that the safety solution meets the safety levels as identified by the risk assessment. Only then do we finalize the drawings, write the code, and install the system.

Follow-up is critical

After the safety system is installed, the customer must do a follow-up risk assessment to evaluate its performance before signing off on it. This follow up risk assessment is critical to ensure that the upgraded machine achieves the required safety levels.

Risk assessment is an ongoing process, to be done every time there is a change made in the safety circuit or re-installation of the equipment. Safety is the most important part of the industrial environment, especially in the U.S.

System integrators always need to talk with their customers and do an evaluation based on the customers’ company-wide safety standards. When customer teams are analyzing these issues, their decision should shade more towards purchasing state of the art equipment. When people add safety, the latest and greatest safety technology solutions would be the wisest decision. They don’t want to be doing this all over again in a few years because they used obsolelete technology.

Tim Gidcumb, senior programmer, Advanced Engineering

www.usa.siemens.com/safety

This article was submitted for the Siemens Simplified Safety custom newsletter.

See other articles in the Siemens Simplified Safety newsletter.

http://www.controleng.com/newsletter/siemens-simplified-safety.html