Integrating safety in automation requires specialized knowledge
In a typical automation project, maximum efficiency is key. However, undertaking a safety automation project presents additional challenges. First, legal requirements must be met. Second, care must be taken to ensure and validate that the application is truly safe. For example, is safety ensured if the equipment is misused or fails?
Many people—including health and safety engineers, consultants, and safety equipment suppliers—are involved in performing a risk assessment and creating a safety plan. Fewer have a role in determining, commissioning, and implementing safety technology. In fact, often it is up to the integrator alone.
During typical automation projects, PLC programmers must adapt the system to the real-world environment and makes changes “on the fly” to get the project up and running. They often use “OR” function blocks (parallel contacts in ladder logic) and active low sensors. Both are highly critical when doing safety programming.
Consider, for example, a safety application where material has to get out of a machine, but neither a person nor any material is allowed in. This arrangement can be accomplished by “muting” a light curtain using additional sensors. Muting refers to an automatic, temporary suspension of the safety device. When the sensors detect a pallet, the safety light curtain is muted.
This configuration can be done with a safety PLC. A safe and pre-certified muting function block ensures safe suspension of the light curtain (see the diagram). Assume that the muting sensor is a reflector switch where the output is “HIGH” when it sees a reflector and “LOW” when an object is in the light path. The muting function block needs to be “HIGH” on the sensor input to mute the light curtain.
An inexperienced integrator might see no problem with this situation, saying, “Let’s negate the signal by using a ‘NOT’ function block.” Although such a solution works well and appears safe, what happens if the common power supply for the two sensors breaks down? Both sensors will switch off and it will appear to the safety PLC that they both see the object. As a result, the light curtain will be suspended.
An unsafe situation can occur easily by adapting a safe concept (function block) to a real-world scenario. An integrator needs to know his/her determination methods will influence the application. In this case, use of active “HIGH” sensors in combination with additional control signals or time monitoring functions would be required.
Integrating safety technology in a practical way requires extensive knowledge, application expertise, and many years of experience. Just as you would carefully select a qualified integrator for an automation project, make sure to do the same for the safety portion of the project by keeping them separate. This approach will ensure that safety is left to the experts. Experienced safety integrators will reduce the likelihood of safety risks during all phases of a project. Making use of simple and easy-to- use tools for design, simulation, and test also will help validate the safety functions throughout the life cycle of a machine or project.
– Juergen Bukowski is safety program manager, Sick Inc., Minneapolis, MN. Edited by Jeanine Katzel, consulting editor, Control Engineering, www.controleng.com
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