All you need is a little PLC

2019-11-20 2 min read

When Apple released the iPhone X in 2017, some critics suggested the improvements compared to its predecessor were not worth the extra cost. However, if you could benefit from facial identification and an OLED display, the iPhone X may be a good choice. Manufacturers must also consider the specifications they need when choosing between a standard programmable logic controller (PLC) and a safety PLC.

Safety PLCs support all the applications of standard PLCs. The difference is that, while standard PLCs have a single microprocessor, to which manufacturers often add safety functions using relays, safety PLCs use redundant microprocessors. This eliminates the need for safety relays. Each input circuit in a safety PLC has an associated output circuit so that errors in the system, such as short circuits, ground faults and channel mismatches, can be detected.  

Safety PLCs have two core functions; to reduce the chance of a fault and to shut the system down safely if a fault occurs. A safe and systematic shut down is important in circumstances where an instant shut down could cause harm to users or equipment, such as if a robotic arm is holding an item in mid-air. A safety PLC can ensure the robotic arm places the item down securely before shutting down, while the remainder of the system shuts down instantly.

Safety PLCs that offer additional functions are now available. For example, ABB was the first to develop a safety PLC with integrated condition monitoring. This means the system can monitor the status of brakes on moving equipment by measuring vibration levels, which reduces the risk of collisions.

Is it for me?

The upfront costs of safety PLCs are higher than those of standard PLCs. Therefore, if the safety system is basic, such as one emergency stop button and a light curtain, it may be more cost effective to use a standard PLC with a safety relay.

However, safety applications become more complex when more than three inputs must be monitored and controlled or when communication over a digital network is required. At this point, engineers should consider safety PLCs, which would integrate all control and safety functions and save costs on hard wiring.

Because safety PLCs don’t need as much hard wiring, they are much easier to modify than standard PLCs. A modification only requires programming changes rather than wiring changes or additional relays. As a result, manufacturers can be more flexible, which makes it easier to adapt to industry developments, internal changes and customer demands.

However, engineers may be less familiar with safety PLCs due to their very recent introduction into the industry. While this should not deter manufacturers from adopting the technology, training should be factored into the implementation plan. Also, like with the iPhone X, manufacturers should consider the features they want from a PLC’s user interface. Because safety is a priority, safety PLCs do not offer high-level language programming features like structured text.