SO why is a safety PLC not always seen in system design? Safety circuits are nothing out of the ordinary to the engineering community. This is not to say that using one is right or wrong. It may simply be an unknown or unexplored entity to many engineers designing a safety circuit. Understanding what they are, and the fundamental differences of a dedicated unit are the first steps to evaluating this technology.
Safety in this text will be defined as relating to electronic safety circuits typically found in industrial, commercial and manufacturing settings. The most basic yet important safety circuit is the emergency stop. It is important to note that the National Fire Protection Agency does not permit electronic components or communication transmissions to be used in an E-stop circuit. It is entirely permissible to use a safety programmable logic controller, for most other safety related applications.
A safety PLC is essentially a redundant computing system. Conventional PLCs typically contains one microprocessor, flash memory and RAM. In direct contrast, a safety system will have more than one microprocessor, flash area and RAM. Depending on the complexity of the specific controller double, triple and even quadruple redundant PLCs are available.
In redundant safety logic controllers the memory and processors are monitored by continuous watchdog circuits. With the various manufactures of safety controllers other proprietary detection methods are also used to verify safe operation. Among these methods synchronous detection circuits are also widely implemented.
With most standard PLCs there is no internal method to test functionality of an input. In A typical safety PLC, output circuits are associated with each input. This allows verification tests during setup and troubleshooting. It will perform constant functionally tests during run time cycles.
Outputs are typically linked to multiple processors as part of the redundancy, which requires every processor to receive the correct operational safe condition for proper operation. In a typical case if one processor, input, output, sensor or switch is not in the proper state the PLC will safely and systematical shutdown the current cycle according to the program.
The differences in methods are plentiful and both can be quite useful in the correct situation. Using this special PLC when one is not entirely needed will add to the complexity, cost and future maintenance of a project. When to use one can be best described with the following example conveyor systems.
In the first example there will be a simple gravity fed roller conveyor, fed upstream from a chute with a normally closed feeder door. There is walking access to the conveyor on both sides for 6 feet to allow for unloading and stacking manually. Conventional light curtains on both sides span the access points. In this case a standard safety relay tied into the existing E-stop circuit is sufficient. The light curtains only need to trip the emergency stop and not allow the feed chute to open when someone is present inside the access point.
In the next example a powered feed conveyor is used to drive empty pallet stacks into a robotic arm bay. The robot picks a pallet then turns 90 degrees to place it on a loading conveyor. It is stacked and driven out of the bay on another powered roller. On each conveyor entering and exiting the bay a set of conventional light curtains are used. The light curtains must be able to shut the system down if a person or foreign object enters the bay. This must be done is a safe and orderly matter, due to the possibility of the robotic arm holding a pallet in mid cycle.
This is a case where a safety PLC would be very useful.The system cannot be simply shut down instantaneously, it must retain grasp of the pallet but stop movement of rollers and robotic arm instantly.When feed pallets enter into the bay on the conveyor, it must break the light curtain plane in order to enter or exit. The safety PLC, with the help of position sensors on the conveyors, can turn off the light curtains on a delay for the pallets to enter without tripping the safety system.
With two simple examples it is easy to see the advantages and disadvantages depending on the system needs. Most systems could be designed using either multiple standard PLCs or many relays and safety interlocks. Even though they cost about 30% more than a standard PLC the extra cost of specialized software, and time added engineering would make it cost prohibitive to not use a safety PLC.
As with any PLC or electronic equipment speaking with a vendor about your individual system needs is typically the best way to choose the right safety PLC. Having an underpowered or incorrect device can be expensive mistake just as an over powered high-priced system that will not be used to its full potential.
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