A PLC (Programmable Logic Controller) is a computer specially designed to operate reliably in harsh industrial environments, such as extreme temperatures, wet, dry or dusty conditions. It is used to automate industrial processes, such as the assembly line of a factory or a wastewater treatment plant. PLCs can be standard or safety.
PLC and personal computer
Although at first glance it may surprise, PLCs share many characteristics with any personal computer. Both have a power supply, a CPU, inputs and outputs (I / O), memory and an operating system, although it is not the same in both cases.
The biggest differences are that a PLC can perform discrete and continuous functions that a computer cannot, and is much better suited to complex industrial environments. You could say that a PLC is like a hardened digital computer that manages the electromechanical processes of an industrial environment.
PLCs play a crucial role in the field of automation as they are part of a larger SCADA system. A PLC can be programmed according to the operational requirements of the process. In the manufacturing industry, to cite one case, there is a need for reprogramming due to the change in the nature of production. To overcome this difficulty, PLC-based control systems were introduced in its day.
As for a safety PLC, it has many similarities to a standard PLC. It can be used to control and automate industrial equipment. In fact, a safety PLC supports all the applications that a standard PLC does. However, a safety PLC contains built-in safety functions that allow you to control safety systems as well.
A safety PLC is designed to achieve two fundamental objectives: not to fail and, if this is not possible in the end, the failure to occur in a predictable and safe manner. It achieves these goals through its redundant microprocessors, eliminating the need for safety relays to create redundancy. It also has built-in diagnostics that allow you to constantly monitor inputs and outputs. If a fault is detected, a safe shutdown of the PLC occurs.
For a PLC to be considered a safety PLC, it must comply with a set of rigorous international standards related to safety itself, notably IEC 61508. This standard covers the design, design methods and testing of hardware and software.
It should be noted that zero risk can never be aspired to, but non-tolerable risks should be reduced as low as reasonably possible. Although it may seem like a truism, since it is very important that safety PLCs have a high level of diagnostic coverage, it is essential to determine whether a safety PLC is, in fact, safe.
For a safety PLC to achieve a SIL or integrity level of three, it must be able to detect more than 99% of potential errors. Some of the tests required to determine diagnostic coverage involve a series of tests such as data verification and program flow control. These tests ensure that the PLC stores critical data and verifies that the internal functions that the PLC executes are in the correct order. Additionally, safety PLCs must undergo software defect injection testing. It is in these tests where programs are corrupted and downloaded to the PLC to verify that it responds safely.
Advantages of a safety PLC over a standard one
A safety PLC allows for safety control and standard control, while a standard PLC only allows the latter. By using a safety PLC to control a safety system, you save time and money on wiring as there is no need for safety relays. As a result, a safety system designed around a safety PLC is extremely flexible. It is easy to modify because it simply requires programming changes but does not require additional wiring or relays.
Once the security system has been tested and validated, it can be locked and password protected in the security system to prevent unauthorized variations. Some models even support Integrated Motion over Ethernet and can be used to initiate the Safe Torque-off feature on different types of VFDs.
Disadvantages of a safety PLC over a standard one
However, there are also some potential cons with safety PLCs. The initial cost is always higher, so this type of PLC would not be ideal for smaller and simpler applications. Another important aspect to keep in mind is that safety PLCs are still a relatively new product with new technology. And not all maintenance engineers and technicians are always familiar with the latest technologies, so investment in additional training may be necessary. In any case, the cost of the investment reports an early return for companies.
Finally, it is not uncommon to meet professionals who do not fully trust its reliability and have doubts about the integrity of the security system. While safety PLCs are still a relatively new concept, they have already blazed a trail in safety applications in industries around the world.
Although they are subject to strict certification processes to ensure reliability and minimize risks, they can also have some drawbacks. By offering several advantages that can make the design of a safety system much easier, they also have drawbacks, it is important to determine, via an exhaustive preliminary analysis, if the use of a safety PLC is appropriate for each of the existing applications.