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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 a factory assembly line or a wastewater treatment plant. PLCs can be standard or safety PLCs.
PLC and personal computer
Although it may seem surprising at first glance, PLCs share many features 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 major differences are that a PLC can perform discrete and continuous functions that a computer is not capable of performing and is much better suited to complex industrial environments. You could say that a PLC is like a ruggedized 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 a case, there is a need for reprogramming due to change in the nature of production. To overcome this difficulty, PLC-based control systems were once introduced.
Safety PLC
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 integrated safety functions that allow it to control safety systems as well.
A safety PLC is designed to achieve two fundamental goals: not to fail and, if this is ultimately not possible, for 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 it 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 meet a set of rigorous international standards related to safety itself, most notably IEC 61508. This standard covers design, design methods, and hardware and software testing.
It should be noted that zero risk can never be aspired to, but non-tolerable risks should be reduced as low as reasonably achievable. Although it may seem 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 necessary 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 executed by the PLC are in the correct order. In addition, safety PLCs must undergo software defect injection testing. It is in these tests that programs are corrupted and downloaded into the PLC to verify that the PLC responds in a safe manner.
Advantages of a safety PLC over a standard PLC
A safety PLC allows both 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, since 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 no additional wiring changes or relays.
Once the safety system has been tested and validated, it can be locked and password protected in the safety system to prevent unauthorized variations. Some models even support integrated motion over Ethernet and can be used to initiate the safe torque-off function on various types of variable frequency drives.
Disadvantages of a safety PLC over a standard PLC
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, 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 engineers and maintenance technicians are always familiar with the latest technologies, so it may be necessary to invest in additional training. In any case, the cost of the investment brings an early return for companies.
Finally, it is not uncommon to encounter professionals who are not fully confident in their reliability and have doubts about the integrity of the safety system. While security PLCs are still a relatively new concept, they have already made significant inroads into security applications in industries around the world.
Although they are subject to stringent certification processes to ensure reliability and minimize risk, they can also present some drawbacks. While they offer several advantages that can make the design of a safety system much simpler, they also have drawbacks. It is important to determine, via a thorough analysis beforehand, whether the use of a safety PLC is appropriate for each of the existing applications.
