Engineers have long worked to incorporate automatic safety features into automation systems. Historically, safety devices built with pneumatics and relay technology were developed. The idea behind the "safe protection system" was that it would fail in a way that interrupts power supply, ensuring the system defaults to a safe state when power is lost.
As automation equipment advanced, the number of components used also increased. This caused control cabinets to become larger, with more complex circuits involving relays and timing devices. As a result, the reliability of these systems decreased significantly, leading to instances where identical units operated differently.
It was common for engineers to use discrete electronic components when building control and protection systems in the late 1960s, during their widespread emergence. Many of these systems were banned in process or machine protection applications because they had high failure rates and were very sensitive to changing operating environments.
The development of semiconductor devices that are sensitive to temperature and operating conditions, as well as methods for synthesizing electronic circuits, has created a need to ensure the reliability of electronic circuit operation, which inherently depends on circuit design. Additionally, efforts have been made to verify device reliability statistically, a natural outcome of large-scale production of control systems intended for protection functions.
Engineers have therefore created standards to evaluate the reliability of electronic systems in a clear and straightforward manner. Additionally, safety-related functions have been defined, with the safe shutdown function serving as the foundation.
In the following decades, the foundations for safety-related standards were built through the publication of many normative documents in Western European and American countries. At the end of the 1990s, the first version of IEC 61508 was developed, laying the foundations for the standard electronic systems used today to perform safety-related functions. IEC 61508 defines statistical terms used to assess the reliability of electronic devices. Basic concepts such as the frequency of damage over time or the structure of the system based on redundancy have been in place for many years. Safe and dangerous failure, damage caused by a common cause, allows for a reliable assessment of the device's reliability. Frequent and infrequent recall combined with SIL (Safety Integrity Level[1]) provide a measure of reliability in the operation of equipment. The number B10, derived from Weibull's statistical analysis, is utilized in devices that perform switching functions structurally. All of the above parameters have been defined and described in the IEC 61508 standard. Additionally, the standard provides methods and examples of reliability analysis for electronic devices, which today constitute the "state of the art" catalog for analyzing the reliability of electronic devices.
Reliability in the factory automation systems
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