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Safety Programmable Logic Controller Market: Introduction to programmable logic controllers

A programmable logic controller is an industrial digital computer that has been toughened up to adapt for the control of diverse manufacturing processes such as robotic devices or assembly lines, or any other activity that necessitates ease of programming, process fault diagnoses and high-reliability control. All controllers available in the programmable logic controller market essentially serve the same purpose i.e. controlling some mechanical process. The most common programmable logic controller functions are control, sequencing and timing of actions such as turning pumps, valves or motors on or off.

Safety Programmable Logic Controller Market: Difference between standard and safety programmable logic controllers

A safety programmable logic controller is a dedicated controller specially built to meet certain safety requirements such as the ISO 13849-1, IEC 62061 and IEC 61508 functional safety directives that define the Safety Integrity Level or the risk reduction level enabled by a particular safety function. There are several important differences at the hardware level between the physical programmable logic controller and the safety logic controller. The main difference between the two is the presence of self-checking and redundancy mechanisms in the safety programmable logic controllers that are not found in the standard programmable logic controller.

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Another key difference is in the input and output stage. In the case of the former, a safety programmable logic controller continuously keeps track of the input status to identify and malfunctions or failures that could indicate a problem in the machine itself or sometime in the future. In the latter case, there is additional safety circuitry between a connected device and the output meant to limit the damage to the external device in the event of any fault of malfunction.

So how can it be decided if an organisation needs a regular programmable logic controller or a safety programmable logic controller? Typically, the machine safety requirements are understood in one of two ways; by the safety regulations governing machines in the country where it is going to be used or on the basis of the machine type designed. For e.g.- there are substantial machine safety requirement differences between the U.S. and the E.U. or between the E.U. and China. Even machines deployed in the food and beverage industry, a stamping press controller, or an oil refinery controller have different safety requirements that must be taken into account.

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The four main categories available in the safety programmable logic controller market are –

Category 1 – Lowest category safety circuit. Single channel relay to safety
Category 2 – Two channel safety circuit. The first channel is hardwired to an e-stop chain while the second channel can be either the PLC monitoring on safety switch auxiliary contact or another hardwire
Category 3 – Two hardwired channels in a safety circuit. Those drives that have a ‘safe off’ feature are included in this category
Category 4 – Two hardwired channels for a safety circuit with a safety relay that is self-monitoring. Category 4 is the toughest category and is the most popular in the automobile industry.