Safety PLCs

More Information about Safety PLCs

Safety controllers are built to perform functional safety-critical tasks in control systems. If a PLC is performing a safety-critical task, and it stops working or gives a wrong answer, you're in trouble. Hopefully, your safety PLC has enough redundant hardware and diagnostic features to recognize that it has a problem and will act in a safe and predictable manner (with an emergency stop, if necessary). But life isn't always that simple, and sometimes you want to integrate a safety PLC with other safety systems, so that all your controls work together as one big system.

FAQs

What are the internal functions of a safety programmable logic controller compared to a standard programmable logic controller ?

A safety PLC is a specialized type of PLC that is designed to provide enhanced safety functions and meet safety requirements and standards, such as IEC 61508 and ISO 13849. Compared to standard PLCs, safety PLCs have several internal functions that are specifically designed to enhance safety and reliability which include: dual processors, self-checking, safety functions, safety communication protocols, diagnostic coverage and data verification. Both standard and safety plcs serve important functions in a safety system.

What is program flow control?

Program flow control is the process of controlling the sequence of instructions executed in a program. It refers to the way in which a program directs the flow of execution from one instruction to another based on certain conditions or events.

What is a Safety Integrity Level (SIL) in a safety instrumented system?

A Safety Integrity Level (SIL) is a measure of the reliability and effectiveness of a safety instrumented system in reducing risk, with each specific safety integrity level (ranging from SIL 1 to SIL 4) representing an increasing degree of risk reduction and reliability required for specific safety functions in hazardous operations.

PLCs

Programming Logic Controllers (PLCs) are industrial computer that are designed to monitor and control a series of inputs and outputs.

The program running the PLC evaluates the inputs, and changes the output signals based on the program response to the input signals. Several programming languages are available to program most PLCs. The most common of these is called ladder logic. Ladder logic is a visual programming language that evolved from relay logic diagrams, which visually look like ladders because they consist of two vertical rails with runs between them that contain programmed logic. While ladder logic is the most common programming language used within PLCs, there are others that are also common, including function block diagrams, structured text and sequential function charts. 

The hardware included in a PLC includes: a CPU, memory, I/O connections, a power supply and a programming device.

The CPU is a microprocessor that acts as the brains of the PLC. It performs logic operations, controls instruction and performs various other tasks that help keep the PLC running effectively. The memory in a PLC allows data to be stored that is needed to run the program, as well as any data that is collected by the inputs. Depending on the the type of PLC, the section for I/O may have a fixed number of points or it may be modular, allowing for additional I/O to be added. Many PLCs require some type of power supply and it is common to see more modular based systems integrate the power supply. Lastly, the PLC provides a programming device that feeds the program into the PLC's memory, so that it can perform the desired operation. This is most commonly some type of computer. 

The PLC is designed to run a program repeatedly over and over again, normally many times a second. Each time a program runs, the PLC goes through some very specific steps.

While the order and the specifics may vary from manufacturer to manufacturer, they all typically follow a pattern similar to the following: 

The PLC performs internal self checks to make sure that it is functioning correctly.

The PLC reads the inputs that are connected to it. It will then store the status of each of those inputs in its memory.

The PLC will execute the program logic.

The PLC will write the new values to the hard wired outputs to match those determined during the program execution. At some point in the cycle, communication requests will also be processed.

Following the execution of the last step, the PLC will then cycle back to the beginning of the process and begin all over again.