What is PLC Redundancy?

In this article, we will discuss about PLC Redundancy.

In engineering systems, redundancy means providing dependability and a process alternative to a failing condition.

In process control systems each industry has its own definition of reliability. Some automation systems require PLC redundancy to keep people and equipment safe, depending on the type of interruption, the severity of the consequences, or the governing regulations.

In case of small investment, extra hardware control and intelligent software can reduce the damage and inconvenience when a controller fails.

All contributing factors and statistically possible outcomes must be considered by instrumentation and control engineers. These may include:

Continuous process - Food processing, product assembly lines, chlorine-based pulp, and paper processing require a combination of several steps. Stoppage in one section may lead to bottleneck in others and loss of unfinished product across the assembly line. Instrumentation engineers need to investigate the extent to which a controller or subsystem malfunction can adversely affect the in-process material.

Batch Processing: Surface Mount Technology (SMT) circuit boards are a very good example of how a large number of products are handled or processed simultaneously, sent to inventory, and then the production line is shut down. PLC redundancy will help to ensure that the line is operational when order output is resumed.

Critical Industries: Mining, nuclear and gas control cannot interfere with operation and safety monitoring. A full back up control system requires 100% uptime to prevent fatal and costly incidents.

Reduction of PLC components

PLC redundancy options are available in various components of PLC.

The different PLC redundancy factors are as follows -

CPU redundancy:

If the CPU fails, standby maintains the CPU plant

Power supply redundancy:

In the event of a power supply failure the power supply assumes control of the situation.


Multiple communication channels are provided to resolve communication failure

I / O Frequency:

Multiple I / O channels are provided to detect input or output failure.

There are four ways to improve reliability even most PLC vendors provide units with built-in redundancy for processor control and power supplies. They are:

Special mode (independent operation) - Again, two PLCs are used at a time, but each operates separately, and the inputs and outputs are split between the two processors equally (50/50). If a PLC fails (along with its internal backup systems), only half of the capacity will be lost instead of the full load. This is the easiest redundant controller system to implement but requires duplicate assembly lines, controllers, sensors, and actuators.

Shadow mode: Two identical PLCs share the same inputs and outputs and run the same software. The first serves as the primary, while the second serves as a backup. If the second unit does not receive a heartbeat signal from the first, the backup unit takes over control of the automation system, ensuring uninterrupted operation. These require a little more design, and an arbitration circuit for the sensors and actuators to avoid inconsistencies.

Split Mode (Dependent Operation): The two controllers**,** as used in telecom systems and banking mainframes, share the same inputs and compare independent decisions before setting a setting output. If a mismatch is noticed, a special system response is applied. In automation systems, the appropriate intervention may be simple reporting or slowing down or slowing down the assembly line until human intervention occurs.

Voting: An odd-number of independent control systems make self-governing decisions, as popularized by NASA, and a ballot is posted before a decision is made (majority rule). These systems are the most costly to build because they require redundant control systems, which may be too bulky and costly for some applications. And PLC programming necessitates a more thorough design.

Types of PLC Redundancy:

There are several types of redundancies to consider when designing control systems for fault conditions:

Cold Redundancy,

Warm Redundancy,

And Hot Redundancy

Cold Redundancy

It is for non-critical processes in which time is not critical factor and human intervention is acceptable.

For example: if a system fails and the automation system will inform the operator and give an alarm. A simple response will be displayed by electricity supply and a red alarm lit. The operator can be resumed by requesting the service to start another unit and requesting the service.

Warm Redundancy

When the time and response to a failure is more important, a temporary accumulation is sufficient if a temporary accumulation is acceptable.

As an example, if a valve fail to work in a liquid exchange system, the pump can be disabled and system shut down. Depending on the process, the product may have a limited term for the product, it will not be damaged or worse.

To avoid any integrity issues the process can endure a few seconds or minutes quickly.

PLC redundancy is typically run in shadow mode, where the hot systems can run, and there are a heart rate signal is sent from the primary to secondary then to secondary to primary.

Disruption to controlling the primary can cause secondary control. Process Train & Depending on the components, Mux control signals require additional arbitration circuit for depending on the elements.

The things that may happen at any moment of a program - because of a program per program, may take some program scans for change. The standby processor may have incomplete or stale data, which may cause a process and flows into the product.

Hot Redundancy

The hot redundancy is similar to warm redundancy, but the hot redundancy will be offered to correct instant process editing.

For example, if a repair or an elementary controller fails, a Backup Transfer should be governed to avoid any delays.

Plenty of programming software and hardware coordination should be allowed to access the continuous messaging and access to public data between processors to allow smooth transformation.

The data can be transferred between processors or accessible through a typical database located on the network. Anyway, the secondary process should have knowledge of all logic and all logs.

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