What is the best way to implement redundancy in a PLC system?

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Redundancy is a key feature of any reliable and robust PLC system, especially in critical applications where failure is not an option. Redundancy means having backup components or systems that can take over the control functions in case of a fault or malfunction in the primary ones. But how do you implement redundancy in a PLC system effectively and efficiently? In this article, we will explore some of the best practices and methods for designing and programming a redundant PLC system.

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Mahesh Nakade

PCS7 Lead Automation & Control Engineer | Pharma | F&B | Water | Chemical | PCS7 | GAMP | ISA-88 | 21CFR | Batch | TIA…
Ganesh Kumar G.

Principal Software QA Engineer @ Cognizant | Software testing | Industrial Automation | PLC, SCADA | AVEVA System…
Jamshid Bayat Mokhtari

Manager | Leader | Senior Engineer | (PMP)? | MBA

1 Benefits of redundancy

Redundancy can improve the availability, reliability, and safety of your PLC system by reducing the risk of downtime, data loss, or hazardous situations. By having redundant components or systems, you can ensure that your control logic and communication will continue to operate even if one of them fails. Redundancy can also help you perform maintenance, testing, or upgrades without interrupting the normal operation of your PLC system. Redundancy can also increase the performance and scalability of your PLC system by distributing the workload among multiple components or systems.

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Jamshid Bayat Mokhtari

Manager | Leader | Senior Engineer | (PMP)? | MBA
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An old school trick is to use an internal timer (counter) for the critical functions so in case of an input failure, the function is triggered by the timer (counter). Another way is to apply redundant mechanisms and/or sensors.

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Hardik Shah

Innovating Industrial Automation: AVEVA? Certified Professional | PLC & SCADA Specialist | Proactive Problem-Solver | Team Leader
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Implementing redundancy in PLC involves ensuring that critical components have backup counterparts to enhance system reliability. One common approach is to employ a redundant PLC configuration, where two identical PLCs operate in parallel. These PLCs constantly exchange information, and if one fails, the other seamlessly takes over to maintain continuous system operation. Redundant power supplies, communication networks, and I/O modules further enhance reliability. Additionally, employing a hot standby configuration allows for online maintenance, where one PLC can be updated or replaced without system downtime.

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2 Types of redundancy

When considering redundancy for your PLC system, you have several options to choose from depending on your needs and budget. These include hardware redundancy, which involves having duplicate or spare hardware components, such as PLCs or I/O modules, that can switch over automatically or manually in case of a failure. This can be either hot or cold depending on whether the backup components are active or standby. Software redundancy means having duplicate or redundant software functions, such as control algorithms, that can run on the same or different hardware components. This can be either parallel or serial depending on whether the redundant functions run simultaneously or sequentially. Lastly, system redundancy involves having complete or partial duplication of the entire PLC system, including the hardware, software, and network. This can be either homogeneous or heterogeneous depending on whether the redundant systems are identical or different.

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Mahesh Nakade

PCS7 Lead Automation & Control Engineer | Pharma | F&B | Water | Chemical | PCS7 | GAMP | ISA-88 | 21CFR | Batch | TIA | Step 7
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PLC redundancy involves using multiple PLCs to ensure that if one fails, another can take over without disrupting the process and data. This is crucial in industries where continuous operation is essential for safety, productivity. Types: 1. Cold Redundancy: The backup PLC is not running until the primary PLC fails. 2. Warm Redundancy: The backup PLC runs in a standby mode, ready to take over if the primary PLC fails. 3.Hot Redundancy: Both PLCs run simultaneously, with the backup PLC ready to take over instantly if the primary PLC fails. Components: CPU Redundancy. Power Supply Redundancy. Communication Redundancy. I/O Redundancy.

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Mohammed Hifzur Rhman

E&I planning Engineer at Maaden
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Control transfer from a primary programmable controller to a redundant one in case of failure.When the primary PLC comes back on line, control can be transferred from the redundant PLC back to the primary so mminimal loss of data

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3 Factors to consider

When implementing redundancy in your PLC system, you need to consider several factors that can affect the design and performance of your system, such as fault detection and isolation, synchronization and data transfer, switch over and recovery, and cost and complexity. For fault detection and isolation, you must have a mechanism to detect and isolate faults, as well as criteria for triggering the switch over to the backup components or systems. Synchronization and data transfer require a method to transfer the data and configuration between the primary and backup components or systems. Switch over and recovery involve determining how fast the switch over will be, how to recover failed components or systems, and the impact of the switch over on operations. Cost and complexity involve balancing the cost of implementing redundancy with the benefits of your application, considering initial costs, operational costs, programming efforts, testing efforts, and maintenance efforts.

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Ganesh Kumar G.

Principal Software QA Engineer @ Cognizant | Software testing | Industrial Automation | PLC, SCADA | AVEVA System Platform | AVEVA Communication Drivers | InTouch | Certified Scrum Master.
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Since redundant systems have many advantages, some considerations need to be taken depending upon the system. Cost consideration: To check whether the cost is worth it, the cost of redundancy is compared to the loss due to the failure of the system. Complexity: The complexity is due to additional programming or the handing over of control from the original controller. Synchronization and data transfer: Standby PLC should always be in sync with Master PLC. Data should be transferred between the controllers so that there is no data loss when failover happens. Switch over condition detection: Redundant system should detect the fault condition so that standby PLC will take in charge of the system.

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4 Best practices and methods

To implement redundancy in your PLC system effectively and efficiently, you can use standard and proven redundancy solutions from manufacturers and vendors. Additionally, you can employ modular and flexible architectures that allow for the addition or removal of redundant components or systems. Furthermore, clear and consistent redundancy logic should be established, which includes roles, responsibilities, and behaviors of redundant components or systems. Moreover, test and verify the system before and after deployment with simulation tools, test equipment, or live scenarios. It is also important to monitor and audit the system during operation with logging, reporting, or visualization tools. Redundancy is an important feature of any PLC system; however, it requires careful planning, design, and implementation. By following these best practices and methods discussed in this article, you can implement redundancy in your PLC system effectively and efficiently while achieving desired results for your application.

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Mohamed Abdelsalam

Industrial Automation Engineer at Ezzsteel | PCS7 | TIA | WinCC | Step7 | C# | python | JavaScript
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One rule of redundancy is both redundant modules' firmware must be identical, so you need also to keep your spare modules' firmware identical to the installed modules to reduce your repair time. Note: The firmware version is not the marking on the module, but the version that gets from the online connection with the module since it could be previously updated.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What is the best way to implement redundancy in a PLC system?

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1 Benefits of redundancy

2 Types of redundancy

3 Factors to consider

4 Best practices and methods

5 Here’s what else to consider

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