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SIS Logic Solvers – more choices needed

Author : Bob Myles, Moore Industries

06 July 2024

Industrial processing and manufacturing industries are not new to the concept of safety processes, methods, and designs. In fact, since the Industrial Revolution, layers of security have been developed to safeguard workers in factories and mining regions. Over several decades, more effective safety regulations emerged, and the accessibility of safety-related equipment significantly increased.

Image: Moore Industries
Image: Moore Industries

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IEC 61511 Functional Safety - Safety Instrumented Systems for the Process Industry Sector appears to have recently gained acceptance across all significant manufacturing and processing sectors.

At the heart of the IEC 61511 standard is the SIS or Safety Instrumented System which is implemented to mitigate and prevent unacceptable risk by an organization to protect its personnel, facility and/or surrounding community and environment. Each SIS is made up of one or more SIFs, or Safety Instrumented Functions that bring a process or loop to a desired safe state. The basic elements of a SIF are the sensor, logic solver and final element. The sensor monitors the process and transmits that information to a logic solver where in turn that data is compared against predetermined settings to determine whether the final element should be adjusted, activated or engaged.

Since Logic Solvers are at the epicentre of every SIS decision, this article will primarily focus on logic solver capabilities and considerations. Currently, several vendors are offering Functional Safety logic solvers, but there can be a large price and functionality gap that exists between single-loop logic solvers and larger safety systems.

The key considerations, capabilities and features that should be part of the decision process in selecting a suitable logic solver for your Functional Safety application will be discussed and reviewed in the following sections. In addition, we will introduce the concept of the multichannel and multiloop logic solver that effectively fills the large price and functionality gap between single-loop logic solvers and safety PLCs or larger safety systems.

The logic solver gap

Image: Moore Industries
Image: Moore Industries

Of the three main components typically contained in the SIF, the logic solver is the most critical. The logic solver is responsible for determining whether dangerous conditions have been met and is responsible for the final element’s ultimate effect on the mitigation function or strategy.

Two types of products have become widely accepted tools in implementing the logic solver component in Functional Safety applications. They are the Programmable Logic Controller, or Safety PLC, and the Single Loop Logic Solver. The Safety PLC, which is the generic name given to larger point count logic solvers, offers much more flexibility but does so at a significantly higher price and with greater complexity, while the Single Loop Logic Solver is more limited in its capability but can adequately reduce risks and meet safety system requirements with less expense and complexity.

Safety PLCs certainly fill key requirements within Functional Safety. Large point and loop counts, TMR (Triple Modular Redundant) applications or where the need exists to sync or network multiple safety logic solvers together to address a complex safety function. Safety PLCs are very capable but come with an extremely high price tag and often require sophisticated programming, maintenance and documentation.

Conversely, there is the fully capable but smaller Single Loop Logic Solver that handles one loop and just a few points. Like Safety PLCs these are often IEC 61508 certified but have a much smaller footprint and cost far less than Safety PLCs. Additionally, the programming is less complicated and does not require any software licensing.

This is where the logic solver gap lies – functionality, complexity, and cost between these two types of logic solver options are vast. While each certainly has its place, many Functional Safety applications require just two loops, or three loops with six inputs and six outputs, and perhaps some simple 1oo2 or 2oo3 voting or maths. The Safety PLC could certainly handle this, but is it overkill? Alternatively, Single Loop Logic solvers might be able to handle this with output relay wiring for voting, but point counts are limited and voting architectures can become convoluted with relay inter-wiring.

Bob Myles, Moore Industries
Bob Myles, Moore Industries

What is needed to fill this gap is a less expensive, less complex, multipoint, voting capable and IEC 61508 certified logic solver that allows safety practitioners an option that meets the functionality below that of the Safety PLC but above the capabilities of the Single Loop Logic Solver. This is where the new Multiloop Logic Solver comes in that may be more effective and a better fit for many SIFs. Like the larger safety systems and PLCs, the standalone Multiloop Safety Logic Solver can accept multiple I/O points, handle one to three loops, performs logic and maths equations, and offers significant flexibility for voting architectures at a fraction of the cost and complexity of larger safety systems and PLCs. Multiloop Logic Solvers typically harness the cost and configuration simplicity of Single Loop Logic Solvers, but also offer much of the advanced functionality of the larger safety systems and PLCs, albeit at smaller loop and point counts. These hybrid logic solvers can potentially meet many of the SIS applications that once required safety PLCs, but also offer an advancement in functionality and capability at an attractive price point for those smaller SIF loops that utilized Single Loop Logic Solvers.

Final thoughts

Today’s logic solvers come in many different shapes and sizes with a wide range of capabilities. The fundamental requirements of your Safety Instrumented System will ultimately determine what type of SIS logic solver best fits your needs. Large safety PLCs are not necessarily a requirement, or they could be only a part of the total SIS solution. Your final implementation may be a hybrid of a full-blown safety PLC and standalone multiloop capable logic solvers strategically dispersed to best meet the requirements for your safety applications, as well as fit your budget. Whatever you decide, choosing a logic solver that has full third-party approval to the IEC 61508:2010 standard can save you significant time and money and will also give you the confidence that you’ll meet the SIL requirements for your SIFs that are part of your SIS. Safety requirements, implementation cost, long-term cost of ownership, ease of programming, operational characteristics, protocol support and security are all key parameters to consider when making a final decision on which logic solver you choose.

About the author:

Bob Myles is Director of Engineering for Moore Industries-International, Inc. He is an exida-certified functional safety practitioner (FSP) with nearly 40 years of experience in development of safety-critical systems for commercial aerospace (DO-178C/ DO-254/ARP4761/ARP4754), military (DO-160), and process monitoring industries (IEC-61508).


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