Safety Systems: To Integrate or Not to Integrate

Combining safety and production can save money, space in many applications.

Safety has always been an important aspect for industrial facilities, but tighter regulations have pushed it even higher in the requirement lists for industrial automation systems. At the same time, the need to maximize efficiency has prompted developers to turn away from architectures that use discrete systems for production and safety functions.

Dedicated safety equipment adds significant cost and complexity. It also takes space, which is becoming more expensive as enterprises use lean manufacturing, just in time and other strategic plans that help them get the most out of every inch of real estate.

For many industrial automation systems, it now makes sense to use integrated safety systems. They use the same hardware for standard functionality such as on/off and modes as well as for relevant safety functions. That saves costs and simplifies installations.

This level of integration is made possible by faster, more powerful electronic controls as well as by networks that offer enough bandwidth to carry safety and functional communications on a single cable. Safety Programmable Logic Controllers (PLCs) and Safety Rated Variable Frequency Drives (VFDs) are among the components that facilitate the use of integrated safety.

Though there’s a definite trend towards integrated safety, the old approach with segregated control systems for standard functions and safety will sometimes make perfect sense. Some examples include companies that build smaller systems and those that always build identical systems. In the latter case, the segregated architecture may make sense even for complex systems.

Siemens meets these requirements by selling non-PLC safety systems. Such components include safety relays, a sensor level bus with safety relay (ASi-Safe) and the Modular Safety System, which includes parameterizable and scalable safety relays. These are all perfectly viable products for the right applications.

In these systems, the standard control typically provides flexible logic and very strong diagnostics with reporting and trending capabilities. The safety systems typically have hardwired functions and limited diagnostics and reporting capabilities. However, diagnostics and reporting can be improved by using measures such as connecting additional safety sensor channels back into the standard control.

Integrated systems

For most other types of equipment, it makes much more sense to integrate safety and standard control functions. System designers should consider it when they’re establishing architectures for many different environments. These include approaches that are:

• Modular: Sometime sections A, B and C are all provided, but other times only A and C are used.

• Flexible: The safety system needs to support various operational modes, reacting differently in the different modes.
• Complex: Often, safety decisions are fairly complicated, but the control may be kept very simple in order to avoid complexity. For example, an entire machine may be shut down when a door is opened even though it would be more appropriate to have different responses to different doors, such as shutting down just a part of the machine, or going into a limited speed mode.
• Difficult Environment: Machines such as a pick-and-place, a crane, or rotating robot have significant safety requirements. Due to limited communication methods, such as brushes or slip rings, the information crossing the boundary is kept to a minimum from both a control and a diagnostic information standpoint. With a Safety PLC, wireless safety communications would allow much more granular safety control, while significantly improving diagnostics.
• Integrated to other systems: One machine often needs to control the safety operation of upstream and downstream systems, and the control of choice is a single shutdown contact. This may be what is really needed, but perhaps it’s better to provide safety communications between each machine’s Safety PLC so the system responses could be better controlled.
• Distributed: The machine is very large, or it just does not make sense to wire all the safeties back to a central location. With a distributed I/O architecture, the I/O can be located logically to match the machine design instead of safety system limitations. This approach may also allow segmented startups and maintenance functions, shorter wire runs and smaller wire bundles. All of these can lead to shorter startups and less downtime.
• Lots of devices: As the number of safety functions grows, system complexity also rises. A Safety PLC can provide many safety functions and build up the relationships via Safety Logic instead of hardwires.

Basically, if you compare the benefits of PLCs to relay control, the same benefits may be realized using Safety PLCs as compared to just safety. When standard and safety controls are integrated into the same system, the additional information and decision options increase significantly compared to attempting to integrate two separate systems.

Some broad benefits available through Safety Integrated are as follows:

• Safety technology and standard automation integrated in an overall system
• Safety communication over standard field buses
• Lower engineering costs due to the use of standard software – even for Safety technology
• Simple and practical to handle – high acceptance by operating personnel
• Effective and fast diagnostic functions – for highest availability of machines and plants
• Cost reduction due to less hardware requirements, faster installation, and more efficient engineering
• Products, systems, solutions, and service – made by Siemens.

When equipment developers establish their architectures, they need to determine whether to combine standard controls and safety management into one system or to use two discrete systems. For many, combining the two can yield substantial savings.

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