Safety buses: More common than you think

Thursday November 17, 2011
When you talk to safety system and control engineers, many say that they do not have faith in the communications reliability of buses and prefer to use conventional analog signals for all safety applications and SIS (Safety Instrumented Systems). Why? Because, they say, conventional analog signals have been proven in use for so many years. It seems as though these engineers have conveniently forgotten that there was a time when these systems were new as well.

The Fieldbus Foundation demonstrated SIF (Safety In Field) technology in May 2009. We are still waiting for a range of products to become commercially available, in part because of the rigours of the approval process. A SIF device must obtain two safety approvals - one from the sponsoring fieldbus organization that the device complies with the requirements associated with the Safety Function Block, and the other from an approval agency such as EXIDA or TUV for the affiliated SIL (Safety Integrity Level) rating. The Fieldbus Foundation Safety Analog Input Function Block received its "check mark" earlier this year, so that hurdle in product availability has been passed. Now we need to obtain similar approvals for a Logic Solver host system.

A number of field trials are planned for installation in early 2012. This will start to satisfy the requirements of most control engineers, especially those who want to see the technology in use before adopting it.

What I find amusing is that many people use safety buses everyday without even realizing it. Don't believe me? Do you use one or more of the following?

• ABS brakes in your vehicle?

• Airbus or Boeing aircraft?

• Express elevator?

If you do, then I would argue that you are already using safety buses.

Most vehicles use a CAN (Controller Area Network) bus for communications between the various microprocessors in a vehicle. And since ABS brakes use a microprocessor to detect brake "lock up" and then activate/control the rapid application and release of the brake, this is effectively a "safety bus."

The majority of commercial airlines are "fly by wire," which means that rather than use pneumatics or cables to send commands to the engines, flaps, elevators, etc., a network is used to send the commands to the motors operating these various components.

Building automation uses BACNet as its communications protocol, linking elevator controls on every floor to the motors and switches to transfer the elevator up and down the shaft, while the switches confirm that the elevator is properly aligned and positioned with the floor you want.

A number of people may try to argue that none of the above are "safety bus" applications because they are not in the process industry; however, I am sure that at least some of you drive your vehicles inside the boundaries of a process plant, and if something malfunctioned the result could be just as catastrophic as a failed valve.

I have also heard the argument that unless the bus is actually being used as part of a safety shutdown system, it is a protective system rather than a safety system. Balderdash. In all of the cases where I come across this, it is hair splitting to avoid having to meet the project requirements of a safety system, including the associated operational maintenance requirements.

In closing, you may not believe that you are ready to use safety bus systems, but these systems are all around you already. Spend a few minutes next time you are travelling from A to B and really look at how embedded processors are all around you controlling and protecting your environment.

 

This column originally appeared in the November/December 2011 issue of Manufacturing AUTOMATION.

 

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