Eight tips for the technical side of safety systems

July 11, 2013
From new standards for functional safety to making sure that an E-stop will actually stop everything on a machine, these practical recommendations address the details of a safety system:
1. Functional safety. Functional safety, as defined in IEC 61508, is a totally new methodology in automation. It's based on two main principles: 1. It's a systems approach. Sensor - controller - actuator, every component should have the same SIL/PL. Otherwise, it makes no sense. 2. System/product lifecycle idea. Today's safety does not mean tomorrow's safety. Functional safety needs a procedure to implement it continuously.

2. Validate the safety code. Determine each failure mode and validate that it fails to a safe state. Just because the logic tells you to turn an output on or off when running, does not necessarily mean it will return to the correct state if there is a fault. It needs to be verified.

3. Key features for safety systems. Low PFD (probability of failure on demand), equivalent to the long-accepted TMR (triple modular redundant) benchmark standards, in either simplex or redundant configurations. Low STR (steps to reproduce) that meets or exceeds the TMR standard when implemented in dual configuration. Exceptional hardware fault tolerance in dual configurations. Safety modules independent from the logic solvers. By choosing safety systems with these features, availability has been estimated as high as 99.9999 percent.

4. Identify risks. Deep dive into Process Hazard Analysis (PHA) to identify the risks underlying a process. Apply LOPA (layer-of-protection) analysis to rate the SIL level and guide the SIF design according to IEC 61508/61511. Understand all aspects of the safety lifecycle before proceeding with design and implementation.

5. Send robots home. When programming a robot that has the potential for a crash situation and relies on an operator to manually move the robot out of position, program in flags or position steps. Then create a program called "home." After each critical move/step of your robot in the program, to avoid other machinery or tooling, use a variable, such as "pos," and assign it a numeric or other value like zero as the home value. When you create the home program, you can run it and have a series of "if" statements. For example, if "pos" = 10, then do this. Now you know where the robot was positioned when it crashed and you can provide a safe route automatically to exit its current position and into a safe home position. Make sure to clear your "pos" values or set them to zero once you make it to the "home" position. Now the main program can be run from the beginning, without any worries.

6.Consider magnetic locks. Magnetic lock systems have come a long way and they really simplify the way you can design and safeguard a machine. Interlocks are built in and they are a little harder to bypass than previous generations of the technology. They can be very useful for maintenance and for troubleshooting because they can be installed in a manner that will satisfy everyone, from the operator to OSHA.

7. Discharge capacitors. On older AC and DC drives, even when locked out, make sure capacitors are fully discharged before removing the front panels and working within the drive.

8. Stop everything. Don't allow OEMs to place E-stop buttons on their consoles if they do not stop all equipment in the immediate vicinity.

Don't overlook safety distance calculations

The safety of a machine is not guaranteed by simply using a safety laser scanner in an application. A safety distance calculation needs to be considered to ensure the correct use and functionality of the scanner device.

The safety distance calculation will help determine the size of the safety field or the distance a scanner plane is mounted from a hazard. How will the device be mounted? Will it be scanning vertically or horizontally? If it is vertical, the scanner plane may have to be mounted farther from the hazard, similar to a light curtain.

More commonly, if it is mounted with a horizontal plane, there may be a chance for reach-over, which would require a larger-sized safety field. These two possibilities are called the Depth of Penetration and are a part of the safety distance calculation.

Consider also the stop time of the entire system. Realize that the stop time may include more than just the stopping of the machine. The time it takes for the scanner to react, for the stop signal to travel to the machine, for the machine to actually come to a stop and any other delays must be considered. These delays may be small, but can have an impact on the overall size required for the safety field.

The stop time of the machine will also be used in calculating the safety distance. By implementing this calculation into your design of a safety laser scanner, you will be on your way to properly complying with standards and achieving greater employee safety.

IEC 61508 explained

For a comprehensive look at functional safety and the IEC 61508 standard, visit http://awgo.to/016
Source: International Electrotechnical Commission

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