How to Apply a Safety Edge to a Machine Guard – Part 1: Pressure-sensitive devices

Safety Edges are often forgotten as safeguarding devices. Most machinery engineers and designers are familiar with interlocking devices and light curtains, but once we step away from the familiar, our understanding of how to apply safeguarding devices like a safety edge becomes a bit foggy.?

Full disclosure: I use examples from both Rockwell Automation and Pepperl + Fuchs in this article. Neither firm has any relationship with me, and no financial or other considerations were offered or solicited in relation to this article or any other work on this blog.

What is a Safety Edge?

Safety Edge devices, more correctly called “pressure-sensitive edges,” are used on power operated?guards; these are applications where a power-operated actuator, like a pneumatic cylinder or an electric drive, opens and closes a movable guard. The leading edge of the guard?is fitted?with a pressure-sensitive?edge to prevent inadvertently crushing an operator’s hands or arms. For example, older passenger elevator (passenger lift) doors will often have a movable rubber edge, called a mechanical door bumper, which is set up to sense impact with people entering or exiting the elevator car. Bumping the edge causes the doors to re-open. Modern elevators use?door sensors that offer additional safety functionality, sometimes in addition to mechanical door bumpers.

When guarding machinery, the pressure-sensitive edge fits on the leading edge of the guard and is designed to sense contact with an obstruction and signal the control system to stop or reverse the motion of the power-operated guard. The photo above shows a typical example of a power-operated sliding guard [1]. The guard pictured does not appear to be fitted with a pressure-sensitive edge. The lack of a pressure sensitive edge on the guard is not necessarily wrong, as there may be other safeguarding devices protecting the operator’s hands that are not pictured. In risk reduction, there are almost always multiple ways of reducing the risk to the worker.

Standards

There are a few standards that provide guidance on the installation of pressure-sensitive edges. I’ve referenced them at the end of this article, but I want to introduce them to you here too.

ISO 13856-2 [3] and ISO 13856-3 [4], are the standards that govern the design of pressure-sensitive edges and bumpers. Knowing about the governing standards is important when selecting a product to specify in new machine design, but detailed knowledge of these standards is only relevant if you are designing and manufacturing these products for sale to others.

The positioning of the pressure-sensitive device is covered by ISO 13855 [5], although this standard does not provide much guidance to designers on the topic. More on that later.

Finally, the integration of the device into the safety-related control system is covered by ISO 13849 [6] and [7] or IEC 62061 [8], depending on which you select for your system. Since pressure-sensitive devices are intended for use in the machinery sector, the majority come with a specified ISO 13849 Performance Level (PL) rather than the IEC 62061 Safety Integrity Level (SIL). Regardless, either can be used. There is a table in ISO 13849-1 that maps IEC SILs to PLs in case you need to purchase a device that is SIL rated.

Definitions

It’s important to get the language used to discuss safety edges right so that everyone is talking about the same things. The following definitions come from ISO 13856-2 [2]. The standard divides safety edges into two distinct groups: devices where the shape of the device is deformed at the point of contact and devices where the whole device contact surface moves when contact occurs. In this article, the term “pressure-sensitive device” will be used as the general term to describe both types; if a distinction is necessary, the specific term will be used to indicate the distinction.

3.1 pressure-sensitive edge

sensitive protective equipment of the ?mechanically activated trip? type comprising a sensor (3.3) or sensors and a control unit and one or more output signal switching devices (3.5) intended to detect contact with a person or body part of a person and where the effective sensing surface (3.9) is deformed locally to actuate the sensor(s)

Note 1 to entry: The sensor or sensors generate(s) a signal when pressure is applied to part of its surface. The control unit responds to the signal from the sensor and generates an output signal(s) to the control system of a machine.

Note 2 to entry: The length of the sensor(s) is greater than the width. The cross-section throughout the pressures sensitive area is constant and its width is usually within the range from 8 mm to 80 mm. Note 3 to entry: For the definition of sensitive protective equipment see ISO 12100:2010, 3.28.5.

3.2 pressure-sensitive bar

sensitive protective equipment of the ?mechanically activated trip? type comprising a sensor (3.3) or sensors and a control unit and one or more output signal switching devices (3.5) intended to detect
contact with a person or body part of a person and where the effective sensing surface (3.9) moves as a whole to actuate the sensor(s)

Note 1 to entry: The sensor or sensors generate(s) a signal when pressure is applied to part of its surface. The control unit responds to the signal from the sensor and generates an output signal(s) to the control system of a machine.

Note 2 to entry: The length of the sensor(s) is greater than the width. The cross-section throughout the pressure sensitive area is constant and its width is usually within the range from 8 mm to 80 mm.

Note 3 to entry: For the definition of sensitive protective equipment see ISO 12100:2010, 3.28.5.

Safety Edges for Machinery Safety

The most common type of pressure-sensitive device has two conductive strips embedded in a soft rubber profile. Compressing the profile causes the strips to touch, generating the stop signal. One approach is shown in the figure below, taken from the A-B 440F installation manual [10].

Another approach uses an optical sensor that shines a light through a tube in the contact profile. A sensor on the opposite end of the tube receives the light as long as the profile is not compressed. When an obstruction compresses the profile, the light is blocked, and?the stop signal is generated. Failure of the light source will also generate a stop signal. Pepprl and Fuchs make a product like this, and there are other manufacturers with similar products.

Ready for more? Read Part 2…

Credit

Thanks to one of our readers, Mr Philip G Horton, for asking the questions that inspired this article, and for being patient with me while I carved out the time to write it.

References

[1]? ? ?”SafeInd Custom Machine Safety Guarding – cprsafe.com.au”, cprsafe.com.au, 2018. [Online]. Available: https://www.cprsafe.com.au/products/guards/custom/. [Accessed: 23- Apr- 2018].

[2]? ? ?Safety of machinery — Pressure-sensitive protective devices — Part 2: General principles for design and testing of pressure-sensitive edges and pressure-sensitive bars.?ISO 13856-2. 2013.

[3]? ? ? Safety of machinery — Pressure-sensitive protective devices — Part 3: General principles for design and testing of pressure-sensitive bumpers, plates, wires and similar devices).?ISO 13856-3. 2013.

[4]? ? ?Safety of machinery — Positioning of safeguards with respect to the approach speeds of parts of the human body. ISO 13855. 2010.

[6]? ? ?Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. ISO 13849-1. 2015.

[7]? ? ?Safety of machinery — Safety-related parts of control systems — Part 2: Validation. ISO 13849-2. 2012.

[8]? ? ?Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems. IEC 62061. 2005.

[9]? ? ?Safety of machinery — General principles for design — Risk assessment and risk reduction. ISO 12100. 2010.?

[10]? ?Guardmaster? Safedge? Pressure Sensitive Safety Edge System Installation and User Manual 440F, 3rd ed. Milwaukee, WI: Rockwell Automation, 2015.

[11]? ?”Safety Edges”,?Pepperl+Fuchs, 2018. [Online]. Available: https://www.pepperl-fuchs.com/global/en/classid_2794.htm. [Accessed: 27- May- 2018].

[12]? ? A. MUHD, “How to Read Pneumatic Schematic Symbols….”,?Amzardabest.blogspot.ca, 2018. [Online]. Available: https://amzardabest.blogspot.ca/2011/01/how-to-read-pneumatic-schematic-symbols.html. [Accessed: 27- May- 2018].

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[16]? ?Safety edge PSE4-SL-01. Mannheim, DE: Pepperl+Fuchs Group, 2016.

[17]? ??Sensors for Safety Applications Product Overview. Mannheim, DE: Pepperl + Fuchs GmbH, 2017.

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[19]? ? W. Karwowski, T. Plank, M. Parsaei, and M. Rahimi, ?Human Perception of the Maximum Safe Speed of Robot Motions,? in Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 1987, pp. 186?190.

[20]? ? S. Haddadin, A. Albu-Sch?ffer, M. Frommberger, and G. Hirzinger, ?The role of the robot mass and velocity in physical human-robot interaction – Part I: Non-constrained blunt impacts,? in Proceedings – IEEE International Conference on Robotics and Automation, 2008.

[21]? ? Y. Chinniah, B. Aucourt, R.?Bourbonni?re.?Study of Machine Safety for Reduced-Speed or Reduced-Force Work R-956, no. March. 2017.

[22]? ?S. Haddadin, A. Albu-Schaffer, and G. Hirzinger, ?Requirements for Safe Robots: Measurements, Analysis and New Insights,? Int. J. Rob. Res., vol. 28, no. 11?12, pp. 1507?1527, 2009.

[23]?Industrial, commercial and garage doors and gates – Safety devices for power operated doors and gates – Requirements and test methods.?EN 12978. 2003.

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