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

CNC machine with sliding doors and safety edges
This entry is part 5 of 7 in the series Guards and Guard­ing

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.

Vertically sliding power-operated guard door on the front of a small hydraulic press.
A power-oper­ated slid­ing guard on a hydraul­ic press. image: CPR SafeInd. Australia.[1]
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].

Section view through an A-B 440F profile showing the principle of operation.
A-B 440F Safedge™ Pres­sure Sens­it­ive Safety Edge [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.

Pepperl & Fuchs Pressure Sensitive Edge and Monitoring Module
Pep­perl & Fuchs Pres­sure Sens­it­ive Edge and Mon­it­or­ing Mod­ule [11]
Diagram showing pulse train travelling between the emitter and receiver in an optical sensing pair.
Dia­gram show­ing optic­al pulse train and con­trol con­nec­tions used by Pep­prl and Fuchs. Image: Pepperl+Fuchs Group. [11]
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].

[13]   “Solen­oid Valve – STC Valve”, Stcvalve.com, 2018. [Online]. Avail­able: https://www.stcvalve.com/Solenoid_Valve.htm. [Accessed: 30- May- 2018].

[14]   Safety edge PSE4-RUB-01. Man­nheim, DE: Pepperl+Fuchs GmbH, 2017.

[15]   Safety con­trol unit PSE4-SC-01. Man­nheim, DE: PPepperl+Fuchs GmbH, 2017.

[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.

[18]    Y. Beauchamp, T. J. Stobbe, K. Ghosh, and D. Imbeau, “Determ­in­a­tion of a Safe Slow Robot Motion Speed Based on the Effect of Envir­on­ment­al Factors,” Hum. Factors J. Hum. Factors Ergon. Soc., vol. 33, no. 4, pp. 419 – 427, 1991.

[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-Schäf­fer, 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. Bour­bon­nière. 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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Series Nav­ig­a­tionISO 13857 – Safety Distances”>Canada Adopts ISO 13857 – Safety Dis­tancesHow to Apply a Safety Edge to a Machine Guard – Part 2: Design Con­sid­er­a­tions

Author: Doug Nix

Doug Nix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Senior Editor of the Machinery Safety 101 blog. Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity. For more see Doug's LinkedIn profile.