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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Acknowl­edge­ments: SafeInd, IEC, ISO, Rock­well Automa­tio more…
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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.