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].

[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.

Series Nav­i­ga­tion

ISO 13857 — Safe­ty Distances”>Canada Adopts ISO 13857 — Safe­ty Dis­tancesHow to Apply a Safe­ty Edge to a Machine Guard — Part 2: Design Con­sid­er­a­tions