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, 3^rd 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.