How to Apply a Safety Edge to a Machine Guard — Part 3: Stopping Performance

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

In Part 2 of this arti­cle, I looked at the pres­sure-sen­si­tive devices (safe­ty edges) them­selves. This part explores the stop­ping per­for­mance require­ments that engi­neers and tech­nol­o­gists need to con­sid­er when apply­ing these devices.

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.

Stopping Performance

Pres­sure-sen­si­tive devices need phys­i­cal deflec­tion to detect the pres­ence of an object. For instance, the P+F PSE4 needs 12.8 mm of deflec­tion for ini­tial detec­tion of an object.

Pepperl + Fuchs PSE4 profile
Pep­perl + Fuchs PSE4 pro­file [14]
Since we are deal­ing with the clos­ing speed of the guard, and not the speed of the human approach­ing the haz­ard, ISO 13855 and by exten­sion, CSA Z432, do not pro­vide any guid­ance. Instead, we need to focus on the behav­iour of the guard itself.

Below is the force-path dia­gram from the P+F PSE4 man­u­al [14]. Using the force-path dia­gram, we can deter­mine how much force is required to deflect the edge pro­file enough to trig­ger the sen­sor.

P+F Force Path Diagram for the PSE4-SL-01 Safety Edge
P+F Force Path Dia­gram for the PSE4-SL-01 Safe­ty Edge [15]
We need to under­stand the rela­tion­ship between the clos­ing speed of the guard, the deflec­tion of the pres­sure-sen­si­tive edge, and the force with which the guard is clos­ing. If we assume that the guard is clos­ing at the “safe speed” of 250 mm/s, we can assume that a per­son might be able to avoid an impact from the guard as it clos­es. Cal­cu­lat­ing the reac­tion time, we take the detec­tion dis­tance from the force-path dia­gram and the clos­ing speed:

 

Detection distance of 12.8 mm divided by the "safe" speed of 250 mm/s = 0.0512 s

Fifty-one mil­lisec­onds is excep­tion­al­ly fast. The safe­ty relay used to mon­i­tor the pres­sure sen­si­tive edge requires 32 ms to react on its own. Here is the reac­tion time bud­get for a sam­ple appli­ca­tion:

Table 1 — Reac­tion Time Bud­get
Device Reac­tion Time (ms)
Total avail­able reac­tion time 51 
 PSE2-SC-02 Safe­ty con­trol unit 32
SMC SY3000 sole­noid valve 16
Mechan­i­cal Sys­tem (cylin­der and guard) 3

 As you can see, there is only 3 ms remain­ing for the mechan­i­cal sys­tem. Three mil­lisec­onds is sim­ply not enough time for a mechan­i­cal sys­tem like a slid­ing guard to stop and reverse direc­tion. Most mech­a­nisms take some­where between 50 and 250 ms to stop, let alone reverse direc­tion. The mass of the mov­ing parts, the veloc­i­ty and the effi­ca­cy of brak­ing method cho­sen makes a big dif­fer­ence in the time required.

CNC machine with sliding doors and safety edges
CNC machine with slid­ing doors and safe­ty edges [17]
Although the edge can deflect past the detec­tion point, called the “cush­ion zone,” there is sig­nif­i­cant­ly more force required as the deflec­tion increas­es past the detec­tion point. The table below shows the increas­ing force nec­es­sary in the “cush­ion zone.” Refer­ring to the table, you can see that a force of 250 N (56 lbf.) is required to cre­ate 22 mm of deflec­tion or about three times more force than is required to reach the detec­tion point.

 

Deflec­tion & Defor­ma­tion Force
Point   Deflec­tion (mm) Force (N)
A Detec­tion Point 12.8 80
B Resis­tance 22.0 250
C Resis­tance 23.0 400
D Resis­tance 24.0 600

Con­sid­er­ing these fac­tors it is clear that it is very unlike­ly for the sys­tem to be able to react quick­ly enough to pre­vent any injury from the guard. Slow­ing the clos­ing speed of the guard down to the point where ade­quate time is avail­able for the sys­tem to detect  — and react — to the pres­ence of a per­son or an object is need­ed.

The clos­ing speed must be set so that the con­trol sys­tem and the mechan­i­cal sys­tems have ade­quate time to react under the worst case con­di­tion — the point at which the guard clos­es against an obstruc­tion and a fixed object like the frame of the machine, or against an oppos­ing door.

The “safe speed” used in our first cal­cu­la­tion comes from the robot safe­ty stan­dards. This speed is used as the max­i­mum speed that a robot can move at when a human is inside the reach of the robot. The assump­tion is that this speed is slow enough for a per­son to rec­og­nize that they are about to be hit by the robot, and move out of the way. There is sig­nif­i­cant research [18], [19], [20], [21], [22], going back to 1991 show­ing that even 250 mm/s is too fast for most peo­ple and that speeds in the 140–170 mm/s range are more avoid­able.

Reduc­ing the clos­ing speed of the guard makes it eas­i­er for peo­ple to avoid being hit by the guard as it clos­es, with­out mak­ing the clos­ing time exces­sive­ly long. For exam­ple, if the guards on the CNC machine in the pho­to above have to close a 1.5 m (60″) wide open­ing, each door needs to move 750 mm. Using 140 mm/s we get a clos­ing time of

Closing distance of 750 mm divided by closing speed of 140 mm/s = 5.36 s

Round­ing up, five and a half sec­onds is fair­ly slow, so some increase in speed is prob­a­bly okay. A decrease to 3.5 s for clos­ing yields a clos­ing speed of about 215 mm/s. Using the reac­tion time cal­cu­la­tion,

Detection distance of 12.8 mm divided by reduced closing speed of 215 mm/s = 59.5 ms

Again round­ing up, 60 ms gives the fol­low­ing reac­tion time bud­get.

Table 2 — Reac­tion Time Bud­get
Device Reac­tion Time (ms)
Total avail­able reac­tion time 60 
 PSE2-SC-02 Safe­ty con­trol unit 32
SMC SY3000 sole­noid valve 16
Mechan­i­cal Sys­tem (cylin­der and guard) 12

Twelve mil­lisec­onds is still not enough time for a mechan­i­cal sys­tem to react. Addi­tion­al mea­sures will be required.

Additional Measures

Real­is­ti­cal­ly, 100–250 ms is like­ly required for the mechan­i­cal sys­tem to react. As we can’t slow the guard clos­ing speed enough to pro­vide 300 ms of reac­tion time, we need to address the clos­ing force. If the clos­ing forces are lim­it­ed by restrict­ing the air pres­sure so that no more than 80–90 N (18–20 lbf.) can be cre­at­ed by the door clos­ing mech­a­nism, then based on the force-path dia­gram, we can see that we have suf­fi­cient pres­sure to trig­ger the device with­out a high prob­a­bil­i­ty of injur­ing the per­son. Even if the per­son is briefly trapped, they are unlike­ly to suf­fer a sig­nif­i­cant injury.

As you can see, there is a need to bal­ance the oper­at­ing speed of the guard with the clos­ing force of the actu­a­tor, while ensur­ing min­i­mal impact on the pro­duc­tion time of the machine. If there is a risk of some­one forc­ing the guard open once it is closed, you may need to con­sid­er guard lock­ing, or per­haps a dual pres­sure clos­ing sys­tem where a low pres­sure is used to close the guards and a high pres­sure is used to hold the guard closed through the pro­duc­tion cycle. There are many ways to design a sys­tem that is both safe and func­tion­al.

Type-C Standards

Numer­ous type-C (machine spe­cif­ic) stan­dards antic­i­pate the use of pres­sure sen­si­tive devices and will pro­vide some guid­ance in rela­tion to their appli­ca­tion. One exam­ple, EN 12978 [23], applies to garage doors and gates, pro­vid­ing appli­ca­tion and test­ing require­ments for this type of machin­ery; but there are lots of oth­er exam­ples of machine spe­cif­ic stan­dards that antic­i­pate the use of pres­sure sen­si­tive devices for reduc­ing risk to peo­ple.

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 Nav­i­ga­tionHow 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.