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.

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.