Guards and GuardingHierarchy of ControlsSafeguarding Devices

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