How to Apply a Safety Edge to a Machine Guard — Part 2: Design Considerations

CNC machine with sliding doors and safety edges
This entry is part 6 of 7 in the series Guards and Guard­ing

In Part 1 of this arti­cle, I looked at the pres­sure-sen­si­tive devices (safe­ty edges) them­selves. This part explores the design 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.

Design Considerations

The infor­ma­tion you need to design this kind of appli­ca­tion includes:

  1. The clos­ing speed of the pow­er-oper­at­ed guard
  2. Stop­ping time of the pow­er-oper­at­ed guard
  3. Force exert­ed by the pow­er-oper­at­ed guard when clos­ing
  4. Your choice of a pres­sure-sen­si­tive edge device
  5. Ini­tial deflec­tion dis­tance of the pres­sure-sen­si­tive device
  6. Intend­ed reac­tion to con­tact with an object — stop and hold or stop and retract?

Test­ing is the best way to find the clos­ing speed and stop­ping time of a pow­er-oper­at­ed guard. Whether you use a stop­ping per­for­mance test set or use video to deter­mine the stop­ping time, you can read­i­ly obtain the required infor­ma­tion. DON’T GUESS as this is crit­i­cal infor­ma­tion for the design.

The clos­ing force of a flu­idi­cal­ly oper­at­ed (hydraulic or pneu­mat­ic) guard can be eas­i­ly cal­cu­lat­ed based on the applied pres­sure and the bore of the cylin­der. If you are using an elec­tri­cal actu­a­tor of some kind, you may need to devise a means to mea­sure the applied force.

Your choice of pres­sure-sen­si­tive edge device is a design deci­sion that may be dri­ven by pur­chas­ing agree­ments or by the pref­er­ence of the end user.

The ini­tial deflec­tion dis­tance can be found in the device man­u­al.

Intend­ed reac­tion to con­tact with an object is a design deci­sion that needs to be made when devel­op­ing the safe­ty require­ments spec­i­fi­ca­tion.

Safety Functions

Depend­ing on the out­come of the risk assess­ment, there are two sep­a­rate safe­ty func­tions that need to be con­sid­ered:

  1.  Pre­ven­tion of unex­pect­ed start­ing of the haz­ard BEHIND the guard; and
  2.  Pre­ven­tion of injury from the motion of the guard as it clos­es.

Unex­pect­ed start­ing is pre­vent­ed based on the inter­lock­ing sys­tem used with the mov­able sec­tion of guard­ing. Just because you have a pres­sure-sen­si­tive device on the lead­ing edge of the guard doesn’t mean that you can get away with­out a guard inter­lock. The interlock’s PL should be deter­mined based on the risk pre­sent­ed by the machine haz­ards behind the guard.

The sec­ond func­tion, pre­ven­tion of injury from the clos­ing of the guard, is rel­e­vant to the design of the safe­ty-relat­ed con­trols for the motion of the guard itself, which is the haz­ard in the case. The open­ing motion is rarely a con­sid­er­a­tion, it’s the clos­ing motion that mat­ters. The risk assess­ment should con­sid­er the fail­ure modes of the mechan­i­cal aspects of the guard mechan­ics, like the cou­pling between the prime mover and the guard itself, the con­nec­tions between the prime mover and its sup­ports, etc. The speed and mass of the mov­ing sec­tion of the guard must also be con­sid­ered, as well as any shear­ing haz­ards cre­at­ed at the point of clos­ing.

Electrical Installation Requirements

Pres­sure-sen­si­tive device man­u­fac­tur­ers spec­i­fy safe­ty mod­ules that are designed to work with the spe­cif­ic char­ac­ter­is­tics of the sens­ing device. One of the crit­i­cal con­sid­er­a­tions are the fail­ure modes inher­ent in the sens­ing device. The fail­ure modes in the A-B prod­uct are clear­ly dif­fer­ent from those of the P & F device. As part of the func­tion­al safe­ty design process, an FMEA or an FTA may be use­ful for the devel­op­ment of a list of fail­ure modes. This list is used to deter­mine the required mit­i­ga­tion mea­sures, and may also be need­ed to deter­mine Diag­nos­tic Cov­er­age.

The design con­straints on the elec­tri­cal side start with the PL or SIL. If you are not sure about what I’m talk­ing about here, I’d sug­gest read­ing my series on How to Do an ISO 13849 Analy­sis. Once you’ve got the archi­tec­ture select­ed, you can then look at what you need to do with the out­put sig­nals from the pres­sure-sen­si­tive device’s safe­ty mod­ule.

At the end of the elec­tri­cal design chain, you will come to the inter­face with the mechan­i­cal por­tion of the sys­tem. This inter­face could be a revers­ing con­tac­tor arrange­ment for an elec­tri­cal­ly actu­at­ed guard or a flu­idic valve for a hydraulic or pneu­mat­i­cal­ly actu­at­ed guard.  

Mechanical Design Considerations

Remem­ber that con­ven­tion­al stop­ping time cal­cu­la­tions won’t work for appli­ca­tions where the edge is being used to stop a pow­er actu­at­ed guard. To deter­mine the stop­ping per­for­mance require­ments, you need to con­sid­er how much deflec­tion the edge requires before detec­tion occurs, and use that as the stop­ping dis­tance. The stop time is then the time it takes the guard to tra­verse the detec­tion dis­tance at the clos­ing speed. More on this lat­er.

How should the pres­sure-sens­ing mech­a­nism react when an obsta­cle is detect­ed?  If the guard should stop and hold posi­tion, then there is the pos­si­bil­i­ty of trap­ping a per­son between the guard and the sur­round­ing struc­ture. A more com­mon approach is to have the guard stop and then reverse to an open state. If the guard is flu­idi­cal­ly actu­at­ed, then a 5/2  sole­noid valve, that has a spring return to the raised posi­tion is prob­a­bly the best choice. (See fig­ure below.) This selec­tion will cause the guard to return to the raised posi­tion in the event that elec­tri­cal pow­er is lost.

Example schematic symbol for a 5/2 spring-returned solenoid valve.
Exam­ple schemat­ic sym­bol for a 5/2 spring-returned sole­noid valve [12].
The down­side? If the return spring should fail, then the guard will con­tin­ue down until it clos­es or traps the per­son. At that point, the full avail­able force will be applied to the trapped body part.

Alter­na­tive­ly, a 5/3 cen­tre-blocked sole­noid valve like that shown below, can be used.

5/3 Closed Center Spring-returned Solenoid Valve Schematic Symbol
5/3 Closed Cen­ter Spring-returned Sole­noid Valve Schemat­ic Sym­bol [13]
Using this type of valve, with spring return to cen­tre and dual sole­noids, the con­trol sys­tem can select RAISE, LOWER or STOP. If elec­tri­cal pow­er is lost, the valve spool will return to the cen­tre blocked posi­tion, stop­ping the motion of the guard. The down­side to this approach is that you can still end up with a trapped per­son; how­ev­er, you can design a par­al­lel man­u­al actu­a­tor that can be used for res­cue pur­pos­es or, if the valve is select­ed with man­u­al over­rides, you can man­u­al­ly over­ride the con­trol sys­tem to select the raise con­di­tion for res­cue. If this is the design deci­sion, then the valves need to be locat­ed in a pro­tect­ed loca­tion, and the RAISE over­ride needs to be clear­ly marked.

Read Part 3 on Stop­ping Per­for­mance…

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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Acknowl­edge­ments: As cit­ed in the text.
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Series Nav­i­ga­tionHow to Apply a Safe­ty Edge to a Machine Guard — Part 1: Pres­sure-sen­si­tive devicesHow to Apply a Safe­ty Edge to a Machine Guard — Part 3: Stop­ping Per­for­mance

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.