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.

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.