Guards and GuardingHierarchy of ControlsSafeguarding Devices

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