Presence Sensing Devices — Reaching over sensing fields

This entry is part 2 of 3 in the series Guards and Guard­ing

I recent­ly heard about an appli­ca­tion ques­tion relat­ed to a light cur­tain where a small gap exist­ed at the top of the sens­ing field, between the last beam in the field and the sur­round­ing struc­ture of the machine. There was some con­cern raised about the gap, and whether or not addi­tion­al guard­ing might be need­ed to close the gap. To answer this ques­tion, we need to split it into a few small­er pieces that we can deal with using the tools in the stan­dards.

The first piece to con­sid­er is the gap at the top of the sens­ing field. For this part of the analy­sis, I’m going to assume that the light cur­tain is a fixed bar­ri­er guard, and we’ll analyse the gap based on that idea.

The sec­ond piece of the puz­zle is the place­ment of the light cur­tain, and we’ll look at that sep­a­rate­ly. Once we under­stand the two pieces, we’ll put them togeth­er to see if there are any oth­er issues that may need to be addressed.

The Application

For the pur­pose of this arti­cle, I’ve sketched up the fol­low­ing fig­ures to illus­trate the ideas in the arti­cle. These draw­ings don’t rep­re­sent any actu­al robot cell or appli­ca­tion. Note that the light cur­tain in the sketch is shown with zero safe­ty dis­tance to the robot enve­lope. This is NEVER per­mit­ted.

Cell Elevation View
Fig­ure 1 — Cell Ele­va­tion View show­ing Gap above Light Cur­tain

 

Cell Plan View
Fig­ure 2 — Cell Plan View

Analyzing The Gap

Light cur­tains are treat­ed the same way that mov­able guards are treat­ed, so the answer to this ques­tion starts with deter­min­ing the size of the gap. I’m going to ref­er­ence two sets of stan­dards in answer­ing this ques­tion: CSA and ISO.

Safety Distances for fingers reaching through an opening
Fig­ure 3 — Fin­ger-to-Knuck­le Reach­ing through a Reg­u­lar Open­ing [1, C.4]
Z432 Reaching Through Regular Openings
Fig­ure 4 — Arm-up-to-Shoul­der Reach­ing through Reg­u­lar Open­ing [1, C.4]
Ref­er­enced Stan­dards
CSA Z432 2004 [1] ISO 13857 2008 [2]
Table 3 — Min­i­mum dis­tance from haz­ard as a func­tion of bar­ri­er open­ing size Table 4 — Reach­ing through Reg­u­lar open­ings
Open­ing Size (e) Safe­ty Dis­tance (sr) Open­ing Size (e) Safe­ty Dis­tance (sr)

11.1– 16.0mm [0.376″–0.625″]

Slot­ted >= 89.0 mm [3.5″] Square >= 66 mm [2.6″] Slot
10 < e <=12 Square/Round
10 < e <=12
>= 100 mm >= 80 mm
49.1–132.0 mm [1.876–5.000″] Slotted/Square <= 915.0 mm [36.0″] Slot/Square/Round 40 < e <= 120 mm <= 850 mm

The first thing to notice is that CSA and ISO use slight­ly dif­fer­ent open­ing sizes (e) and safe­ty dis­tances (sr). These dif­fer­ences have their ori­gin in slight­ly dif­fer­ent anthro­po­met­ric data used to devel­op the tables. In both cas­es, the max­i­mum val­ue for e defines the largest open­ing per­mit­ted with­out addi­tion­al guard­ing.

Let’s look at the appli­ca­tion to see if the gap between the top-most beam and the edge of the phys­i­cal guard falls into the bands defined for e.

Cell Elevation Close Up
Fig­ure 5 — Cell Ele­va­tion Close-Up

Based on the sketch­es of the appli­ca­tion, we have a prob­lem: The gap shown above the light cur­tain is right at the edge of the robot enve­lope, i.e., the dan­ger zone. We are going to have to either, a) Move the fence back 915 mm to get the nec­es­sary safe­ty dis­tance or, b) close the gap off com­plete­ly, either with hard guard­ing or by extend­ing the light cur­tain to close the gap.

Know­ing the size of the gap, we can now decide if the gap should be reduced, or the light cur­tain moved or enlarged. Since light cur­tains run about $125/linear inch, adding an addi­tion­al plate to reduce the size of the gap is like­ly the most cost effec­tive choice. We also need to know the dis­tance from the top-most beam of the light cur­tain to the haz­ard behind the guard. If that dis­tance is less than 915/850 mm, then we have anoth­er prob­lem, since the guard­ing is already too close to the haz­ard.

Analyzing the Light Curtain

The light cur­tain posi­tion­ing is dri­ven by the stop­ping per­for­mance of the machine. Again, let’s ref­er­ence both CSA and ISO for the rel­e­vant cal­cu­la­tions.

Ref­er­enced Stan­dards
CSA Z432 2004 ISO 13855 2005 [3]
5.1 Over­all sys­tem stop­ping per­for­mance
The over­all sys­tem stop­ping per­for­mance com­pris­es at least two phases.Thetwophasesare linked by Equa­tion (1):

T = t1 + t2                             (1)

where
T is the over­all sys­tem stop­ping per­for­mance;
t1 is the max­i­mum time between the occur­rence of the actu­a­tion of the safe­guard and the out­put sig­nal achiev­ing the OFF-state;
t2 is the stop­ping time, which is the max­i­mum time required to ter­mi­nate the haz­ardous machine func­tion after the out­put sig­nal from the safe­guard achieves the OFF-state. The response time of the con­trol sys­tem of the machine shall be includ­ed in t2.

t1 and t2 are influ­enced by var­i­ous fac­tors, e.g. tem­per­a­ture, switch­ing time of valves, age­ing of com­po­nents.

t1 and t2 are func­tions of the safe­guard and the machine, respec­tive­ly, and are deter­mined by design and eval­u­at­ed by mea­sure­ment. The eval­u­a­tion of these two val­ues shall include the uncer­tain­ties result­ing from the mea­sure­ments, cal­cu­la­tions and/or con­struc­tion.

Clause 10.11 — Safe­guard­ing device safe­ty dis­tanceThe­cal­cu­la­tion­formin­i­mum safe dis­tance between a safe­guard­ing device and the dan­ger zone of a machine shall be as fol­lows:

S = [K (Ts + Tc + Tr + Tbm)] + Dpf

where
Ds = min­i­mum safe dis­tance between the safe­guard­ing device and the haz­ard

K = speed con­stant: 1.6 m/s (63 in/s) min­i­mum, based on the move­ment being the hand/arm only and the body being sta­tion­ary.
Note: A greater val­ue may be required in spe­cif­ic appli­ca­tions and when body motion must also be con­sid­ered.
Ts = worst stop­ping time of the machine/equipment

Tc = worst stop­ping time of the con­trol sys­tem

Tr = response time of the safe­guard­ing device, includ­ing its inter­face
Note: Tr for inter­locked bar­ri­er may include a delay due to actu­a­tion. This delay may result in Tr being a deduct (neg­a­tive val­ue).

Note: Ts + Tc + Tr are usu­al­ly mea­sured by a stop-time mea­sur­ing device if unknown.

Tbm = addi­tion­al stop­ping time allowed by the brake mon­i­tor before it detects stop-time dete­ri­o­ra­tion beyond the end users’ pre­de­ter­mined lim­its. (For part rev­o­lu­tion press­es only.)

Dpf = max­i­mum trav­el towards the haz­ard with­in the pres­ence-sens­ing safe­guard­ing device’s (PSSD) field that may occur before a stop is sig­naled. Depth pen­e­tra­tion fac­tors will change depend­ing on the type of device and appli­ca­tion. See Fig­ure 5 for spe­cif­ic val­ues. (If applic­a­ble, based on the style of safe­ty device.)

Clause 6.2.3 — Elec­tro-sen­si­tive pro­tec­tive equip­ment employ­ing active opto-elec­tron­ic pro­tec­tive devices with a sen­sor detec­tion capa­bil­i­ty of  < 40 mm  in diam­e­ter

6.2.3.1 Cal­cu­la­tion

The min­i­mum dis­tance, S, in mil­lime­tres, from the detec­tion zone to the haz­ard zone shall not be less than that cal­cu­lat­ed using Equa­tion (2):

S = (K x T ) + C                             (2)

where

K = 2 000 mm/s;

C = 8 (d — 14), but not less than 0;

d is the sen­sor detec­tion capa­bil­i­ty of the device, in mil­lime­tres (mm).

[Author’s Note — T comes from 5.1, above]

Then

S = (2 000 x T ) + 8(d-14)               (3)

Equa­tion (3) applies to all min­i­mum dis­tances of S up to and includ­ing 500 mm. The min­i­mum val­ue of S shall be 100 mm.

Where the val­ues for S, cal­cu­lat­ed using Equa­tion (3), exceed 500 mm, Equa­tion (4) can be used. In this case, the min­i­mum val­ue of S shall be 500 mm.

S = (K x T ) + C                          (2)

where

K = 1 600 mm/s;

C = 8 (d - 14), but not less than 0;

d is the sen­sor detec­tion capa­bil­i­ty of the device, in mil­lime­tres (mm).

Then

S = (1 600 x T ) + 8(d — 14)

ISO 13855 Fig. 3 a) Normal Approach
Fig­ure 6 — ISO 13855 Fig. 3 a) Nor­mal Approach

Key

1 haz­ard zone

2 detec­tion zone

3 fixed guard

S min­i­mum dis­tance

a Direc­tion of approach

The two cal­cu­la­tion meth­ods shown above are essen­tial­ly the same, with the pri­ma­ry dif­fer­ence being the val­ue of K, the “hand-speed con­stant”. ISO uses a high­er val­ue of K for light cur­tain instal­la­tions where the field is ver­ti­cal or angled as low as 45º. If the cal­cu­lat­ed val­ue of S is >500 mm, then the val­ue of K is reduced to 1600 mm/s. Using the high­er val­ue of K for a North Amer­i­can instal­la­tion is not wrong, and will result in a more con­ser­v­a­tive instal­la­tion result. Use of 1 600 mm/s for machines going into inter­na­tion­al mar­kets is wrong if S is <500 mm when cal­cu­lat­ed using 2 000 mm/s.

Let’s assume some val­ues so we can do a rep­re­sen­ta­tive cal­cu­la­tion:

Stop­ping Time of the sys­tem (T) = 265 ms [0.265 s]

Light cur­tain res­o­lu­tion (d) = 30 mm [1.2″]

Cal­cu­lat­ing Dpf

Dpf = 8 x (d — 14) = 8 x (30 — 14) = 128

Using K = 2 000 mm/s

S = (2000 x 0.265) + 128 = 658 mm

Since appli­ca­tions where S > 500 mm can be recal­cu­lat­ed using K = 1600 mm/s

S = (1 600 x 0.265) + 128 = 552 mm

So, from the above cal­cu­la­tion, we can see that the dis­tance from the plane of the light cur­tain to the edge of the robot enve­lope (i.e., the dan­ger zone) must be at least 552 mm [21.75″]. That dis­tance is enough that some peo­ple might be able to stand between the light cur­tain field and the fix­ture in the cell, so we should prob­a­bly add a hor­i­zon­tal light cur­tain to pro­tect against that pos­si­bil­i­ty. See Fig­ure 7.

Figure 7 - Vertical Light Curtain with Horizontal segment
Fig­ure 7 — Ver­ti­cal Light Cur­tain with Hor­i­zon­tal seg­ment [1, Fig. B.15 ©]
Anoth­er alter­na­tive to adding a hor­i­zon­tal sec­tion is to slope the light cur­tain field, so that the plane of the light cur­tain is at 45 degrees above the hor­i­zon­tal, with the high­est beam as far away from the haz­ard as pos­si­ble. See Fig­ure 8.

Figure 8 - Sloped light curtain installation [1, CSA Z432 Fig B.15 (c)]
Fig­ure 8 — Sloped light cur­tain instal­la­tion [1, CSA Z432 Fig B.15 ©]
This type of instal­la­tion avoids the need to replace the exist­ing light cur­tain, as long as the field depth is enough to meet the cal­cu­lat­ed Ds.

The field could also be laid hor­i­zon­tal­ly, with no ver­ti­cal com­po­nent. This will change the Dpf cal­cu­la­tion as high­light­ed by the note in Fig­ure 8. Dpf for a hor­i­zon­tal field is cal­cu­lat­ed using the fol­low­ing equa­tion:

Dpf = 1 200 mm [48″]

there­fore

S = (1 600 x 0.265) + 1200 = 1 624 mm

Note also that there is a height restric­tion placed on hor­i­zon­tal devices based on the object res­o­lu­tion as well, so the 0.3 m max­i­mum height may not apply to an exclu­sive­ly hor­i­zon­tal appli­ca­tion. Note that ISO 13855 allows H a max­i­mum val­ue of 1 000 mm, rather than cut­ting the val­ue off at 990 mm as done in CSA Z432. Using either the 14 mm or the 30 mm res­o­lu­tion cur­tains yields a min­i­mum height of 0 mm and a max­i­mum of 990 mm (CSA) or 1 000 mm (ISO). Note that the 3rd Edi­tion of CSA Z432 is like­ly to har­mo­nize these dis­tances with the ISO cal­cu­la­tions, elim­i­nat­ing these dif­fer­ences.

Also, note that field heights where H > 300 mm may require addi­tion­al safe­guards in con­junc­tion with the Pres­ence-Sens­ing Safe­guard­ing Device (PSSD) field.

Figure 8 - Calculating "H" [1, Fig. B.15 (g)]
Fig­ure 8 — Cal­cu­lat­ing “H” [1, Fig. B.15 (g)]
Going back to our orig­i­nal ver­ti­cal field instal­la­tion, there is one more option that could be con­sid­ered: Reduce the object res­o­lu­tion of the light cur­tain. If we go down to the small­est object res­o­lu­tion typ­i­cal­ly avail­able, 14 mm, the cal­cu­la­tion looks like this:

Dpf = 8 x (14–14) = 0

S = (2 000 x 0.265) + 0 = 530 mm

Since S > 500,

S = (1 600 x 0.265) + 0 = 424 mm [16.7″]

While we have sub­stan­tial­ly reduced the safe­ty dis­tance, it looks like we will still need the hor­i­zon­tal light cur­tain to ensure that no one can stand behind the cur­tain with­out being detect­ed.

If the design of the machin­ery allows, it might be pos­si­ble to reduce the stop­ping time of the machine. If you can reduce the stop­ping time, you will be able to short­en the safe­ty dis­tance required. Note that the safe­ty dis­tance can nev­er go to zero, and can nev­er be less than that deter­mined by the object res­o­lu­tion applied to the reach­ing-through tables. In this case, a 14 mm open­ing results in an 89 mm [3.5″] min­i­mum safe­ty dis­tance (CSA). Since the stop­ping time of the machine can nev­er be zero, 89 mm works out to a stop­ping time of 44.5 ms using K=2 000 mm/s, or 55.6 ms if K= 1 600 mm/s. Very few machines can stop this quick­ly.

The cal­cu­lat­ed safe­ty dis­tance is about half of the safe­ty dis­tance required for the gap, at 915 mm. Clear­ly, clos­ing the gap with the light cur­tain or hard guard­ing will be prefer­able to mov­ing the fence away from the dan­ger zone by 915 mm.

Here’s one more fig­ure to help illus­trate these ideas.

Z432 Figure B.14 a
Fig­ure 9 — CSA Z432 Fig­ure B.15 a)

Fig­ure 9 shows the dif­fer­ence between the reach­ing-through or reach­ing-over light cur­tain appli­ca­tions. Notice that with­out a restrict­ing guard above the cur­tain as we have in our exam­ple, the Dpf val­ue goes out to 1 200 mm [48″], rather than the 915 mm val­ue used in our exam­ple.

The low­er fig­ures show light fence appli­ca­tions, where two or three beams are used, rather than a full cov­er­age light cur­tain.

Summary

Here are some of the more impor­tant con­sid­er­a­tions:
1) Is the field of the light cur­tain placed cor­rect­ly, based on the stop­ping per­for­mance of the machine?
2) What is the object res­o­lu­tion of the sens­ing field? This dimen­sion may be used to assess the size of the “open­ings” in the field if this becomes rel­e­vant.
3) What is the height of the low­est and high­est beams or the edges of the sens­ing field?
4) What are the dimen­sions of the gap above the field of the cur­tain, and the dis­tance from the open­ing to the clos­est haz­ard?

ed. note: This arti­cle was reviewed and updat­ed 28-Aug-17.

Acknowledgements

I’d like to acknowl­edge my col­league, Chris­t­ian Bid­ner, who sug­gest­ed the idea for this arti­cle based on a real-world appli­ca­tion he had seen. Chris­t­ian works for OMRON/STI in their Toron­to office.

References

[1]     Safe­guard­ing of Machin­ery. CSA Z432. Cana­di­an Stan­dards Asso­ci­a­tion (CSA).  Toron­to. 2004.

[2]     Safe­ty of machin­ery — Safe­ty dis­tances to pre­vent haz­ard zones being reached by upper and low­er limbs. ISO 13857.International Orga­ni­za­tion for Stan­dard­iza­tion (ISO). Gene­va. 2008.

[3]     Safe­ty of machin­ery — Posi­tion­ing of safe­guards with respect to the approach speeds of parts of the human body. ISO 13855. Inter­na­tion­al Orga­ni­za­tion for Stan­dard­iza­tion (ISO). Gene­va. 2010.

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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.