Safety Light Curtains

 

Safety light Curtains: characteristic elements

Light Curtains are electro sensitive devices using one or more light beams, emitted by an Emitter and received by a Receiver, to create an intangible controlled area. Fundamental characteristics are:

Safety type

  • Defines the self-monitoring and safety principles contained in the device
  • It must be chosen as a function of the risk level characterising the machine

When the chosen safety device is a photo-electric barrier (AOPD Active Optoelectronic Protective Device), it shall necessary belong to TYPE 2 or TYPE 4 as established by the International Standard IEC 61496 1-2.

Why “Type” and not “Category”?
When talking about light curtains and laser scanners, we normally refer to their “safety type”; while for all other safety devices the term of choice is “safety category”. This distinction is due to the International Standard IEC 61496, in which the term “type” is introduced to determine the safety level of optoelectronics protective equipment. In practice, “type” adds some optical requirements to the requirements which define categories for non-optical safety devices. Therefore, a Type 2 light curtain is a light curtain which complies with the requirements for category 2 safety electronics and furthermore whose beams have certain characteristics, among which a given aperture angle, immunity to light interference and so on. The same applies for Type 4 light curtains and Type 3 laser scanners.

Resolution


Minimum size of an object that, placed into the controlled area, will obscure the controlled zone and hence stop the hazardous movement of the machine. Single beam light barriers: their resolution R is the same as the diameter of the lens.
R = D
Multibeam light curtains: their resolution R is the same as the sum of the lens diameter + the distance between two adjacent lenses.
R = P + D
R = P + D R = D
R : resolution R : resolution
D : beam pitch D : lens diameter
P : lenses A : optical axis

Protected height


This is the height controlled by the light curtain. If it is positioned horizontally, this value shows the depth of the protected zone.
L : protected area length
H : protected area height

Range


This is the maximum working distance that may exist between the Emitter and the Receiver. When deflection mirrors are used, it is necessary to take into account the attenuation factor introduced by each of them, which it is about 15%.

Response Time


This is the time it takes for the light curtain to transmit the alarm signal from the time the protected zone is interrupted.
ressolution image

 

 

Advantages of light Curtains

 

  • Effective protection in the event of fatigue or distraction of the operator
  • Increase in the productive capacity of the machine as the light curtain does not require the manual handling of physical guards or waiting for them to open.
  • Faster machine loading/unloading operations.
  • Reduced times of approach to the working areas.
  • Elimination of the risk of tampering since any irregular intervention on the light curtain stops the machine.
  • Simple and quick installation, with greater flexibility of adjustment on the machine, even in the case of subsequent repositioning.
  • Possibility to build up large sized protections, either linear or along a perimeter, on several sides, at greatly reduced costs.
  • Facilitated and fast maintenance of the machine, as there is no need to remove physical guards, such as grids, gates, etc.
  • Improved appearance and ergonomic effectiveness of the machine.

 

Condition of use

For the photoelectric safety protections to be effective, it is necessary to verify that:

  • It must be possible to electrically interface them to the control unit of the machine.
  • The time taken to reach the hazardous point must be greater than the time necessary to stop the hazardous movement.
  • The machine must not create secondary dangers due to the projection or fall from above of materials. If this danger exists, additional protections of a mechanical nature have to be provided.
  • The minimum size of the object to be detected must be equal to or greater than the chosen light curtain resolution.

Selection criteria of a safety light curtain

  • Definition of the zone to be protected.
  • Definition of the parts of the body to be detected:
    - fingers or hands;
    - approaching body of a person;
    - presence of a person in a hazardous area
  • Definition of the safety distance between the light curtain and the hazardous point
  • Definition of the safety category Level/Type to be adopted according to ISO 13849-1, IEC 62061, IEC 61496.

Definition of the zone to be protected

  • Take into account the configuration of the zone:
    - shape and dimensions: width and height of the access area;
    - positions of hazardous parts;
    - possible access points.

The light curtain must be positioned so as to prevent the access to the dangerous area from above, from below, and from the sides without having. intercepted the field protected by the light curtain. It is possible to install one or more deflection mirrors in order to protect areas with access from several sides. This results in a considerable reduction in costs, as this solution eliminates the need of installing many separate light curtains.

 

 

Definition of type of detections

detection characteristics advantage
finger
or hand
Detection necessary when the operator must work close to the danger.
Barrier resolution must be between 14 mm and 40 mm.
Possibility to lower the dimensions by reducing at the top the space between the protection and the dangerous zone.
Short time for machine charging and discharging.
Less operator fatigue, more productivity
body
(use as trip device)
Ideal detection for access control and protections of several sides, also for long scanning distances.
The barrier must be placed at least at 850 mm from the danger.
Barrier normally composed by 2, 3, 4 beams
Protection costs reduced by the restricted number of beams.
Possibility to protect zones with big dimensions by using deflection mirrors.
see note below
presence in a
dangerous zone
Detection realized by positioning the light curtains horizontally to control continuously the presence of an object in a definite zone.
The light curtains resolution depends on the height of the detection plane, anyway it cannot be higher than 116 mm.
Possibility to control zones not visible from where the machine’s push button controls are located.

 

Note
Accidental start-up of the machine shall not be possible when anyone crosses the sensitive area and stays undetected in the dangerous area.
ways of eliminating this type of risk include the following:
  • Use of start / restart-interlock function positioning the command so that the dangerous area is in full view and so that the command cannot be reached by anyone from inside the dangerous area. The Restart command has to be safe in compliance with IEC 61496-1
  • Use of additional presence sensing detectors for the detection of the operator inside dangerous area
  • Use of obstacles preventing the operator from remaining undetected in the space between the sensing zone of the protective device and the dangerous area.

Determination of the safety distanced

The effectiveness of the protection depends greatly on the correct positioning of the light Curtain with respect to the danger. The light Curtain must be located at a distance greater than or equal to the minimum safety distance S, so that reaching the dangerous point will be possible only when the dangerous action of the machine has been stopped. The light Curtain must be positioned so that:

  • It is impossible to reach the dangerous point without going through the zone controlled by the light curtain
  • A person cannot be present in the dangerous zone without his/her presence being detected. To this end, it might be necessary to resort to additional safety devices (i.e..: photoelectric light curtains arranged horizontally).

European Standard EN ISO 13855 provides the elements for the determination of the safety distance. If the machine in object is governed by a specific C type Standard, it shall be taken into two account. If the distance S determined in this manner is too big, it is necessary:

  • a) to reduce the total stopping time of the machine,
  • b) to improve the detection capability (resolution) of the light curtain.

 

General formula for the determination of the minimum safety distance

 

S = K x T + C

S

minimum safety distance between the protection and hazardous point, expressed in mm

K

speed of approach of the body or parts of the body, expressed in mm / sec

T

total stopping time of the machine, consisting of:
t1 reaction time of the protective device in seconds
t2 reaction time of the machine in seconds, until it stops the hazardous action

C

additional distance in mm

 

C takes into account:

Possible intrusion of parts of the body in the sensitive area before they are detected. For example:

  • C = 8 x (d-14) if D (light Curtain resolution) ≤ 40 mm.
  • C = 850 if D (light curtain resolution) > 40 mm and for 2, 3, 4 beams Curtains.
  • C = 1,200 - (0,4 x H) for horizontal light Curtains.

The dangerous point can be reached by leaning over the upper edge of the sensitive area of a vertical light Curtain.
In this case C, called “CRO” is obtained from the following Table 2 of EN ISO 13855 / EN 999.

height of Hazard zone "a" (mm) height "b" of upper edge e della zona protetta della barriera fotoelettrica (mm
900 1,000 1,100 1,200 1,300 1,400 1,600 1,800 2,000 2,200 2,400 2,600
alternative distance CR0
2,600 0
2,500 400 350  300  250  150  100  -
2,400  550  500  450  400  300  250  100  -
2,200  510  750  700  650  600  550  400  250  -  -
2,000  950  850  800  750  700  550  400  -  -  -
1,800  1,100  950  850  800  750  550  -  -  -  -
1,600  1,150  1,100  900  800  750  450  -  -  -  -
1,400  1,200  1,100  1,000  900  800  650  -  -  -  -  -
1,200  1,200  1,100  1,000  850  800  -  -  -  -  -  -
1,000  1,200 1,150  1,050  960  750  700 
800 1,150  1,050  950  800  500  450 
600 1,050  950  750  550 
400 900  700 
200 600 
0 -


(Tab.2 ISO 13855/EN 999)

 

  • Interpolation is not allowed
  • If distances a, b or CRO fall between values listed in the table, use the higher.
  • CRO (reaching over) calculated using Table 2 of EN ISO 13855 / EN 999 must be compared to C as conventionally calculate (see paragraph 1).
    Always select the higher value.

For combined mechanical and electrosensitive protections (as shown), where it would be possible to lean against the mechanical protection and bypass the light curtain, for the calculation of the parameter C should use the Table 1 (for low risk applications) or the Table 2 (for high-risk applications) of ISO 13857:2007 (formerly EN 294) in place of the table on the previous page. In this catalog the two tables of ISO 13857:2007 (formerly EN 294). Safety distances to prevent danger zones being reached by upper and lower limbs - are not mentioned.

Direction of approach perpendicular to the protected plane with α = 90° (± 5°)

 

Direction of approach parallel to the protected plane with α = 0° (± 5°)

 

Direction of approach angled to the protected plane with 5°< α < 85°

 

Muting function

The Muting function is the provisional and automatic cut-out of the light curtain protective function in relation to the machine cycle. Muting can only occur in a safety condition. Two types of applications are envisaged:

1. Enabling personnel access inside dangerous area during the non-dangerous part of machine cycle

Positioning or removal of workpiece


Depending on the position of the tool, which is the most dangerous part, one of the two curtains (the one facing the tool working area) is active whereas the other is in Muting mode to enable the operator to load/unload the workpiece. Muting mode of the light curtains is subsequently reversed when the tool works on the opposite side of the machine.

 

2. Enabling access to material and preventing access to personnel

Pallet exit from dangerous area


The safety light curtain incorporates Muting sensors able to discriminate between personnel and materials. Only the material is authorized to pass through the monitored area.

 

The essential requirements regarding the Muting Function are described by the followings Standards:

IEC TS 62046 “application of the protective equipment to detect the presence of persons”
EN 415-4 “Safety of the Machinery - automatic palletizing systems”
IEC 61496-1 “Electro-Sensitive Protective Equipment”

General Requirements

  • Muting is a temporary suspension of the safety–related function and it must be activated and de-activated automatically
  • The safety integrity level of the circuit implementing the Muting function shall be equal to that of the safety function temporarily suspended, so that the protection performance of the entire system is not adversely affected.
  • Muting should be activated and de-activated only by means of two or more separate hardwired signals triggered by a correct time or space sequence
  • It shall not be possible to trigger Muting while the ESPE outputs are in the off state.
  • It shall not be possible to initiate Muting by turning the device off and then on again.
  • Muting shall be only activated in an appropriate point of the machine cycle, i.e. only when there is no risk for the operator.
  • Muting sensors shall be mechanically protected to prevent mismatch in case of impact.

 

Muting: palletizers and materials handling systems

Requirements for the monitoring of the openings

  • Monitor the load, not the pallet, otherwise the operator might go into the hazardous zone being dragged by the pallet.
  • Muting time must be restricted to the actual time taken by the material to pass through the opening.
  • Muting must be time-restricted.
  • Sensor mismatch with effect similar to their actuation shall not allow a condition of permanent Muting.
  • The configuration and positioning of the Muting sensors shall ensure reliable differentiation between personnel and material.
  • The layout of the opening, the positioning of the Muting sensors and the additional side protections shall prevent personnel access to the dangerousarea for all the time the Muting function is activated and throughout the time the pallet crosses the opening.

 

 Therefore it is necessary to realise a safety system able to distinguish between:

The Muting function can be present on both Type 2 and Type 4 Safety light Curtains

 

Common solutions for Muting sensor positioning

Muting with 2 crossed-beam sensors
Configuration type T with timing monitoring and two-way pallet operation:

  • The point of intersection of the two beams shall lie in the segregated dangerous area beyond the light curtain
  • A fail safe timer shall be provided to restrict Muting to the time needed for the material to cross the opening
  • The Muting function shall be activated only if the Muting sensors are contemporaneously intercepted: (t2(S2) – t1(S1) = 4 seconds max.).
  • The two beams shall be continuously interrupted by the pallet throughout the transit through the sensors.
  • A matt cylindrical object D=500 mm (simulating the size of a human body) shall not trigger the Muting function.

 

Muting sensor beam intersection shall be positioned the higher up or equal than level of the lower light curtain beam  to avoid possible tampering or accidental triggering of Muting.

 

Muting with 4 parallel-beam sensors
Configuration type T with timing and/or sequence monitoring

Two-way pallet operation:

  • The 4 Muting sensors shall be all actuated together for a brief moment (sequential actuation and de-activation of the 4 sensors).
  • The distance between sensors and the sensing field of the light curtain shall be:


dl and d3 < 200 mm

To prevent undetected personnel access by preceding or following immediately after the pallet
during Muting.

 

d2 > 250 mm

To prevent personnel limb, garment, etc. from enabling Muting by triggering two sensors
simultaneously

 

Muting with 2 crossed-beam or parallel-beam sensors
Configuration type L with timing monitoring and oneway only (exit from dangerous area) pallet operation:

 

  • Muting sensors shall be positioned beyond the light curtain in the dangerous area
  • Muting shall be disabled as soon as the light curtain is cleared and not later than 4 seconds max. from the instant the first of the two Muting sensor is cleared. The timer monitoring the 4 seconds shall be a safety-related item.

 

Possible functions associated with Muting are:

  • Muting sequence: in case Muting inputs are not activated within the right temporal sequence Muting function is excluded.
  • Maximum Muting Time-Out: if barriers remains in Muting condition longer than the prefixed time muting function is automatically stopped.
  • Override function: if a wrong muting sequence occures or any material remains blocked inside protected area it’s not possible to restart the machine.
    Override function allows emergency forced temporary restart of the machine by activiating a dedicate signal on the barrier. That allows to free the control area from the obstacle.

 

Blanking function

Blanking is an auxiliary function of safety light curtains for which the introduction of an opaque object inside parts of the light curtain’s protection field is allowed without causing the stoppage of the machine. Blanking is only possible in the presence of determined safety conditions and in accordance with a configurable operating logic. The blanking function is therefore particularly useful when the light curtain’s protection field must be inevitably intercepted by the material being worked or by a fixed or mobile part of the machine. In practice, it is possible to keep the light curtain’s safety outputs in an ON condition, and the machine working, even if a pre-determined number of beams within the protection fields are being intercepted.
Fixed Blanking allows a fixed portion of the protection field (i. e. a fixed set of beams) to be occupied, while all the other beams operate normally.

Floating Blanking allows the object to move freely inside the light curtain’s protection field occupying a given number of beams, at the condition that the occupied beams are adjacent and that their number is not higher than the configured one.

Floating Blanking with compulsory object presence makes the light curtain work in a reverse way within the blanked portion of the protection field. That is, the blanked beams must be occupied during blanking and therefore the object has to be inside the protection field for the light curtain to remain in the ON state. In this case too the object can move freely within the protection field if the above conditions are respected.
Requirements for the blanking function can be found in the Technical Specification IEC/TS 62046 describing additional means that may be required to prevent a person from reaching into the hazard through the blanked areas of the detection zone.

Warning
  • The use of the blanking function can be allowed depending on the characteristics of the application to be protected. Based on the risk analysis of your application, check whether the use of the blanking function is allowed for that particular application and with what features.
  • M.D. does not assume responsibility for the improper use of the blanking function nor for the possible damages deriving from it.
  • M.D. does not assume responsibility for the improper use of the blanking function nor for the possible damages deriving from it.

 

Minimum distance from reflecting surfaces

The optical beams of the projector, having a beam angle that is not null, can partly be diverted by reflective surfaces located near to the safety light curtain. This may mean that a break in the direct path of the optical beam is not detected, which is why all reflective surfaces and reflective objects (in any position they may have with respect to the controlled area, above, under, inside or outside) must respect a minimum distance from the direct path of the beams of the Safety light Curtain.

 

Safety light Curtains respect the maximum beam angle defined by IEC / EN 61496-2.
The safety distance D is calculated considering the entire beam angle and the safety light curtain reciprocally orientated towards the reflective surface by an angle a, in this way we consider the case of alignment at the limit of reciprocal visibility between the emitter and receiver, but which is more dangerous due to the effects of the reflection.
For ranges less than 3 m the value calculated at 3 m applies:
D = tan(A) * P/2
For P ≥ 3 m the safety distance D to take is calculated as follows:
Type 2: A = 10° tan(A) = 0.1763 Type 4: A = 5° tan(A )= 0.0875

The following chart shows minimum distance from reflective surface for Type 2 and Type 4 safety light curtains with a maximum operating distance of 10 m.

 

Instructions for component installation

The angular opening of a safety light curtain is extremely narrow (±5° for Type 2 and ±2,5° for Type 4), therefore the alignment procedure must be carried out with great care

  • The brackets must make it possible to position both the emitter and the receiver, and at the same time they must be strong enough to maintain the alignment under working conditions. Furthermore, remember that eventual vibrations or shock could misalign the Safety light Curtain.
  • If beam deflecting mirrors are used, it is necessary to keep in mind that the alignment procedure will be more difficult. As a matter of fact, considering that each mirror will involve a capacity reduction of about 15% (depending on the quality of the mirror) and that the mirror brackets may not be particularly steady, the system will be easily misaligned (for example when trying to clean the mirror).
  • The presence of shiny or reflective surfaces can cause the safety light curtain to be ineffective. It is therefore necessary to position the safety light curtain at a suitable distance from any reflective surfaces.
  • Any control unit which does not provide at least an IP54 protection degree must be installed within a switchboard, in order to guarantee this minimum protection.
  • The safety light curtain must be positioned so as to avoid anyone accidentally leap frogging it. When determining the minimum distance at which the safety light curtain must be positioned in the danger area, it is necessary to take into account.

the safety light curtain’s response time and the machine’s shutdown time, as previously described.

  • The start-up or restart-up devices must be positioned so as to prevent the function in question from being activated by an operator within the protected area.

 

Integration of the ESPE

As the ESPE will be integrated in the machine safety-related control system, the choice of its safety level will depend on the result of risk analysis and, consequently, on parameters such as PL, SIL or Category resulting from this analysis. Product Standards (Type C) usually recommend the most suitable ESPE type for each safety-related function involved. If type C Standards are not available, adopt the recommendations of ISO 13849-1 and IEC 62061. Also consider that the overall safety integrity of the serial connection: input – control unit – actuators, shall necessarily be equal to or lower than that of the weaker device

Rules for correct interconnection of protection devices to machine control system

The interconnections between safety outputs of ESPE (OSSD) and the machine primary control elements, the positioning and selection of reset push buttons shall not reduce or eliminate the extent of safety integrity assigned to the safety-related machine control system. Figure A shows the most common example, i.e. where the machine control and monitoring system (e.g. the PLC) has no safety-related function. In this case, the safety-related control system monitoring the protective devices connected to it must operate autonomously and must be inserted between the machine control system and the machine primary control elements.


MPCE = Machine Primary Control Element

If the machine is equipped with an integrated safety-related control and management system (safety-related PLC), see figure B, machine operational functions and safetyrelated functions should be governed through the centralized safety-related system.

 

 

Glossary

 

symbol detection characteristics
Warning to avoid danger! A warning indicates real or potential hazards. Its task is to indicate procedures and behaviour that can avoid accidents. Read and follow these instructions carefully.
Indication Indications that can help achieve better performance.
Emitter symbol This symbol identifies devices that have the function of a projector
Receiver symbol This symbol identifies devices that have the function of a receiver.
Finger protection This symbol marks devices designed to detect fingers entering a protected area. It refers to safety light curtains with a resolution of 14 mm, this value enables using the minimum safety distance and therefore reducing the loading and unloading times to a minimum and the least fatigue for the operator.
Hand protection This symbol marks devices designed to detect a hand entering a protected area. It refers to safety light curtains with a resolution less than or equal to 40mm; these resolutions allow safety distances compatible with short loading and unloading times and a low level of operator fatigue.
  Presence control This symbol marks devices designed to detect limbs entering a protected area or detect human presence in a protected area. For presence detection, with light curtains in a horizontal position, resolutions of between 50 and 116mm are to be used, the height off the ground is calculated in relation to these values.
  Body protection This symbol marks devices designed to detect a body entering a protected area. It refers to multi-beam safety light grids with 2, 3 or 4 beams. These light curtains are usually cost-effective and feature a long range, they enable creating protection for extensive areas and on more than one side, using diverter mirrors.

 

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