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), the latter shall necessary belong to
or TYPE 4
as established by the International Standard IEC 61496 1-2.
|NOTE: 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 optoelectronic 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.
“Type” can be used in the same way as “category” is used to determine which device should fit into a given protection circuit. For example, a type 2 light curtain
connected with a category 2 control system forms a category 2 protection circuit. If the control system is category 3, a type 2 light curtain will downgrade it to category
2, while a type 4 light curtain or a type 3 laser scanner will keep it in category 3. Only a type 4 light curtain will keep a category 4 safety circuit in category 4.
The resolution of a light curtain is the 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
- Multibeam light curtains: their resolution R
is the same as the sum of the lens diameter + the distance between two adjacent lenses.
R = D
R = P+D
• 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.
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.
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.
|CONDITIONS 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.
• It must be possible to stop the hazardous movements
of the machine at once. In particular, it is important to know
the machine stopping time to place the light curtain at the
• 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.