Light curtains, optical protection against dangerous situations
Safety light curtains, also known as light curtains or optical protection, are the ideal solution for protecting operators from dangerous machines and situations. Light curtains ensure that the process is automatically stopped when someone enters the protected area.
What are safety light curtains used for?
Safety light curtains shield dangerous areas with a protective field of infrared light beams, creating a safe area and making light curtains the ideal protection for many automated processes. Because light curtains can optically shield dangerous areas, they can in some cases replace physical shielding methods such as Machine guarding. In comparison with a physical barrier light curtains are more efficient for maintaining machines and for carrying out semi-automatic procedures. The use of light curtains saves both space and costs. Simultaneously, safety curtains offer more freedom and flexibility for the user.
Light curtains are the ideal solution when active intervention in the production process is required, for example for packaging machines, presses, robotic systems, assembly lines and conveyor systems.
Light curtains help to ensure the safety of the operator when setting up, maintaining and repairing the machines. Light curtains can also be used to protect a specific point in a hazardous process or to define the hazardous area around a machine.
When to use a light curtain
Light curtains are optical safety sensors that can be used for area protection, access protection and interception protection. A light curtain protects an opening against the unauthorised passage of body parts. The opening that needs to be safeguarded can vary from a few centimetres to several metres.
When there is a risk of hands, fingers or arms of operators being in the direct proximity of machinery, light curtains can be used. There are light curtains specially designed for finger detection, hand detection or body detection. On our webshop we offer the MLG light curtains by Mechan Controls. These are light curtains with a resolution of 14, 30 or 70 mm.
What is the resolution of a light screen? The resolution is the distance between the light beams. The smaller the resolution, the shorter the distance between the light rays. A lower resolution means that smaller parts of the body are detected by the light curtain. The light curtain detects when a finger, hand or arm breaks the light beams and stops the machine in order to avoid a dangerous situation and injury.
With light barriers for walk-in detection, an optical shield is created around the machine. When someone enters the guarded area, the machine is switched off. Light curtains for walk-in detection are light curtains with 2, 3 or 4 light beams. The number of beams you need depends on the detection height. In our product range we have the Multi-Beam light curtains by Mechan Controls. These light curtains are safety type 4 and this makes them suitable for SIL 3, Cat. 4 and PLe applications.
Which type of safety light curtain can best be applied to your machine installation, depends on:
- The resolution (number of beams) of the light curtain
- The safety category that must be achieved
- The detection height
- The maximum distance between transmitter and receiver
- Requirement for muting
How does a light curtain work?
A light curtain consists of a transmitter and a receiver. The transmitter contains several LEDs that send infrared light pulses to the receiver. The transmitter and receiver communicate to receive the pulses in the right order and at a specific frequency. This eliminates the risk of interference from other light sources.
When an opaque object breaks the signal between the transmitter and the receiver. The receiver will not receive all the pulses transmitted, this means that the light curtain has been interrupted. This interruption causes the connected machine to stop. In the event of an error, an automatic stop signal is sent to the monitored machine, bringing it to a standstill.
Points of attention when installing a safety light curtain
Before choosing a light curtain to prevent unsafe situations, it is important to make a risk analysis. A laser light curtain is only suitable if unintentional entry or sudden intervention has no undesirable consequences. It is also important that no dangerous parts can come loose from the machines; is that the case? Then it is better to opt for physical Machine guarding.
A light curtain is only applicable to machinery that can stop at any point in its working cycle. If a machine, e.g. a hydraulic press, still has to complete its cycle when the light curtain sends a stop signal, it is not a suitable machine for protection by a light curtain. For machines where process disturbance is undesirable, are process interlocks (in combination with machine guarding) a better alternative. If the machine has a run-down time, always use a safety interlock or trapped key system.
When installing a light curtain, the reactivation and reset switch of the machine must be located outside the safety zone. This allows the machine to be reactivated safely. A mechanical blocking device with a safety key can be installed on the light curtain to prevent entrapment and unintentional restart of the light curtain.
How do you calculate the safety distance for safety light curtains?
When light curtains are installed, first it is necessary to determine the necessary safety distance. The safety distance can be calculated in with EN ISO 13855. To determine the correct distance, a distinction is made between area protection, walk-in protection and protective measures. EN ISO 13855 states that the safety distance (S) is mainly determined by the following factors.
Safety distance (S)
The reaction time of the safety device (T1)
The reaction time of the machine (T2)
Constant value depending on the type of protection (multi-jet, finger protection, hand protection) applied [mm] 8 x (d -14) (C)
The approach speed of parts of the body or the whole body. This factor is defined in EN ISO 13855 as 1600 mm/sec for walking speed and 2000 mm/sec for hand speed. (K)
EN ISO 13855 states that the safety distance (S) can be calculated using the following formula: S = (K x T ) + C
For more information on determining safety distances the safety norms EN ISO 13857 on safety distances for hazardous areas and EN ISO 13855 on the positioning of protective devices in relation to the approach speeds of the human body can be consulted.
How to mount safety light curtains
How safe a light curtain is, a light curtain only protects against an unsafe situation if a light curtain is installed at the correct height and safety distance.
To ensure safety, the transmitter and receiver must always be mounted at the same height, with their axes aligned. Misalignment of the light curtains must be avoided, as this creates an unsafe situation.
There must be no reflective surfaces within the “field of vision” of the protective light curtains. Reflective surfaces within the range of the light curtains can lead to dangerous situations, since obstructions are not detected and thus the machine is not stopped.
When using light curtains, it is important to avoid interference when different sensor units are used within each other’s detection range. Therefore, it must be ensured that a transmitter is not within the field of vision of the receiver and vice versa.
ISO standard EN13855:2010 provides guidelines for the correct positioning of photoelectric safety devices on industrial machines. The three main considerations to be made are:
- Determining the correct detection height.
- Determining the correct minimum safety distance.
- Access to the dangerous parts of the machine must, where the light curtain is not located, be covered by physical safety fencing.
To prevent unsafe situations with a light curtain, the height of the light curtain must be high enough that access to all dangerous parts, from every possible direction of approach, is only possible through the light curtain. All possible approaches by the operator must be taken into account. The light curtain must be installed at an appropriate safety distance from the hazardous parts of the machine. Consideration must be given to the braking power of the machine, the size of the light curtain and the detection capability of the light curtain.
There are 3 different types of mounting brackets available for the MLG light curtain systems from Mechan Controls.
- The adjustable mounting bracket, combines a robust design with quick installation. Light curtains can easily be changed without having to re-align them.
- The clamping bracket, offers versatile installation possibilities. The clamping bracket is specially designed for installing the light curtain at an angle.
- There is also a possibility to install light curtains in limited spaces. The Endcap brackets can be used to install the light curtain easily on a support post or on the outside edge of a fixed structure.
Muting of a light curtain
Light curtains often have a muting function. Muting means the temporary interruption of the safety function, muting can be used to supply machines with materials and to transport produced products without the machine making a safety stop.
Muting sensors are used to automatically trigger an interruption to allow material to pass through. Muting sensors have to fulfil strict conditions, which are described in IEC 62046. Muting sensors differentiate between passing materials and persons.
This distinction is made by the sequence and time interval in which the sensors are activated. And this determines whether or not the output of the safety light curtain is bridged.
When transporting, for example, a stacked pallet along a roller conveyor, the muting sensors ensure that, if all muting requirements are achieved, the safety light curtain enters muting mode.
For example, when a pallet on a roller conveyor passes a light curtain, this will not lead to a machine stop. The moment a person passes the light curtain on the roller conveyor, the muting sensors are not activated in the correct order and an emergency stop is triggered.
The use of muting is extensively described in the standard IEC 62046. This includes information on how to handle an override of the muting function, the placement of muting sensors and the correct bridging cycle.
Are you looking for the right safety solution to create a safe situation? Do you want to know whether a light curtain is the right solution? Our safety experts are always at your service and will gladly answer all your questions about safety light curtains.