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Safety light curtains: going back to basics

21 June 2024

Martin Kidman explains what light curtains can contribute to machinery safety, factory control and productivity, looking at the functions and features that are available today.

Cross beam and 4 sensor muting.
Cross beam and 4 sensor muting.

The light curtain has become so ubiquitous in factories today that it is all too easy to regard it as a simple on/off device –a fail-safe, fit-and-forget substitute for protective fencing. However, an expanding and versatile range of light curtains has evolved to perform multiple functions and they can contribute positively to productivity and integrate with factory local area networks to support better plant-wide automation.

In 1951, Erwin Sick presented the first wooden model of his light curtain at the ‘German Inventor and New Development Trade Fair’ in Munich. Later the same year a patent registration of the light curtain based on the autocollimator principle was regarded as the technical breakthrough that began the development of technology we know today. Over the years hardware and software development has transformed the versatility and use of safety light curtains. 

The starting point of any safety application is always the danger you want to guard against and the space that it is in. Based on this, there are different attributes to consider regarding the physical properties of the light curtain. Currently, the document EN ISO 13855 is the harmonised standard for positioning safety light guards, and it can assist in determining which of these physical attributes to go for. However, there are also additional properties such as IP rating, current consumption, visible indications etc. that are not covered in the standard.

Range
The range is the maximum distance between transmitter and receiver. Products are available with either a fixed range, a selectable range or they an auto-adjusting range (at startup, the sensitivity is adjusted based on the distance between sender and receiver).  Choosing the correct range ensures that the light curtain is best fit for the application. For example, if a high power, long range, device is used over a short distance, reflections off nearby surfaces can make the installation unsafe. On the other hand, a long range device may be required for protecting large areas which require high beam strength and sensitive detectors. 

Height
The height of a safety light curtain must be chosen based on the access to the hazard. For example if a light curtain is protecting against ‘point protection’ then it must cover the whole aperture. However, if the safety light curtain is for access protection then different heights can be used based on the number of beams required and the height from the floor plus possible reach over. 

Resolution
The resolution of a light curtain can also affect the mounting position with respect to the hazard. The terms ‘finger protection’ and ‘hand protection’ are often used often which refer to resolutions of 14mm and 30mm respectively. however, light curtains can also have different resolutions. The higher the resolution the better the detection capability is for a safety light curtain and therefore the device can be mounted closer to a hazard. EN ISO 13855 contains lots of different calculations for mounting distances and resolution can have a large impact but the basic equation is:

S = (K x T) + C

Where S is the distance in millimetres of the device from the hazard; K is a parameter in millimetres per second based on approach speeds of the body or parts of it; T is the stopping/run down time of the overall systems; C is a supplement in millimetres based on a number of factors.

The standard also covers additional things such as direction of approach and possible reach over.

For larger distances and access protection a resolution of 400mm is normally sufficient, but then additional devices may be required for presence detection if the machine can be reset while a person is behind the light curtain. A thorough risk assessment, according to the harmonised standard EN ISO 12100, should be performed to take these points into consideration.

Higher resolution light curtains are usually more costly than lower resolutions but switching to a lower resolution means that the distance between the safety light curtain and the hazard needs to be increased which may be impractical or not possible on the factory floor. 

Type
When choosing a safety device for an application, the harmonised standard EN ISO 13849 or IEC 62061 can be used to design the safety related parts of the control system (SRP/CS). Following the definition of a safety function (For example, initiating a stop using an light curtain) it is important to determine what performance level (PL) or Safety integrity level (SIL) the associated SRP/CS should achieve, based on the risk. For electrosensitive protective equipment, the safety light curtain ‘Type’ according to IEC 61496 is directly related to what PL/SIL that the safety function can achieve. 

Active/passive
To save on wiring and to keep costs down for access protection applications, active passive systems can be used. An active/passive system consists of two devices. One contains both LED senders and photo-diode receivers and the other device contains mirrors that are used to deflect the LED beams back to the receivers.

Mirrors reduce the strength of a light beam and therefore active/passive systems will normally have a much shorter distance than sender/receiver systems.

Functions
The simplest function of the light curtain is that light beams are broken and the outputs turn off which then stops any hazardous movement. If no incident has occurred and the area is safe then a reset can be performed. This stop/start style of safe operation can be disruptive to factory flow and productivity and therefore the light curtain has evolved to perform multiple functions such as internal reset, external device monitoring (EDM), beam coding and blanking. Some of the most common functions are:

Restart interlock: The two main interlocking modes that can be set in are Automatic – Output Signal Switching Devices (OSSD’s) go high when the beams are free. Manual – OSSD’s go high when the beams are free and then a reset button is pressed. It is important that an automatic restart can only be used in specific case when it is not possible to stand behind the light curtain without being detected. 

A manual reset can be performed using either an external safety relay, safety PLC or internally if the light curtain has the capability. Before safety PLC/relays the reset was performed using relay logic.

Eternal Device Monitoring (EDM): The level of safety of a safety function can be increased if fault detection is implemented. The harmonised standard EN ISO 13849, which is used for calculating performance level of a safety-related parts of controls systems (SRP/CS) describes the term diagnostics coverage (DC) which measures this capability. It is a measure of how many dangerous failures are detected. 

One method which is listed in this standard and is used extensively in industry to assume a diagnostics coverage of 99% (Can detect 99% of dangerous failures) is ‘External Device Monitoring’, or EDM or ‘Check Back’.

This monitoring function is a means in which a safety device actively monitors the state of external devices that it controls such as machine primary control elements (MPCE's), contactors or relays. This is needed because the external devices usually have no diagnostics of their own and are pretty much ‘Dumb’ devices. 

If an unsafe state is detected in the external device then the safety device can lock out. This is performed by monitoring complimentary channels of the external devices which are physically linked internally. So that if the outputs of a safety device go high then the returning EDM signal from the final switching devices must go low if the system is healthy. Otherwise there is a fault. 

Blanking: In some applications, it might be desirable to permit certain objects to protrude through the light curtain field, for example a support table or rack required to feed the machine with a heavy work-piece. In this case, a blanking facility allows the user to ‘turn off’ some of the beams and hence not interrupt the work flow. It can either be fixed or floating to allow a variable position for the object.

Enabling material throughput – muting: In many light curtain applications of light curtains, particularly on conveying lines with goods passing from one area of production to another, or in automated warehousing operations where staff are excluded from the automated stacking and shelving areas, a light curtain must be able to allow objects through while still reacting to people. Sensors can be used to identify the object and allow passage, while not allowing an operative through due to their shape and size. This is automatic and is known as muting. The two most popular methods of muting are cross beam and 4 sensor muting.

However, there are many different methods of muting, including the use of photocells, ground inductive loops, the use of safety laser scanners and partial muting (partial blanking). The technical specification IEC/TS 62046 can assist in the application of protective devices to detect persons. It is important to understand. However, that muting is not the same as bypass. Muting is automatic while bypass is a mode selected by an operator.

Another term related to muting is ‘override’ which is a manual triggering of the muting after an error in the muting conditions. This is needed to make a system clear for example when a pallet is stuck in a safety light curtain which has stopped the conveyor and the conveyor temporarily needs power to remove the pallet.

Enabling material throughput - pattern recognition: There are some safety light curtains which can perform complex algorithms on individual beams. This means that it can detect if the beams are broken or not but also which beams and in which sequence.

This means that rather than muting, the safety light curtain can safely detect patterns, objects and direction. This means that the safety is always active while allowed known patterns can enter and leave the light curtain. This function can increase availability but at the same time can increase safety by protecting at the same time as allowing material to pass through. 

Beam coding: Because safety light curtains work over similar optical frequency in the infra red spectrum, if several safety light curtains are used in close proximity interference can occur between the systems. To overcome this, some safety light curtains offer beam coding so that a receiver can distinguish the beams from a specific sender and not be affected by another device in close proximity. When beam coding is activated though, this may affect the response time and/or range of the safety light curtain and should be taken into account as this may affect the mounting distance.

Today’s light curtain installations are much more attuned to production needs. They are capable of a wide range of manual and automatic actions that ensure a light curtain assists safety and productivity rather than sacrificing efficiency.

However, it is important to know what features and functions are available on a safety light curtains and also if the application requires them or not? It is extremely common for light curtains to have many features which are not used and therefore there may be a more cost-efficient solution.

Martin Kidman is Product Manager - Safety (S&W Europe), SICK UK.


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