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Light Curtain Specification and Purchasing

By Lisa Eitel

Contributed By Digi-Key's North American Editors

Light curtains are safety devices that gate dangerous machine axes to prevent the injury of operators and other plant personnel who work near or with the machine. Light curtains employ a bar-shaped emitter that generates photoelectric light beams to be detected by a receiver (Figure 1). Interruption of any beams serves as a signal to machinery controllers that there’s been motion into the workspace and that dangerous operations should either cease or slow to speeds that pose no risk to the human operator. Use of safety light curtains is on the rise, as they’re less obtrusive and far more configurable than fence-based boundaries.

Image of today’s light curtains impart unprecedented safety to machineryFigure 1: Today’s light curtains impart unprecedented safety to machinery. Many are designed to simplify the process of replacing safety systems based on area sensors. (Image source: Design World)

This article provides an overview of the considerations when specifying and installing light curtains. Because this is a safety critical activity, all relevant national and international standards must be consulted before attempting installation. Even better would be to use the services of a professional installer who is used to working in accordance to these standards. This article should be treated as an introduction and guide to some of the relevant standards as well as provide some background knowledge to help in discussing design requirements with a professional safety system installer.

Identifying hazards and risks

Before specifying any safety system, a full risk assessment should be carried out for the hazardous machinery being safeguarded. This will establish hazardous areas of the machinery and potential entry points. The level of risk that a hazard presents will form an important part of the specification of appropriate safeguards.

Minimum sensor detection capability

The minimum sensor detection capability is the minimum object size which will trigger the light curtain. Different light curtains have beams arranged at different spacings. If the distance between beams is a 5 mm diameter, then a finger inserted anywhere through the light curtain will result in a stop command. However, a minimum sensor detection capability of 150 mm would make it possible for plant personnel to reach inside the curtained area with an entire arm without prompting the machinery to stop. The object size that a light curtain can detect will determine the distance that it needs to be placed from the hazardous parts of machinery.

Minimum distance

Unlike guards, light curtains do not physically stop a person from reaching into a hazard zone. Instead, the light curtain issues a command for a hazardous operation to stop or reduce to a safe speed. This means that the light curtain must be positioned far enough from the hazard to give machinery time to stop before the person reaches it (Figure 2). This distance depends on three parameters – the overall system stopping performance, the intrusion distance, and the approach speed.

Diagram of proper specification of a light curtainFigure 2: Proper specification of a light curtain requires accurate calculation of the safety distance between the curtain’s sensing area and the dangerous machinery area — and then maintenance of a distance equal to or greater than that value. Standards exist to help define such distances, and in fact, the EN ISO 13855 safety distance standard has been shortened to leverage increasingly capable curtains while enabling more compact production machinery. (Image source: Panasonic Industrial Automation Sales)

The overall system stopping performance is the total time between a person passing the light curtain and the machine actually stopping. It includes the delay in the electrical control system and the inertial effects that determine how quickly moving machinery can stop. Overall system stopping performance is represented by the variable T.

The intrusion distance is the distance that a small part of the body, such as a finger or hand, could move past the light curtain towards the hazard zone before the light curtain is activated. This is most important when the minimum sensor detection capability is large, perhaps meaning that only a person’s body would trigger the stop command. In this case, a person could reach through the light curtain and their arm would not stop the machine.

According to the EN ISO 13855 safety distance standard — established by the International Organization for Standardization and used globally — the minimum distance to the hazard zone S is calculated (in mm) using the equation:

Equation 1

where K is a maximum human (arm or body) approach speed in mm/sec, T is the overall stopping performance in seconds, and C is the intrusion distance in mm. In regard to K, EN ISO 13855 defines K as 2,000 mm/sec for the fastest a human arm can move and 1,600 mm/sec for the fastest a human body can move.

For example, assume that a work cell has a light curtain capable of detecting objects as small as 40 mm in diameter. The minimum distance should first be calculated using K = 2,000 mm/sec. If this results in a distance of less than 100 mm (as defined by EN ISO 13855) then this lower resultant value should be ignored — and a value of 100 mm used. If the resultant value exceeds 500 mm (as defined by EN ISO 13855) then the distance should be calculated again using K = 1,600 mm/sec (to prevent injury from personnel walking into a hazardous area) and the new lower distance can be used; provided that it’s not less than 500 mm. Values for other sensor detection capabilities are also given in ISO 13855.

When calculating the minimum distance from a robot, consideration must be given to the maximum reach of the robot. In doing this, the robot program should not be relied on to limit the robot to a subset of its full working volume. However, limit switches may be used on the robot’s axes as part of an interlock system which safely limits the robot’s reach.

Is a light curtain appropriate?

During the initial stage of safety system specification, it is important to establish whether a light curtain is appropriate. Here, International Electrotechnical Commission (IEC) and ISO classifications of standards are indispensable. These categorize safety standards as basic type-A safety standards, general-function type-B safety standards, and machine-specific type-C safety standards.

The first consideration is whether a type-C standard exists for the machine operation in question — and exactly what type of safeguard it requires for that particular application. In fact, type-C standards are often specific to both the machine type at hand and the industry — and can be defined by NFPA, BN, ANSI, RIA, or other regulatory bodies that have over the years adopted the EN ISO system of classifying standards according to an A-B-C system. If a type-C standard applies to the machine operation, then the guidelines in that standard (including those regarding light curtains) must be followed. That’s because type-C standards clearly quantify all hazards and required risk-mitigation measures for the machine function at hand — and supplants use of all less specific standards. If there is no type-C standard that applies to the machine function at hand and a light curtain appears to be a good option, the minimum distance should be calculated. If the minimum distance is practical, then the design engineer can continue with the specification of a light curtain. However, if space is limited or a machine takes a significant amount of time to stop, then another form of safeguard (such as a physical guard) should be considered.

Types of light curtain

The internationally employed IEC 61496 standard classifies light curtains as Type 2 or Type 4. Type 2 light curtains are lower cost with slower and less reliable electronics. If a fault occurs in the safety circuits of a Type 2 light curtain, there may be a period of time before the fault is detected when it provides no protection. In contrast, Type 4 light curtains employ continuous automated cross-checking for faults and errors. If an error occurs, it immediately issues a stop signal, meaning there is continuous safety protection.

Point of operation control (POC) light curtains are designed to be installed close to a hazard, where operators frequently interact with a machine. They are therefore designed for finger, hand, and arm detection. It is common for physical guarding to be used at the sides of a machine and a POC light curtain installed at the front.

Perimeter access control (PAC) light curtains create a safety perimeter around a machine that does not need operators to approach it. They typically only offer full-body detection.

An area access control (AAC) can also be used to perform this function. PACs often use mirrors to reduce the light curtain hardware required to create a full perimeter.

International standards related to light-curtain installation

There are a number of standards relevant to the correct installation of light curtains. If a plant is installing a light curtain for a safety-critical application, then it must refer directly to these standards. ISO 13857 is concerned with establishing safety distances to prevent people’s limbs reaching hazard zones. The safety distance depends on a risk estimation. For example, when reaching upwards, a person should not be able to get within 700 mm of a hazard, or 500 mm if there is a low risk of harm. ISO 12100 is about establishing the severity and the probability of harm. A low level of harm would be reversible damage such as bruises or broken fingernails, or temperature and contact duration which doesn’t reach a burn threshold value. Burn thresholds are given in ISO 13732, and ISO 14121 gives more details of risk estimation.

As detailed above, ISO 13855 describes the positioning of safeguards considering the approach speeds of different parts of the human body. This is generally more relevant than ISO 13857 for light curtains, because light curtains do not prevent a person from entering the hazard zone but rather stop hazardous operations. It is therefore important to consider how far a person might travel in the time between the light curtain sending the stop signal and the machine actually stopping. The overall system stopping performance and maximum possible human approach speed together dictate the minimum safe distance of the light guard from the dangerous process.

ISO 14119 covers the specification and design of interlocking devices associated with physical guards. Although it does not explicitly mention light curtains, many of the principles are relevant — such as designing to minimize defeat possibilities.

ISO 14120 covers the specification and design of physical guards themselves and is therefore less relevant to light-curtain installations. However, light curtains are often combined with physical guards. For example, physical guards may be used to prevent access to a hazard zone from the sides; with a light curtain at the zone’s front. ISO 14120 may therefore also need to be consulted.

Conclusion

Light curtains can greatly improve the convenience of operating machinery. They allow a clear view into the working volume of the machine and enable operators to reach inside to remove parts and install tooling without the inconvenience of opening guards.

However, light curtains don’t always offer the same level of protection — especially against projectiles being ejected from machine operations.

It is also important to remember that due to the need to allow for overall stopping performance, light curtains are usually fitted further away from the hazard zone than guards. These are all important considerations when selecting an appropriate safeguard.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Lisa Eitel

Lisa Eitel has worked in the motion industry since 2001. Her areas of focus include motors, drives, motion control, power transmission, linear motion, and sensing and feedback technologies. She has a B.S. in Mechanical Engineering and is an inductee of Tau Beta Pi engineering honor society; a member of the Society of Women Engineers; and a judge for the FIRST Robotics Buckeye Regionals. Besides her motioncontroltips.com contributions, Lisa also leads the production of the quarterly motion issues of Design World.

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Digi-Key's North American Editors