Light curtain or safety laser scanner?

Safety light curtains and safety laser scanners are the two most common machine protection technologies, and both can be used in many safety applications.

From machine building to parts manufacturing to industrial vehicles, safeguarding machinery is a critical step to ensuring the safe and productive operation of any manufacturing facility. Safety light curtains and safety laser scanners are two common safeguarding devices, and both can be used for many safety applications, including point-of-operation guarding, area guarding, perimeter protection and entry/exit safeguarding.

But how do you choose the right technology for your safety application? While it is strongly recommended to consult a safety professional about your specific application requirements, below are some general considerations that can help identify an appropriate safety technology.

Point-of-operation guarding

The point of operation refers to the area of the machine where work is performed. In protecting points of operation, a person’s approach is detected within a very close proximity to the hazardous point. The advantage of using an optical device for point-of-operation protection is that it may allow for a short minimum distance and the operator can work more ergonomically (eg, during loading work on a press).

Using a light curtain for point-of-operation guarding

With a safety light curtain, the sender unit emits pulses of infrared light beams to a receiver unit. The light curtain can then detect interruptions to one or more light beams within the sensing field. Safety light curtains are ideal for point-of-operation protection due to their slim profile that can easily be mounted close to the hazard and their small resolution between beams. For example, the resolution can be set small enough to detect a hand or even one finger. This allows the safety system to respond quickly when the sensor is tripped.

Using a safety laser scanner for point-of-operation guarding

A safety laser scanner is less likely to be used for this type of application, since laser scanners generally require a greater minimum distance from the hazard, due to the longer response time of the device. Another possible option for point-of-operation guarding would be a safety-rated vision system.

Important considerations

When choosing a safety technology, you must always consider the minimum safety distance calculation.¹ This is especially important for point-of-operation applications because people are working so close to the hazard that you must ensure that the machine can be stopped before injury occurs. It is also important to consider the size and discreteness of the optical safety device. The device should be able to protect a worker from injury while also maintaining an ergonomic workspace.

Area protection and perimeter guarding

Area guarding and perimeter protection are sometimes used in similar applications. There are important differences that can help determine an appropriate optical safety device.

Area protection refers to safeguarding a hazard point by monitoring an area in the zone by which a person would approach a hazardous point. The approach of the person is detected by sensing the person’s presence in an area. Area protection can sometimes be necessary for machines that have a hazardous area that cannot be viewed completely from the position of the reset device. If the hazard area is entered, a stop signal is initiated — stopping the machine and preventing restart. Hazard area protection is also important for AGVs (automated guided vehicles), transfer cars and for cranes and stackers, in order to protect people while the vehicles are in motion, or docking to a fixed station.

Perimeter guarding refers to safety measures that guard one or more sides of the machine. If the perimeter is breached, a stop signal is initiated and the machine is stopped. Typically, safety laser scanners are used more in area protection and safety light curtains or multiple beam systems are used more in perimeter guarding applications. You should always discuss your specific application needs with a qualified safety professional.

Figure 1: Safety laser scanners being used for perimeter guarding.

Using a light curtain for area and perimeter guarding

Since the light curtain uses the sender/receiver concept, it can easily be used in conjunction with mirrors to guard multiple sides of a machine. Depending on the set-up, you could also potentially mount light curtains very close to the machine, saving valuable floor space.

Using a safety laser scanner for area and perimeter guarding

Using time-of-flight technology, a safety laser scanner emits a pulse of light that is then reflected by a target, if present. The time required for the pulse to travel between the scanner and the reflector and back again is proportional to the distance. This allows the scanner to detect objects in the defined safeguarding area.

Because the safety laser scanner is based on software-configured freely defined fields, it may be able to be tucked into the machine for discrete mounting. Additional benefits of a safety laser scanner include the ability to switch between zones and the ability to segment field types for different outputs — eg, safety (protective) field or warning field.

Important considerations

As with all safety applications, a minimum safety distance calculation is an essential step. In addition, for both area and perimeter guarding, it is important to consider whether optical guarding or hard-guarding (completely preventing access to the area) is the most appropriate safety measure. You should also weigh the benefits of fenceless safety, while taking into account foot and AGV traffic in the area. For example, in some factory floor configurations with high traffic and narrow separation between guarding areas, hard-guarding may be more suitable to minimise tripping the optical safety devices and causing the machines to come to a stop.

Entry/exit guarding

In many applications (such as palletising) objects must enter the guarded area via conveyors while the machinery remains in motion. It is therefore necessary to allow entry and exit of objects while still protecting personnel from danger.

Using a light curtain for entry/exit applications

Traditionally, if differentiation between goods and a person is required to allow goods to access a hazard zone, muting is used in conjunction with light curtains or multiple beam devices to allow entry/exit of the material or goods, but not a person. Muting is defined as the ‘temporary, automatic suspension of the safety function’. It can be used to temporarily deactivate the protective function of a protective device, allowing material or goods to pass through without initiating a stop of the machine. This can be done using two or four muting sensors, such as inductive sensors or photoelectric sensors, which are logically evaluated to determine the sequence of the object moving through. If the sequence is correct, and all other parameters within the logic are met, the object will be allowed through the safety light curtain without stopping the work routine.

Figure 2: Light curtains in an entry/exit guarding application.

Another way of moving material into or out of a safeguarded area with a light curtain is active differentiation between person and goods. For this application, horizontally arranged software-configurable safety light curtains are applied. The ability to evaluate each light beam individually allows the light curtain to differentiate the interruption pattern of the material or material carrier (such as a pallet) from a person. This differs from muting on two levels. Muting sensors are not required, so there is less mechanical mounting effort and less wiring needed; and because the light curtain is now continuously active, (ie, there is no suspension of the safety function) the application no longer involves ‘muting’.

By using self-teaching dynamic blanking, as well as other differentiation criteria such as direction of movement, speed, entry and exit in the protective field etc, a safety-relevant distinction can be made. In this way, undetected entry into the hazard zone can be prevented.

Using a safety laser scanner for entry/exit applications

Active switching of protective fields provides another way of moving material into or out of a safeguarded area. For this application, safety laser scanners are normally used with a vertical protective field.

The appropriate protective field, from a series of preconfigured protective fields, is activated by corresponding signals from the machine controller and suitable, appropriately positioned sensors. The contour of the protective field is designed so that passage of the material does not cause the protective device to activate and any unmonitored areas are small enough to prevent undetected access to the hazard zone.

Important considerations

We most often see safety light curtains in entry/exit applications because it is an effective and economical solution in most cases. However, there are certain instances where you might want to use a laser scanner instead. For example, if you do not have an appropriate space to mount two perpendicular devices, a laser scanner would be the more feasible choice.

Fenceless safety

In fenceless safety set-ups, the laser scanner monitors hazardous areas of machines by scanning its surroundings in two dimensions. As soon as the safety laser scanner detects an object or person in the protective field, safe machine shutdown is initiated. Warning fields can be used to detect an intrusion before impacting the protective field, signalling an audible or visual alert, or machine slowdown.

There are four benefits of fenceless machine safety: increased productivity, customisability, out-of-the-box solutions and affordability.

Increased productivity

Hard guards or mechanical fences limit access to machines and make changeovers more complicated and time-consuming. Safety mats, light curtains and other perimeter guarding also consume extra space for installation and the required safe stopping distances. In contrast, fenceless safety is a non-contact solution, saving floor space and improving access to machines for quicker changeover.

Customisation

With fenceless safety, the laser scanner is used to make the safety solution fit the application, rather than force the application to conform to the safety system. The scanner’s monitoring field conforms to the unique shape of the machinery and hazardous areas, without impeding worker accessibility and process productivity.

Out-of-the-box solutions

Unlike their hard-and-fast counterparts, fenceless safety is an out-of-the-box solution. With safety laser scanners, you simply disconnect power to the scanner, move it to the new location and reconfigure the new safety zones using the built-in software.

More affordable

The cost of technology has decreased over time, and the flexibility, robustness and configurability offered now means that fenceless safety is a viable, cost-effective option.

Conclusion

Machine builders and end users should consult with a qualified safety professional to discuss the most appropriate safety system for their application. It is best to work with a trusted supplier who has demonstrated competency with a variety of safety sensors and protective devices, and who can also provide your staff with certified functional safety training and other safety services such as risk assessment and system validation.

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