How Safety Laser Scanners Can Protect People and Machines

Safety laser scanners can contribute to safety in industrial and logistics facilities. They can protect people from unsafe interactions with machines, and they can protect machines from unintended interference by people.

Several factors are needed to maximize the effectiveness of safety laser scanners. The first is to determine if a laser scanner is the best solution or if another technology, like a light curtain, may be better suited for the application.

Once it's been determined that a scanner is the best choice, there are important decisions to make, including the:

• Selection of the optimal safety protection fields and field sets
• Use of a standard like the International Organization for Standardization (ISO) 13855 for positioning safeguards with respect to the approach of a person
• Selection of the correct multiple sampling value
• Selection of a scanner with the optimal feature set

This article starts by reviewing the factors that affect when to choose a scanner and when to choose a light curtain. It then presents the important selection criteria for using scanners and looks at representative safety laser scanners from IDEC, Omron, SICK, and Banner Engineering.

Scanner or curtain?

One of the first considerations is: what physical space is being protected? Both scanners and light curtains can protect people from harm and protect machines from interference. While there is some overlap in the protective capabilities of scanners and light curtains, they are generally suited to different applications, such as:

Point-of-operation protection, which refers to the active area of a machine where work is performed. Light curtains are suited for this application since they can be positioned in an optimal location, and their resolution can be set up to detect fingers, hands, or feet/legs and provide the required level of protection. Scanners usually need a longer minimum distance from hazards because of their longer response times and are not generally used for point-of-operation protection.

Perimeter protection, which guards multiple sides of a machine. Like point-of-operation protection, light curtains are well suited to provide compact solutions for perimeter protection. If a person crosses the perimeter, a stop signal can be sent, and the machine is stopped. While both light curtains and scanners can be used in perimeter protection, safety light curtains are used more often in perimeter protection, and scanners are used more often in area protection.

Access control and area protection, which can be implemented with a light curtain or scanner depending on the specific application needs. Light curtains are suitable when there's a single point of entry. For example, individually monitoring and evaluating each beam of light enables a light curtain to differentiate between a "tall" person and a "short" material carrier, like a pallet crossing a threshold, and modify its response accordingly.

Scanners can be set up to monitor a 275° area to create a user-defined two-dimensional protected zone (Figure 1). They can also establish multiple protection zones based on the distance between a person and the protected machine and slow or stop it as appropriate.

Figure 1: A safety laser scanner like this can monitor a 275° area to create a two-dimensional protected zone and send an alarm if any unexpected person or object enters that area (red lines). (Image source: Banner Engineering)

Mobile systems like autonomous mobile robots (AMRs) and automatic guided vehicles (AGVs), which can benefit from using multiple scanners. Those scanners can be battery-powered and installed to work cooperatively, simultaneously monitoring dozens of safety zones around the vehicle. Different zones can be activated based on vehicle speed, position, and anticipated changes in direction. The data from the scanners can be supplemented with encoders on the wheels and other sensor inputs to support AMR navigation.

What level of safety?

Once the physical space being protected has been defined, the next consideration is the needed level of safety. In addition to having different application benefits, light curtains and safety laser scanners support different levels of safety as defined by various international standards. For example, ISO 13849-1 defines the reliability of safe control functions using performance levels (PLs) from "a" to "e", with PLe representing the highest level.

Safety laser scanners meet the criteria of PLd and are suitable for use in applications where safety is a major priority. A PLd rating is granted to systems with the probability of a dangerous failure every 1 to 10 million hours (141 to 1,141 years based on continuous operation). Light curtains are available with a wider range of options from PLc to PLe.

IEC 62061, Safety of machinery: Functional safety of electrical, electronic, and programmable electronic control systems is another important standard. It's based on a risk assessment and reduction strategy for safety control functions like light curtains and safety laser scanners. It includes functional requirements specifications and safety integrity level (SIL) requirements.

Examples of functional requirements include frequency of operation, response time, operating modes, duty cycles, operating environment, fault reaction functions, and so on. The resulting SILs are measured on a scale from 1 to 4 (Figure 2).

Figure 2: Safety laser scanners meet the criteria of PLd and SIL3 and are suitable for use in applications where safety is important. (Image source: SICK)

ISO 13855 defines how to place scanners with respect to a person's approach. For example, if a scanner is mounted at a height of 300 mm, a resolution of 70 mm is sufficient to detect a human leg. At lower mounting heights, the recommended minimum resolution is 50 mm.

Scanner specifications

Once it's been determined that a safety laser scanner satisfies the application requirements and can support the needed level of safety, it's time to consider specifications. Examples of important scanner specifications include:

Scan angle. Several scan angles are available, such as 190°, 270°, and 275°. The scan angle and its structure determine how the scanner gets mounted into the system to monitor the needed area(s).

Protection zones. Safety laser scanners have multiple protection zones, including the primary protection zone and one or more warning zones. Some can use scanned data for automatic configuration to ignore stationary objects in the warning zones (Figure 3). In some cases, a safety laser scanner can scan multiple zones sequentially or simultaneously. For example, one scanner can support up to 70 unique safety zone sets. On an AMR, this feature enables the scanner to adjust the scanned areas based on the surroundings and speed of movement.

Figure 3: Commissioning can be sped up by using a scanner that uses scanned data for automatic configuration to identify stationary objects in the warning zones. (Image source: IDEC)

Protective field range. It is the maximum distance that the protected area can extend from the scanner. Typical values range from 3 to 10 m. The required protective field range depends on local regulations and the scanner's response time and resolution.

Resolution. It is measured in mm and determines the minimum size object the scanner can accurately detect. Typical values range from 30 mm to 200 mm.

Response time. Also called detection time, it measures how quickly a scanner can recognize the approach of an object. Typical values range from 60 to 500 ms.

Sampling. Indicates how many times an object must be scanned in succession to be recognized by the scanner. By default, it usually takes at least two sampling scans. However, for some scanners and under some circumstances it can require ten or more successive sampling scans to recognize an object.

Dual protection zones

Safety laser scanners have various features and functions that suit different application needs. For example, IDEC's SE2L safety laser scanners include master/slave functionality and dual protection zones. The master/slave function enables one scanner to communicate with up to three other scanners. That can significantly simplify system design and allow the use of a lower-cost controller since the safety controller only needs to communicate with the master, which relays the instructions to the slave scanners. The model SE2L-H05LP can be installed using 2 to 20 m long cables, further enhancing flexibility.

These scanners have a scan cycle time of 30 ms and can include 32 patterns in the scan area. Using the dual-zone function, a single SE2L unit can independently scan two adjacent zones simultaneously, eliminating the need for a second scanner and simplifying system design.

Low power for battery-operated safety

Extending the run times of AGVs and AMRs can be an important consideration. Those applications can benefit from using Omron's compact (104.5 mm) OS32C-SP1-4M safety laser scanner. It consumes a maximum of 5 W (3.75 W in standby mode) and has 70 sets of safety zone and warning zone combinations available, making it suitable for use in complex environments (Figure 4). Other features include:

• Minimum resolution can be set to 30, 40, 50, or 70 mm.
• Safety zone varies with resolution:
  • 1.75 m (30 mm resolution)
  • 2.5 m (40 mm resolution)
  • 3.0 m (50 mm resolution)
  • 4.0 m (70 mm resolution)
• Warning zone radius up to 15 m
• Configurable response time from 80 ms up to 680 ms.
• Zone switching time can be set from 20 ms to 320 ms

Figure 4: This low-power safety laser scanner supports 70 sets of safety zone and warning zone combinations, making it suitable for AMRs operating in complex or dynamic environments. (Image source: Omron)

Triple fields with selectable sampling and resolution

SICK's S300 Mini Standard safety laser scanners feature selectable detection resolutions and sampling levels. For example, model S32B-2011BA supports 30, 40, 50, and 70 mm resolution diameters. Multiple sampling and resolutions can be individually defined for each field, including simultaneous protective fields (Figure 5). These scanners support up to 48 freely configurable fields and 16 switchable field sets. The triple field function enables a protective field and two warning fields to be used simultaneously.

Figure 5: The S300 Mini Standard safety laser scanners can implement multiple sampling levels and different resolutions for each scan field. (Image source: SICK)

Eliminating the need for a safety PLC

The SX5 safety laser scanners from Banner Engineering feature an integral muting function that can monitor and respond to signals and automatically suspend the safeguarding function to allow an object to pass through the safety zone without generating a stop command. The muting function allows the suspension of the entire safety zone (total muting) or just a section of the safety zone (partial dynamic muting).

An SX5 master unit, like the SX5-ME70, can control up to three remote units like the SX5-R; the scanner can also read incremental encoder inputs to modify the safety zone based on the vehicle's speed. These functions can eliminate the need for additional control hardware like a safety PLC.

Conclusion

If properly specified, configured, and integrated, safety laser scanners are well suited to protect people and machines in applications like access control, area protection, and on mobile systems, including AGVs and AMRs. They meet the requirements of PLd and SIL3 and are suitable for use in applications where safety is an important consideration. These scanners are available with various combinations of features and functions to suit a range of application needs.