Seven (potentially) deadly sins when setting up safety laser scanners

Introduction

SICK invented a Safety Laser Scanner back in the early 1990s and the organisation today has the broadest application experience and portfolio in the industry. Most recently, SICK pioneered the transition to outdoor safety with the introduction of the world’s first outdoor Safety Laser Scanner, outdoorScan3. SICK has developed a wealth of knowledge regarding the successful application of this technology and you will find below expert advice on the ‘Seven (potentially) Deadly Sins when setting up Safety Laser Scanners’, to assist you with successful implementation of this technology.

Installation and mounting — Thinking about safety last

If you are going to remember just one point then this is it! Too many times we have seen an “almost finished” machine and someone then posing the question, “Right, where can I stick this scanner?” Inevitably, what ends up happening is that blind spots (shadows created by obstacles) become apparent, requiring mechanical alterations and maybe even additional scanners, to cover the complete area when one scanner may have been sufficient if the cell was designed well in the first place. In machinery safety, designing something out (eg: blind spots) is by far the most cost effective and robust solution. If you know you are going to be using a safety laser scanner then design for it from the beginning. This could save you a world of pain. Consider blind zones, coverage, potential field obstructions and the location of hazards. This also goes for automated guided vehicles/carts. For example, the most appropriate position to provide complete coverage for an AGV/AGC is to have two scanners diagonally opposite each other on the corners, integrated into the vehicle — figure 1.

Figure 1: Typical AGV scanner mounting and integration

However, for an AGV on a fixed track that only ever moves forward, a single scanner on the front in the centre could also be perfectly acceptable. The important thing is to “Design it in”, rather than “Add it on” at the end.

Incorrect Multiple Sampling values configured

An often misunderstood concept, multiple sampling indicates how often an obstruction has to be scanned in succession before a safety laser scanner reacts. By default and out of the box, this value is usually x2 scans, which is the minimum value, giving you the quickest response time. However, this value may vary from manufacturer to manufacturer. A higher multiple sampling value reduces the possibility that insects, weld sparks, weather (for outdoor scanners) or other particles infringing the protective field, cause the machine to shut down. Increasing the multiple sampling can make it possible to increase a machine’s availability, but it can also have negative effects on the response time in the application. Increasing the number of samples is akin to adding an OFF-Delay to the system, meaning that your protective field may need to be larger due to the increase in the total response time. If a scanner has a robust detection algorithm then you shouldn’t have to rely on this feature so much to improve the ‘availability’ of the device. Recognise that increasing this value increases the response time of the device. This may impact the ‘safety distance’ calculation and hence the required size of the protective field. If it is changed, you should make a note of the safety laser scanner’s new response time and adjust the minimum distance from the hazardous point accordingly to ensure it meets requirements. Furthermore, in vertical applications, if multiple sampling is set too high then it may be possible for a person to pass through the protective field without being detected, so care must be taken! You can see an example of advice below provided for the SICK microScan3 Safety Laser Scanner — figure 2.

Figure 2: Recommended multiple sampling values

Incorrect selection of safety laser scanner

The maximum protective field that a scanner can facilitate is an important feature but this value alone should not be a deciding factor on whether the scanner is suitable for an application. A safety laser scanner is a Type 3 device according to IEC 61496 and an Active Opto-Electric Protective Devices responsive to Diffuse Reflection (AOPDDR). This means that it depends on diffuse reflections from objects within its field of view. Therefore, to achieve longer detection ranges, scanners must be more sensitive. In reality, this means that sometimes, scanning angle but certainly detection robustness can be compromised, to achieve a longer detection range. This could lead to a requirement for an increasing number multiple samples and maybe lack of angular resolution. The increased response times and lack of resolution could mean that larger protective fields are required and even additional scanners — even though you bought the longer range scanner in the first place. A very important point to remember is that a protective field should be as large as necessary but as small as possible. In addition, do not configure the scanner to monitor areas, which are not necessary. This only leads to the possibility of detection in space which is not required to be monitored. A shorter range scanner may be more robust than a longer range version and hence keep the response time down, reduce the footprint, reduce cost and eliminate annoying false trips.

Incorrect resolution selected

The harmonised standard EN ISO 13855 can be used for the positioning of safeguards with respect to the approach speeds of the human body. Persons or parts of the body to be protected may not be recognised or recognised in sufficient time, if the positioning/configuration is incorrect. The safety laser scanner should be mounted so that crawling beneath, climbing over and standing behind the protective fields is not possible. If crawling under could create a hazardous situation then the safety laser scanner should not be mounted any higher than 300 mm. At this height, a resolution of up 70 mm can be selected to ensure that it is possible to detect a human leg. However, it is sometimes not possible to mount the safety laser scanner at this height. If mounted below 300 mm then a resolution of 50 mm should be used. It is a very common mistake to mount the scanner lower than 300 mm and leave the resolution on 70 mm. Reducing the resolution may also reduce the maximum protective field possible on a safety laser scanner so it is important to check these values and their impact.

Ambient/Environmental conditions were not considered

Sometimes safety laser scanners just aren’t suitable for an application. A Safety Laser Scanner is a piece of ‘electrosensitive protective equipment’ and infra-red light can be a tricky thing to work with. Scanners have become very robust devices over the past decade with increasingly complex detection algorithms (Safe HDDM by SICK) and there are even safety laser scanners certified to work outdoors (outdoorScan3 by SICK). However, there is a big difference between SAFETY and AVAILABILITY and expectations need to be realistic right from the beginning. A scanner might not maintain 100% machine availability if there is significant dust, steam, wood chips, or other material constantly in the field of view. Even though the scanner will continue to be safe and react to such situations, trips due to environmental influences may not be acceptable to a user. For extreme environments, the following question should be asked: “What happens when the scanner is not available due to extreme conditions?”, which can be especially true in outdoor applications in heavy rain, dust, snow or fog. A full assessment of the ambient conditions and potentially proof testing should be carried out. This particular issue can become very difficult to resolve after the event.

Non-safe switching of field sets

A field set in a safety laser scanner can consist of multiple different field types. For example, a field set could consist of 4 safe protective fields (Field Set 1) or it could consist of 1 safe protective field, two non-safe warning fields and a safe (contour) detection field (Field set 2) — figure 3.

Figure 3: Safety laser scanner field sets

A scanner can store lots of different fields which can be selected using either hardwired inputs or safe networked inputs (CIP Safety, PROFISAFE, EFI-pro etc.). This is a feature that industry finds very useful for both safety and productivity in Industry 4.0 type applications. However, the safety function (as per EN ISO 13849/EN 62061) for selecting the field set at any particular point in time should normally have the same safety robustness (PL¹/SIL²) as the scanner itself or the PL required in the application. A safety laser scanner can be used in safety functions up to PLd/SIL2 and if we look at AGV’s for example, usually two rotary encoders are used to switch between fields achieving field switching up to PLe/SIL3. There are now also safety rated rotary encoders that can be used alone to achieve field switching to PLd/SIL2. However, sometimes the safety of the mode selection is overlooked. For example, if a standard PLC or a single channel limit switch is used for selecting a field set then this would reduce the PL/SIL of the whole system to possibly PLc or even PLa! An incorrect selection of field set could mean that an AGV is operating with small protective field in combination with a high speed and hence long stopping time, creating a hazardous situation.

(¹PL = Performance Level, ²SIL = Safety Integrity Level)

Setting the safety field too tight

Operating instructions can be quite boring, let’s be totally honest! At best, most people flick through them to find the bits they are interested in and never read them front to back. The “Ctrl-F” function in PDFs now makes this even easier! However, important information regarding availability can be missed. For example, when drawing a protective field in a cell area, it would be easy to assume that the field should be configured as closely as possible to the surrounding contours (cell walls). This is a consideration which is usually not known or obvious to someone who has little experience applying this technology. Safety Laser Scanners use the quo;Time-of-flight” principle i.e. if you know how long a pulse of light takes to travel to and from an object then using the speed of light (299792458 metres per second) you can calculate the distance. We are working with very high speed and very short time differences and therefore there is a statistical error that must be taken into account. This will also vary from scanner to scanner depending on the scanning technology. For example for the SICK microScan3, we recommend that a gap of approximately 65 mm is ensured to maximise availability — figure 4.

Figure 4: Distance of protective field from a wall

Scanners may well work most of the time below these recommended values but may also trip randomly and this can be a hard thing to identify if you don’t know what you are look for.

Summary

Safety Laser Scanners are complex devices requiring good application knowledge for successful implementation. Nowadays, there is lots of choice in the market concerning scanning range, connectivity, features, size and robustness. There are also many variables to consider when designing a safety solution using scanners. If you are new to this technology then it is a good idea to contact the manufacturer for advice on the application of these devices. SICK has complementary services available to our customers such as consultancy, on-site engineering assistance, risk assessment, safety concept and safety verification of electrosensitive protective equipment (ESPE) and we are always happy to answer any questions.