Safe material flow – an issue for muting?

On many machines, electro-sensitive protective equipment (ESPE), like safety light curtains and safety laser scanners, are used as safeguards where a physical guard would be unsuitable. This could be, for example, on conveying lines with goods passing from one area of production to another. These applications, more often than not, require the ESPE to allow certain objects through the protective field while still responding to the presence of people. The most common implementation of safe material flow is to temporarily “mute” the ESPE using additional sensors while the material is passing. There are other methods, however, that don’t involve muting but use safe algorithms to differentiate between the material and a person.

This article explores this topic and presents some examples of methods of safe material flow through an ESPE along with some guidance on how to choose the best solution for your application.

EU regulations: ensuring the safety of machinery

It is a legal requirement in the European Economic Area to ensure that machines are safe.

For manufacturers intending to supply machinery to the European Economic Area, the Machinery Directive (2006/42 / EC) specifies the essential health and safety requirements relating to the design and construction of machinery. The Work Equipment Directive (2009/104 / EC) applies to those who operate the machinery.

To ensure the safety of machinery, there are practical solution approaches for complying with the safety requirements in the harmonized EN standards. When the requirements of a harmonized EN standard are fully taken into consideration, the so-called presumption of conformity (to the respective guidelines) takes effect.

 

Harmonized standards in three categories

These harmonized standards are split into three categories:

• A-Type standards – Basic safety standards containing basic terminology, principles of design and general aspects that can be applied to all machinery (e.g. EN ISO 12100 for risk assessment)
• B-Type standards – Group safety standards that address safety aspects or a particular type of protective device. They can be used for a wide range of machinery (e.g. EN ISO 13855 for calculating minimum safety distances or EN ISO 14119 for guard interlocking etc.)
• C-Type standards – Contains all of the safety requirements for a specific machine or type of machine

If a C-Type standard exists, it has priority over the A-Type and B-Type standards. A C-Type standard will refer, however, to any relevant A-Type and B-Type standards instead of recreating the content.

However, the use of a single C-Type standard might not provide the intended conformity with the essential health and safety requirements (EHSRs) of the EU Machinery Directive, and it is common nowadays to see many machines (each of which may or may not have a C-Type standard) working together which can make things a bit more complicated, e.g. a CNC robot tending machine comprising a lathe, robot, automated drawer system, and conveyor belt (see Figure 1).

Functional safety

For machines that commonly incorporate electro-sensitive protective devices (ESPEs), the relevant C-Type standards will often contain guidance regarding their application. For example, the EN 415 set of C-Type standards that cover the safety requirements for packaging machines contains lots of useful information.

EN 415-6 for pallet wrapping machines contains Annexes covering topics such as ESPEs in a vertical position, dynamic cell positioning of EPSE, and EPSE muting. However, this standard was published in 2013 and since then, newer technologies exist that can be used in these applications. Standards describe the state of the art and unfortunately, the development of standards can be slower than the progress of technology.

EN 415-6 refers to EN ISO 13849 & EN 62061 (now replaced by EN IEC 62061) with regards to the robustness of the control system that the ESPE is part of. These “safety-related” parts of the control systems (SRP/CS) need to be designed and constructed accordingly and must have a certain level of robustness appropriate for the level of risk reduction required.

EN ISO 13849 and EN IEC 62061 provide safety requirements and guidance on the principles for the design and integration of SRP/CS, including the design of software and specifies characteristics for carrying out safety functions (SFs). While either of these standards can be used, this article focuses on EN ISO 13849.

EN ISO 13849 provides examples of a number of typical safety functions, and it references other standards that are also relevant. For ESPEs and muting, it references EN ISO 13855 and IEC/TS 62046.

EN ISO 13855 covers the positioning of protective devices with respect to the approach speeds of the human body. The general formula for calculating the minimum distance is:

S = (K × T) + C

Where:

S = Minimum distance of the ESPE from the hazardous point

K = Approach speed of the human

T = Stopping/run-down time of the entire system

C = Intrusion distance (a supplementary value that changes based on both the resolution of the protective device and the possibility of reaching over)

SICK can provide help and guidance on this topic and has many documents that it can supply to customers. SICK also offers safety services such as stop time measurements to obtain a value for “T” in order to calculate the required minimum distance “S”.

EN IEC 62046 is a standard which was released after the publication of the current EN ISO 13849 standard. It is always advised to use the latest version of a standard. EN EC 62046 specifically covers the application of protective devices to detect the presence of persons and contains guidance on muting.

For a pallet wrapping machine, the following standards contain useful guidance on ESPEs:

• EN 415-6
• EN ISO 13849 / EN IEC 62061
• EN IEC 62046
• EN ISO 13855

However, this is not a comprehensive list and other standards will also be relevant, for example, EN 60204 for electrical safety, and EN ISO 14118 for the “prevention of unexpected start-up” to name a few.

C-Type standards normally contain information on muting and will refer to the other standards. If no C-Type standard exists, all relevant B-Type standards must be taken into consideration for risk reduction. If you are unsure with regards to which standard is applicable, then it is best to speak to an expert to get advice.

Standards cannot, however, contain guidance on technologies that have been developed since they were published. In this case, the standard can be used in conjunction with EN ISO 12100.

 

Standards relevant to muting

With regards to muting, C-Type standards will have guidance, but it would be a large task to go through all of the relevant C-Type standards. So, what do EN ISO 13849 and IEC 62046 say on this matter?

 

EN ISO 13849

EN ISO 13849 defines muting as a “temporary automatic suspension of a safety function or safety functions by the safety-related parts of the control systems (SRP/CS)”. There are a few important words here:

Temporary – How long is the safety function (SF) suspended?

• Automatic – How does the control system decide when to suspend the SF?
• Safety function – What SF is being suspended and how?

Let us consider a safety-related stop function initiated by a safety light curtain protecting the entrance to a pallet handling machine. When the material approaches the light curtain (full pallet), the control system must automatically detect the material and suspend the light curtain safety function (detection of people) to allow the material through (see Figure 2).

 

This can be done with sensors. The system must be able to reliably recognize the material and thereby differentiate between the material and a person. During muting, safe conditions must be provided by other means (blocking of the entry by the material being transported) and when finished, all safety functions of the SRP/CS must be reinstated.

A common misconception is that all muting systems must have a muting lamp. Even though some C-Type standards prescribe the use of a lamp, EN ISO 13849 only says however that “In some applications, an indication signal of muting is necessary”. Furthermore, EN IEC 62046 says that when an indicator is provided to show that the muting function is active, it should be considered whether the provision of the mute indicator can lead to improper attempts to access the hazardous zone. So it might be better not to have one.

 

EN IEC 62046

EN IEC 62046 contains informative annexes to provide guidance on the application and positioning of protective equipment and sensors for material detection for the purposes of initiating the muting function, and also provides illustrative examples of this. However, the examples are not intended to be the only solutions to an application. They are also not intended to restrict innovation or advancement of technology. The standard also presents the following figure to show its relationship with other standards (see the  figure below).

Clause 5.7 in the EN IEC 62046 standard covers muting in detail, and covers similar topics to EN ISO 13849 but goes into much more detail. The standard provides requirements that apply when the muting function is provided, such as:

• Muting must be initiated by two or more independent muting signals
• The muting function must be terminated when any of the muting signals maintaining the function is deactivated
• Timing and/or sequence control must be used on the muting signals to ensure correct operation
• There must be protection against inadvertent initiation of muting or continuous muting due to mechanical damage and/or misalignment of muting sensors
• Measures must be provided to prevent circumvention of the protective equipment
• There must be protection against foreseeable misuse including manipulation

The standard also gives recommendations and many examples of measures that can be used, including advice on muting to allow access by persons or material, and a mute dependent override function.

 

Examples of safe material flow solutions

EN IEC 62046 – Muting examples

Annex D of IEC 62046 contains a number of examples for the arrangement of photoelectric muting sensors when used to allow automatic access by material. However, as already pointed out, this is not an exhaustive list, and it is not intended that only these should be used. However, they do cover the most commonly used configurations (see Figure 4).

 

The most important requirement for the muting sensor configurations is that they should ensure that a person cannot pass through the opening towards the hazardous zone while the system is in a mute condition. This could occur, for example, by riding a pallet or walking alongside/behind the material. The annex also provides guidance on this (additional measures, swinging doors, etc.) as well as presenting some methods to avoid manipulation.

It is also important that the muting configuration does not cause any additional hazards (e.g. crushing) between the material being transported and any fixed structures. So, there is also guidance on maximum/minimum distances for the following:

• Distance between the material and fixed parts of the enclosure
• Distance between the muting sensors
• The height of the muting sensors from the conveyor plane

The current version of IEC 62046 was published in 2018 and replaced the IEC/TS 62046 technical standard which was originally published in 2008. This standard contains limited examples, however, and there have been new technologies and new muting solutions developed since this standard was drafted. From a safety technology perspective, what is important in the end is that the protection goals in EN IEC 62046 in relation to muting are met. This means that any alternative solution approaches that lead to an equivalent (or even higher) safety level are, of course, also permissible.

The following section provides additional muting examples, but also presents novel methods that take advantage of smart algorithms and do not require the suspension of an ESPE.

 

Safety laser scanner – protective field adaptation

Like safety light curtains, safety laser scanners are ESPEs that can be used to detect a person. You will normally expect the protective field of a laser scanner to be in a horizontal plane, where common applications include preventing a restart in a robot cell, or as a protective stop on an automated guided vehicle (AGV) (see Figure 5).

Laser scanners can, however, also be used vertically like a safety light curtain and it is also possible to dynamically change the protective field using input signals. When using a safety light curtain, you can increase safety by only muting a certain number of beams. With a safety laser scanner, however, you can actually change the shape of the protective field to surround the contour of the material when in the muting state (see Figure 6).

This removes the need for additional protective equipment or sensors to detect if a person is trying to gain access to a machine while the ESPE is muted.

Additionally, if you have a safe data interface with a scanner (e.g., CIP Safety, PROFISAFE, SICK – EFI Pro), you can access multiple protective fields at the same time. This is called simultaneous field evaluation and allows engineers to develop complex applications using just one sensor. The Safe Portal Solutions from SICK uses these features to safely detect material using separate “muting fields” that dynamically adjust the protective fields to allow the material to pass through while still protecting the rest of the opening. This eliminates the need for muting sensors, which provides more flexibility and can reduce the footprint (see Figure 7).

Alternatives to traditional muting

The Safe Entry Exit safety system is a TÜV-certified alternative to traditional muting. This system takes a single signal from the process controller as the first signal, and the light curtain itself as the second muting signal. Combined with time and sequence checks, this enables it to reliably detect material and thus initiate the muting function. Like the Safe Portal, this can eliminate the need for additional muting sensors and reduce the footprint of the muting system (Figure 8).

True human-material differentiation

Safety devices have become incredibly versatile, and powerful microchips mean that complex algorithms can be implemented at high speeds. Individual beams can be evaluated, thereby enabling pattern recognition in a permanently active safety sensor. This means that a safety device can safely differentiate between a human and the material that is allowed to enter a hazardous area. A system like this does not mute the person detection safety function at any point in the process. This provides constant protection because the uninterrupted beams continue to remain active.

The C4000 Fusion safety light curtain can be used horizontally, and it allows the user to configure different patterns by specifying parameters such as:

• Allowed number of objects
• Object widths
• Distance between objects
• Direction
• Sequence
• etc.

This means that only certain silhouettes are allowed to pass through the protective field of the safety light curtain without triggering the safety function (= initiating a stop), which enables, for example, a car skid to be distinguished from a human (see Figure 9).

Another example of pattern recognition is the SMART Box Detection feature on the deTec4 safety light curtain. This feature evaluates the beams to identify objects that appear rectangular from the two-dimensional perspective of the light curtain, e.g. cardboard boxes or even drums. This could be particularly helpful in applications that have traditionally used tunnel guards to increase the distance from a hazardous point. This eliminates the need for muting sensors and, depending on the stopping time of the machine, can significantly reduce the safety distance. Beams that have not been interrupted remain active and thereby continue to prevent undetected reaching in over the object (see Figure 10).

Implementation of a solution

Another topic that should be considered is the implementation of a solution. For example, it is possible to purchase pre-configured, wired, aligned, and pre-certified solutions as described in EN IEC 62046 (see Figure 11).

Purchasing a complete system instead of individually purchasing all of the components, brackets, and cables reduces the effort involved in design, configuration, and installation. It can also reduce the effort required to verify the system's performance (PL/SIL).

If muting is implemented in a safety controller, then pre-certified function blocks are available that give the user much more flexibility with a full logic suite available. There are also stand-alone solutions and devices with integrated muting/safe algorithms.

It is important to understand the pros and cons of how a muting system is implemented because this can affect different factors such as cost, availability, robustness, specification, and footprint among other things. Table 1 shows the most common implementations.

In reality, some systems may differ from this table but in general, these implementation examples are based on many years of experience in this field and can be a good yardstick.

Summary regarding safe material flow: every application is different

Electro-sensitive protective devices are commonly used on machines to protect access to hazardous areas, and it is often the case that the system needs to allow material in and out of the machine while safely protecting people. The traditional muting feature of protective devices has developed over time into an increasingly versatile method. There are many solutions on the market, however, including solutions that do not mute the safety function (= initiating a stop) but use smart pattern detection to dynamically allow material through while still detecting access by a person.

Every application is different and some solutions may offer particular advantages over others, so the designer should take into consideration what is important to them. If a C-Type standard exists for a machine, then it may offer advice and guidance on how to implement safe solutions. Technology, however, often moves quicker than the development of standards. EN IEC 62046 recognizes this and states that other solutions may be possible. When selecting the best solution, the only thing that matters is whether the protection goals have been achieved, and whether the solution is robust against manipulation.

Manufacturers of ESPEs have lots of experience in the application of their protective devices, and offer many different solutions for implementing the safe passage of material on a machine. If you are unsure, the safety and sensor specialists at SICK are happy to provide advice and help you find the right solution.

 

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