Electrosensitive protective devices for safe machines — Part 2

The optoelectronic technologies available for machine safety protection are nowadays very diverse and provide advanced functions not only to protect workers, but to improve productivity at the same time.

The measures and products for implementation of machine safety requirements have become more diverse over the years. The goal is ever better integration of the functional safety in machines and systems for safeguarding. Various technologies for implementation of protection measures are now available. In Part 1 of this article the main types of electrosensitive protective devices (ESPDs) were introduced. In this part we look at problems associated with multiple ESPDs in close proximity and how to prevent interference between them, and examine some more advanced functions of ESPDs that enable greater productivity.

Important factors that influence reliable ESPD protection

In Part 1 of this article, minimum distance and stopping/run-down time were explained. But there are some other serious factors that also need to be taken into account when implementing ESPDs.

Preventing reflections from AOPDs

For AOPDs, the light beam is focused from the sender. The aperture angle of the lens is reduced as far as possible such that an operation free of false trips can be ensured even in the event of small alignment errors. The same applies to the aperture angle of the receiver (effective aperture angle according to IEC 61496-21). Nevertheless, even with smaller aperture angles, there is the possibility for the sender’s light beams to be deflected, leading to detection failure (Figure 1). It is therefore necessary that all reflective surfaces and objects (eg, material containers, reflective floors) must be at a minimum distance from the protective field of the system (see Part 1, Figure 3). This minimum distance depends on the distance D between sender and receiver (protective field width). It must be maintained on all sides of the protective field.

Figure 1: How a reflection can nullify the protective effect of an ESPD.

Prevention of mutual interference between AOPDs

If several AOPDs are operated in close proximity to each other, the beams from one system (S1) can affect the receiver of the other system (R2), creating the risk that the affected AOPD provides no protection. Installation situations of this kind must be avoided or suitable measures must be taken, such as mounting opaque partitions or reversing the direction of transmission of a system. Type 4 AOPDs either have to have suitable extraneous sender detection and change to a safe state (outputs in the OFF state) when affected or have technical means to prevent the interference. Beam coding is normally used, so that the receiver only responds to light beams from the assigned sender that is coded the same (Figure 2).

Figure 2: Avoiding mutual interference by encoding or by adequate spatial arrangement.

Automatically ignoring material passing through ESPDs

The following safety functions can be supported through the logic unit or directly through a suitable ESPD.

Temporary deactivation (muting)

The muting function allows temporary deactivation of the ESPD’s protective function. This is necessary when material must be moved through the protective field of the protective device without stopping the machine operation. It can also be used effectively to optimise the machine operation (eg, muting a safety light curtain during the safe run-up of the die in a power press, making it easier for the operator to remove work pieces).

Muting is only allowed when access to the hazardous point is blocked by the material passing through (Figure 3) or when no hazardous machine functions are occurring. This condition is assessed by muting sensors and muting signals.

Figure 3: Muting function with a safety light curtain and muting sensors on a wrapping machine.

For the muting function, great care is necessary when selecting and positioning the muting sensor. The following conditions are to be met to implement a safe, standardised muting function:

• During muting, a safe state must be ensured by other means, ie, the hazardous area must be inaccessible.
• Muting must be automatic, not manual.
• Muting may not depend on a single electrical signal.
• Muting may not entirely depend on software signals.
• An invalid combination or sequence of muting signals shall disallow the muting state.
• The muting state must be ceased immediately after the material passes.
   

To improve the quality of differentiation, additional limits or signals can be used, including:

• Direction of movement of the material (sequence of the muting signals)
• Limiting of the muting duration
• Material demand by the machine controller
• Operational status of the material handling elements (conveyor)
• Material identification by additional properties (such as barcode label)

Safety light curtains with entry/exit function

Another possibility for transporting material in a protected area is through active differentiation between man and material (known as an entry/exit function). For this application, horizontally arranged safety light curtains are used. The possibility of evaluating each light beam is used to differentiate the interruption pattern of the material or material carrier (eg, pallet) from a person. By using self-teaching dynamic blanking, as well as other differentiation criteria such as direction of movement, speed, entry and exit position in the protective field, a safety-relevant distinction can be made. In this way, undetected entry into the hazardous area by persons can be reliably prevented (Figure 4).

Figure 4: Entry/exit function with horizontally arranged safety light curtain.

Safety laser scanners with protective field switching

An additional possibility for transporting material through a protected area is via switching the protective fields. For this application, safety laser scanners are normally used with vertical (and slightly angled) protective fields. The appropriate protective field is activated from a series of preprogrammed protective fields, by adequately positioned sensors and appropriate signals from the machine controller. The contour of the protective field is programmed so that passage of the material does not cause the protective device to activate, but unmonitored areas are small enough to prevent undetected entry into the hazardous area by anyone.

Additional functions of ESPDs

Blanking

For many AOPDs, configuration of the detection capability or protective field can be set so that the presence of one or more objects within a defined section of the projective field does not trigger the safety function (OFF state). Blanking can be used to allow specific objects to pass through the protective field (eg, hoses for cooling lubricant, a slide or carrier for work pieces).

For fixed blanking, the blanked area is precisely defined in size and position. For floating blanking, only the size of the blanked area is defined, but not the position in the protective field (Table 1).

Table 1: Criteria for fixed and floating blanking.

To prevent gaps in the protective field, the presence (or, in some cases, a change in the size or position) of an object can be used to trigger the safety function (OFF state).

Presence-sensing device initiation (PSDI) mode

Use of the protective device to trigger the machine function (cycle reinitiation) is described as PSDI mode. This mode has its advantages when work pieces are manually loaded or unloaded at each machine cycle. Conforming to standards, PSDI mode can only be executed with Type 4 AOPDs with an effective resolution d ≤30 mm. In PSDI mode, the machine waits at a defined position for a specified number of interactions by the operator. The AOPD releases the dangerous movement automatically after this specific number of interruptions.

The ESPD has to be reset under the following conditions:

• When the machine starts
• On restart when the AOPD is interrupted with a dangerous movement
• If no cycle initiation was triggered within the specified time
   

It is necessary to check that no danger to the operator can arise during the work process. This limits the use of this mode to machines in which the hazardous area is only accessible through the protection field of the AOPD or through interlocked guards, and it is not possible for the operator to remain undetected between the protective field and the machine.

Single-break PSDI mode means that the AOPD triggers the machine function (next cycle) after the operator has completed one intervention (interruption).

Double-break PSDI mode means the AOPD locks the machine function after the operator’s first intervention (eg, removal of a work piece). Only after the operator has completed the second intervention (interruption) will the AOPD release the machine function (eg, feeding of a billet).

PSDI mode is often used on presses and stamps, but can also be used on other machines (eg, rotary tables, automatic assembly systems). When using PSDI mode, it shall not be possible to trespass the safety light curtain. For presses, special conditions apply for PSDI mode.

Conclusion

Due to their mode of action, their functional flexibility and the various application possibilities for safeguarding machines, electrosensitive protective devices have many advantages. Special optoelectronic protective devices have been established in the automation world for many years. While their design requirements are defined in product standards, their application is stated in different machinery-specific standards. Due to the optical principle, the design engineer has to take special care when planning the application of AOPDs with a machine. The support of unhindered workflow and the resulting positive impact on productivity are important arguments for using optoelectronic protective devices.

Because a slowdown in the work process is virtually non-existent, the manipulation of protective devices by the machine operator is not very common. Therefore, the creation of a potential risk of injury due to such manipulations — the manipulating person being aware of the risk or not — is hardly relevant for electrosensitive protective devices. In addition to the required safeguarding of operators, the achievable productivity improvement is an important advantage of electrosensitive protective devices.

References

1. International Electrotechnical Commission 2013, IEC 61496-2:2013: Safety of machinery — Electro-sensitive protective equipment — Part 2: Particular requirements for equipment using active opto-electronic protective devices (AOPDs).

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