Light curtain options for high-efficiency pallet handling

Automated palletising processes have done much to improve efficiency, speed and flow but, at the same time, have increased the potential safety risks for personnel. That is because there is still a need for a degree of operator involvement in one way or another, to oversee the operations and intervene for maintenance.

In a typical palletising process, an automated machine, or perhaps a robot, picks items from bulk and places them on a pallet for wrapping and shipping. Palletisers might be stationed where goods are shipped out or accepted in, wherever products are packed and loaded onto pallets, stored or moved from one place to another.

Palletising operations are normally safeguarded by fencing with access only possible via safety interlocked gates or the entrance, usually called infeed, and exit points. The infeed and exit points have goods passing through them more or less continuously but must simultaneously be safeguarded to prevent unsafe personnel intrusion. If access is required for setting, teaching, process changeover, fault-finding, cleaning or maintenance and it is necessary run any part of the machine, safety should be achieved using a specific control mode and meet the requirements of EN ISO 13849.

Light curtains for palletising

The safety light curtain is one of the oldest forms of active safety, pioneered in the 1950s by Sick, and offers excellent protection for palletising. Light curtains are essentially an array of optical emitters and photo sensors that are mounted vertically or horizontally either side of the access.

The simple principle is that beams of light create a circuit. When broken, an alarm is triggered and the machinery is shut down. The safety system must be reset and machinery restarted once the beams are restored and the operator is satisfied that all is safe.

A light curtain has to provide protection at the same time as allowing safe passage of material so as not to stop the continuous flow of goods and frequent stoppages are costly, both in plant profitability and in long-term wear and tear of machinery.

The safety of packaging machines is covered by the set of standards EN 415 including palletisers and the infeed and exit points are normally required to be protected using Electro Sensitive Protective Equipment (ESPE).

Moving materials in or out of a hazardous area and automatically differentiate between material and people can be achieved using the safety function known as 'muting'.

Muting explained

The muting function is used to deactivate the protective function of a protective device temporarily and must only be possible if the hazardous point is blocked by the passing material. This is the first and most widely-used method in the industry. There are a number of conditions that need to be met when implementing safe muting according to the standards:

• During muting, a safe state must be ensured by other means (material blocking access).
• Muting shall be automatic.
• Muting shall not be dependent on a single electrical signal.
• Muting shall not be entirely dependent on software signals.
• An invalid combination of muting signals shall not allow any muting state.
• The muting status shall end immediately after the material has passed through.

Additional limits can also be applied to improve the quality of differentiation such as detecting the direction of movement (muting sequence), limiting the muting duration (maximum mute timer) and identifying the material by additional properties, for example bar code reader or RFID. A muting system can become quite complex very quickly.

Further information can be found in the technical specification IEC/TS 62046 that specifies the requirements for the selection, positioning, configuration and commissioning of ESPEs to detect people in industrial applications. There is also information in the EN 415 set of standards for particular machines.

Muting is achieved by installing additional sensors such as photocells to detect material and mute the light curtain. Integrated with a safety controller PLC such as Sick's Flexi Soft or dedicated UE403 muting module, the muting sensors are usually installed in two-beam cross formation (figure 1a) or four-beam sequential arrangement (figure 1b).

Figure 1a:

Figure 1b:

Other muting methods include exit only, parallel muting and muting using inductive loops.

Laser scanners

Using safety laser scanners instead of safety light curtains for PLd/SIL2 applications in a vertical plane can also enable further discrimination. The protective field which is drawn can be changed when material passes using muting sensors rather than completely ignoring the device. This enables the passage of specific shapes while still providing protection around it.

Bypass/override

To reduce downtime and prevent false trips, bypass and override can be utilised which are manual operations as opposed to automatic (muting). Bypass is a mode that is selected by the operator to disable the safety function while other protection measures are in place. This would be used, for example, in robot teaching, in which the robot can only be operated in slow speed and using a pendant or teaching control device with a three position enabling device in accordance with IEC 60204-1:2005.

Override is a manual triggering of muting after an error in the muting conditions so that power can be applied to the conveyor in order to get the material out of the ESPE and clear the system. Both must only be operated by a key-operated switch. The switch must be a positive-action resetting device and with two switching contacts and its integration must comply with EN ISO 12100 and EN 60204-1:2005 and must be installed in such a location that the entire hazardous area can be seen.

Blanking

Sometimes when using light curtains, an object needs to be allowed to break the curtain; this is when blanking can be used. There are two main different types of blanking: fixed blanking and floating blanking in which fixed beams are ignored or a number of beams which can 'float' (move) in the field (figure 2).

Figure 2:

Care should be taken since in the blanked area the resolution capability of the ESPE is enlarged - and therefore deteriorates - which should be taken into account when calculating the minimum mounting distance. Partial blanking, sometimes called partial muting, can also be used if only certain beams are to be ignored on passage of material.

Reducing cost and risk

All of the muting configurations above use additional sensors to deliver added functionality, which can be tuned to the application. However, as well as cost and space, additional sensors can introduce complexity and increase the risk of downtime due to false trips. It can also be difficult to get robust muting when working with tricky materials that can change shape and size.

Dynamic guarding

Sick now provides light curtain technology that uses pattern recognition to offer customer-specific dynamic blanking in order to allow passage of material without muting. The C4000 Fusion is a multi-functional safety light curtain that can differentiate between complex objects in the protective field and other objects - in particular, people. Process flow is no longer affected by restrictive protection, while installation cost, and the risk of malfunction with additional instrumentation, are minimised for further benefits.

The SICK C4000 Fusion can also provide standard protective operation and fixed/floating blanking and so can be used in conventional muting systems. The dynamic blanking option allows the configuration of specific objects such as feet on pallets, mesh crates, wheels on a trolley and direction monitoring all without the need for muting sensors, figure 3 (right).

In addition, the C4000 Fusion can check the number of objects in the field, the size of the objects and the gaps between them, with the option of reduced resolution for ignoring small objects like wires and wrapping that may cause false trips.

The C4000 Fusion has a maximum 20mm resolution between beams, to prevent anything hand-sized or larger getting through the protection, and a range of up to 19m to enable large expanses to be guarded, often to replace static screen guarding. It can be deployed vertically or horizontally, with protective heights available between 300mm and 1800mm, and has a built-in laser to simplify alignment.

Safety and productivity in balance

To achieve the safety functions described, while meeting PLe/SIL3, light curtain safety is now much more versatile in adapting to changing production, logistics and storage environments. The features offered by the C4000 Fusion make this light curtain a a neat option for applications in which material has to pass into a hazardous area and ensures that the light curtain assists safety and productivity rather than sacrificing efficiency.

Follow the link for more information about the Sick C4000 Fusion and safety control equipment.