AGV protective fields – does size matter?

07 August 2018

Dr Martin Kidman looks at safety laser scanner technologies which are a vital component of AGVs.

Without the safety laser scanner, Automatic Guided Vehicles (AGVs) would not have become the integral part of production and logistics that they are today. Using the time-of-flight measurement principle, a safety laser scanner can measure distances around a radius in a horizontal or vertical plane. The requirement for today’s Type 3 devices are contained in the harmonised standard EN 61496-1. When a safety function includes a Type 3 safety laser scanner, the maximum performance level (PL) or Safety Integrity Level (SIL) that can be achieved is PLd (EN ISO 13849) or SIL2 (IEC 62061). There is also a Technical Specification for safety laser scanners, IEC 61496-3, which covers the particular requirements for Active Opto-Electric Protective Devices responsive to Diffuse Reflection (AOPDDR).

When integrated with encoders on an AGV, it is possible to measure speed so that safe motion control can be implemented, rather than emergency stopping at every detection. Warning fields can be used to ensure that AGVs slow down and speed up with respect to the distance of a person or object so that optimum availability is ensured and stoppages are minimised. However, the field must be large enough to ensure halting before a person or object is reached.

Multiple field configurations on a scanner enable a system to recognise permanent features such as machinery, fences and docking areas or temporary objects like people and other AGVs

Size isn't everything

A recent development in safety laser scanner capability has been the ability to scan longer protective fields. But is bigger always better? The maximum protective field that a scanner can facilitate is an important feature to consider if you design and build AGVs and AGCs – or if you want to purchase one. However, field size alone should not be the deciding factor.

The response times of the safety devices and associated control system are also important values to consider and not taking care to ensure that these values are reasonable can have a big impact on an application.

There are four types of field that can be configured on a modern safety laser scanner, as show in Figure 1. Fields are drawn using associated software and a combination of these fields can be combined to create a ‘Field Set’. Any combination of fields can be used in a field set, as shown in Figure 2.

Safety laser scanners can now create many field sets, and a field set is chosen based on a set of input conditions which form a ‘monitoring case’. However, switching between cases can add to the response time of a system, so the use of simultaneous field evaluation capabilities (for example, looking at six protective fields in one field set) can be advantageous.

Multiple sampling

By default, a safety laser scanner must see an object twice (two revolutions) to detect an object but, in difficult environments such as dirt, dust, welding sparks or heavy vibration, multiple sampling may be a useful option to ensure reliability. This increases the number of times an object must be scanned before a detection signal is created but also adds to the response time of the device.

There is trade-off between the need for multiple sampling and the total response time, as the response time is increased for every additional scan.

Safe AGV control

EN ISO 13855 is a harmonised standard dealing with the positioning of safeguards with respect to the approach speeds of parts of the human body and assists in the creation of protective fields, creating the safety footprint of an AGV. As the protective field length gets bigger, the footprint increases because the scanner needs to see further ahead to compensate for its reaction time so it is advantageous to keep the protective field length as small as possible so that AGVs can work in closer proximity to each other; do not need large clearance spaces, and can move faster with smaller fields and increase productivity.

Usually the largest value, and the one that varies the most, is the stopping distance of the vehicle. This comprises the braking distance for the vehicle and the distance covered during the response time of both the safety laser scanner and safety control system.

The transmission times to the control system can be improved by hard wiring output signal switching devices to inputs on safety controllers or by using a safe communication method. Using simultaneous field evaluation to monitor multiple protective fields and not having to switch field sets will minimise the response time of a system.

A smaller AGV or AGC’s protective field footprint is better suited to high output efficiency and space utilisation in a modern production environment. Rapid system safety response times in an integrated safety system enhanced by devices such as safety encoders to control acceleration and braking, can achieve the required safety in AGV operation in a smaller footprint.

Therefore, when specifying a safety laser scanner, it is more important to consider the total response time and detection robustness so that protective fields can be kept to a minimum.

Dr Martin Kidman is product specialist - Machinery Safety at SICK (UK).

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