Posted on 27 May 2017
When selecting a suitable light curtain for your lift door safety application it is important to consider several factors before making your decision. Diode count, number of beams and response time all play a part in determining the relative performance of a particular detector, and no feature should be judged in isolation.
The most important feature of a light curtain is the number of diodes it contains. This number is related to the smallest target that can be detected, particularly at the detector’s edges.
The number of beams can also be used as a marketing ploy to demonstrate higher performance. If implemented correctly, diagonal beams will improve detection capability for smaller objects. Using a criss-cross pattern with four or five scans per diode will easily provide detection of 12mm (a child’s finger) virtually anywhere between the detectors, except at the detector’s edges.
However, the stated number of beams is only available at wide separations and beams sometimes have such poor sensitivity that they are effectively redundant. Increasing beam counts further (three up and three down) will not improve performance.
Some light curtain manufacturers claim that offset diagonal beam systems and faster scanning provide performance benefits over rival technology. This is not actually the case.
Imagine two detectors that use the same diagonal scanning method, right up to when the doors close. Detector A has 32 diodes while Detector B features 24 diodes.
When the doors are approximately 600mm or more apart, detector A switches to an up-and-down two-beam pattern, giving four scans per diode. This dramatically increases the beam pattern density.
The 50mm target is used as the benchmark for detector testing. The scans below compare performance with a 50mm target for Detectors A and B. A continuous red line indicates complete detection, while a broken line shows areas where targets are missed. As can be seen, there are many gaps for Detector B, but none at all for Detector A.
Detector B is particularly weak when the doors are near to closing – it has very poor close-in sensitivity with a relatively large target.
The impact of these large gaps can indirectly extend the response time of the detector to several seconds. The doors will have to move quite a distance before an object, such as a hand, is detected. Detector B may scan very quickly, but that is pointless if it can’t detect a person’s hand for several seconds.
A detector’s response time is a secondary factor. It has never been considered important to reduce response time in either the design or specification stages, as it is not critical to the detector’s performance.
Lift doors travel at slow speeds to avoid physical damage to passengers or equipment. Detectors are mounted between the inner and outer doors. The depth of the doors (front to back) means that there are several centimetres of unguarded space.
Doors that close relatively quickly have a higher risk of hitting people or equipment (prams, wheelchairs, trolleys, etc.). A typical closing speed of a lift door is 0.5m/s. This means in 100ms the door will only travel 5cm until the detectors are triggered. The doors themselves have inertia, so they will continue to travel a short distance even after a trigger.
Detectors with improved scan or response times have very little perceived benefit to the end user. It is unlikely anyone would notice the door travel difference between a detector which can respond in 50ms and one which has a response time of 100ms.
Avire detector products typically scan at around 1.5ms per diode, plus some overhead at the end of the scan for processing. This means that a standard 32 diode detector will have a scan rate of approximately 70ms. This is a worst case time though. As soon as any beam is obstructed, the hardware output is triggered within a few milliseconds. The average response time will therefore be much less, in the order of 40ms.
The information above can be condensed into a few key points:
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