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What sensors do I use to prevent 2 gantry cranes colliding?

It is often necessary to have a system of warning when two gantries on a single crane system are close enough to run the risk of collision. Although it sounds a simple task, it is complicated by a number of issues:
The distance at which the alarm needs to be raised can be considerable, greater than many forms of proximity sensor can manage, typically 2.5 to 4 metres. Many sensors capable of working reliably at these distances are not suitable for judging distances, only for detecting a presence, or not, and not indicating the actual distance.

Ultrasonics are potentially a possible solution as these have ranges up to 8 metres and can be of analogue type, therefore can actually measure the distance, or of digital on/off type where the the trigger distance can be set quite precisely. There are drawbacks with these solutions. Firstly expense, they are typically 2 to 3 times, sometimes more, the price of optical sensors. Secondly they have a relatively wide angle of view, so over a large distance can easily be interfered with by other equipment which is not the target. Also, if analogue sensing is used, it will be necessary to feed the signal into a plc or similar first, to interpret the signal. Due to the above limitations, photo electric (optical sensing) tends to be the method of choice.

Retroflective, or reflex, sensors use a reflector to bounce a beam back to the sensor and can determine if the beam is broken or not. These have the advantage of having potentially long range (can be up to 20 metres or more), are relatively lower cost, and if a polarised reflector system is used it prevents stray reflections triggering the sensor, only a reflection from the reflector itself will be sensed.

The next question is how to to sense when the sensor on one gantry is at the critical distance from the reflector on the other. Many attempts have been made to vary the trigger point by aligning the two and then adjusting the sensitivity of the sensor so that the sensor will not react until the reflector is within a certain distance of the sensor. This is not a suitable method. It is not accurately repeatable and tremendously effected by atmospheric conditions and dust and dirt on the reflector and/or sensor lenses. The purpose of the adjustable sensitivity is to set the sensor up to respond to different target types/materials (eg semi transparent bottles) etc, not to adjust the sensing distance.

A better method for this application is to set the gain to maximum in the case of sensors with adjustable sensitivity (not all have this facility) and position the sensor at an angle to the reflector so that the beam only hits the reflector once the gantries reach the required distance. Please refer to the attached diagrams.

It should be noted that this is not fail safe system, ie failure of the system will not give an alarm or stop the gantries colliding, therefore should only be used as an indication or warning. Machine safety should not be dependent on such a system in a critical safety application. If a design of this type is used to prevent a collision by switching in safety systems then suitable back up and/or redundancy systems must be provided to achieve whatever safety levels are required.

NOTE : These notes and diagrams provide a schematic method of achieving the control but it remains the responsibility of the installer to ensure that any safety requirements, local legislation and directives, circuit and personnel  protection are met and/or provided. Installation of any moving or motorised industrial equipment should only be carried out by qualified persons familiar with electrical and machine safety requirements.

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