I was just thinking about the state of the sea bed, it is not always going to be flat sand and therefore the ROV may not be so close to the bottom. In which case you may need more sensitivity than you can get with a 'detector on a stick' type system and have to move to a 'twin box' approach which would require two coils for a PI system separated by at least one metre. That should give you a detector depth of about three feet for a coke can sized object, five feet for a gallon can, seven feet for a two gallon can and up to 20 feet for a car. On the negative side it will be insensitive to small objects such as a single coin or a ring pull. It does have the advantage that the metal mass of the ROV between the coils is not going to screw up operation.
On a practical note, ROV's cannot stop instantly, so unless the target is really large, it will be detected and past before the human operator can analyse the signal. For that reason I think I would include an SD card data logger so that target acquisition can be 'played back'. I might also do some experiments to see what colour LED's show up best against different sea beds .... with the thought that one could fit the ROV with a blinking LED marker dispenser. The ROV could then automatically mark the target on the sea bed ..... on the fly.
I am thinking that the LED could be made to blink at 5 kHz. That would allow electronic acquisition of the marker under water and it's direction. Sounds like an interesting project !.
jk
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