Hidden sensors to use for small grid

Question

I'm a programmer, working on designing a project, but I have very little experience with sensors other than RFID and radar technology. After doing my fair share of research, I've found myself stumped, I can't figure out which type of sensor I'm looking for. I apologize if I'm using incorrect terminology, this is not my field of expertise.

About the project: I'm building a 1.5 foot x 1.5 foot grid system. A 324 square inch grid, if you will. I'd be looking to have an accuracy of location down to the inch, so I'm thinking I'd probably need 324 sensors spaced out evenly. The sensors will be encased in no more than a half inch of painted plastic, some under rubber, and/or some under artificial grass material laid out on a flat plain. The sensors would ideally be no more than a half-inch tall and a half-inch wide. I'd like the sensors to either detect a small electronic signal or a quarter sized piece of metal, for the lack of a better word, I will call this the target item. The target item will be passing right over the material holding the sensors or possibly a quarter-inch to half-inch above. The sensor in which the object is closest too would ideally be able to output the highest reading or some indication that the target item is closest to it than any other sensor. I'm also worried about interference of the sensors, say if a phone was sitting next to the grid system.

Any thoughts or help on the sensors and ideal target items for this would be greatly appreciated. Thank you.

Answer 1

For any type of object, proximity sensors are your best bet I think. Like a bunch of LEDs, likely IR though may not be eye safe with so many just to provide light to reflect and then a phototransistor grid that actually picks up the light reflected off anything near enough. Then you would need to ideally get visible blind phototransistors (opaque black packaging) so they ignore ambient light and perhaps modulation to filter out ambient IR noise if there is any.

If you can magnetizing the object then you can use a grid of hall-switches instead which is cheaper and easier, not by much though since they would be wire up a lot like phototransistors except you do not need the LEDs for illumatinion. If you use hall effect sensors then you could get the position more accurately via interpolation since they will have continuous outputs rather than just binary outputs.

If it's just plain steel then you have to start getting into induction stuff where you induce stuff in the steel to pick it up and it is probably not worth it.