Alternatives to IR array or PIR for 360 degree sensing

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I need to sense proximity 360 degrees around something at a range of 0 to 100cm (around 3 feet). I would like to know how close someone is, but will settle for binary input if necessary. This is for an outdoor sculpture and thus overhead video tracking solutions are not an option. I have 9 of these things so cost is an issue.

An array of typical Sharp IR sensors is the standard solution, but you need 12 of them to have enough overlap to ensure no blind spots. 12 x $14 = $168 per unit. Too expensive, plus you need a Mega or multiplexer for extra analog input pins.

Two wide-angle PIR sensors (http://www.parallax.com/product/28032) are cheap $12 x 2 = $24 per unit. But PIR has a fatal flaw: it switches off if somebody is still in range, but has stopped moving.

I have also tried the horribly documented AniBat ultrasonic sensors (http://www.robotshop.com/en/hagiosonic- … 40dai.html), they are supposed to see 180 degrees AND give analog distance feedback. Sounds ideal. $26 x 2 = $52 per unit. But they produce terribly unstable feedback and at least in my tests, are not usable.

Anyone have ideas for alternative methods?

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eforman:
But PIR has a fatal flaw: it switches off if somebody is still in range, but has stopped moving.

Move the PIR...
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Is this to go into the sculpture or be part of some base under it or ??? Is there some size and power requirement ? The general idea is that when someone gets < 100 cm away from the sculpture, he/she is detected and the sculpture does something vs a burglar alarm ?

Instead of an array of Sharp sensors (or sonar/ultrasonic), could you put one on a slowly spinning base and scan for people (to follow ski’s idea above) ? Which leads to another question, how quickly must the person be sensed and the action (or whatever) happen ?