PIR, Ping, or camera for motion tracking?

Using Arduino Project Guidance
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Hello,
I'm, I hate to say it, a "newb" to the world of Arduino and programming and am looking for some guidance.
I'm a sculptor (and art teacher) and I love that my artwork has me constantly learning new things but I'm a bit over my head on this one, http://www.studio407.net/irobot.htm

I've included photos of the sculpture so far (not sure the order in which they will appear, sorry). The painting is my concept drawing of the finished sculpture, complete with cell phone spine. The photos are the finished cast (it's been polished since this pic) and the cell phones in their vertebra holders. And finally the sketch (not arduino sketch, lol) is one of the issues I will ask about.
This is the 5th installment in my series "The Classics" temporarily titled "iRobot" and I would very much like the head/tablet/face to track folks as they move around the sculpture.
The head-neck joint is a high quality turntable lazy susan bearing (one ring rotates inside the other, larger ring, not the usual one on top of the other), the pedestal (not pictured) will be a simple, although custom, rectangular form, about 22x22x48 inches and will house the electronics. And now, because the sculpture hasn't been challenging enough already (sarcasm), I want the torso to also rotate on a turntable, 360 degrees (head rotation about 160 degrees). So in theory, as a patron enters the gallery, the torso would move to face their direction and once the patron is x number of feet from the artwork, the face would also turn to face directly at them. The face-tablet will display all sorts of things including short clips of human faces which can be seen, although distorted, through the transparent face.

I subscribed to an online Arduino course (no live instructor) to learn the programming, I've been researching options and watching videos for weeks, and reading until my eyes can no longer focus, haha. I've even written a sketch that passed the compiling stage (not a bid deal but I was thrilled). But I feel more confused (although more informed) than when I started.

My questions:

  1. It would be great if the Android tablet behind the face could be the sensor that tracks the viewer; would it be possible to hack the camera and use a facial identification software such as openCV without needing a full time laptop inside the pedestal? If it would work, I'd be willing to work out the laptop issue.

  2. Assuming #1 won't work or is too far out of my league, I will need two sensor arrays; one for the torso to point towards the viewers and one for the face to track said viewers. I started designing a PIR array but was told that I need the ranging capabilities of Ping sensors, otherwise the sculpture might shake it'self apart if more than one person is viewing the artwork. Can a PIR sensor be set to stay high as long as there is activity in front of it and ignore other high sensors? Are Ping sensors a better choice?

  3. Approximately how close together can I place the sensors, I would like the smoothest operation possible. The HC SR-04s I purchased have a cone of 30 degrees (I think); does than mean I can only place one per 30 degrees of arc? If closer, will they interfere with each other? Same question for the Ping sensors.

  4. Assuming two sensor arrays are used, one for the torso, one for the face, and assuming they are spaced one over the other with maybe a few inches between, will the two arrays interfere with each other? Would a foot or two space be better?

  5. This is a big one: referring to the color sketch of the torso from above; If I could center a large servo motor in the neck it would greatly simplify things but as you can see I have a spine running up the center. Can a stepper motor be programmed to complete x number of revolutions for sensor A, x number from A to D, x number back to sensor B, and so forth? I don't have the torso/neck gearing worked out yet so I can't be specific.

If you made it this far, many thanks. I can't go forward on this artwork until I work some of this out. All opinions are welcome. I have about a year's work in the head, torso, spine and would rather it not shake apart at this point.

Again, many thanks for reading and feel free to ask questions. I will answer them the best I can.
-Mac

cast.jpg
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concept drawing.jpg
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mechanism.jpg
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spine.jpg

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That's a tough one. The image processing required to "follow the red ball" with a camera is pretty well-defined these days. But "follow a random person among other people and don't switch between people too often" isn't well-defined. If you're following them by looking for the brightest face, then what happens when that person turns around? That might be the effect you want but then someone turns up with a bright hat and it wants to look at the back of their hat instead of anyone else.

If you can somehow limit the path of people accessing the artwork? Like have them always walk past left-to-right and then the tracker follows the leftmost person? That way it will appear to respond to new people entering the room and everyone thinks that it did actually look at them when they entered.

Most of the ideas for this are beyond the processing power of the Arduino. You would probably get better answers if you asked the same question on a Rasberry Pi forum or an Android developers forum.

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Maybe it's a very interesting hat?
If it's beyond Arduino then it's beyond me. Maybe it's time to simplify. I didn't realize when I first saw the angry bird head following a guy around his room or the tracking webcam security gizmo (both PIR based), there was always just one person in the vicinity.
I'll keep reading. Thank you for the reply.

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The chances are pretty good that not everyone in the gallery will be at the same distance from the sculpture. Using Ultrasonic (Ping) sensors, I could possibly set the sensors to track the closest object/person, or at least limit the range of influence, say, three feet for the head, maybe four for the torso. Or would I want them the same? I'll have to think about that one. If I place the Ping sensors around the neck (for the head movement), they might not interfere with the sensors in the pedestal (for the torso movement). It might look a little.....uhh......punk?

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Ultrasonic sensors can be triggered in sequence so they don't interfere with each other, but that may slow down the acquisition rate.

You might also try some form of sensitive (pressure resistive or capacitive) carpet in front or under the sculpture itself.

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