Plans for the Desk

EDIT AGAIN: Scratch the whole array idea entirely. I just found a method at NUI group that uses photoresistors (i.e. fast scanning), but only requires them along the edge. Even though they are on the edge there are no occlusion problems as one might expect because of some magic with polarizing film. Will post updates soon.

EDIT: Well it seems that the rise time on photoresistors is generally about 60 ms. This is a hit on the response time of the screen. Basically what that means is no matter how fast we sample the matrix, true response is limited to about 16 fps. So the goal is to either find a photoresistor which is cheap and has a rise time of ~10ms -> 100fps, or else think of a new sensing system.

First of all, we (or maybe just I) plan on finishing the first desk by the end of Christmas break, but I have some ideas for the next version of the desk that I'll talk about now. The major improvement I want to make is to the size. The first version of the desk is about a foot and a half deep because of the distance requirement of the camera from the screen. So the first step in making a thin desk is eliminating the camera from the design.

Obviously, we need some sort of visual sensor to replace the camera, so the option we will try is to make a large array of analog light sensors (photocell network) underneath the screen. A micro-controller will then read the value of each sensor sequentially and use an algorithm to deduce the location of blobs. The circuit underneath the screen will look something like this. To select a column, the MCU puts a voltage on one of the top pins, and a high impedance on the others. Then, to read a specific sensor, the MCU puts one of the row outputs through an analog-to-digital converter (using a multiplexer in between).


So far it seems that diodes are required on every photocell in order to prevent current from flowing through the other resistors. I simulated the circuit and it seems that the back-flow was somewhat negligible, especially in larger matrixes, but diodes are cheap, and precision is good, so I will keep them.

The other concern is the cost and difficulty of construction. For the first version of the desk I want to use a 32x24 matrix of photocells, so there are 768 cells that need to be purchased and soldered. So far the cheapest I have been able to find are $0.32 for large volume orders. I would be very glad to find one closer to $0.20, bringing the cost for all resistors down to $150. The diodes and resistors needed don't total to be more than $10-$15, so they aren't a problem. I was thinking I might be able to use my lab's rapid prototyping machine to fabricate the PCB and solder all of the components onto it.

No comments:

Post a Comment