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From: hibbett@prcs3.decnet.philips.be
Newsgroups: sci.virtual-worlds
Subject: RE: sci.virtual-worlds
Message-ID: <1991May21.220642.23708@milton.u.washington.edu>
Date: 21 May 91 09:41:15 GMT
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Organization: University of Washington
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>From: lance@motcsd.csd.mot.com (lance.norskog)

>As for tracking, this Polhemus stuff is for the birds.  It should
>be easy to put a few ultrasonic or infrared transmitters in the
>corners of the room, and place 2 receivers each on your head and hands.

I dont know about infrared, but I have developed a simple system for detecting
location and orientation of two objects (ie, head & hand) using ultra-sonics.
It consists of 3 transmitters (in my case, fixed to a wall) and three receivers
per object to give x-y-z and orientation. You are welcome to the circuits 
diagrams; They are very simple to build. Contact me if you'r interested.

At the moment the system comprises of a master clock which provides the 40KHz
signal to drive the  transmitters, and a synchronous 5ms pulse every 30ms to 
enable each transmitter in turn. When a transmitter is enabled, 6 counters are 
enabled (1 for each receiver) running at 500KHz. Each counter stops when the 
coresponding receiver get the signal. When all 6 counters have stopped, the 
control system (a microcontroller) is informed that new time-of-flight values 
are available. This is then repeated for the other two transmitters.

This information allows the control system to calculate x-y-z values for each 
receiver, and to determine the orientation of the object (The 3 receivers must, 
of course (IMHO), form a rightangled triangle). 

Reaching for a calculator, one finds that this system takes 30ms x 3 for each 
set of x-y-z values; This allows for only 11 samples to be made each second.

Now, due to the inevitable jitter on the signal, it would be nice to take n 
samples and produce an average. If we took 5 samples per update, we will only 
be able to generate 2 updates every second. Not bad, but not good.

Lance mentioned multi-frequency transducers. Does anyone know if alternatives 
to 40KHz transducers are available? 

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Mike Hibbett  Software engineer   |   Philips Radio Communication Systems Ltd
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