Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: How can you 'track' something back and forth?
Message-ID: <1991Jan27.023401.22157@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <304@fxrs.intel.com>
Date: Sun, 27 Jan 1991 02:34:01 GMT

In article <304@fxrs.intel.com> jmasters@fxrs.intel.com (Justin Masters) writes:
>I was wondering how one might go about tracking something back and forth...
>IR.  Utilizing a diamond grid of two receptive elements (recessed, or shielded
>from stray IR energy), I would use some kind logic to simulate the following
>way a human centers his eyes on a light...

Barring expensive things like imaging sensors, you're going to have to move
the sensor somehow.  One slightly eccentric way to do this is the way used
in satellite sun sensors.  You use one sensor behind two intersecting slits
(usually forming a V shape together), and rotate the whole assembly on the
vertical axis ("vertical" with the V standing upright, so to speak).  The
light source is visible to the sensor twice during a rotation, once through
each slit, so you get two pulses per rotation.  The spacing between the
pulses gives you vertical direction of the light source, while the position
of the pulses with respect to the rotation gives you horizontal direction.

>FM.  Somehow receive a pulsed signal on an FM signal to two different points,
>and combining the signal to each other to see which pulse 'arrived' first.

At the speed of light, you're going to have to measure fractional nanoseconds
to get any useful accuracy with this.  Also, unless you observe certain
somewhat troublesome restrictions, your FM transmitter will need to be
licensed.  This sounds like more trouble than it is worth, if IR is suitable.

>How would you go about doing a comparison to see which receptor/receiver had
>received its signal first or was closer to the point desired?

One way of getting a pointing system with optical sensors is to put a baffle
between the two -- a "nose", to speak -- and just use a differential
amplifier to compare the outputs of the two sensors.  The "nose" will partly
shadow one of the sensors, reducing that sensor's output, unless the whole
assembly is pointing directly at the light source.

Note that there is a fundamental contradiction between being able to acquire
your light source over a wide range of starting angles and being well shielded
against stray light, unless your IR emitter is doing something distinctive
like pulsing at a known frequency.

>What kind of motor do you try to use (12v or less) to turn towards the
>originating signal, and how do you control it?

Almost any kind of motor can be used.  Far more information is needed to
answer this question.  How rapidly does it have to track?  How heavy is
the tracking assembly?  Does it have to track in one axis (horizontal
only) or two?  What other requirements are present?

>How do you keep from
>having constant 'over-compensation'?  In otherwords, how would you keep the
>motor from going back and forth in slight increments while trying to center on
>a stationary point?

Add a bit of hysteresis ("snap action") or a central "dead band", so that
it takes a substantial deviation to make the tracking assembly move.
-- 
If the Space Shuttle was the answer,   | Henry Spencer at U of Toronto Zoology
what was the question?                 |  henry@zoo.toronto.edu   utzoo!henry
