Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!olivea!orc!inews!fxrs!jmasters
From: jmasters@fxrs.intel.com (Justin Masters)
Newsgroups: sci.electronics
Subject: Re: How can you 'track' something back and forth?
Message-ID: <310@fxrs.intel.com>
Date: 28 Jan 91 19:16:25 GMT
References: <304@fxrs.intel.com> <1991Jan27.023401.22157@zoo.toronto.edu>
Reply-To: jmasters@fxrs.intel.com (Justin Masters)
Organization: Intel FMD, Folsom, CA
Lines: 64

In article <1991Jan27.023401.22157@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
|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


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