Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!uunet!munnari.oz.au!uhccux!hale!whinery
From: whinery@hale.ifa.hawaii.edu (Alan Whinery)
Newsgroups: sci.electronics
Subject: Re: How can you 'track' something back and forth?
Keywords: alpha damping servo loop
Message-ID: <11206@uhccux.uhcc.Hawaii.Edu>
Date: 30 Jan 91 18:55:56 GMT
References: <304@fxrs.intel.com> <418@adpplz.UUCP>
Sender: news@uhccux.uhcc.Hawaii.Edu
Organization: Institute For Astronomy, Hawaii
Lines: 57

In <304@fxrs.intel.com> jmasters@fxrs.intel.com (Justin Masters) writes:
>
>What kind of motor do you try to use (12v or less) to turn towards the
>originating signal, and how do you control it?  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?

(Oh goodie -- all those servo loops and stabilized platforms I did in 
my LAST job might be useful.)

In a servo system, such as the loop you describe, the problem of "jitter"
or "popping" back and forth across the line where you want to be -- due
to the mechanical inertia of the motor and the electronic inertia of the
servo loop -- is kind of the whole problem. Once you get an acceptable
solution, you're there. 

First of all there's the loop gain, or how hard you drive the motor
towards target (assuming you know which way to go). If your target
(assuming only one axis of rotation for simplicity's sake) is
30 degrees away from your sensor, and you turn the motor full on, 
you will get there quickly, but probably overshoot. If your target 
is at 1 degree away, you certainly wouldn't want to go "balls to the 
wall" towards the target. There are two methods to use in slewing
(moving) the sensor at a proper rate to reach the target with minimum
jitter at the end -- damping and ramping. Damping is simply good ol' 
servo loop theory, the alpha factor which determines how quickly the 
loop responds to a impluse change in feedback, or how perky/sluggish the 
loop's reaction is.  You can simply limit the maximum slew rate, or 
add a capacitve element to the drive voltage on the motor to damp the
motor's response, which will of course slow down the speed with which 
you acquire the target, but that will be the tradeoff -- loop response 
for jitter. 
	In "ramping", you simply drive the motor with a ramping voltage,
say, ramp up in the first 5 degrees (to avoid a jerk at the start),
and when you are within 5-10 degrees of the target, ramp down the speed,
minimizing the overshoot -- then drive back at a lower speed, until your 
target is aquired. It would be good to have as wide an "acquisition beam"
as possible, such as 5-10 degrees, so that there is a hysteresis effect,
allowing the target to wander slightly without your loop popping on and  
off constantly. You may need 3 sensors, the "acquired" sensor in the middle,
and leading and lagging sensors on the right and left, so that while doing
target acquistion the leading sensor can say "OK HERE IT COMES, SLOW DOWN",
and after acquisition, when the target is lost, the appropriate sensor 
will tell you which way the target went.

	Servoing around is a business of error management, not of 
error avoidance. You also might try the "rotating radar dish" effect, 
and just keep track of the position of the target in memory. 

Stepper motors would give you more precise control of overshoot and 
positioning, but they're basically just motors in the paradigm of the
servo loop.

Wake up, I'm done talking.
Alan
whinery@hale.ifa.hawaii.edu