From: cmcmanis@Sun.COM (Chuck McManis)
Newsgroups: comp.robotics
Subject: Re: hobby servo motors
Date: 3 Aug 1994 18:59:00 GMT
------------------------------

Rick Baer 7505 (baer@hplwave.hpl.hp.com) wrote:
:  What sort of signals are required to drive the servo motors used in model
: cars and planes?  I have one that specifies a 4.8 volt power supply and
: positive pulse control (1520 us neutral).  Three leads (red, black, white)
: are attached to the motor.  I had expected to find something like 5 leads
: (3 for the potentiometer and 2 for the motor).  Thanks in advance for your
: help! 

Red = V+
Black = Gnd
White = Control Signal

The control signal is a pulse whose width varies from approximately 500uS
to 2500us, with 1520 us in your case being "dead center". The pulse should
be repeated once every 20 mS or so. (note the units change)

Another thing to note is that different servos have different ranges of
travel so all you "know" is the center pulse width, you can then decrease
the width until the current used by the servo jumps up (this defines
one end) and then widen the pulse until the same thing happens (this defines
the other extreme) Now using those numbers and the center number you can
calculate the range of pulse widths that will move the servo through its
entire length.

Since the relationship is nearly linear (based on the linearity of the pot)
you can normalize the value using the formula:
		(center - lower_bound) + (upper_bound - center)
	C =     ----------------------------------------------
				       2

Width = position * C + center

Where position is -1 to 1.

The good thing is that most are centered right around 1.5mS. 

--
--Chuck McManis			     All opinions in this message/article are
FirstPerson Inc.                     those of the author, who may or may not
Internet: cmcmanis@firstperson.COM   be who you think it is.