Newsgroups: comp.robotics
Path: brunix!cat.cis.Brown.EDU!agate!overload.lbl.gov!ames!enews.sgi.com!decwrl!netcomsv!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: hobby servo motors
Message-ID: <nagleCu0r7I.Kz7@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <Ctyv0J.BKI@hpl.hp.com> <MAFM.94Aug4133451@parma.cs.uwa.edu.au>
Date: Thu, 4 Aug 1994 16:30:50 GMT
Lines: 39

mafm@cs.uwa.edu.au (Matthew McDonald) writes:
>    What sort of signals are required to drive the servo motors used in model
>   cars and planes?  

>	Chuck McManis has already posted an explanation of which wires
>are which and what the signal should be.

>It might be worth adding a couple of things.
>	(1) Although the on-time of the pulse determines the angle the
>	    servo goes to, the off-time isn't critical at all -
>	    provided it's constant.

	The off-time doesn't even have to be constant, and it isn't
	constant in the usual R/C application, where the full-cycle
	time is the sum of the ON times of all the servos plus a
	delay at the end of the cycle.

	If the off-time is long enough, the servo times out and
	turns off, becoming "limp" and no longer drawing motor current.

>	(2) If you're driving servos from software, note that since
>	    the on-time is never more than 2.5ms, and the off-time is
>	    at least 17.5ms, your code can do a lot during the off-time,
>	    there's no need to sit in a timing loop during the
>	    off-time.

	Even on a 68HC11 in FORTH you can run several of these things
	using interrupts and timers, without using a big fraction of the 
	CPU.

>	(3) If you plan to drive servos from the same power supply as
>	    a computer, it's probably worth putting a diode and a
>	    500MFD 12V capacitor between the +ve and ground lines.

	Yes.  The V+ currents drawn when you command a big servo change
	are large.  But the control line of most servos can be driven
	directly from a CMOS or TTL output, and isn't that noisy.

					John Nagle
