On May 6, 2007, at 8:25 PM, Robert Rolf wrote:

> Actually, in most early (non digital) R/C setups the pulse
> DOES drive the motors directly. The leading edge triggers
> a monostable whose period is set by the current servo position.
> an exclusive-or of the incoming and position pulses go to an
> H bridge...

That's what you call "directly" ?!  I think the poster
meant that PWM of the motor power (traditional for some speed
control sorts of things) is not driven off of the "servo output"
PWM channels...

>
> The receiver demultiplexes the PWM (not PPM) train using a simple 
> counter and monostable to detect end of frame

I see AM, FM, and "PCM" radios advertised, so it's not entirely
obvious that they all encode the bitstream the same way...
The output of the RADIO is some sort of bitstream that includes
the position of each of (12? many?) possible servos and some
synchronization scheme that allows you to tell which bits go
with which servo.  This goes into a decoder of some kind (these
days frequently a microcontroller, though SSI logic of the sort
described here used to be popular) that demultiplexes the bit
stream and provides a signal to each of the servos supported

Radio output:  SSSS___XX___XXX__X____XXXX______
Servo 1 out:   _______XX_______________________
Servo 2 out:   ____________XXX_________________
Servo 3 out:   _________________X______________
Servo 4 out:   ______________________XXXX______

HOWEVER, My impression was that the original poster was more
interested in the problem: "given a servo pulse of 1 to 2ms
as input, is there a standard way to drive a stepper that
maintains "current position" and needs as output "direction"
and "number of steps.""  My guess is "no", cause it's not
too complicated except for taking into account the variables
that exist in range of servo travel and degrees per step for
the stepper motors.  (It seems an unfortunate side effect of
electronics economics that relatively beefy stepper motors
are cheaper, more standardized, and easier to find for hobbyists
than geared motors.  I'm not sure you're accurately estimating
the relative torque of big steppers and smaller servos, though.)

BillW

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