Sean Breheny wrote:
> Hi Ariel,
> 
> It IS actually PWM, just PWM where the maximum "on" time is not 100%
> of the period. The frequency is usually around 50Hz, and the "on" time
> varies between 1 and 2 milliseconds (which would be 5 and 10% duty
> cycle).
> 
> The signal received by the receiver is PPM encoded, but the receiver
> converts this into PWM for each servo. Note that unlike regular PWM,
> this is not used to directly drive a motor but rather to send a
> command.

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, whose direction is set by which edge fell first.
The result is pulses to the motor (at the frame rate) that are portional
to the position error.

I know this because when we were developing C-Leg (prosthetic knee) I
raised the pulse rep rate to 300 Hz to get a faster response from the
R/C servo we were using to position the hydraulic valve controling the knee.

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

See figure 2 for a schematic of a decoder.
http://www.seattlerobotics.org/encoder/200009/Servos.html

Robert

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