Re: Choosing RC Servo Controller

John Mianowski wrote:

On Jul 16, 9:41 am, Frank Buss <f...@xxxxxxxxxxxxx> wrote:

http://de.wikipedia.org/wiki/Pulsphasenmodulation

It says that the pulse width is constant, which is not the case for the
animation in the page I referenced. For me it looks like PWM, but the
length of the full cycle is not important, only the length of the high
impulse, so it is not the usual PWM, where the duty cycle is important.

They're talking about Pulse PHASE Modulation, not Pulse WIDTH
Modulation or Pulse POSITION Modulation. Alsok known as Phase
Modulation.

This is a bit confusing. The German word for pulse-position modulation is
Pulsphasenmodulation.

The entry notes that this modulation technique is rarely
used in the field, which isn't quite right; PPM is widely used in R/C
systems & has been for about 50 years. Phase Modulation, though,
isn't widely used. Here's the PM entry in English:

http://en.wikipedia.org/wiki/Phase_modulation

This looks like an analog modulation concept, but I think it would be the
same like pulse-position modulation, if you use square waves, because then
the phase is simply the position. And after some googling I think you are
right, that pulse-position modulation is used in R/C systems. But looks
like it is interpreted a bit different, e.g. on this page:

http://www.mftech.de/ppm_en.htm

you can see high pulses of different lenghts, but it is really just an
inverted pulse-position modulation, with different positions of low
impulses of the same length :-)

PWM is typically used WITHIN a hobby servo, by the
controller to drive the motor, while the input TO the hobby servo is
PPM.

You mean PPM from this page?
http://en.wikipedia.org/wiki/Pulse-position_modulation
I think this is nearly correct: The page says, that multiple channels can
be transfered and this page:
http://www.mp.ttu.ee/risto/rc/electronics/radio/signal.htm
shows the scope diagrams for the output of the NE5044 where you can see how
it looks like.

But the signal of each single channel is a PWM signal and fed into the
control input of the servo motor. This is the reason for the strange
definition of about 20 ms cycle time and 1 ms to 2 ms for controlling the
angle of a servo motor: I think it is very simple to decode a PPM signal
and fed the different PWM signals to the servo motors, without any
postprocessing.

Last but not least, my PIC program, which generates a PWM signal, should
work with a servo motor and should do what the OP needed, if the
description of the pulse length and pause between pulses is right.

--
Frank Buss, fb@xxxxxxxxxxxxx
http://www.frank-buss.de, http://www.it4-systems.de
.

Relevant Pages

  • Re: Dick Pierce is WRONG.
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  • Re: cell phone jammer
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  • Re: Simple mosfet question
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