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Thread: PWM? Servo Pulses? PPM?

  1. #1
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    PWM? Servo Pulses? PPM?

    Following on from another post of mine (Post and a later post), im still working on my project.

    I've been researching various methods/products to deliver a PPM signal.
    I know there is lots of code in Arduino to do this, but it just doesn't quite fit what im trying to do. Anyway, here are my questions:

    1. I'm looking at these servo drivers from pololu. Now when it says it can drive a servo, does that mean it's outputting a PWM signal from each pin?
      1. If not, what's the difference between the signal this is outputting versus the signal an RC receiver is outputting from its pins?
        1. To convert from servo pulse to PWM, can I use this method?
        2. If not, can it be done?
        3. If yes, is there a better way?
      2. If yes, then why in it's documentation does it say that only one pin can output a PWM signal to drive an LED or motor or whatever?
      3. So can I connect 8 of the PWM/servo pins to this PWM to PPM converter to connect to a transmitter module?
    2. Is there anything else I could look at as an "off the shelf item" that might be able produce some PWM or PPM signal? I know Adafruit have some PWM/servo drivers, but they don't have a nice GUI for it.

  2. #2
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    1- Yes, standard 1000 to 2000usec servo pulse with 1500usec center.
    a. This is the same as a Standard RX's servo output pulses.
    a. In the RC world the pulses used to control servos are incorrectly called PWM (pulse width modulation). The correct engineering term for these servo control pulses is PPM (Pulse Position Modulation).
    Further, in the RC world what they call PPM is really a steam on PPM pulses that sequences each of the RC channels with a gap to indicate the beginning the channels. This is how the TX sends the stick data to the RX.
    That device is to control a DC brushed motor from a standard Servo pulse (the terms they use in the description is actually correct).

    In the RC world there are a few different PPM protocols, CPPM is one.

    What exactly are you trying to do?
    Control the speed of DC Brushed motors from an RC Radio (TX/RX)?
    Or just control RC Servos (Note that i didn't say "Servo" but "RC servo")??

  3. #3
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    Thanks for the reply waltr1

    I want to:
    Combine 8 RC-PWM pulses into a single CPPM stream
    Send that stream to my aircraft
    Use an RC transmitter to send the RC-PWM pulses to control my autopilot.

    Im starting to wonder if I might be able to program the Maestro to output a CPPM stream?
    This might be my task for today. If I can, then that will be awesome.

  4. #4
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    I am not sure what a Maestro is. But if its a micro-controller/processor then this is the way to go.
    Which RC TX are you going to use? Some use a PPM/CPPM steam from the control board to the RF module so maybe best to dig into the exacte protocol it uses then write your code to do this. Some also have the option of injecting a PPM/CPPM stream into the Trainer port.

    This way you can use a standard RC RX that matches the TX which will then output standard Servo PWM pulses.
    You did look up the protocol for CPPM?

  5. #5
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    Link for maestro

    I'll likely be using a frsky or a DTF-UHF module only, I won't be using the whole hand unit. I can connect +ve, gnd, CPPM to any of these and it will transmit to their corresponding receiver.

    My idea with the Maestros is to have them output 8 RC PWM signals, then combine them into a CPPM with the converter I mentioned above (LINK). Now I have started thinking about trying to code the Maestro to output a CPPM pulse, but it only has a pulse range of 64μs to 4080μs. For a CPPM pulse I would need it to go down to 1μs.
    Click image for larger version. 

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    Obviously the frame length will need to change for the different brand.

  6. #6
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    Oh wait. A CPPM stream needs 1ms (milliseconds)?
    The maestro can go as low as 67μs (0.067milliseconds), with a resolution of 0.25μs (0.00025milliseconds)

  7. #7
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    If I understand this correctly here, for a CPPM stream I need a pulse of width 300μs to define the channel. The time between channels is the value of the channel.
    Then I need a larger gap (of 10.5ms) to signal the start of the next stream.

    This shouldn't be too hard to code.

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