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Re: [Paparazzi-devel] Electrical connections with the Tiny and PPM Enco

From: Chris Gough
Subject: Re: [Paparazzi-devel] Electrical connections with the Tiny and PPM Encoder Board
Date: Wed, 14 Jul 2010 12:31:25 +1000

Hi Ana,

> Another question, what is the PPM Encoder Board function? I don't  have one,
> and after reading about it in the other hardware section and looking for
> something in the devel-list, I'm not sure I have understood.

Let me explain; to make my explanation coherent, I'll start with
background information that you probably already know.

Many RC transmitters send a PPM signal through the air, which is
received by the RC receiver then demultiplexed into multiple PWM
signals (which branch out to each of the actuators in parrallel). The
demultiplexing happens within the receiver, it's a black box with an
antenna (airborn PPM input) at one end and a bunch of PWM outputs at
the other.

In a paparazzi system it is the autopilot that sends the PWM signals
to the various actuators, not the RC receiver (how else could it
control the vehicle?). In manual control mode, the autopilot passes
the input from the receiver directly to the actuators (relatively
unmolested). When the pilot is in the loop (semi-manual mode), the
autopilot takes the receivers input into consideration, but decides
for itself what to send to the actuators. In autonomous mode, the
autopilot basically ignores what the receiver has to say (except for
the channel dedicated to selecting control mode; the pilot is allways
in command, even when she is not in control).

The "normal" way to configure a Paparazzi system (with no PPM encoder
board) is to hack into your RC receiver and intercept the PPM signal
from the appropriate place on the circuit board, attach your own cable
to carry PPM from the receiver. In other words, ignore the bunch of
PWM outputs. The autopilot has a single PPM input, that's where the
other end of the custom cable from the hacked RC receiver goes.

The PPM encoder board is kind of back-the-front receiver. It has
multiple PWM inputs, and a single PPM output. I don't like to hack
into my receivers because I don't trust myself not to break them.
Instead, I have a bunch of PWM cables (servo extension leads with
severed female ends) that plug into the unmodified receiver as though
they were normal servo leads. The other end of these cables (bare
wires where the female ends used to be) are soldered to the
appropriate places on the PPM encoder board. Then PPM encoder board
takes it's multiple PWM inputs and makes a single PPM output
(undemultiplexes? remultiplexes? encodes multiple PWM inputs to a
single PPM output), which then feeds to the autopilot.

If I was using RC equipment that didn't send PPM through the air (for
example, PCM or some 2.4 GHz system), extracting PPM from a
vivisectioned receiver might not even be possible. In this situation,
something like a PPM encoder board is the only option.

I can't make pictures right now, but if you still need them ask me on
the weekend and take some and publish them in the Wiki.

Chris Gough

>It's said it
> plugs into the servo output of a receiver(like another servo?), and then you
> can connect from the Encoder Board to the Tiny? Is it a way of not modifying
> the receiver? Is it powered directly or can it be powered by the BEC?
> Does anyone have a picture showing how the wirings go from the receiver to
> the board and finally the autopilot? Sorry, but I am not very used to this,
> and it's difficult for me to understand it.

On Tue, Jul 13, 2010 at 5:05 PM, Ana García <address@hidden> wrote:
> Hello,
> I've been reading the latest posts. I also have doubts.
> I have thought about connecting the RC receiver in the "RC Receiver
> classix.jpg" way. I mean, with the servos connected to the receiver. This
> way, the servos can be powered by my motor BEC(which is connected to the
> receiver). Then, the connections between the receiver and the autopilot will
> be only PPM_IN, SERV_RST, SERV_CLK. Am I right? Is it a good choice?
> Thanks
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