You are right, it is easy to use an arduino for that, you do not even
need a shield. DF robots cheapduino would be great for that. You could
use an analog port to listen to the Telemetrum and the PWM port to talk
to the servo.
Spacetec (http://www.spacetecrocketry.com/) is also offering a
commercial solution for the problem.
Which will convert the igniter signal of any altimeter to an servo
signal. It also simulates the behaviour of an igniter to the altimeter.
But that is introducing another electronic, with another power supply,
which can fail.
Best
Thomas
Am 28.05.2013 21:33, schrieb Peter Hackett:
I'm no expert, but if the goal is to "get it done" I would use an
Arduino and a daughter board ("Shield")
that knew how to talk servo. If you have some technical experience,
the Arduino is very easy to
use. You hardly need to understand what "programming C" means. My 15
year old son pick it up
in a few days without any help from me.
Given the above, you'd just need to have a very simple way to
communicate from the TeleMetrum to the Arduino.
E.g. pyro output(?)
Not to say it wouldn't be cool to be able to do everything from the
TeleMetrum/Mega/NextGen.
I'm interested in experimenting with a steerable parachute for rocket
recovery. (Just for the challenge
and fun of it.)
On Sun, May 26, 2013 at 9:55 PM, Bdale Garbee <[email protected]
<mailto:[email protected]>> wrote:
Thomas Müller <[email protected]
<mailto:[email protected]>> writes:
> there is one thing I would really like to see in future versions of
> TeleMetrum or TeleMega. It is the possibility to control a
> servo.
> It is very useful for controlling servo releases for serial dual
deploy
> like spacetecs SRM2. They are pretty common in Europe.
Interesting .. I've never actually seen someone using an RC-style
servo
for deployment before. As you say, there's no conceptual reason we
couldn't do that... but in practice there are some issues I'll address
below.
> In principle you need only a analog/digital output which can be
freely
> controlled.
To drive an RC-style servo, you need to be able to PWM an output pin,
which is easiest to do if that pin is connected to a timer with PWM
support, but of course also possible to do with a GPIO and
software PWM
generation. That part isn't hard. The difficult part is that all the
servos I know of run at 5V, and all of our products are 3.3V using
single-cell LiPo batteries with 3.7V nominal and 4.2V fully
charged. So
at minimum, you'd need a separate battery / regulator to generate
the 5V
that is required, and you'd need a level shifter (transistor) on
the PWM
output to enable it to drive a 5V input.
My son and I are actually working right now on a board to control 12
such servos for a robotic arm project using the same STM32L151 part
that's on TeleMega. I haven't loaded the first prototype boards yet,
but once we do, we'll be generating an RC servo PWM driver for AltOS
that could potentially be used in our rocket firmware on boards like
TeleMega using that chip.
> Is there some output accessible? As far as I understand the
companion
> port it is only a SPI interface.
The SPI clock, miso, and mosi pins on the companion connector are
shared
with other devices on the board, but the chip select line is a unique
GPIO (P1_2) that could be re-tasked with a firmware modification.
> Port 3 and 4 on the debug port seem for me to be regularly unused
> digital IO ports. Can they be used?
Yes. We've avoided using them for anything "active" simply
because it's
nice to be able to do source-level debugging of the firmware without
having to compete with some active function... but there's no hard
reason you couldn't re-task these. They are P2_1 and P2_2 on the
cc1111.
Any of these three pins would require software PWM generating
code, and
the biggest problem with that is that the existing TeleMetrum
build uses
almost all of the available memory, and so anyone wanting to create
custom firmware to drive servos would have to choose something to live
without...
It would certainly be easier to implement this in TeleMega where we're
not yet so resource constrained... but even there some solution to the
5V interface problem needs to be devised.
Thoughts?
Bdale
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