[amsat-bb] Re: inquiry about homebrew az-el systems
karn at philkarn.net
Fri Mar 8 09:46:53 PST 2013
On 03/08/2013 12:08 AM, Gus wrote:
> Still, it sure sounds interesting! What do you think it would cost to
> put one together?
Dunno. I'd have to build one. It would run from a DC supply, because the
inverter would convert that to AC at the necessary voltage and
frequency. Because the rotor motors take a nominal 24V AC the DC supply
would have to be higher. A 24V RMS sine wave has a peak-to-peak
amplitude of about 68V, so that's the DC rail voltage needed if the
rotors have to be driven in a single-ended fashion. This is the case for
the rotors I've seen as they usually have three wires, one being a
common to both windings. If each winding had separate wires, you could
drive each one with an H-bridge and use a 34V DC supply. Either way, a
DC-DC converter could still be used to power the system from 12V or
Its major advantage is in continuously tracking at a low and variable
speed without constant starting and stopping. But this design could
easily go faster than the nominal 50 or 60 Hz speed if the DC rail
voltage is increased proportionately. (The voltage and frequency in a
VVVF AC motor drive are varied together to push constant current through
the inductive reactance of the motor windings and provide uniform torque
at all speeds.) But I wouldn't say *how* fast until I built and tested it.
Also dunno why you would need two IMUs. A single one on the antenna
itself would tell you which way "down" is and which way "north" was.
Platform acceleration (not mere motion) might be a problem but I'd have
to think about how to compensate for it. Other than that, the only thing
I'm concerned about is RFI from the transmitter getting into the sensor.
You could simply not read it when transmitting.
I've also been thinking of using one of these IMU sensors for automating
the setup of my Meade LX-200 telescope -- which I also want to use for
satellite tracking. Not having to center that damn bubble level and find
north would be nice. The IMU would probably be good enough to bring a
pair of bright alignment stars into view so I can fine-tune the
orientation. (I haven't checked but I wouldn't be surprised if scopes
are already available that do all this. But I want to see if I can use
my existing scope.)
A GPS will still be almost mandatory for both satellite antennas and
telescopes for accurate time and location. This is needed not only for
the pointing calculations but also to look up magnetic declination and
inclination to interpret the magnetometer data. Then the magnetometer
and accelerometer together give you a 3-axis orientation in space
without calibration, assuming you don't have anything nearby to distort
the earth's magnetic field.
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