[amsat-bb] Re: inquiry about homebrew az-el systems

Phil Karn 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.

--Phil, KA9Q

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