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

Gus 8p6sm at anjo.com
Fri Mar 8 00:08:22 PST 2013


I'm glad you find the idea of interest, because I'm sure you could 
greatly contribute towards the idea.

Please note that we haven't simply been thinking of designing a better 
5400/5500.  We've also been thinking about a design  that could be used 
in the field and after a disaster/in an emergency. And a design that 
could be replicated in countries around the globe.  Hence 12v automotive 
motors and bicycle sprockets were all part of the brainstormed recipe!

Your 9dof IMU idea is sexy!  Just think -- with TWO of them, you could 
compensate for the motion of the station, when operating from, say, a 
boat or a vehicle under way.  (Nobody say "GPS" please!)  One RTC chip, 
a USB interface to the laptop or bluetooth interface to the Android 
tablet...  But it sounds less and less like you will be able to source 
much of it in the "third world."  Which is where I happen to live.

Still, it sure sounds interesting!  What do you think it would cost to 
put one together?

On 03/08/2013 02:35 AM, Phil Karn wrote:
> Just noticed this thread and caught up.
> While rotor controllers are indeed a dime a dozen, I think we could do 
> a lot better than any of them.
> Your typical Yaesu/Kenpro rotor uses a 24V AC 2-phase induction motor. 
> The control box applies 50/60 Hz AC directly to one winding and to the 
> other through a capacitor. The capacitor creates a phase shift in the 
> current through the second winding, creating a rotating magnetic field 
> within the motor that drags the rotor in one direction or the other. 
> You reverse the motor by applying AC directly to one winding or the 
> other.
> Although this design is extremely common, it has several highly 
> non-ideal features. First, the current through the second winding 
> isn't actually in phase quadrature (90 degrees) with the first. It's 
> somewhat less due to the series resistances of the winding and capacitor.
> Second, the current amplitudes in the two windings are not the same, 
> and for the same reason -- series resistances. This means less torque 
> and more motor heating than could otherwise be produced for the same 
> input voltage.
> Third, the motor has only one synchronous speed: 50 or 60 Hz. Stalled 
> rotor torque is rather low, especially for a non-ideal supply.
> What you *really* want is a variable frequency, variable voltage 
> (VFVV) inverter producing two phases in exact quadrature (same 
> amplitude, 90 degrees with respect to each other). You can smoothly 
> vary the speed from a dead stop to faster than 60 Hz and with more 
> torque at every speed, making it easy to track a continuously moving 
> satellite with a narrow antenna. And you don't wear out the brakes and 
> constantly flex the masts and booms until the clamps all work loose.
> You can even use the motors as brakes by sending a small amount of DC 
> current through them. It doesn't take much, as this essentially 
> creates a DC generator with a shorted output, and that torque is 
> amplified by the gear train.
> The necessary waveforms could be generated with the PWM channels in an 
> Arduino or similar microcontroller and amplified with the power MOSFET 
> H-bridges common in robotics.
> I do see several rotors using DC motors, plus several people 
> suggesting them here. While they're somewhat easier to vary in speed 
> (you just vary the average DC voltage with a PWM drive) you have to 
> remember these motors contain brushes rubbing on commutators, and that 
> makes them far less reliable than AC induction motors, which are 
> famously simple, rugged and reliable. There's a reason AC motors are 
> universal in the modern generation of hybrid and battery electric 
> vehicles even though most hobby conversions still use DC motors.
> As for position feedback, what about one of the cheap, modern IMU 
> devices, like the Pololu MinIMU-9. I've been playing with this 
> particular board, which contains a 3-axis accelerometer, magnetometer 
> and rotational gyro. Just mount one on the antenna boom and directly 
> measure the antenna position. The accelerometer will give elevation 
> without any calibration at all. The magnetometer can read azimuth with 
> a lookup table for your local magnetic declination, and any local 
> magnetic distortions could be removed with a one-time calibration. And 
> the gyro will quickly tell you if the antenna is out of balance or has 
> stalled.
> --Phil
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73, de Gus 8P6SM
Barbados, the easternmost isle.

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