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

Phil Karn karn at philkarn.net
Thu Mar 7 22:35:15 PST 2013

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 

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.


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