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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