Phil,
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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