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CX2SA > SATDIG 08.03.13 21:04l 445 Lines 17213 Bytes #999 (0) @ WW
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To : SATDIG@WW
Today's Topics:
1. Alternate web site for pass predictions (Dobarrows)
2. In 1618... (Bruce)
3. Re: inquiry about homebrew az-el systems (Phil Karn)
4. Re: inquiry about homebrew az-el systems (Gus)
5. Re: We-Wish decay (Nico Janssen)
6. Re: inquiry about homebrew az-el systems (Phil Karn)
7. Re: inquiry about homebrew az-el systems (Gus)
----------------------------------------------------------------------
Message: 1
Date: Thu, 7 Mar 2013 19:13:11 -0500 (EST)
From: Dobarrows <dobarrows@xxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Alternate web site for pass predictions
Message-ID: <8CFE9A8DAA06A9D-B44-A7B0@xxxxxxxxxxxx.xxxxxx.xxx.xxx>
Content-Type: text/plain; charset="us-ascii"
I too found the pass prediction page was missing from the amsat web site. I
did find
www.heavens-above.com provides the ability to predict passes, although it is
a little complicated.
I registered with a userid and password so the site would remember the
location info and
time zone when I returned to heavens-above.
I found that the predictions generated by satscape (the predictor program on
my macbook)
matched heavens-above. Then I found out why I wasn't hearing satellites at
the appropriate
time on 70cm was because something is wrong with my 70cm antenna. Due to
health
issues, I can no longer go up on the roof to do antenna work so I will have
to find
someone to help me fix it.
Dave W8IJ
------------------------------
Message: 2
Date: Thu, 7 Mar 2013 17:02:59 -0800 (PST)
From: Bruce <kk5do@xxxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] In 1618...
Message-ID:
<1362704579.97289.YahooMailRC@xxxxxxxxx.xxxx.xxx.xxxxx.xxx>
Content-Type: text/plain; charset=utf-8
I found this whilest trolling the internet today...
On this day in history....
* 1618 ? German astronomer and mathematician Johannes Kepler discovered the
third law of planetary motion.
?
73...bruce
------------------------------
Message: 3
Date: Thu, 07 Mar 2013 22:35:15 -0800
From: Phil Karn <karn@xxxxxxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: inquiry about homebrew az-el systems
Message-ID: <513986A3.6060805@xxxxxxxx.xxx>
Content-Type: text/plain; charset=UTF-8; format=flowed
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
------------------------------
Message: 4
Date: Fri, 08 Mar 2013 04:08:22 -0400
From: Gus <8p6sm@xxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: inquiry about homebrew az-el systems
Message-ID: <51399C76.4030307@xxxx.xxx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
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
>
>
>
>
>
>
>
>
>
> _______________________________________________
> Sent via AMSAT-BB@xxxxx.xxx. Opinions expressed are those of the author.
> Not an AMSAT-NA member? Join now to support the amateur satellite
> program!
> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
>
>
--
73, de Gus 8P6SM
Barbados, the easternmost isle.
------------------------------
Message: 5
Date: Fri, 08 Mar 2013 15:54:00 +0100
From: Nico Janssen <hamsat@xxxxxx.xx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: We-Wish decay
Message-ID: <5139FB88.4070105@xxxxxx.xx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
As solar activity will likely increase slightly in the coming days,
the decay of We-Wish may already occur on Sunday, March 10 (UTC).
73,
Nico PA0DLO
On 2013-03-05 20:58, Nico Janssen wrote:
> All,
>
> Cubesat We-Wish (38856, 1998-067CS) will soon burn up in the atmosphere.
> Probably the decay will occur on March 13. Of course this depends on how
> solar activity develops.
>
> 73,
> Nico PA0DLO
>
> _______________________________________________
> Sent via AMSAT-BB@xxxxx.xxx. Opinions expressed are those of the author.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
>
>
------------------------------
Message: 6
Date: Fri, 08 Mar 2013 09:46:53 -0800
From: Phil Karn <karn@xxxxxxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: inquiry about homebrew az-el systems
Message-ID: <513A240D.7080202@xxxxxxxx.xxx>
Content-Type: text/plain; charset=UTF-8; format=flowed
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
whatever.
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
------------------------------
Message: 7
Date: Fri, 08 Mar 2013 15:22:22 -0400
From: Gus <8p6sm@xxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: inquiry about homebrew az-el systems
Message-ID: <513A3A6E.6080400@xxxx.xxx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 03/08/2013 01:46 PM, Phil Karn wrote:
> 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.
Willing to have a go at it? I'd contribute towards parts for a prototype...
> Also dunno why you would need two IMUs.
Because I foolishly thought to compare data from the antenna and the
base, to get pointing angles. Only after posting did I realize that one
IMU would give antenna position data in the earth frame of reference
(not the vehicle frame of reference).
> Platform acceleration (not mere motion) might be a problem but I'd
> have to think about how to compensate for it.
Are you familiar with the UAV Dev Board? They do all manner of clever
tricks and don't even have a magnetometer!
> 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.
Won't the IMU work in a Faraday cage? Yes, but power has to get in and
sensor data has to get out, so RF will still be a problem. What about
auto-sensing the RF and delaying the output from the IMU or telling the
CPU not to read them? Could be a problem for big-mouthed rag-chewers
like myself. Also, in a Field Day type environment with several nearby
transmitters operating, your tracker could be offline for an entire
pass. Of course! Fibre optic control cable! Obvious, isn't it? :-)
> 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.
>
I've got a couple uBlox 5's around here somewhere...
--
73, de Gus 8P6SM
Barbados, the easternmost isle.
------------------------------
_______________________________________________
Sent via amsat-bb@xxxxx.xxx. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
http://amsat.org/mailman/listinfo/amsat-bb
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