[amsat-bb] Re: inquiry about homebrew az-el systems
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@amsat.org. 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. ___ Sent via AMSAT-BB@amsat.org. 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
[amsat-bb] Re: We-Wish decay
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@amsat.org. 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 ___ Sent via AMSAT-BB@amsat.org. 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
[amsat-bb] Re: inquiry about homebrew az-el systems
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 ___ Sent via AMSAT-BB@amsat.org. 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
[amsat-bb] Re: inquiry about homebrew az-el systems
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@amsat.org. 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
[amsat-bb] Re: inquiry about homebrew az-el systems
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!) Hi Gus, you asked for it :-) http://www.g6lvb.com/ao40mobile.htm So also here Howard sets the standard, nobody beats him - respect! Btw: here is my 18 year old project, it still works fine after so many years: http://pa3guo.com/zb_rtr_ctrl.html Things I learned and included in my software: - thresholds are needed to avoid the system oscillating around a target position - corrections need to be made due to overshoot (after a stop command the rotor takes time to actually stop -- Henk, PA3GUO ___ Sent via AMSAT-BB@amsat.org. 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
[amsat-bb] Re: We-Wish decay
Here is the altitude plot of WE WISH and other four CubeSats which were deployed from ISS at 4th Oct 2012. http://www.ne.jp/asahi/m-arai/gkz/5CubeSats_Altitude.png Only WE WISH is moving fast toward earth. Its trace is very similar to ARISSat-1. Time line of ARISSat-1 aligns to the five CubeSats deployment date. 73 Masa JN1GKZ Tokyo Japan 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 ___ Sent via AMSAT-BB@amsat.org. 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
