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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/BT4MZF2WCIRDNS56PNHVNA2AXDBTGWT6/",
    "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/",
    "message_id": "[email protected]",
    "message_id_hash": "BT4MZF2WCIRDNS56PNHVNA2AXDBTGWT6",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/CLOQYBU3FX2QD32ZMTRJOW3ZPNOG7XYE/",
    "sender": {
        "address": "karn (a) philkarn.net",
        "mailman_id": null,
        "emails": null
    },
    "sender_name": "Phil Karn",
    "subject": "[amsat-bb] Re: inquiry about homebrew az-el systems",
    "date": "2013-03-08T06:35:15Z",
    "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/E6H3VXIV3ACZPY3IZZKPBYBRRFURFKYH/",
    "children": [
        "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/M4QRPEX2M2YBBLYUD6D6KDCCKVDB6MXN/"
    ],
    "votes": {
        "likes": 0,
        "dislikes": 0,
        "status": "neutral"
    },
    "content": "Just noticed this thread and caught up.\n\nWhile rotor controllers are indeed a dime a dozen, I think we could do a \nlot better than any of them.\n\nYour typical Yaesu/Kenpro rotor uses a 24V AC 2-phase induction motor. \nThe control box applies 50/60 Hz AC directly to one winding and to the \nother through a capacitor. The capacitor creates a phase shift in the \ncurrent through the second winding, creating a rotating magnetic field \nwithin the motor that drags the rotor in one direction or the other. You \nreverse the motor by applying AC directly to one winding or the other.\n\nAlthough this design is extremely common, it has several highly \nnon-ideal features. First, the current through the second winding isn't \nactually in phase quadrature (90 degrees) with the first. It's somewhat \nless due to the series resistances of the winding and capacitor.\n\nSecond, the current amplitudes in the two windings are not the same, and \nfor the same reason -- series resistances. This means less torque and \nmore motor heating than could otherwise be produced for the same input \nvoltage.\n\nThird, the motor has only one synchronous speed: 50 or 60 Hz. Stalled \nrotor torque is rather low, especially for a non-ideal supply.\n\nWhat you *really* want is a variable frequency, variable voltage (VFVV) \ninverter producing two phases in exact quadrature (same amplitude, 90 \ndegrees with respect to each other). You can smoothly vary the speed \nfrom a dead stop to faster than 60 Hz and with more torque at every \nspeed, making it easy to track a continuously moving satellite with a \nnarrow antenna. And you don't wear out the brakes and constantly flex \nthe masts and booms until the clamps all work loose.\n\nYou can even use the motors as brakes by sending a small amount of DC \ncurrent through them. It doesn't take much, as this essentially creates \na DC generator with a shorted output, and that torque is amplified by \nthe gear train.\n\nThe necessary waveforms could be generated with the PWM channels in an \nArduino or similar microcontroller and amplified with the power MOSFET \nH-bridges common in robotics.\n\nI do see several rotors using DC motors, plus several people suggesting \nthem here. While they're somewhat easier to vary in speed (you just vary \nthe average DC voltage with a PWM drive) you have to remember these \nmotors contain brushes rubbing on commutators, and that makes them far \nless reliable than AC induction motors, which are famously simple, \nrugged and reliable. There's a reason AC motors are universal in the \nmodern generation of hybrid and battery electric vehicles even though \nmost hobby conversions still use DC motors.\n\nAs for position feedback, what about one of the cheap, modern IMU \ndevices, like the Pololu MinIMU-9. I've been playing with this \nparticular board, which contains a 3-axis accelerometer, magnetometer \nand rotational gyro. Just mount one on the antenna boom and directly \nmeasure the antenna position. The accelerometer will give elevation \nwithout any calibration at all. The magnetometer can read azimuth with a \nlookup table for your local magnetic declination, and any local magnetic \ndistortions could be removed with a one-time calibration. And the gyro \nwill quickly tell you if the antenna is out of balance or has stalled.\n\n\n--Phil\n\n\n\n\n\n\n\n\n\n",
    "attachments": []
}