Continuous oscillations on pan

Discussion in 'Radian' started by Lyndon Hill, Nov 25, 2014.

  1. Lyndon Hill

    Lyndon Hill New Member

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    Aircraft Type: Octocopter
    Gimbal Type: 3 axis with damped mounting plate
    Radian Type ( 1, 2 , 3 axis) 3 axis
    Gimbal Power source: 11.1 V 1300 mAh
    Gimbal Radio Type: Futaba
    Radian Communication Protocol (S.Bus, PPM, PWM, Spektrum) S-bus

    Connections: Change these to match your setup
    Roll Sensor:
    Port
    1. Sbus in
    2.
    3. Sbus out to tilt
    4. Servo

    Tilt Sensor:
    Port
    1. Sbus in
    2.
    3.
    4. Servo

    Pan Sensor:
    Port
    1. Sbus in from receiver
    2. Power in
    3. Sbus out to roll
    4. Servo

    Symptoms:
    Constant buzzing sound from roll servo.
    Pan is oscillating, sometimes when switching to stabilised slew mode; sometimes after panning slowly.
    Seems to set itself into a feedback loop and keep going forever.
    Even at fast speed gimbal seems to move and "stick" then carry on, but when checking by hand there is nothing physically making it stick

    Steps taken to resolve:
    Checked all connections.
    Drive belts look taught.
    Tried another battery.
    Tried reducing pan gain to 40% (from 85%) and slew rate to 30 (from 60).
     
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Lyndon:
    How well balanced is the gimbal? If you remove the servo belts, will the camera stay in an arbitrary position in which you put it?
    The roll servo sounds like either it is having to work to correct an imbalance, or it may be telegraphing that it's about to become an ex-servo.

    As for the pan oscillation, press your finger horizontally against the pan drive belt at the mid-point between the small and large pulleys and you should see a deflection of 2 to 3 mm and no more.

    Have you also checked that the small pulley on top of the pan servo is on tightly. Back in the day when I had a Radiand gimbal, that was prone to come loose.

    Have you added anything like wheels to the landing gear? Those can act as counterweights and make oscillations happen.

    Hope this helps.
    Andy.
     
  3. Lyndon Hill

    Lyndon Hill New Member

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    Hi Andy,

    Thank you for your suggestions.

    All the drive belts are tight with only a couple of mm of slack.

    We went through the balancing procedure for tilt and roll; it's not perfect but mostly balanced.

    The biggest problem for us is that when using the controller to pan in slew mode; the motion has a lot of judder and then when we stop panning the gimbal goes into oscillation.
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    You will need to tip the gimbal on to its side to balance it in pan, then go back and check tilt and roll balance -- and repeat the whole series of balancing until you can get all three into balance.

    Have you watched: by the way? It shows you how to balance the pan axis by suspending the entire copter by one boom (the copter is tipped over on its side so that the battery plate is pointing at a wall). You can achieve the same effect if you have any kind of a stand (or an assistant) that can hold the copter so the plane of the booms is vertical.

    It can be quite tricky to balance the pan axis -- and you really do need to take the pan belt off to avoid deadening the response of the gimbal. You should be able to rotate the gimbal in the pan axis to any arbitrary position and have it stay there.

    The oscillation is caused only by the gains being too high -- but I recall that I would see a judder when panning if I had the slew rate set too high.

    Sorry I cannot be more specific, but I no longer have a Radian-based gimbal.

    Andy.
     
  5. Lyndon Hill

    Lyndon Hill New Member

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    Hi,

    Yes I've watched this video and the other one where the 'copter is hung by the arm.

    How does one balance the gimbal for pan ?

    The following link should show you a 20 second video of the current pan balance, which is pretty awful. We're tried moving the camera forward as far as we can but even with a relatively small lens we can't seem to affect balance.

    https://drive.google.com/open?id=0B48AqnkmcumTOXpiMEx1UlJLelU&authuser=0

    Does anyone have some tips on how to balance for pan ?

    Thanks in advance.
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Suspend the copter so that it is on its side (one boom pointing at the ceiling, one at the floor).
    You then may need to move the "down tube" on the gimbal (on which the roll servo is mounted) closer to the center of the copter or further away, to bring the gimbal into balance.

    Hope that helps.
    Andy
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sorry, wrote the above without having had time to watch the video.
    You have the transmitter on the rear landing leg. You might want to consider moving that to one of the "front" landing legs -- as well as the battery that powers the gimbal -- there's too much weight at the rear of the gimbal.

    Also consider doing as I mentioned above....moving the whole gimbal assembly towards the center using the down tube...

    Andy.
     

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