Servo droop

edited February 2006 in Control Systems
I built a new rig with the Prinzler 433txrx system.

The camera frame is farily well balanced, but maybe not perfect.

My problem is that after adusting tilt of the camera up, say level with the horizon, the servo (Futaba 3003) slowly droops down until the camera is faciing down. Hitting the up button on the transmitter, instantly puts the camera up again, but then it slowly droops back down.

Any help? Do I need a stronger servo?


  • Sounds like you haven't *balanced* the rig properly. You need the servo arm as close as possible to the center of gravity .. so that

    1. there is much less work for the servo to do, and

    2. It will stop the problem you are having now.

    Richard in Boston, MA, USA
  • edited February 2006
    According to Prinzler's web site:
    The transmitter is transmitting only if a switch or the buttom is pressed. The receiver sends pulse width signals only to the requested servo modules if a transmission is active.
    So most of the time the servos are getting no signal. In this case, most servos will stop maintaining position and there would be nothing to stop the droop but the friction in the servo. So your rig would have to be perfectly balanced to hold position. Frankly, I would expect the servo to move somewhat even if it were perfectly balanced due to the KAP rig's motion.
  • Indeed the signal stops but the power supply isn't cut off. That should keep the servo in position.
    And I've seen rigs on which the tilt frame is way out of balance but still work fine.

    kapDave, can you show us a picture?

  • The way some (most?) analog servos work, they only supply power to the motor in pulses that correspond with the pulses from the receiver. No pulses from the receiver means no power to the motor, even though the servo itself still has power.

    Now I've been looking into digital servos today and it sounds like they completely separate processing input pulses from powering the motor. Maybe a digital servo would work in this application. Then again, maybe certain makes of analog servos would work too. But I know from experience that my own servos send no power to the motor when pulses are not being received. (On my rig, the servo power is separate from the control pulse generation so this situation is easy to produce.)

    It sounds like kapDave's servos behave as mine do.
  • Sorry Peter, but the servo has to have a steady stream of pulses to actively maintain position. This is easily proven by turning on your rig with the TX off. You should be able to manually move the tilt servo or any other servo. If the pulse steam stops the servo will stop at its present position, but any outside torque could cause the servo to turn.

    Balancing the rig on the tilt axis will definitely help, but as David Wheeler mentions, the tilt could still move due to the forces of the wind on the face of the camera. Much of this depends upon the make of the tilt servo. A standard servo like the Futaba S3003 is harder to turn than a mini servo like the Hitec HS-81.
  • Two big downsides to digital servos.

    1. Cost
    2. High current usage resulting in shorter battery life.
  • Yeah. And I didn't mean to imply that digital servos would definitely work in this application. I don't know how they would behave without input pulses.
  • edited February 2006
    I suspect that digital servos would behave just like analog servos. Here is a good article about digital serovs for those interested:
  • On a more practical note, kapDave, since your camera droops until it is pointing down, at least you know it's front-heavy. You need to move it back until it stops drooping. If it starts to rise up spontaneously, you've moved it too far.
  • edited February 2006
    All my systems use servos on the basis you are discussing. Get them to move when wanted but don't waste energy when they should be resting. And my tilt servos have never drooped - but then to get the balance point I use 4 hands 2 compass points and I find where the thing spins.

    KapDave's question 'do I need a stronger servo' is a good one. The fact that the perfectly ballanced cradle doesn't move as the system swings around is because on the side opposite the servo there is a certain amount of resistance. If you make this adjustable with a wing nut you can find the point at which your camera no longer droops. Now against that resistance what size does your servo need to be to move your camera as you want ?
  • edited February 2006
    In this discussion I keep hearing that shutting off the signal to a servo saves energy. Is this true? I would think that if the servo arm is in the correct position, the motor would be doing no work, even when pulses are sent to the servo. The only way you're saving energy is if the arm is out of position and the motor is not running to correct it. And that just means the servo is not performing its intended function.

    I think only having correct balance saves energy. Stopping the signal to the servos should be neutral on energy demand if the balance is correct.
  • edited February 2006
    Your right. And maybe it's only a smallish part of the way to powering off completely which must be the the ideal. Why insult your cradle by constantly nagging it to stay in position ? Why keep holding something when you can put it down ? From my point of view it's a step in the right direction.

    But also, I am able to add an hour later thanks to the leniency of a Vanilla whose charms are undisputable, with systems like Prinzler's the energy saving is proven by a transmitter which doesn't have to stream pulses all the time and can be small enough to slip into your pocket.
  • For what it's worth, I've used a Prinzler 433 for over a year with a well-balanced Pentax S4, and I've never had the tilt change position spontaneously in any kind of wind. I think balance is the important factor.

  • Hi Folks,

    you will have the same effect with normal transmitters, if they are shut off.

    With unbalanced rigs (camera holder) the receiver has to correct the position continiously and that will not save battery power. With my receiver you see imediately the result.

    If someone needs to have continious pulse output for vertical direction control, a special software change could be possible. But the best way is to have a good balance with the camera.

  • Either the rig has to be carfully balanced, or you can change to a worm wheel design. (wich has it's own disadvantages...) But using a worm wheel allows to use very small and light servo's, and there will be no strain to the servo's motor even if the rig is out of balance.

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