Conference vmszoo::rc

    How about using a geared down servo like a winch servo designed
    for trimming sails on boats?  You definately want to use a DC type
    motor for efficiency reasons.  Maybe one of the cheap robot arm
    drives like radio shack has would be usable as well.
    
    Charlie
    

    Does a winch servo do more than 180 degrees or so?  Good ideas,
    keep them coming!
    
    I got something kludged together last night, but the tilt pivot
    is not great, it only pans about 90 degrees (though I can see almost
    180 degrees side-to-side), and it needs more range on the tilt.
     I think I'm going to get a small hinge from a hardware store to
    replace the pivot and see how much that helps.
    
    One thing I have noticed is that while the S-138 servos only turn
    90 degrees, you can turn them by hand over 180 degrees.  I suspect
    that by adding some resistance to the ends of the position_feedback
    potentiometer I can get more range out of it, does anyone have any
    idea if this will work?
    
    Willie

    re: .3
    
>        One thing I have noticed is that while the S-138 servos only turn
>    90 degrees, you can turn them by hand over 180 degrees.  I suspect
>    that by adding some resistance to the ends of the position_feedback
>    potentiometer I can get more range out of it, does anyone have any
>    idea if this will work?
 
    
    To answer your question, yes you can get more travel out of an S-138
    servo.  Open it up and look at the wires coming from the pot to
    the main board, there will be one each of brown, white, and blue.
     Put a 3.3K ohm, 1/8 watt resistor in series with the brown lead
    and a 1.0K ohm, 1/8 watt resistor in series with the white lead
    and the full travel range (including trim) is a bit over 180 degrees.
    There's a spare pad next to where the white lead comes into the
    board you can use to add the 1K resistor, but the 3.3K has to stick
    straight up and use a flying tie point.  You have to be fairly good
    with a soldering iron and itty bitty parts, but it works pretty
    well!
    
    Willie

    Yes, a winch servo will go over 360 degrees with a large gear reduction
    which is what you want, I think.  The reason the servos only go
    90 degrees or so is that the transmitter does not normally vary
    the pulse over the entire useful range.  Changing the pot the way
    you did will work, but you have changed the servo's loop gain by
    reducing the voltage swing at the pot wiper for a given angle of
    rotation.  This is ok if the servo does not chatter or ocsillate.
    You might want to monitor the current to be sure that the servo
    is behaving in a stable mannor. If so, you are ok.  The other 
    reason the servos normally limit travel is to prevent running
    past the pot ends under any circumstances.  If this happens, they
    will do a quick 360 degree rotation on you because the voltage will
    suddenly go from one polarity to the other with respect to the center
    point.  You must be careful of this when you increase the travel
    by adding resistors to the end.  If the servo chip trys to obtain
    a pot voltage outside the 360 degree range, the servo will just
    spin forever without stopping.
    
    Charlie
    

    Thanks, Charlie, I'll look into a winch servo (any reccomendations?),
    and I'll also see what happens when I build my own encoder, maybe
    I can get the pulse width to vary a bit more.  The servo does tend
    to 'hunt' at the very ends of its' travel, but panning 180 degrees
    instead of 90 is a lot nicer.  Do sail winch servos go a little
    slower than normal servos?  Being able to pan so fast is making
    me motion-sick...  :+)  Now all I have to do is increase the tilt
    range, but that's going to take some more work to get the swivel
    a bit more stable.  I'm thinking about a small hinge from a hardware
    store.
    
    Thanks much for the feedback!
    
    Willie
    

    Another slow and precise movement technique might be some sort of
    lead screw or jack screw driven by a servo.  I saw an article in
    Model Aviation, I think, where a guy made a swing wing plane.  He
    used a modified servo and limit switches to swing the wings with
    a cable and pulley scheme.  He took the pot out of the servo and
    mounted it on the movable part so the servo turned multiple
    revolutions, turning a drum to wind up the cable.  You might be
    able to use this idea to get a slower, more precise movement.
    	I have not used winch servos, but I bet the tower catalog (Big
    One) would have specs such as torque and transit time.  I think
    that they are fairly slow compared to standard servos due to the
    much increased gear ratios and therefore much higher torque.
    
    Charlie