L = 10cm
Rot = 180 degrees
therefore circumference = 20cm
diameter = 20 / Pi = 6.37cm
So you attach a capstan head of 6.37cm diameter to your servo and when it turns 180 degrees it pulls a cord 10cm
And if your servo isn't man enough for the job then you get a more powerful servo
viola
jackrae:
L = 10cm
Rot = 180 degrees
therefore circumference = 20cm
diameter = 20 / Pi = 6.37cm
So you attach a capstan head of 6.37cm diameter to your servo and when it turns 180 degrees it pulls a cord 10cm
And if your servo isn't man enough for the job then you get a more powerful servo
viola
There are plenty of sites on the web showing how to convert a servo to continuous rotation, so you can just spin the servo until the cord is pulled as far as you want it.
CrossRoads:
There are plenty of sites on the web showing how to convert a servo to continuous rotation, so you can just spin the servo until the cord is pulled as far as you want it.
oh my - but I had understood this wasn't a win win situation and my servo would lose its intelligence if I did so. For instance, I imagine I would lose any ability to be precise - and this is important for my application. I need it to move the cord 100mm, then move the cord back to the original position (cord is sprung).
My current question is how do I know how far a given servo is supposed to be able to move?
A winch/sail servo is what you want; they are designed to rotate multiple turns, but still allow the positional feedback needed. Alternatively, you could "gear up" the output of a regular servo to achieve the same result (albeit with less torque; I'm not sure if a winch servo does this, or uses a multi-turn potentiometer or something else to preserve torque).
continuous rotation hobby servo - not a good idea as doesn't given any positional feedback? Or?
Correct, you don't want to do that.
winch servo - is this the same as a continuous rotation hobby servo? If not maybe this is the answer?
Winch servos are standard servos (not continuous rotation) that the different gear train arrangement that allow for much more travel. Problem is that they don't all move the same number of degrees, check datasheets before purchace.
360° servo? is this the same as a continuous rotation hobby servo? If not maybe this is the answer?
No, not a modified continuous rotation (unless mistakenly listed as such), but rather different gear train arrangement to allow 360 degree total travel.
DC motor - not a good idea as is essentially a continuous rotation hobby servo?
Correct, you would require some form of external feedback to determine actual travel position.
adding a long arm to the hobby servo - doubt the H.S. has the pluck for this?!
That's a simple solution and often used by the original users of radio control servos. Changing the servo arm length or using an external offset bell-crank control arm is a method. As far as pluck, that depends on the force your load requires and the torque force of the servo. Servos are available in many torque values.
'bigger' hobby servo - this wouldn't give me any more 'pull' (right?) but might tolerate a longer arm?
Might not give more travel, but would most likely have greater torque which would then allow longer control arm length. Have you measured required push/pull force your application requires?
something else I haven't thought of
No I think you have thought through the issues and possible solutions pretty well. Lefty
You also need to consider that a simple radius arm will give a non-linear pull per degree of rotation (sine function or cosine, depending upon where you start) and an inverse non-linear torque whereas a capstan drum will give a linear relationship of movement and a constant torque.