i have a standard continuous rotation servo driving a gearbox. i need to control the position of the output gear of the gearbox. how can i do this?

I'm pretty drunk atm, so I'm not quite certain what you mean by "standard continuous rotation servo". I was unaware that servos were standardized. ;) have any technical specifications for it?

Anyway, you can determine its relative rotational displacement using a rotary encoder, and you can decypher said position and control said servo using a microcontroller. I, Spatula Tsar, and possibly a cuple others understand the PIC microcontrollers, and we would recommend them for such a project.

this is the servo im using

http://www.parallax.com/Store/Sensors/CustomKits/tabid/134/txtSearch/servo/List/1/catpageindex/2/ProductID/102/Default.aspx?SortField=ProductName%2cProductName

ive got 12 servos that i need to control this way.

for control im using a bs2 with a servo controller.

im not an expert on electronics, but isn't a servo controlled with a potentiometer on one of the gears inside the servo? couldn't i just move the potentiometer to the gear i need controlled?

Some of them use potentiometers, others use encoders, and some don't even use position feedback at all. (We have some at the lab that are open loop with no position feedback and are powered with a 900W controller.)

http://www.kronosrobotics.com/an116/GAN116.shtml

It seems that the pulses are just encoded positions that the circuitry inside the servo interpret to a position. Neat idea.

I'm not clear on what you're asking. Standard hobby servo motors take in a PWM pulse, and move the armature to an absolute position based on the pulse. It can only rotate ~180 degrees, but can accurately slew to any position.

A common modification is to convert it to continuous rotation, making it more suited to drive the wheels of a small robot. So common, some companies sell them this way. In this case, the feedback potentiometer is removed, and the motor is free to spin continuously. The PWM controls the speed and direction of the rotation. Since the only sensor is removed, it has no way of telling where the armature is positioned. All you can do is make it spin faster or slower.

To get the best of both worlds, you need a rotary encoder, similar to what old mechanical mice use. This way it can rotate continuously, but you can still keep track of where it is. Unfortunately, not many cheap hobby servos have this. You'll either have to add one yourself, or spend lots of money.

i have a servo that was designed to rotate continuosly. the output shaft of the servo is connected to an external gear box (to reduce speed/increase torque). the output of this gearbox is driving the leg mechanism for a walking robot. i need to acurately control the position of the output shaft on the external gearbox within a 180 degree radius.

if i understand correctly the 180 degree servos have a potentiometer on their output shaft that allows this. suppose i took a 180 degree servo and moved the potentiometer to the output shaft of the external gear box, would i not be able to control it's position within 180 degrees? it would work the same way but just more gears....except since i can get the continuous rotation servos for dirt cheap, it would be more cost effective to add the potentiometers in myself.

the original plan was to just put a limit switch at either extreme of the leg's movement, and hope for the best, but i figure it'll be much better if i can control whatever degree it goes to.

i have a cad model, but i can only open it at school; ill post pictures tommorow.

Yes, it will work. The servo you linked can probably be modified back to an absolute position servo. Simply place the potentiometer wherever you want, and it should be fine.

any hint as how to what potentiometer is best and how to wire it to work with the motor?

Simply follow the continuous rotation mod in reverse. I don't think the potentiometer value is very critical.

What a bizaare concept. Modding a continuous servo back to a stock.

Can you really get continuous servos cheaper than normal onces?

whats really weird is that the stores nearby only sell the continuous servos.

also, i cant just use a stock absolute position servo for this, because the servo is powering an external gearbox.

1 turn of the servo's output moves the output of the external gearbox only a small fraction of a rotation, however i need 180 degrees of movement out of it.

hopefully by moving the pot to the gear i need controlled, i can get the necessary range of movement

i ended up acquiring an absolute position servo and cutting out the potentiometer and extending it outside the casing with some wire. then i connected the pot to the output on the external gear train, and connected the servo output to the gearbox input (hopefully that makes sense)

it worked, i can now control the position of the output gear within a range of 180 degrees.