Can you do this with a continous rotation servo? ...tell it to turn to 45 degrees and stop there for instance?
no
Hmmm, too bad.
A stepper motor looks like you can control it's exact position with code, but it lacks gear reduction for my application.
You didn't say what your application is but stepper motors excel in low speed applications. Unlike a brushed DC motor they develop their highest torque at the lowest speed.
Yankee:
You didn't say what your application is but stepper motors excel in low speed applications. Unlike a brushed DC motor they develop their highest torque at the lowest speed.
Sure enough??! Well, that might be exactly what I need then. Tell me, would SparkFun's stepper motor:
work to open a drawer in this setting:
If I put a small rubber wheel on it (say, 1-1/12") and mounted it under the drawer. The drawer would be the equivilent in weight of a silverware drawer or something, with new, quality drawer glides. Right now I can slide it open and closed with 1 finger. So I would be able to accurately open the drawer 8" without having to have some kind of limit switches?
This would be great.
You could always have the motor run timed - run for 3 seconds or as determined experimentally to stop at the distance you want.
Or put the equivalent of a string out of sight on the back of the drawer that pulls a switch the arduino reads to know when to stop.
I really like the stepper idea. You say it's got good low RPM torque, but is this too slow? I'd need about 2 revolutions to go 8 inches. To move the drawer, it'd take 1-2 seconds. Can a stepper motor work like that?
KE7GKP:
You can get stepper motors with reduction gear trains. You didn't mention whether you think you want gear reduction for low speed or for increased torque.
I suppose to open a drawer 8" with a 1 1/2" diameter wheel, @ diameter times pi, I'd only need 2 revolutions, and it would have to have a little time for the opening process, just roughing timing a normal open/close, I'd say 1.5 seconds. So that a speed of 90 RPM. Does a stepper motor have enough torque for this little job at that speed? Can you run a stepper motor at that speed?
thanks.
"2 revolutions to go 8 inches"
so you're going to have a 4" circumference wheel to move the drawer in/out?
circumference = Pi x D, so a 4"/Pi = 1.27" diamater wheel. Do you have that much room under the drawer?
What happens as the wheel wears, or slips a little on a wet spot, or someone nudges it by accident as it starts?
Inkjet printers use stepper motors to slide the cartridge back & forth, right? Probably not as heavy as a silverware drawer, but you can get motors in all capacities.
I saw a youtube video of a stepper motor spinnning something arouund at quite high speed. I bet 90 RPM is not a problem.
CrossRoads:
Inkjet printers use stepper motors to slide the cartridge back & forth, right? Probably not as heavy as a silverware drawer, but you can get motors in all capacities.
I saw a youtube video of a stepper motor spinnning something arouund at quite high speed. I bet 90 RPM is not a problem.
...I was refering to 90 RPM as a 'low speed' not a 'high speed'. So they must have good torque at, say, 60 RPM or something. Slower would be better for the drawer. 2 seconds to open it would ensure no slipping.
Getting bumped out of sync won't be an issue.
...so, SparkFun's stepper motor wouldn't have enough power (torque) to open a drawer?
Dejavue
zoomkat:
Dejavue
...sorry if I'm repeating myself. To state the obvious, I'd like to get away from limit switches and sensors if possible. I'm new to this stuff, and just wondering about the power of a stepper motor. I didn't know a motor could have power at low speeds. I'm used to motors running 1600-1700 RPM, what you'd find on a bench grinder. I had no idea a motor could run this slow, but if a printer head motor doesn't use gear reduction, then it must be able to run pretty slow, which sounds like a perfect solution to me--a nice slow drawer opening with digital control as to it's where abouts, so no need of sensors or limit switches.
Steppers usually have a data sheet that will include a graph of the torque curve. You can use this to find the torque at a given speed. Unless the drawer is very heavy it shouldn't take a very large motor.
Since no one gives an answer to the question of the topic, I have to step in and show you some methods to hack a servo to get both regular and continuous rotation:
-
Replace the electronics in your servo and get full positional control. The perfect solution, but costly:
http://letsmakerobots.com/node/18615
Nowadays you can buy digital servos that already have built in these electronics. -
Modify your servo for continuous rotation as usual, then open up your potentiometer, cut off the stop, then add a transistor triggered relay that replaces the potentiometer with 2 resistors; use a separate wire to trigger the relay to turn your servo into a continuous rotation servo or a regular servo:
http://letsmakerobots.com/node/24705
I hope these links will help other people that are looking to use a servo in both rotation modes.
Since no one gives an answer to the question of the topic, I have to step in and show you some methods to hack a servo to get both regular and continuous rotation:
Good answer, but you did not answer the OP's origional question below: 
Can you do this with a continous rotation servo? ...tell it to turn to 45 degrees and stop there for instance?
Good answer, but you did not answer the OP's origional question below:
Quote
Can you do this with a continous rotation servo? ...tell it to turn to 45 degrees and stop there for instance?
Actually, both methods allow that. Any unmodified servo is capable of going to 45 degrees and stay there if you continue supplying the pulses for that position. It will not stay there by itself.
With the first method I showed, the servo has a 360o magnetical sensor with a high resolution and it has a built in microcontroller. You can tell the servo to go to ANY position and stay there. Or to just rotate continuously. The way the magnetic encoder works allows the servo to stop at 45 degrees even after a random number of complete rotations.
The second method I showed, is a servo hack, and because of the way the potentiometer is built, you can tell the servo to stop at any degree between about 270 degrees (the range of the potentiometer resistance), for the rest until 360 degrees it is unresponsive. But, there are 360 degrees potentiometers out there (minus about 2-3 degrees), easy to replace the original one. Just look at the video and you will see the servo rotating continuously and then stopping at a certain degree and staying there until a new position was issued.
Of course, if you buy a continuous rotation servo and do not modify it, you can't tell it to go to 45 degrees and stop there!