Hi
I would like to make a motor rotate once every 24h, so very very slow indeed.
I never worked with motors, can somebody please point me in the right direction?
DC Motor with gears? Stepper motor?
Thanks
Fubbi
dead easy I would think
stepper motor usually 200 steps per rev
so 200 steps/day
= 1 step every 432 seconds
unless you want to wait 24hrs then rotate 360 in which case
wait 24 hours then send 200 pulses!
I would like to make a motor rotate once every 24h
then its even better get your self a RTC and wake up the arduino every 24 hours to make the motor go for one revolution ! thats all 
I actually want it to complete the cycle in 24 hours. Slow rotation.
cheers
fubbi
The answer depends very much on what you want the motor to do. Are you planning to make a tabletop clock? Or are you designing a backup system to restart the Earth, in case it suddenly stops rotating?
if thats the case you could go for the first post ! 
but Just in case have a look at interrupts that would save you come battery
if it is to restart the earth you'll need a MOSFET interface as the Arduino won't quite manage enough current
I'm looking at getting a really even tan...
no I want a small object to rotate around its own axis as smoothly as possible and complete exactly one cycle every 24h
If I just wanted to get the job done simply, I'd scrounge the motor from a 24-hour analog clock, and mount the object to the hour-hand shaft.
If I were looking for a "learning challenge", I'd get a motor and a really high-ratio gearbox like Tamiya sells for motorized models. Maybe even two gearboxes, so I could connect them in series to get a ratio of many-thousands-to-one. Then I'd put an encoder disk on the motor, because the RPM is highest there, and use PWM to precisely control its speed.
If you can tolerate a little jerking in the rotation, you could use a fine-pitch stepper with the gearbox(es).
If you can tolerate a little jerking in the rotation, you could use a fine-pitch stepper with the gearbox(es).
In addition to using a stepper with a lot of steps per rotation, one can obtain a stepper motor driver that can perform 'micro-stepping" allowing for decreasing stepping size by /2, /4, and even /8, depending on model driver and motor windings.
Lefty
It is a pupa turning in front of an artificial sun until it hatches.
The arduino is there to keep track of time as I will be following the real sun.
I will track the relation between the subject and the artificial sun.
I guess really small steps are okay.
fubbi
What kind of ride would you like to give a baby butterfly? It deserves some smoothness IMHO
Anyway, I'm just trying to find out what kind of motor I should use. I will work with whatever resolution the best motor i can find will give me.
So the question remains, DC with gearbox, Clock motor, Stepper with high resolution?
fubbi
It just struck me, the clock is just 12h and i need 24h...
Don't use microstepping for reducing the distance moved between cycles.
Microstepping is used for increasing the velocity of a movement while preventing complete torque loss, for fighting harmonic resonance, and for making smooth starts - it is not possible to guarantee a microstep position can be held because the motor is being held between two detents. Normally, microstepping is used in the following sense:
Move 1 rotation at a high velocity (ignoring step timing):
Microstep 16x for certain # of steps
Microstep 8x for certain # of steps
Microstep 2x for certain # of steps
Full step for remaining # of steps
The second issue with microstepping is power consumption. Your only hope to hold a microstep position is that the motor stays at full power to hold the position in braking. This means you will be consuming large amounts of power when not moving, and will be generating lots of heat at the driver and in the motor - which could lead to early component failure.
If you want better resolution out of the stepper for "more" accurate positioning (won't use the term "accurate" alone, it's hard to achieve without a specified level of accuracy =) use a gear chain with minimal friction and backlash. A 4:1 gear chain will take you from 200 to 800 steps per rev. Now, you can use microstepping to "smooth" the transition, just make sure as you transition through microstep phases, you end up at a full step position.
!c
I have to agree with Richard. My CNC router utilizes 1/8 stepping and I have repeatable accuracy of +/- .0015. Most of that error is likely mechanical. Beyond 1/8 step the resolution does become "fuzzy" though.
Drone is correct about microstepping being useful for smoothness of motion and combating mid-range resonance. For this, the smaller microsteps are useful.
Having said all that, it seems the simple thing to do would be the clock motor geared down 2:1. This still leaves the problem of coordinating with the Arduino. For that a stepper would be the easiest IMHO though it would have to be geared down to get smooth motion.
There are also hobby DC motors with encoders available but this would be a much more complicated solution.
Apologies,
Yes, obviously microstepping can increase resolution as part of a larger system design. I clearly worded my statement wrongly as an absolute, when there should have been an 'or' indicating that you -can- use it to increase resolution (at the cost of accuracy without a lot of additional work), but that it would have no positive impact on his project given the parameters. (Leaving that stepper powered 24x7 will be problematic.)
!c
It would be easy enough to paint half the gear black and the other half white. Then you could use a photo detector to control "sun" on/off times. With a clock motor driving it, it doesn't even need an arduino. That would make me want to re-design the project.
Best excuse for feature creep I ever heard!
"But, seriously, we've got all these MHz, let's put 'em to work!"
!c