DC motor or Stepper motor????

Hi,

I'm new to Arduino and have been working on a project where I want to be able to make two objects rotate simultaneously, clockwise at 15 degrees per 60 minutes. Does anyone know a way i can achieve this with a DC motor? ....or would i need to order stepper motors?

Also, if i do need stepper motors, which one is the best one to buy for Arduino Uno board??

"Two objects" is way too vague

The reason it is too vague is anything motor related requires load calculations and the power devices used must be chosen to accommodate the desired load. The mass and size of the load effects the torque and power requirements since the larger the load, the greater the inertia and resulting start-up current. Please post a photo of a hand drawing taken with your cell phone or some other type of illustration with dimensions and mass specifications. In addition we would need to know more about the speed or nature of the motion. For example, would you need to do micro-stepping ?

If you want two objects to rotate together, the easiest approach is to mechanically connect them e.g. two spindles connected by a chain or belt. If you want to control the speed then it's important to know the mechanical characteristics of the things you're trying to rotate. In the simplest case you could just use a free-running motor and gearbox to give you the required output speed. In more complex cases you would need some positional feedback and a closed loop control system to maintain the target speed. An Arduino might be used to implement that controller.

mass ? (EACH)

You can buy small, synchronous motors that turn as slowly as 1 revolution per day. They are as accurate as household AC clocks. http://www.autotrol.com/model-150-ac-gear-motor.aspx

jremington:
You can buy small, synchronous motors that turn as slowly as 1 revolution per day. They are as accurate as household AC clocks.
http://www.autotrol.com/model-150-ac-gear-motor.aspx

Thanks, but is there an alternative one I could use, if i wanted to increase or decrease the angle of rotation for testing purposes, one in which i could control the input?

In that case I'd drive one of the spindles, and apply a sprung torque to the other one so that it maintained a constant tension on the connecting string. Given that you want the rotation to stay synchronised to real time you need the speed to be controlled pretty accurately - jremington's suggestion seems like a good way to achieve that without any complex electronics or wiring.

raschemmel: mass ? (EACH)

the cylinder poles will be very light...less than 50g each

You could get a very low-geared DC motor (look up Pololu's website for examples) and, perhaps, add even more reduction gearing as well as using the Arduino analogueWrite() to further control the speed.

However it seems to me this is a case where a stepper motor with a suitable stepper motor driver board that allows microstepping would be the easiest to control. A motor with 200 full steps per revolution would have 3200 with 16 times microstepping. This assumes that the fact that the motor moves in steps, rather than smoothly isn't a problem.

You need also to be aware that the Arduino's 16MHz crystal clock is not all that accurate - nowhere near as good as a time clock.

...R

You can't rotate a dc (non-stepper motor) in specific incremental steps without a feedback encoder and even with an encoder you would have a hard time rotating exactly 15 degrees and then stopping. You would have no way of counting specific steps unless you used a stepper motor.