Step motors


I am new to robots but my friend and I decided to go for small project.

I will now describe here the whole project, but I have a few questions that I hope you guys can give me the answer to:

  1. I need a step motor that can give me about 5-10W. how much such thing should cost?

  2. how accurate the step motors are? I need it to be able to move on a track with the accuracy of 1-2mm (about 1 degree). Are there any step motors with this accuracy with a sane price tag? what is the “standard”?

  3. can you give me a recommendation for a controller that connects to a computer and has the ability to control 3 step motors? If I use motors of 5-10W, will i need an external power source or the controller will be able to use power from the computer?

That’s all for now. I know that the questions are very general, but I have now clue where to start and i think you guys can help me.

thanks in advence,

  1. You can get 5-10 Watt stepper motors for $20-$30 dollars, or for free if you scavenge them. Whenever I see a printer or scanner headed for (or already in) a dumpster I always grab the stepper motors out of it. Some are junkey, but I’ve found nice motors this way as well, some even have manufacturer’s markings on them so I can look up the specs:

    You can also salvage other useful parts like timing belts, mounting brackets, linear bearings, etc…

  2. Nice little stepper motors like these usually have either 200 or 400 “steps” per revolution, so 1.8 or 0.9 degrees per step respectively, but depending on your controller you can get even more accurate than that. With a simple dual-H bridge controller you can alter the switching order to take half-steps rather than full steps, so you would double the positioning accuracy. Some more advanced stepper motor controllers modulate the coil current to move the motor in “microsteps”, which divide up the motors normal steps, usually by powers of two. You can get good positioning to around 32 or 64 microsteps per step, but beyond that, you’re really just smoothing out the motion without increasing your positioning accuracy. Microstepping also reduces the holding torque of the motor, so its a trade off.

  3. The terminology is used very loosely, but what you really want to run a stepper motor (or three) from a computer is a combination controller and driver. The driver handles the high-power switching for the motor coils, and the controller receives computer commands and generates the switching control signals for the driver. They’re a little pricey, more industrial than hobby grade, but I’ve had good luck with stepper motor controller+driver combos from Lin Engineering. They let you do neat things like position/speed control, acceleration profiles, pre-loaded programs, daisy chaining controllers, etc.

I don’t have any hobby or kit drivers to recommend from personal experience, but this one is pretty popular. It will only control one motor though, and it’s just a driver, so you’ll need to work out your own controller to set the step and direction inputs.

If you have a beefy computer power supply you may be able to draw enough power from its 5V or 12V lines to run your motors without crashing your computer, but you’ll probably want a separate one to be sure. Scavenged ATX power supplies from old computer towers are great for this sort of thing though!