[Chugalug] [OT] Best way to rotate a motor precisely

James Nylen jnylen at gmail.com
Mon Mar 4 02:52:32 UTC 2013


I'll be using an Arduino at first, then probably a standalone ATMega
eventually.  For the driver circuit I should be able to either wire up some
transistors or a driver IC.  I'm thinking this stepper will work nicely:

On Sat, Mar 2, 2013 at 7:29 PM, Dan Lyke <danlyke at flutterby.com> wrote:

> On Sat, 2 Mar 2013 18:54:22 -0500
> Stephen Kraus <ub3ratl4sf00 at gmail.com> wrote:
> > Stepper motor and some sort of optical sensor.
> Unless you expect that the motor will encounter load beyond its torque,
> you can do away with the sensor. Steppers gonna step, it's what they
> do, man. At most you need a home switch.
> For James' benefit, 'cause I assume Stephen knows this stuff...
> Stepper motors are super easy to drive. It doesn't matter if you stall
> them (they have a maximum current draw which you usually just arrange
> to drive them at), they hold a position, and a common configuration for
> them is 400 steps per circle, so often you don't even need to gear
> them. And you don't have to reverse polarity on them to drive them
> backwards, so driver circuitry is easy (not that an H-bridge is hard,
> but...).
> The down-side is that they draw current even when static (because they
> hold), so they can be kind of power hungry. There are strategies around
> this. They can also be noisy, because you're driving them a step at a
> time, so they can buzz a lot.
> So, yeah, I'd do this with a stepper. If you don't have anything trying
> to move it when it's still, you can probably de-power it when not in
> motion, just to save power.
> The way a stepper works is that it has a series of coils, usually 4.
> Energize them in sequence, 1, 2, 3, 4, to make the motor step one way,
> energize them in the reverse sequence, 4, 3, 2, 1 to make it step the
> other. Half-step it by energizing two coils, so you can double the
> resolution by 1, 1&2, 2, 2&3, 3, 3&4, 4, 4&1 to step by half.
> If you're super smart you can do finer resolution by doing proportional
> coil steps; take apart a consumer product that uses steppers (like a
> *cough* popular paper cutting device that's on the shelves at Michael's
> and Wal*Mart that it would be wise if I didn't talk too much about the
> internals of) and you might notice that there are 8 driver transistors,
> but 4 of them are going through some monster resistors. This is to do
> quarter stepping.
> Many modern stepper drivers do much more fractional stepping than this,
> primarily to reduce noise, using PWM on 4 drive transistors.
> And there are also ICs that do this stuff for you, usually they
> keep track internally so you have two pins, one for direction,
> one that you pulse to step, although unless you're really jonesing
> for I/O pins it's often cheaper to just do discrete components: A
> resistor to limit drive current from your microcontroller to the
> collector of the transistor, a Darlington transistor like the TIP120
> (which has a 1000 to 1 gain, so you only have to give it 1 milliamp to
> drive an amp of output), and a diode in series to prevent the inductive
> kick from the motor coils from blowing out the transistor.
> In at least one driver circuit I did, I was paranoid and used a PS2501-4
> opto-isolator IC to protect the microcontroller. This was probably
> paranoia, I wouldn't do that today.
> Whatcha gonna drive this baby with? Parallel port on a PC? Arduino?
> Stand alone AVR ATMega? An Arduino with a shield?
> Dan
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