Re: stepper vs. servo

Harvey,

What I wrote is correct. Fullstep can be done with one or two coils
on. This means either 100% or 200%. Half stepping is alternating
between these two "fullstep" techniques. So instead of having only
one current level (100 or 200); you now have both, which is the added
current level that I wrote of. Note that since you don't want to ruin
your motors, drives often will use 70% when running two coils, which
add up to 140. This is the vector correct value for running with even
step torque. (so in this example the half stepping uses two current
levels, 100% and 70%.) Again, matching what I wrote...

My mistake was saying that you "drive the motor" with stepped square
waves. I was trying to keep things simple.

Actually the DRIVER adjusts the current levels into 2 multi-
stepped "square" waves, 90 electrical degrees apart; which causes the
motor to move in microsteps. As the number of current steps increase,
finer microsteps are possible. As we increase the number of current
levels available (from the drive), we can more closely approximate
the sinewave "ideal". The values used for the current levels are
taken from the sin/cos table. They may be "adjusted" from these
theoretical values due to motor construction/response. (FWIW, the
Allegro A3977 spec sheet--type a3977 in a yahoo search-- does a
pretty good job of explaining this. And allegro uses the sin/cos
values exactly)

Look at ANY stepper driver waveforms and you'll see the steps I'm
talking about. If the drive is more than halfstep, you'll see the
multiple levels which make the sine wave approximation.

Ballendo


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Harvey White <madyn@i...>
wrote:

> On Thu, 12 Feb 2004 09:52:06 -0000, you wrote:
>
> >Ray,
> >
> >A bit more:
> >
> >A stepper is just a syncronous motor run using square waves (At

the

> >fullstep resolution). In other words, the square waves are as

close

> >as you can get to a sine wave using only on and off.
> >
> >When you go to 1/2 step, you add another level of current control,
> >and the input waveform gets another step, and is a little closer

to

> >the sine wave ideal. As you go up in microsteps, you keep refining
> >the input waveforms by adding steps/levels towards ideal sine

waves.

>
> Actually, this is not true for half step, or at least, the half step
> that I know.
>
> The trick in half step is to have two adjacent pole pieces actuated,
> so that the balance point for the rotor is half way between. The
> waveforms are still square waves, they just overlap in time.
>
> You can do quarter stepping by providing four points, full 1,
> intermediate A, full 1 and full 2, intermediate B, and full 2. This
> sequence will repeat. The amount of overlap, time wise, or if the
> waveform is continually chopped, the duty cycle of the waveform,

will

> give intermediate A and B the relative strengths of pole 1 and pole

2

> needed to position the step halfway between full 1 and (full 1 and
> full 2).
>
> While you can run the motor with stepped square waves, I'm not sure
> that there is an advantage, since the drivers will dissipate the

least

> current when saturating, and the scheme above has the drivers
> saturating. It seems to me that a stepped square wave, while
> possible, would be a more complicated driver.
>
> Harvey
>
>
> >
> >At some point, you're back running a synchronous motor with sine
> >waves...
> >
> >Ballendo
> >
> >P.S. Our typical steppers will run just fine at 72 rpms, using a
> >capacitor on one coil, and fed by 60 cycle ac. (infinite
> >microsteps<G>)
> >
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Raymond Heckert"

<jnr@a...>

> >wrote:
> >> Has anybody ever designed a micro-stepping driver that outputs
> >> approximations of sine/cos to two adjacent coils. I understand
> >that you
> >> develop max torque at any rpm, up to the max (which is higher

than

> >the
> >> square-wave input). Is this true? How much would it take to
> >develop,
> >> assuming no one already has? BTW does anybody know anything

about

> >Don
> >> Lancaster's (tenaja.com) Magic Sine Waves, and if they can be
> >applied to
> >> this application.
> >>
> >> RayHex
> >>
> >> ----------
> >> > From: Jon Elson <elson@p...>
> >> >
> >> > Given the Gecko 320 as the driver for the servos, I think

there

> >are a
> >> > couple of significant advantages over steppers. First, the
> >servos are
> >> > usually capable of a greater speed range, while steppers

rapidly

> >lose
> >> > torque at higher speeds. With servos, you rarely have to

worry

> >about
> >> > resonance issues. You don't say whether you will use belts,

rack

> >and
> >> pinion
> >> > or leadscrews to drive this machine, but the servo will likely
> >provide
> >> > smoother motion, and be less perturbed by some backlash in the
> >mechanics.
> >> > Steppers can go wild when rack and pinion drive is used, and
> >rattle the
> >> > gears
> >> > something awful. Finally, if you lose a step with a stepper
> >drive, you
> >> > won't know it until you measure the finished part. With the
> >gecko 320,
> >> > it won't allow small errors to build up, and will fault if the
> >drive
> >> binds
> >> > up or crashes. This can be sensed by the computer and cause

it

> >to stop
> >> > all axes.
> >> >
> >> > Jon
> >
> >
> >
> >
> >Addresses:
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