stepping motor with high resolution

Contact Hurst and ask for a list of distributors or area reps, who are
in the business of selling in smaller quantities.

Also consider a standard motor with a microstepping drive. This is the
more common approach to the problem. Some of the drives provide for
1/256th of a step. So given a motor with a 1.8 degree step the effective
step resolution is about 0.007 degrees.

-- Gordon

Cosmin,

What about a standard motor and a gear (or belt) reduction?

Jeff.


--
Jeff Shirley
lotta-spam@mindspring.com
"Bill Gates is filthy rich, but that doesn't mean I want to be married to him."

    --Gooogle on the term "differential screw" (I think that's right);
it's a sneaky way to get high resolution with low tech stuff.

--
        "Steamboat Ed" Haas         :  I'll have the roast duck      
        Hacking the Trailing Edge!  :  with the mango salsa...
                          www.nmpproducts.com
                   ---Decks a-wash in a sea of words---

While I think some of the mechanical means suggested make some sense for
DC motors, for steppers a $150-200 circuit can provide finer control
with no backlash or mechanical setup. What's especially nice about
microstepping drives is that (most) also function in full-step mode.
You're not limited to a set top speed because nothing is geared down.
Under software control the motor will drive at full speed (e.g. 200
cps), so you don't have to sit there for a blue moon as your motors
re-home. Then, under microstep mode they move in fractions of a step.

For a CNC rig, which is my , geared down stuff means $$$ wasted in
operating time.

-- Gordon


steamer wrote:

Gordon:

While I am no expert on micro-stepping, but I have implemented
my own CNC 4-axis drive electronics using microstepping.

You can get some additional stepper resolution using
microstepping, but you do eventually reach a point of
diminishing returns.  At some point the friction in the
system will be greater than the torque required to move
a microstep.  It is extremely unlikely that you will be
able to divide a step into 1/256.  1/32 is probably
pushing it.

For me the primary reason for implementing micro-stepping
is to avoid resonance issues.  If you use ripple-step,
full-step, or even half step drive electronics to drive
a stepper motor, there are some speeds at which the
stepper motor will not want to run; instead, it freezes
up and makes a horrible noise.  Determining the speed
at which resonance occurs is difficult because it depends
upon many system wide factors such as friction, axis inertia,
etc.  The typical solution is to use micro-stepping electronics
to avoid these resonance effects.  I am at a loss of words
to describe why micro-stepping avoids resonance effects,
but suffice to say that is one of the primary reasons people
go through the hassle of doing micro-stepping electronics
for stepper motors.

For the original application that started this thread, I
suspect that resonance effects are not an issue.

-Wayne
430 No such article
Gordon McComb wrote:

Gordon:

While I am no expert on micro-stepping, but I have implemented
my own CNC 4-axis drive electronics using microstepping.

You can get some additional stepper resolution using
microstepping, but you do eventually reach a point of
diminishing returns.  At some point the friction in the
system will be greater than the torque required to move
a microstep.  It is extremely unlikely that you will be
able to divide a step into 1/256.  1/32 is probably
pushing it.

For me the primary reason for implementing micro-stepping
is to avoid resonance issues.  If you use ripple-step,
full-step, or even half step drive electronics to drive
a stepper motor, there are some speeds at which the
stepper motor will not want to run; instead, it freezes
up and makes a horrible noise.  Determining the speed
at which resonance occurs is difficult because it depends
upon many system wide factors such as friction, axis inertia,
etc.  The typical solution is to use micro-stepping electronics
to avoid these resonance effects.  I am at a loss of words
to describe why micro-stepping avoids resonance effects,
but suffice to say that is one of the primary reasons people
go through the hassle of doing micro-stepping electronics
for stepper motors.

For the original application that started this thread, I
suspect that resonance effects are not an issue.

-Wayne

I think you'll be hard-pressed to find a commercial drive that offers
less than 1/16.


It avoids resonance while accelerating the motor by providing a smoother
transition between steps; the motor is not snapping to multi-degree
angles and initiating a wobble which will develop into resonance at the
right frequency.


However there is another issue. The original poster wanted a stepper
motor with 0.1 degrees resolution. The fact of microstepping is that the
intermediate "steps" can be wildly inaccurate. Adding any load to the
system makes things worse. No, I agree with an earlier poster that
gearing is the answer. Not only does it allow high resolution, but it
provides greatly improved torque to prevent the load from making the
position droop or the motor skip steps. Stock Drive Products has a
number of methods for doing this, with backlash essential nil.

The original poster also specified that this will be run in full step
mode, so it appears they are already aware of microstepping and do not
wish to use it, either for simplified drive electronics or the
unreliable microstep resolution I mentioned above.

Wait a minute! The OP said 0.1 degree resolution, not accuracy to 0.1
degree. Different stuff. I'm of the school of thought that considers
these two words, while interrelated, don't mean the same thing, from an
engineering standpoint.

I'm impressed that industry is able to achieve *resolutions* for things
like 1200 dpi inkjet printers, and 2400x4800dpi flatbed scanners, while
still providing for fast re-homing speeds, and while using the cheapest
steppers China manufactures. They achieve these resolutions with
microstepping. Actually a microstepping driver and clever software.

The original poster didn't indicate the load requirements, and while he
did say full step mode, that could have been just from the spec sheets
for the motor he was interested in. Most stepper motors list specs for
full step mode, as a matter of consistency. Otherwise it's like
comparing apples to oranges.

One would assume most people know the concept of a gear system, being
familiar with driving a car. To me that's the obvious approach to
something like this, while a solution in software is not. This is why I
suggested it. As he didn't mention an application, the OP will have to
decide if the particulars of his system merit one approach or another.

-- Gordon

I really don't think that's a good example. Inkjets and scanners only
have to maintain a constant speed across the imaging area, and the
resolution (in direction of travel) comes from how fast they shoot ink
or sample the CCD. They are not homing the imaging head to each pixel
location on the page at 1200 dpi. A microstepping driver merely makes it
easier to keep the speed smooth, and not have annoying ripples in the
image where stepper resonance appeared.

The constant speed you refer to still has a minimum step angle
resolution, which directly gives the product its higher dpi
functionality. They go hand-in-hand.

The OP did use the term "resolution" and not accuracy, and it's not
difficult or expensive to achieve 0.1 degree resolution with a standard
stepper and a microstepping drive. Highland, AllMotion, AMS, and several
others sell programmable step drives that support 1/8 to 1/256
microsteps, some with encoder feedback to improve accuracy. These are
fairly commonplace in factory automation applications where certain
resolutions are needed yet without sacrificing rehoming speeds.

If the OP really does need *accuracy* to 0.1 degrees, and doesn't care
about the overall speed of the system, then I agree certain geardown
approaches may be preferred (i.e., for an astrodrive for a telescope).
In any case, the OP now has several suggestions to pick through.

-- Gordon

Ok, I've seen a few of your suggestions, but there are many problems, I
think.

First, I cannot buy a microstepping driver, someone said $150-$200, I
don't have the money, I'm way past my limit, I had to buy other stuff.

Someone commented about 0.1 value being "0.1 degree resolution" or
"accuracy to 0.1
degree". I actually need to position a laser module (a small one, looks
like an R6 batt) accurately, so this means at least that accurately.

 Anyway, I can't really see microstepping doing a fine job at doing
this, because of reasons others already mentioned, and also because of
stepper motor quality. I need an advised opinion on this, but I've
measured the stepper motor coils, and 3 of them have 80ohms, one has
77ohms, so I'm figuring if these are their error standards, then I
can't possibly hope to obtain that accuracy through microstepping
(maybe even the geometry inside the stepper isn't that precise either).

With the information you have given us so far, I would suggest
gearing down your stepper motor using either gears or pulleys.
At some point in time the friction, backlash, and slop in your
system will overcome your ability to accurately position the laser.
Whether or not you can get it down to .1 degree accuracy is going
to be more of a testiment to your mechanical construction abilities.

If you give us a little bit more information about your application,
we might be able to give you better suggestions.

-Wayne

CosminB [BRT] wrote: