Hi Fr. tom,
Thank you for taking the time to post such a long reply.
I am familiar with microstepping and have read the "Jones"
chapters thoroughly. I know I need non-linear DAC to get
quarter steps that will give good linear machine travel.
I have some good linear CC drivers that do whole/half
steps, but will need some re-engineering to give me good
quarter stepping.
I was hoping someone could point me in the direction
of a simple one-chip solution so I can control it from simple
digital port and get perfect non-linear DAC in quarter or eighth
stepping.
The chips I seem to find on the net are not really
suitable, most of the bipolar output driver chips have stupid
voltage drops like 4v at 1amp, I am used to transitor stages
with 0.4v at 1 amp. And even then they need a second chip
to do the DAC stuff. If the worst comes to the worst I will
update my linear CC drivers to have four separate current
levels, not just two, and be done with it. I just thought there
were some new whiz-bang chips that would do everything I need.
I have never used microstepping before, normally half-steps
have been all I needed.
:o)
-Roman
Thomas McGahee wrote:
>
> Roman,
> Not really sure if what you want is a tutorial on
> microstepping, but if so, I will attempt an
> explanation.
>
> In microstepping you are really using a form of pulse width modulation.
> The usual sequencing used to achieve half-stepping is:
>
> A AB B BC C CD D DA ... A AB etc....
> Now imagine the following sequence:
>
> A A+.5B AB .5A+B B B+.5C BC .5B+C C C+.5D etc....
> The above sequence yields quarter steps.
>
> A A+.25B A+.5B A+.75B AB .75A+B .5A+B .25A+B B B+.25C ....
> The above sequence yields eighth steps.
>
> It is easy to expand the concept to 16 or 32 or 64 or 128 or 256
> microsteps per step.
>
> OK, so how do you get something like .25A? Simple. You pulse-
> width modulate with a 25% duty cycle. For .5A use a
> 50% duty cycle.
>
> You should realize that microstepping has reduced torque, and the
> accuracy is not 100%, but it is still a useful technique.
>
> The technique is easiest to implement with binary units such as
> 1 2 4 8 16 32 64 128, but you CAN implement a number such as
> 40 if you wanted to.
>
> Generally when you are moving long distances you would move in
> half steps whenever possible, and use microsteps at the beginning and
> end of the move to get the position correct. That is for straight
> line motion. If you are drawing curves, then you might want to stay in
> microstep mode the entire travel time. Note that you must go slower
> when microstepping. This is due to the reduced torque, and also to the
> fact that when moving in microstep mode you cannot move faster
> than the rate at which you can smoothly move from one pulse-width
> modulated value to the next. Think of motion as being directly
> related to the diferences in PHASE. You should never attempt
> to move faster than the rate at which you can smoothly adjust the
> pulse width modulation.
>
> Think of a PWM table that goes from one microstep to the next. To
> move from one point to another with the physical stepper motor
> you must smoothly traverse this table without skipping any
> steps. And remember that some microsteps require things like
> A+.25B You must "linger" at this point long enough to produce
> at least one 25% pulse. You should only move on to the next
> "position" at the exact end of the previous PWM period.
>
> So in practice the PWM period has an influence on the practical
> speed that can be obtained.
>
> Use microstepping only where it is appropriate.
>
> If you want a quickie look at what the effects of microstepping
> are, then energize the winding of a stepper with, say,
> +5 volts. Now apply a variable 0-5 volts to the winding.
> You will see that the stepper motor will micro-move through
> the half-step distance. (Use a reasonable supply that can
> give the necessary current!). Try gluing a paper clip
> that has been straightened out to the flat part of the shaft.
> This will act as a pointer, and because it is long, it will
> be much easier to see the micro-movement. Plot the rotary
> positions at 0, 1, 2, 3, 4, and 5 volts. You will see
> that it is not perfectly linear, but close enough to be useful.
>
> If you look carefully at the microstepping sequence you will
> notice that it can be "folded" into two parts: A part that
> is at a fixed voltage, and a part that has a partial voltage.
> If you look carefully at just the variable sequence you will
> see that it goes: 0 .25 .5 .75 1 .75 .5 .25 0 .25 .5
> etc. So think of the total sequence like this:
>
> A with B going from 0 to 100%
> B with A going from 100% to 0
> B with C going from 0 to 100%
> C with B going from 100% to 0
> C with D going from 0 to 100%
> D with C going from 100% to 0
> D with A going from 0 to 100%
> A with D going from 100% to 0 and then it repeats.
>
> I hope you "see" what I am getting at. By looking at the
> waveforms in this light it is not too difficult to come
> up with a circuit and software that makes this sequence
> happen. Just remember, don't jump more than one step
> in the sequence at a time, otherwise you will get a very
> jumpy response.
>
> Fr. Tom McGahee
>
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