Re: [CAD_CAM_EDM_DRO] Re: PWM vs Constant current

Alan, thanks for taking the time. Don't worry, I'm not offended by someone
waving a BS flag , I'd rather learn this now vs, later after I've spent the
$$.

I had settled on a cnc4pc break out board and a charge pump run through
Gecko 202's and a 34 size motors but this kind of side-tracked me. I'm
looking to build the simplest [ as far as figuring out electronics] system
by purchaseing all the pre-made boards and what not that I can off the
shelf. Yet I didn't want to buy a complete off the shelf system as that was
to big of a jump in price.. somewhere in between was the plan. I've been
reading cnc sites and posts for 2 weeks straight [ at least 5 hrs per night
as well as weekends and coffee breaks @ work ] so.. I'm getting a grasp but
still missing some key points which will hopefully be filled in over time.
I'm wondering about the Rex board by Gecko, what does that do vs, a
break-out board? Any recommendations for breakout boards that work great w/
Mach 3? I've got many more questions but I'll leave those for later for
now I'm off the the yahoo file section for more reading!

Thanks
Jarrett Johnson

----- Original Message -----
From: "Alan Rothenbush" <alan@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, October 24, 2005 3:56 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: PWM vs Constant current


> On Monday 24 October 2005 11:55, Jarrett & Heidi Johnson wrote:
>> Alan, I truely don't know [ hence why I asked]. Below is a paste from him
>> which is off a website where he purchased his equipment. I'm sure they
>> are
>> trying to promote their product however I just want the final answer that
>> isn't bias in any way :-)
>
> I'd wait for more in the group to comment, but what's written below
> strikes me
> as .. not completely accurate .. to be as polite as I can be.
>
> Others might suggest "total BS" as an accurate description, but I'll
> remain
> polite.
>
> Driving stepper motors is a complex process, but it's also reasonably well
> understood. I'd advise you to go to the files section of this group and
> study all that's there, paying particular attention to the "white papers"
> produced by Mariss Fremanis, but studying all that's there.
>
> To get you started though, I'll give you a GREATLY simplified view of a
> couple
> of points, that while not being completely technically accurate, will be
> close enough to help you being.
>
> Steppers are current operated devices. That is, you must shove electrons
> (current) into them to make them work.
>
> Shove more current into them, they make more power.
>
> But you can't put too much current into them or they overheat and fail.
> (Typically, the insulation on the wires melts). But even before they
> fail,
> extra current may not help as the current works in conjunction with the
> magnetic materials in the stepper and if you've "exhausted" the magnetic
> materials, extra current just goes to heat.
>
> So, there is a maximum current for any particular motor, it's based upon
> wire
> size and magnetic materials, and manufacturers are pretty good at
> optimizing
> the two.
>
>
> Now, shove current into a stepper motor and it will move to a particular
> angle
> and stay there, resisting attempts to move it. Larger motors (with larger
> amounts of current shoved into them) resist harder. This resistance is
> the
> "holding torque" and is a measure of motor's ability to retain its
> rotational
> position. That is, it's a measure of the motor's performance when
> stopped.
>
> For any given motor, this torque is proportional to the current going into
> it,
> the "resistance to change" increasing as the current increase, right up to
> the maximum current.
>
> (The above statement is a generalization and one of those "not completely
> technically accurate" referred to above, but it's close enough for this
> level
> of discussion)
>
> So, you want more power, feed it more current, and if you can't feed it
> more
> current because it will melt down, buy a bigger motor.
>
>
> Now, all the above implies a means of supplying an optimal amount of
> current
> without supplying too much current, and there are a number of ways of
> doing
> so. The first is to read the manufacturer's spec sheet, where a motor is
> typically listed with a voltage and a current spec, that basically says
> "this
> is the maximum current this motor can take, and if you give it this much
> voltage, that's the current it will draw".
>
> One such spec might be 2 volts @ 2 amps, so put 2 volts on a pair of leads
> and
> 2 amps will be drawn.
>
> The question then becomes, if a 2 volt power supply will cause this motor
> to
> draw the full 2 amps it can take, why not just build a 2 volt power supply
> and be done with it ?
>
> And the answer is, if all you want to motor to do is sit there, building a
> 2
> volt supply is the right thing to do. But if you want it to rotate ...
>
>
> At this point, we need to talk a bit about how a stepper motor rotates.
> (Again a simplification) Steppers have two windings, and if we shove
> current
> into one of the windings, the rotor moves to one position and if we shove
> current into the other winding, it moves to another. The other key point
> is
> that the direction we "shove" the current in is important .. shove the
> current into winding one "east to west" it moves to one position, shove it
> into winding 1 (same winding) "west to east" and it moves to another
> position.
>
> So, take the simplest possible stepper motor and here's the
> position/current
> table
>
> Winding 1 west to east 12:00
> Winding 2 north to south 3:00
> Winding 1 east to west 6:00
> Winding 2 south to north 9:00
>
> Two coils * two current directions = 4 positions.
>
>
> OK, you say, how about just building a pair of 2 volt supplies,
> electronically
> connecting them up as required, and in fact, that would work.
>
> However, here's important technical concept number 1 .. it takes time to
> shove
> that current into the winding. (I'm going to assume from here on that we
> want
> the motor to rotate.) If we want full power out of motor, we have to wait
> until we've filled a winding up before going onto the next winding.
>
> The faster we can get current into a winding (and out again .. more on
> this
> later), the faster a motor will turn. Now we could just stuff less
> current
> into a motor and it would turn faster, but at lower power, and we want all
> the power we've paid for.
>
> The ONLY way to get that current into a winding is to push harder, which
> electrically means using a higher voltage. But if we use a higher voltage,
> we'll get more current (current is directly proportional to the voltage
> applied) and we'll melt the motor down.
>
> So there's our quandry .. how we both use a higher voltage while not
> exceeding
> the current ? There are three solutions, a resistor, a chopper source and
> a
> constant current source.
>
> The resistor is the cheapest solution, but one providing only a slight
> advantage .. it does have its place, but not in the context of machine
> tools.
>
> Both chopper and constant current are more complex than a single resistor,
> but
> both work very well at using a high voltage to shove just the right amount
> of
> current in, but no more, as quickly as possible.
>
> A basic Constant Current (CC) method is very simple to design and very
> reliable, but has the disadvantage of being extremely inefficient,
> requiring
> a much bigger power supply and turning all of that extra power into heat.
>
> It's no surprise that a CC design would appear as a kit .. they don't have
> to
> include the power supply and the cost of that supply does not appear in
> the
> calculations.
>
> Chopper supplies are somewhat more complex than a CC supply.. but only a
> bit .. and are WAY more efficient, easily saving in power supply costs
> what
> they eat up in extra parts.
>
> Really, the market has spoken .. I would guess that 99.9% of all
> commercial
> stepper controllers for the machine industry built in the last ten years
> are
> chopper designs.
>
>
> HOWEVER, getting the current into the coils is only half the problem ..
> you
> have to get the current OUT of the coil as well before shoving current
> into
> the next one.
>
> And it is here that the K142 design appears to fall down. I say "appears"
> because I have not seen a schematic for it, only a pic of the board and so
> I've got to guess as to what's going on.
>
> Suffice to say (that's shorthand for "I'm tired of typing, so I'm going
> with
> short explanations") getting the current out .. aka, "current decay" is
> just
> as important in the whole scheme of performance as getting the current in,
> and it's not really related to CC or chopper designs. Either can have
> sophisticated or primitive approaches to the question of getting the
> current
> out.
>
> Do some reading on this topic.
>
>
> Finally, the big issue, microstepping, which I'll cover VERY briefly. The
> position of a rotor is related to the direction and magnitude of the
> current
> in the coils .. it's a vector quantity.
>
> Take our example above
>
> Winding 1 west to east 12:00
> Winding 2 north to south 3:00
>
> What would happen if we did both ? That is, supplied current to both
> windings
> at the same time, in the manner shown above ?
>
> The rotor would point to 1:30 !
>
> So if, using our primitive stepper above, we supplied current to one
> winding,
> then both, then one, then both, and so on, in the right sequence and the
> right direction, we would get 8 positions, not just 4.
>
> In fact, the position of the rotor is (roughly) determined by the formula
>
> angle = sin(current in winding 1) * cos(current in winding 2)
>
> Say, with our 2 amp motor, we shoved 2 amps into winding 1 W->E but only 1
> amp
> into winding 2 N->S, where would the rotor be ? Roughly 1:00
>
> Why is this important ? NOT because of what you're thinking, greater
> positional accuracy, because you don't get that (I said "roughly" earlier)
> but because of an annoying flaw in stepper motors known as "resonance".
>
> At some rotational speed, a stepper makes no power .. it is unable to move
> from position to position. Won't go into why, it just does.
>
> The RPM at which this happens is roughly related to the size of the steps.
> In
> our simple example, steps are 90 degrees wide, but in example two, they
> are
> only 45 degrees wide and so the RPM at which the motor becomes a problem
> is
> doubled.
>
> "Microstepping" .. the 1:00 scenario .. increases the number of
> intermediate
> steps, often to the point where resonance is no longer an issue.
>
> Microstepping is an absolute requirement in all modern stepper systems,
> and I
> see no evidence (from staring at the PCB) that the K142 supports this in
> any
> way.
>
> Read about microstepping.
>
>
>
> Hope this helps .. gotta go.
>
>
> Alan
> --
> Alan Rothenbush
> Academic Computing Services
> Simon Fraser University
> Burnaby, B.C., Canada
>
>
> Before me things create were none, save things
> Eternal, and eternal I endure.
> All hope abandon ye who enter here.
>
>
>
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