Re: fickle y-axis

Hi Peter,

Assume the NNN and SSS below are the A phase coils (no B phase), and
the S and N are permanent magnets on the rotor (this would be an
example of a two-step motor rather than 200).

NNN

S
|
|
O
|
|
N

SSS

Now, switch the polarity of NNN and SSS. The rotor will want to turn
to match up again. Which way will the top 'S' go, clockwise, or
counter clockwise? Who's to say? If the 'S' is a little closer to
the clockwise position before swaping polarity (due to a little
maechanical torque from a ballcrew, or because of momentum in that
direction, then it will go clockwise. The same is true for counter
clockwiese. An example: Put your foot on a tricycle pedal (while
standing directly over it) that is all the way at the top and push
down. Which direction will it go, forwards or backwards? Who's to
say? But if you are already pedaling you know. A stepper motor
without one of the phases is like trying to ride a tricycle with only
one pedal. Once you get going (forwards or backwards) all you need
to do is to keep adding power, the momentum will carry the pedal to
the proper position so you can add more power in the next stroke.
That's where the momentum comes in.
From a standstill, when you start giving pulses to the stepper,
momentarily, the A phase will go off when the missing B is
energized. Since there is no B, the rotor is then under external
mechanical influence only (torque from the ballscrew etc.) This can
be enough to move the rotor to one side or another. When A is
energized again, it can now have a prefernece on which direction to
move (the shortest route to get to where is wants to go). That can
start the momentum going.


Jeff
http://www.xylotex.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "forumtvm" <forumtvm@y...>
wrote:

> Hi all,
>
> To make it a little clearer, N for the rotor's north pole, S for

the

> stator's South pole. Cycle start with rotor's north pole at A2
>
>
> A1-B-A2-B-A3-B-A4-B-A5-B
> ------N
>
> ..........S
> A1-B-A2-B-A3-B-A4-B-A5-B
> ----------N
>
> ...............S
> A1-B-A2-B-A3-B-A4-B-A5-B
> ---------------N
>
> So even with B non-functional, A3, A4 strong enough to attract
> rotor's north pole to jump past B and move to A3, then to A4, say,

in

> a clockwise direction......
>
> Let's say, Cycle stop and start again with rotor's north at A4.
>
> .....................S
> A1-B-A2-B-A3-B-A4-B-A5-B
> ---------------N?
>
> How would a missing B phase cause N to rotate in a counter-

clockwise

> direction?
>
> .....................S
> A1-B-A2-B-A3-B-A4-B-A5-B
> ----------N?
>
> Where would momentum figure in this case? I just sense something
> still lacking in my understanding of stepper motor's working and

hope

> someone will fill it in.
>
> Thanks.
>
> Peter
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "forumtvm" <forumtvm@y...>
> wrote:
> > Thanks Jeff,
> >
> > Very clearly explained.
> >
> > A1-B-A2-B-A3-B-A4-B-A5-B
> > ------X--------------
> >
> > A1-B-A2-B-A3-B-A4-B-A5-B
> > -----------X----------
> >
> > A1-B-A2-B-A3-B-A4-B-A5-B
> > ----------------X------
> >
> > Momentum causes rotor to bypass B and fly all the way to the next

A

> > coil. Just a little bit more for complete understanding. Starting
> > from A2, How would starting a cycle cause the rotor X to go to A1
> > instead?
> >
> > Thanks
> >
> > Peter
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp"

<xylotex@h...>

> > wrote:
> > > Hi Peter,
> > > Why a stepper can turn with only one phase hooked up.
> > > Have you ever seen those old cartoons where two guys (A & B)
> are
> > > on the railroad tracks with one of those hand pump types of
> cars?
> > > The two take turns pushing down on the lever, first A pushes

down

> > > (while B's handle goes up), then the B pushes down (while A's
> > handle
> > > goes up). The two phase of your stepper motor are kind of

like

> > that
> > > (in full step mode). Now if one if the people (B for example),
> > isn't
> > > there, you'll have half the power, but as long as A's handle
> wasn't
> > > all of the way down, he can start some motion by pushing down

on

> > his
> > > handle. If he pushes enough so that his handle, after reaching
> the
> > > bottom, will cycle all of the way to the top, he can start a

full

> > > cycle by pushing down again. As long as each down stroke
> provides
> > > enough power so that momentum will carry the handle back to the
> > top,
> > > he can continue pushing, and moving the car. Your stepper can
> move
> > > similarly, as long as there is enough momentum to get the rotor
> to
> > > the next cycle.
> > >
> > > The direction your stepper takes with only a single phase
> attached
> > > will be pretty much arbitrary though. (If you've ever seen an
> old
> > > foot-powered treddle sewing machine, the pricnciple is also
> > > similar).
> > >
> > > I hope that helps a little.
> > >
> > > Jeff
> > > http://www.xylotex.com
> > >
> > >
> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "forumtvm"
> <forumtvm@y...>
> > > wrote:
> > > > Hi,
> > > >
> > > > Did some detective work - tried swapping motor cables around.
> > > Turned
> > > > out that the problem followed the "y-axis" cable. One of the
> wire
> > > was
> > > > loose, in fact, eventually broked loose from the soldered

pin.

> > > > Changing to a new cable solved the problem.
> > > >
> > > > However, I am puzzled as to how a bad cable can lead to

erratic

> > > > behaviour, that is when a movement in the positive direction

is

> > > > commanded, sometimes it move in the positive direction, and
> > > sometimes
> > > > in the negative. How would a motor behave if, say phase A

wires

> > are
> > > > OK but phase B connection is bad/broken. Would this cause the
> > motor
> > > > to move in the opposite direction to that commanded?
> > > >
> > > > Am more interested in understanding the problem than in

getting

> > the
> > > > problem solved. Hope someone smarter than me will explain.
> > > >
> > > > Puzzled.
> > > >
> > > >
> > > >
> > > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "forumtvm"
> > <forumtvm@y...>
> > > > wrote:
> > > > > Hi,
> > > > >
> > > > > Using DeskWinNC, Camtronics 2A driver, 33VDC power supply,

I

> > was
> > > > > getting only 16-18 ipm previously.
> > > > >
> > > > > Happy to report that after changing to a Pentium 4 and
> DeskCNC
> > > > (with
> > > > > the same old controller board) I managed to squeeze 35 ipm
> from
> > > the
> > > > x-
> > > > > axis before stalling. For the z-axis, I could get 17 ipm.
> > > > >
> > > > > However, the y-axis seem to be acting up. At one stage, I
> could
> > > > only
> > > > > get it to make noise without any movement. After powering
> off,
> > > > trying
> > > > > another day, it was OK again. Now the y-axis can't seem to
> make
> > > up
> > > > > its mind whether to move forward or backward. In MDI
> > > > > G91 G01 Y0.1 F10
> > > > > it is supposed to move forward (same direction) each time

it

> is
> > > > > activated. But what is happening is, sometimes it move
> forward,
> > > > other
> > > > > times it move backward.
> > > > >
> > > > > DeskCNC uses a serial cable from PC to controller board,

and

> > > > parallel
> > > > > cable from controller board to driver. So I try to see if I
> can
> > > > > isolate the problem. I switched to TurboCNC which uses a
> > parallel
> > > > > cable direct from PC to driver. Same problem.
> > > > >
> > > > > What could be the possible cause? Where should I start
> looking
> > to
> > > > > solve the problem?
> > > > >
> > > > >
> > > > > Looks like a lot more work to get a consistent dependable
> > system
> > > > > going.
> > > > > Thanks
> > > > >
> > > > > Peter