Re: fickle y-axis

Hi Jeff,

Thanks for taking the trouble for explaining. I am getting some of
what you are trying to explain for a two step motor, which I
visualise as below:

......................N.........................
................................................
................................................
......................S.........................
......................|.........................
......................|........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................X.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................N.........................
................................................
................................................
................................................
......................S.........................

When the pole reverse, the rotor may rotate in either a clockwise or
counter-clockwise direction. (arrow can be in either direction)


......................S.........................
................................................
................................................
......................S-->......................
......................|.........................
......................|........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................X.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
..................<---N.........................
................................................
................................................
................................................
......................N.........................


Would the diagram below describe a 4 step motor? (A for a second pole
of the same phase) which will become either North or south depending
on whether the command is clockwise or counter-clockwise?

......................N.........................
................................................
................................................
......................S.........................
......................|.........................
......................|........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
A.....................X........................A
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................|.........................
......................N.........................
................................................
................................................
................................................
......................S.........................

Starting from rest, when a command for a clockwise motion is given,
how will the A pole change, to a North pole or a south pole? I just
suspect I am missing something in the construction of a stepper motor.

Somehow, I think, the above diagram is more for a 2 phase motor than
a single phase 4 step motor, only think is, I don't know where I am
missing some information.

Peter



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp" <xylotex@h...>
wrote:

> 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