Re: z axis drive question

Dave,

You are looking for a simple answer to a complex question.

Think of it this way. A servo motor is nothing more than a DC motor
with an feedback device (encoder) attached to it. A servo motor is
nothing like a stepper motor. One way of visualizing it is that a
servo motor is similar to a analog signal(continuos) and a stepper
motor is like a digital signal(discrete).

Now as to how this relates to an encoder. By its very nature, an
encoder outputs a 'digital' signal. If you have a 1000 ppr (pulse
per revolution) encoder, and the servo system uses quadrature
feedback, in effect you have sliced one revolution into 4000 pieces.

To determine your system resolution, use the calculations provided by
the previous gentleman. If you determine that the calculated
resolution is not acceptable, you would add a gear head to the system
to increase the resolution (or use a higher lead ballscrew). (A
gearhead is also used to match inertias between the motor and load).

Now as to your question regarding commutation. The commutation does
not affect the position of the rotor (although you will be subject
to 'cogging' at very low speeds). Why it doesn't affect it is as
follows: If the system is tuned properly, the motion controller will
take the system error (the difference between the commanded position
and the actual (encoder) feedback position) and apply
a 'compensating' voltage to that error using the tuning parameters Kp
Kd and Ki so that the the motor moves to a position where the error
is theoretically zero (you will always have some error in the actual
versus commanded position.........that is know as following error).
The motion controller looks at all these parameters once per loop
update time (high performance motion controllers typically have loop
update times of 62.5 micro seconds). In other words, the drive will
apply compensating voltages to the motor every 62.5 microseconds.
Although you don't see it, the shaft is moving back and forth many
times per second (this is where the 'servo whine' comes from).

A side note about gearheads: General practice in the Industrial
Automation industry is to design a servo system such that the motor
runs on average between 2000 to 3000. Below 500 rpm the motor is
subject to cogging, and most servo motors will not run above 5000 rpm
or so.

Hope I didn't confuse you.

Tommy


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dave Fisher" <dave@d...>
wrote:

> I can see the calculation gives 0.0005", which I am sure would be

fine for

> my
> needs, but I can't seem to visualise how the DC servo motor can

move such a

> SMALL amount - is the motor not governed by its commutators and

magnetic

> pole
> pieces ?
>
> Dave
> ----- Original Message -----
> From: "turbulatordude" <davemucha@j...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Thursday, November 13, 2003 2:03 AM
> Subject: [CAD_CAM_EDM_DRO] Re: z axis drive question
>
>
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dave Fisher" <dave@d...>
> > wrote:
> > > I am in the process of building a Z axis for an X Y table I am
> > converting into
> > > a CNC router. I plan to use Artsoft mach2 software with Rutex
> > servo amps to drive the system.
> > >
> > > Can anyone give me any more info on the following:
> > >
> > > I have a quality 25 mm thick ball nut screw cut to 5 tpi - I

have a

> > choice of rotary
> > > encoders from 400 to 2000 steps per rev. I intend to drive the
> > screw
> > > direct from my DC servo motor shaft.
> > >
> > > Not being that familiar with servo systems (the amps haven't
> > arrived yet !) I am concerned
> > > that when I turn the ball screw by hand, a mere fraction of a

turn

> > seems to move the ball nut a long way !
> > > Can a DC servo system with my proposed setup resolve enough
> > resolution to move the ball a few thou,
> > > typically for fine, milling depth cuts for finishing etc.
> > >
> > > Any info would be most appreciated
> > >
> > > Rgds
> > >
> > > Dave Fisher
> > >
> > >
> > >
> > > [Non-text portions of this message have been removed]
> >
> >
> > Hi Dave,
> >
> > the simple starting point is that 5 TPI means 0.2" per revolution.
> > An encoder that output 400 points per revolution would resolve

that

> > to 0.0005"
> >
> > how fine of a resolution do you want or need ?
> >
> >
> > Dave
> >
> >
> >
> > Addresses:
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