Re: [CAD_CAM_EDM_DRO] Re: How bout this idea??

For good performance in a servo system with linear encoders on the table,
I'd want a tach on the motor shaft as well.

Paul R. Hvidston, N6MGN
ACKSYS Engineering
Upland, CA

----- Original Message -----
From: "deanc500" <deanc@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, March 13, 2002 8:04 PM
Subject: [CAD_CAM_EDM_DRO] Re: How bout this idea??


> --- In CAD_CAM_EDM_DRO@y..., Doug Fortune <pentam@c...> wrote:
> > sspeed33317 wrote:
> >
> > > I came up with this idea , using regular DC motors , not servomotors
> > > mounting linear encoders right to all the axes
> > >
> > > would eliminate the need for Servo motors , would eliminate
> > > the need for Zero lash ball screws ,would eliminate the need
> > > for expensive servo motors , wouldn't have to compensate for any
> > > backlash because it would be reading actual table movement .
> > >
> > > And the benefit of the ACME screws friction to help hold
> > > the table in position .
> > >
> > > And you could tie in for DRO use ,or tie into an existing DRO
> > > for CNC use if you already have one.
> > >
> > > Good idea??? Any body have input??? .
> > > Already in service??
> >
> > Yup, great idea ... been using it for a few months now
> > (see my webpage where I sell this type of acme leadscrew
> > with an encoder nib on the end for mounting the encoder).
> >
> > The only minor catch is that you still have to make sure
> > your motor-coupler-screw (or motor-gearing-screw) interface
> > has very little backlash (compared to the resolution of the
> > encoder mounted on the screw), otherwise a nasty oscillation
> > can be set up (I'm guessing that a few encoder counts of
> > backlash is OK).
> >
> > Now your table will still appear to have backlash if your
> > table nut has backlash (this is a candidate for compensation
> > in software). Luckily I have found using long length nuts
> > (greater than 3:1 length:dia ratio) have under 1 thou backlash
> > (when new & unworn). I use a 6:1 ratio and there is no measurable
> > backlash.
> >
> > Note that (expensive but worth it in some applications) ballscrews
> > have other desirable features beyond just the low/no backlash
> > ballnuts, so just because you can eliminate backlash doesn't mean
> > that ballscrews aren't the perfect answer (for some applications).
> >
> >
> > Doug Fortune
> > http://www.cncKITS.com
>
> Hi Tim and Doug,
>
> Doug, know from our experiments how and encoder on a lead screw can
> cause trouble. There has to be next to no backlash in the system. So
> again, your long acme nuts or spring loaded backlash nuts would have
> to be used.
> I also have an idea that may be crazy, but here it is anyway. There
> have recently been a couple of threads discussing the virtues of
> ballscrews vs acme (and all its grades) vs regular threaded rod. All
> of these methods are bad for reasons of cost or poor accuracy. How
> about if you removed the encoder from the screw altogether. I saw a
> photo somewhere recently of a benchtop mill that had a dro where the
> encoders were driven by a cable. The fellow had even used a pulley
> system to increase the resolution. So what would happen if you had
> inexpensive screws and anti backlash nuts of some kind, and cable
> driven encoders. Seems to me that your accuracy would not be affected
> by the screw at all. Just a thought.
>
> I also like the idea of using just any dc motor and I have tried
> several with good results. They are a dime a dozen, and are good
> enough for hobby use. I just look for motors that have a relitivly low
> rotor inertia. I can usually tell how a motor will act by spinning it
> by hand. A good one is usually long and skinny, and wont spin by hand
> very easily. Hook it up to 1 or 2 volts and see if it cogs. If it
> doesnt, its a keeper.
> Dean