Re: DC motor controller

Hi Mark,

I can follow along with that, But doesn't overshoot & undershoot
still equate to missed steps or even added steps? Ruined work?

The encoders I ordered are these...
http://www.surpluscenter.com/item.asp?UID=2006111209105239&item=10-1834&catname=electric

200 cpr. Is it workable.

I think I understand...It's all in the tuning.

If I used a spindle head at say 1200 rpm one day then a 30000 rpm
router the next, does all this tuning need to be redone.

It can just be slowed down in mach3?

John

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mark Vaughan" <mark@...> wrote:
>
> If you go to the Rutex site they show a graph of output position against
> time to a step response input.
>
> In control systems a step response is when you tell the servo to go
x number
> of pulses in zero time. Obviously this is impossible it will take
some time
> to accelerate, and then decelerate to position.
>
> (To those control systems engineers out there, I am going to try and
keep
> this simple and avoid PID, nyquist, poles and zero's etc)
>
> There are several things that can happen to a step response.
>
> In a slow system you see it move to position, and slow down before
it gets
> there taking a long time to move the last little bit to position.
>
> In a fast system, overshoot (go past it a little), and then pull
itself back
> to position. But it may get to position, or it may oscillate above
and below
> for a while.
>
>
>
> The servo drive has a gain figure which controls how quick it gets there
> when there is an error, more error and it must get there quicker,
but too
> fast and it will overshoot, and too much gain causes this
oscillation which
> you don't want.
>
> You normally up the gain until you get a little oscillation, you
then add
> some damping to stop the oscillation.
>
>
>
> Depending on the servo control there are then other adjustments that
work
> slightly differently with each manufacturers servo drive.
>
>
>
> Remember the step response is not a real situation, it's a standard test
> method, software like mach3 would control the acceleration within
the limits
> of the machine, and then the servo's work is much easier.
>
> The thing is to allow the servo to know when there is a slight error
so it
> can correct it within a few encoder pulses, you give it room to
play, but so
> fine you never see it.
>
>
>
> If you watch commercial machines with controls of the 1980's and early
> 1990's fitted with hand wheels perform a fast G0 move to position before
> slowing to cut, you will actually see the index on the hand wheel
stop and
> reverse a bit. As a control systems engineer, the control is rather
sloppy,
> but a normal cut is very much slower so the system is then much more
> precise. Most of these machines typically use 250 cpr encoders, if
you put a
> 2500 CPR encoder on and run mach3, you won't see any over or undershoot.
> It's real impressing to CNC operators that think our DIY stuff might be
> inferior.
>
> This is also why we tend to rough cut, and then finish cut with a slower
> feed which is more accurate as well as having a better surface
finish, the
> older machines suffered from a lot more positional lag during normal
> operation.
>
>
>
> To get a fast machine running well might take several hours to set
up the
> servo drives. I never really know when to stop. I like to see a fast
> response with very very little overshoot, perhaps 1 to 2 counts max.
But I
> also see machines especially some of the old ones with analogue controls
> that are very sloppy and they still work quite well, it depends how
hard you
> push them.
>
>
>
> Hope this gives you a little insight
>
> It will all make sense when you start to play.
>
>
>
> Regs Mark
>
>
>
> _____
>
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of John Hansford
> Sent: 12 November 2006 11:44
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: DC motor controller
>
>
>
> Thanks Mark,
>
> I'm trying to follow along, but I've never seen a servo setup
> run. I feel like I did 4 years ago when I got my first kit
> from stepperworld with 80 oz motors. The vids I've seen show
> them(servos) zipping around with real authority.
>
> I am greatly appreciative of all this help and advice.
>
> I've got some stuff ordered to get into "Trial & Error" phase
> as soon as possible. I guess a great deal will become clear
> when Hook-up time comes.
>
> I'll refer back to these posts many times.
>
> I usually build a machine 2-3 times before I'm satisfied with it,
> so I'm not counting on getting it right the first time.
> The main reason the machine's 80% MDF.
>
> For anyone interested, I'm going with the treadmill motors &
> cheap encoders from surpluscenter. Gecko G320's.
> Mainly because something similar has worked for Les for years.
> I need to run ANY servo to get an idea of what to expect.
> I can't go wrong having these motors on hand.
> The parts I need(for now) are basic geometric shapes from MDF.
> I won't know what I want to improve til I see something running.
> The Gecko's are akin to currency...If I can't use them somebody
> else can.
>
> I want to do this in a way that the next guy can learn from it too.
>
> We're all very fortunate to have such helpful and informed people
> from all around the world giving their input.
>
> Thanks Mark,
>
> John
>
> --- In CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> DRO@yahoogroups.com, "Mark Vaughan" <mark@> wrote:
> >
> > Les wrote >>. Higher CPR encoders make it easier for the drive to
> > compensate.
> >
> >
> >
> > This is a very good point for diy'ers. Many diy'ers look at the
> output speed
> > of programs like mach3 together with acceptable accuracy/resolution to
> > decide how many count they will select for their encoder. But
> resolution and
> > accuracy are not necessarily the same.
> >
> > Many modern servo controllers have a divider to allow you to set one
> step
> > input pulse to correspond to several encoder output pulses. This
> allows you
> > to use a hi count encoder so the servo drive has very good
accuracy and
> > several counts per step so it can see errors within each step and
> keep those
> > error small to much less than one step resolution. The divider then
> lets you
> > set the step resolution, and maintain speed with your step
> resolution being
> > pretty much your maintained accuracy even with a little over and
> undershoot
> > error. (cogging problems excluded in poor servo motors). Try
selecting a
> > pulse count resolution of at least 4 times your step resolution.
> >
> >
> >
> > Regs Mark
> >
> >
> >
> > _____
> >
> > From: CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> DRO@yahoogroups.com
> > [mailto:CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> DRO@yahoogroups.com] On Behalf Of Leslie Newell
> > Sent: 12 November 2006 09:29
> > To: CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
> DRO@yahoogroups.com
> > Subject: Re: [CAD_CAM_EDM_DRO] Re: DC motor controller
> >
> >
> >
> > Hi John,
> >
> > The servo drive will have to work to reduce the effect of the
cogging.
> > Modern drives are pretty good and can mask most of it. On my mill,
if I
> > watch the pulleys very closely I can just see the motor speed varying
> > slightly at certain feed rates. It appears to have no discernible
> > effect on accuracy or surface finish so I don't worry about it.
Higher
> > CPR encoders make it easier for the drive to compensate.
> >
> > Les
> >
> > John Hansford wrote:
> > > Hi Dave,Ron,
> > >
> > > Pictures...
> > > http://www.flickr <http://www.flickr
> <http://www.flickr.com/photos/26361144@N00/> com/photos/26361144@N00/>
> > com/photos/26361144@N00/
> > >
> > > I took the CW to mean the nut would probably fly off if it ran
> > > the other way. I opened it ...Nothing flew apart.
> > >
> > > What's the Bad news...How _BAD_ could they be as servos?
> > >
> > > I wouldn't have noticed the very slight cogging if I weren't
> > > trying to find it.
> > >
> > >
> > > John
> > >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>
>
>
>
> [Non-text portions of this message have been removed]
>