Re: Homemade CNC 2 spindle lathe question

On 2008-08-09, Richard J Kinch <kinch@xxxxxxxxxxx> wrote:
DoN. Nichols writes:

I don't know whether the Gecko is as elaborate as that, however.

That is your problem. It is more elaborate than that. You can download
the documentation, which includes the theory, at http://www.geckodrive.com/

They've gone with cluttering up their site with Flash and
JavaScript. But the PDF manual still says what I remembered from the
one which I downloaded when I was considering that as a possible thing
to use in my Bridgeport upgrade.

The Gecko G340 *still* uses step and direction inputs, which
means that you can't say to it "Move three inches in 15 minutes" or
something similar and then sit back and let it move on its own. You
have to keep feeding it steps. Yes, you can jumper it internally so it
will *always* step up to ten steps for one input pulse -- but it will do
them in a sort period -- not over a long time. And you can't change
this from the computer control to allow faster motion when you need it
and then finer resolution when you need *that*.

It *still* does not have pins to connect the tach feedback to
allow it to measure (and thus control) the speed. It *only* uses the
encoder for feedback, which limits it to steps of the size of the
encoder's motion.

A look at the schematic says that it is doing things pretty much
as I would do it (and as I described it somewhere in this thread) with
counters and a digital comparator (adder). It can get 127 steps behind
before it gets lost. After that the sign of the error signal is
reversed and the motor will want to move the wrong direction until it
zeros out the difference.

The extra set of 4-bit counters and comparators feeding a VCO
(Voltage Controlled Oscillator) is obviously to change the pulse rate of
the generated pulses when you select 2, 5, or 10 pulses of motion for
one pulse of input. But no matte what, it will not go slow enough to
emulate a DC servo with a very low speed command voltage.

A *real* servo amplifier is an operational amplifier (op-amp)
with a high voltage and high current output stage.

You are describing an *analog* servo. A digital servo is just as "real".
Unless by "real" you mean "what I happen to know in my limited experience".

And the Gecko G340 is designed to allow a DC analog servo motor
to take the place of a stepper motor -- not to drive an analog servo
motor as an analog servo motor is designed to be used. The benefit of a
DC servo motor and the Gecko is that you can have a faster move when you
are not cutting -- real steppers have a limit on how fast you can step
them, as well as how fine a motion you can accomplish.

There are, of course, AC servo motors as well. I have a couple
of machine-tool sized ones but have not yet used them -- no
amplifiers/drivers for them yet. But I have used the tiny 400 Hz AC
servo motors used in aircraft analog computers and such.

Now -- I would expect a true digital servo to accept a command
to move at a given speed -- but with an 8-bit wide direct input instead
of sequential step and direction inputs. The Gecko G340 is a "pretend
I'm a stepper motor, so I can be used with old (or limited) controllers
which expect only stepper motors.

I still say that you can't give it a pulse, wait a minute, give
it another pulse, repeat and have the shaft move at a constant speed.
It will move rather quickly after the step (at a speed set by the
damping adjustment) and then wait still until the next step.

And thus -- I expect the tailstock spindle (which started this
discussion) to move in steps, while the headstock spindle (unless it is
also driven by a similar motor and driver) will work at a smooth
constant speed while not cutting.

Enjoy,
DoN.

--
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