Re: Homemade CNC 2 spindle lathe question

On 2008-08-08, etpm@xxxxxxxxxxx <etpm@xxxxxxxxxxx> wrote:
On 8 Aug 2008 04:03:59 GMT, "DoN. Nichols" <dnichols@xxxxxxxxxxx>
wrote:

On 2008-08-08, etpm@xxxxxxxxxxx <etpm@xxxxxxxxxxx> wrote:
To All,
The control on one of my CNC lathes has failed to the point that
repair is more expensive that replacement. Even though it's going to
be expensive a new control opens up all sorts of possibilities. One
feature that would be really nice would be synchronised dual spindles.

[ ... ]

The problem that I see is that the spindle will be moving at a
constant speed, while the tailstock spindle will be doing a series of
stops and starts which will result in wear on the jaws of the tailstock
chuck and on the workpiece harming its finish. During the time when
both chucks are gripping, you will be subjecting the workpiece to
torsion vibration too.

Note that a servo motor *can* run at a very stable speed, when
controlled by a true servo amplifier and with tach generator feedback.
You could probably even do what you need with an extra tach generator on
the headstock spindle generating the command voltage to the true servo
amp.

The Gecko servo driver, however, does not pay any attention to
the tach generator on the servo motor (if it is present at all), but
moves the motor in steps just like a stepper motor -- paying attention
only to the encoder.

[ ... ]

Greetings DoN,

Are you sure about the stepped motion of the Gecko drive? It is not
driving a stepper motor, but a servo.

But -- it is trying to make the servo motor take the place of a
stepper motor -- driven by a series of pulses instead of an analog speed
command signal.

Having considered the Gecko drives for possible use on my mill
refit I spent some time looking into them.

1) There is no provision for connecting the motor's tach generator
(speed feedback) to the Gecko so it has no way of telling how
fast it is moving -- just where the encoder says that it is at
the moment.

2) There is no analog command voltage input to the Gecko to say
how fast you want it to move the motor -- just step and
direction signals.

So -- assuming that the motor is starting from a stable
position, when it gets a single step input pulse, it causes the motor to
move in the right direction so the encoder tells it that it has done
what is needed.

I don't know the fine details of what it does internally, but if
I were designing that I would include two up-down counter and a
comparator which would output the difference, and use that to feed a D-A
converter to feed an analog voltage to a true servo amp so the more
behind it got, the faster it would run the motor.

I don't know whether the Gecko is as elaborate as that, however.
But I do expect it to move rather rapidly on a single step input, since
it does not know how soon the next pulse will arrive and it doesn't want
to get behind.

A *real* servo amplifier is an operational amplifier (op-amp)
with a high voltage and high current output stage. It takes a command
voltage proportional to the desired speed, and increases the voltage to
the motor until the tach generator produces an output voltage
corresponding to the desired speed. I've set one up with a 10V signal
corresponding to the maximum speed for the motor from the servo amp's
+/- 40V output range (about 2000 RPM). If I input a signal of 0.1mV
(0.0001V) I have to put a piece of tape on the motor shaft to be able to
see its motion, so it has a very wide range of precise speeds. I expect
that speed to be 0.04 RPM, or 1500 seconds per revolution. And if you
add a load to the shaft, it will *still* rotate at the commanded speed,
but just draw more current.

It keeps track of position by
converting the encoder pulses to a + or - voltage. And how is a Gecko
drive not a real servo amp?

Because a real servo amp does not use pulses as input, it uses
an analog voltage proportional to the desired speed. And you just set
the voltage and forget it until you need to change the speed (say when
the encoder tells the controller that it is approaching the
destination).

The Gecko requires a constant flow of properly spaced pulses to
keep it moving.

Your light calibration method is only a
one time thing isn't it?

Yes.

And if the speeds are slow enough, you can hook a differential
to the two spindles, set one motor up to run in reverse, and observe any
rotation of what normally would be the input shaft, but which is now an
output shaft. Tune for zero speed and you are fine for a reasonably
quick transfer from one chuck to the other.

Anyway, if the spindles are turning fairly
slowly when the swap is done I'm thinking that would be easier for the
Gecko to keep track.

I'm mostly worried about the headstock spindle moving at a
constant speed, and the tailstock doing a series of start-stop jumps.
Of course, if the headstock is also driven by a stepper motor with the
same number of steps per revolution, you could drive both from the same
pulse train and you would be fine.

I guess what I need to do is mount an encoder to
the lathe spindle, another to the servo I plan to use, and see how
close I can get. And how long it takes each time for the spindles to
synch.

That -- and some way to see what kind of stresses are applied to
something chucked in both at the same time.

Enjoy,
DoN.

--
Email: <dnichols@xxxxxxxxxxx> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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