On 2010-06-11, Karl Townsend wrote:

O.K. That voltage is probably necessary to get full torque at
full speed from the servo motors you have. (There should be maximum
voltage and maximum current on the motor's labels.)

Yep, max voltage 145 volts, pulse amps 32. (wow)

[ ... ]

I'd suggest your son do a speed and acceleration test to help you decide.

Consider that a certain percentage of CNC milling is rapid moves
-- moving to cut on another edge, or moving clear to change the tool --
and here is where the maximum speed helps you most. Seldom are cuts
actually made at those speeds. (And the rapid moves are also where
servos are a major win over steppers. :-)

this would be fun. Take a 2:1 transformer, a bridge rectifier, and a
electrolytic cap if you have it. Show him how this makes AC into DC. Have
him read a web site on design of these. (Today's reading English lesson) Now
hook it to your X axis with the table at center travel. Bump your power to
it for a second and measure acceleration and velocity. if your transformer
has 4:1 reduction, do that one too. If you need more speed, make a run with
110 to the rectifier. Extra credit, use a variac in the circuit.

One thing worth doing at the same time (or at last once you are
setting up the servo amps if you use other than what is already tuned
for it) is to hook an oscilloscope to the tach output voltage, then feed
a low frequency square wave (perhaps 10 Hz or so) into the command input
to the servo amp (make sure that the square wave goes equally above and
below ground so it will move back and forth around a center point
instead of driving to one end and hitting the stop), and look for proper
damping of the tach output. If it overshoots, you will likely have
hunting with physical overshoot and motion back. If it moves up with a
very rounded leading edge, then your motor will lag far behind the
commanded position. With just the right amount of damping, it will be a
fairly vertical leading edge, a tiny amount of rounding at the corner
and then a flat top. (Very much like tuning an oscilloscope probe to
the input capacitance of the scope using the 1 KHz square wave.)

This tuning should be with the motor driving the table, and a
typical load on the table -- milling vise and something on the large
side of what fits in the vise -- and compare the change when you empty
the vise -- and when you remove the vise and work with a bare table
(sort of like a single plate of aluminum held down with workpiece
clamping sets. :-)

The servo amps which I have include places to plug in extra
capacitors to tune the damping close, and trimpots to fine tune it.
(They also have a built-in transformer and power supply to run from 120
VAC.

O.K. Saving the image of the transformer then zooming in on a
servo amp shows two gray multi-turn trimpots visible. There really
should be more -- I guess accessed via the top or bottom edge instead of
the front edge.

Enjoy,
DoN.

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