On 2008-09-04, Bob Engelhardt wrote:
DoN. Nichols wrote:
On 2008-09-03, Bob Engelhardt wrote:
[ ... ]
I bought a servo amp on eBay last night -...
I also see that it has adjustable current limits. On DC servo
motors you will proably find a "maximum current" spec. Don't exceed
this, as it can partially demagnetize the permanent magnet field
current.
I suppose that's a real problem with servos - where the amp can really
goose the motor when it's falling behind.
Yes -- unless it is one of the exotic magnets such as are found
in modern disk drives. The problem is that servo motors on eBay are
more likely to be ferrite magnets than anything else -- and those can be
demagnetized with excess current.
So -- set the current to minimum current, clamp the shaft of the
servo motor, turn the command voltage up until you get whatever that
current limit is (while measuring the current through the motor), and
slowly turn the limit up until you reach say 90% of the current limit on
the motor's label. This should be safe enough under all conditions.
You may find yourself needing to play with the damping
adjustments as well -- if you find the motor speed jerking above and
below the set speed.
The best way to adjust the damping (with the full load on the
motor) is to feed it a square wave as a command voltage, a high enough
frequency so the carriage only goes an inch or less, and observe the
output from the tach with an oscilloscope (if you don't have one, check
around for a friend with one). Adjust the damping so the leading edge
of the square wave has just a little visible rounding at the top. If it
has a very slow ramp up to speed (speed is what the tach output
measures), it is over-damped and will trail behind your desired
position. If the speed overshoots the set speed, then comes down below
it, then back above it somewhat less -- it is under-damped and you will
want to increase the damping.
O.K. There are two levels of damping in it -- a lower one when
S9 is open, and a higher one when S9 is closed. There are also
provisions for more precise tuning by adding a capacitor (C72) through a
hole in the case of the amplifier. There is apparently a manufacturer's
web page on tuning according to the PDF you (and I) downloaded. The
_Inhibit_In_ is used to shut down the amplifier under bad conditions,
such as when the carriage hits a limit switch at either end of travel.
It should never get there, but if it does, you want to shut the carriage
down *quickly*. I would suggest that you put the tailstock end limit
switch on the tailstock, since that can move from time to time, and a
stationary limit switch will not always be in the right place.
Add a relay and a panic button to shut it down even before it
hits a limit switch (such as when you discover that you are about to try
to turn the jaws of the chuck. :-)
You will still have to build or find an appropriate power supply
for the voltage which your servo motor wants.
Maybe yet another app for the venerable MOT G. I'd only need
buck/boost on the ac line, a bridge, & cap. Yes?
Hmm ... ideally, you want a transformer which isolates between
the primary and secondary since there is no isolation in the servo
amplifier. You can probably get away without this, but it is safer if
you have it -- and it will be mandatory if you eventually wind up
connecting it to a computer.
As for what voltage to make it for -- get your servo motor first
and then you will know. :-) If you don't need the full speed of the
servo motor -- go for perhaps 20% under the maximum voltage of the servo
amplifier or the motor -- whichever is lower -- since you seem to be
designing without a regulator. With a regulator, you can work closer to
the limits of the motor and/or amplifier.
Enjoy,
DoN.
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