Re: [CAD_CAM_EDM_DRO] Son of Servo questions.

no falloff wrote:

> Thanks to all those who responded. just a few more questions if you please.
>
> What is the difference between a Hall sensor, a "Tachsyn" (I'm assuming a tachometer yes?), and a frameless resolver type of primary feedback?

A resolver is primarily a position sensor which works in a manner sililar to a
linear variable differential transformer, except the cycle is made to repeat periodically.
This is similar to a quadrature encoder, with 2 sine waves 90 degrees out of phase as
the encoder is moved. Resolvers have coarse resolution due to the spreading of the
magnetic fields, so they have to be interpolated. The interpolators generally have
phase-locked loops or similar schemes to handle varying motion of the sensor.
These loops can be used to derive near real-time velocity information. A frameless
resolver soulds like it may mean slotless, ie. no slotted stator like an induction motor.

I'm not familiar with the term Tachsyn, but it brings to mind resolvers like the Farrand
controls Inductosyn, a linear resolver (although they make rotary, too). This may refer
to a resolver interpolator box with a tach output - but I'm guessing, here.

> I notice that stepper motors are usually size classified in NEMA sizes but i rarely see references to these sizes when looking for servos. does that make servo frame sizing more proprietary?
>

Somewhat, but many servo motors are made in NEMA sizes, too. Others are not made
to the NEMA spec, but could be described as "similar to NEMA size x" but the bolt holes
might not match.

> I've seen servos that have a torque rating in torque per amp. then give a demagnetization amperage. is that the peak torque for this motor? and likewise is it safe to figure that the holding torque (is holding the right word?) is 1/10th that figure?

The demagnetization current is the absolute maximum instantaneous current the motor
can survive without permanent damage. You would want to set a current limit some factor
below the limit. Pretty much ALL servo motors develop torque proportional to armature
current. Holding torque is not usually applied to servo motors. There are often stall torque
and continuous torque ratings. The stall torque is what the motor can supply continuously,
while not moving. The continuous torque is what the motor can supply WHILE moving,
and is generally higher, as the motion transfers heat better from the armature. 1/10th is
very arbitrary. If you only have a peak current rating, 1/10th would be a safe continuous
rating, but many motors can do better when moving. Many motors do up to 1/4 peak
for continuous. It is better to have all the data from the manufacturer, but you can't
always get it.

> If I plan on using a .5"/rev ball screw, a 500 count/rev encoder will give me a resolution of only .001" per pulse, yes? likewise if I want my accuracy and repeatability to be as close to .001" as I can afford, a higher encoder resolution would be preferable? Say 1000 count or higher (all other things being equal)?

No. .5"/rev is 2 turns/inch, right? a 500 cycle/rev encoder (that's how most are classified) will give
2000 quadrature counts/rev. With direct drive, that is 2000 x 2 = 4000 counts/inch = .00025"
resolution. If the encoder really gives 500 quadrature counts/rev, that is a 125 cycle/rev
encoder, pretty coarse to use in a machine tool.

> lastly: If I lead an armed movement to standardize and consolidate the specs for these things will any of you support me?

No, again. The plethora of standards and special parts makes all of this possible. If everything
was the same, you'd have to go to the servo junkyard, where they'd charge you your firstborn
child to get anything.

Jon