two types of servos was Re: Is a reduction...

Bob,

That was John Rees in Sautee, GA that Don mentions in his
flutterwhumper articles. John used to sell a 20 buck video on how to
turn alternators into steppers... Info at www.tinaja.com in the
flutterwhumpers library.

On to your next item:
I'm thinking that maybe you're confusing the "servos" used in radio
control models with the servos we speak about here in the CNC world?

First, they're BOTH servos. Which means they have a feedback loop to
control the motor movement.

The specialised R/C servos use a pulse train with varying duty cycle
(how much time On versus how much time OFF) to move the motor over
less than a full revolution. Okay, actually the MOTOR moves more,
because there is a gear train, but the output shaft typically moves
less than one revolution. The feedback device in an R/C sevo is a
variable resistor. A potentiometer. Pot for short. So when the duty
cycle of the pulse train is 50%--same time on as off-- the r/c servo
is centered. If the on time gets longer than the off time, the servo
output arm moves one way, and if the on time is less than the off
time, the output moves the other way.
This on to off time pulse train variation is a common electronic
technique(used in things besides servos) and is referred to as PWM
(Pulse Width Modulation). I'm leaving out some things about this R/C
servo to prevent confusion with the CNC servo about to be described...

It CAN sometimes be confusing because you'll see the same term--like
PWM-- used in many different areas. Be aware that this is possible,
and you'll save yourself much aggravation! Back to servos...

The servos we use in CNC generally turn MANY revolutions. We mount an
encoder(which is basically a counter) to the motor, or in more
expensive systems, to the machine axis itself.

The servo motor is connected to a drive which DOES have the ability
to reverse the current to make the motor turn one direction or the
other. It ALSO has the ability to give the motor more or less "juice"
to make it turn slowly or quickly. Instead of a pot for feedback, we
use the encoder (a counter, remember?). So if we want to move 400
counts(or any other value), we start by giving the motor some
electricity. It begins to turn, and with it the encoder counts start
coming in... Now either the DRIVE (like a gecko, which is a step-
servo), OR the controller (the STG card, or TRUE servo amplifiers,
like copleys, or Jon Elson's) pays attention to these counts from the
encoder, and adjusts the electricity amount and direction so that we
get to where we command at the speed we need to get there.

I've left a lot of details out here too, but hopefully these two
short descriptions will help you see the difference in the two types
of "servo"...

Ballendo

> I guess I don 't know how a Servo works. I thought that
> it was based on current reversals. Energize it with one
> polarity and moves and stays. Reverse the polarity and
> it moves. I have not been clear on how it determines which
> way to go. So obviously, I have not a full understanding.
> At one time I experimented with Automoile Alternators as
> stepping motors. As I recall, the stock winding on the stator
> drew too much current and I had them rewound with smaller
> wire and more turns. They were very powerful but also low
> resolution. Don Lancaster wrote and article and there was
> a guy using them to do wood carving. It was a very cheap
> way to get a lot of torque. Due to manufacturing tolerances,
> each step was not exactly the same as the other steps. This
> was no problem when you are making an alternator whose
> requirement is to put out a pulsating DC current.
>
> Bob McKnight
>
> Bob McKnight wrote:
>
> >Peter
> >
> >It looks like you would have to know the number of steps
> >per revolution of both the stepper and the Servo. I am
> >not sure how the servos are rated , but the steppers are
> >usually rated in degrees per step or steps per revolution.
> >I have been led to believe that steppers tend offer a smaller
> >incremental movement than Servos
> >
> >
> Servo MOTORS, themselves, have NO steps. They can move in
> infinitesimal units. The encoder sets the resolution of the

positions

> the motor can be commanded to index to. So, if you have a high
> resolution encoder, you get high positioning resolution. I have

1000

> cycle/rev encoders on my machine, and counting all transitions gives
> 4000 counts/rev, so this would be 20 times the resolution of a
> standard stepper in full step mode. This is not like microstepping,
> where stepper motor steps are broken into smaller units, but there's
> no guarantee the motor can move to those smaller increments. The
> encoder reads actual position to its resolution +/- some small

error,

> and the servo drive will attempt to drive the motor to the commanded
> position.
>
> Jon
>
>
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