Re: [CAD_CAM_EDM_DRO] Is a reduction drive necessary for servo motors?

On Thu, 15 Jan 2004 11:20:34 -0700, you wrote:

>Organization: Pico Systems
>To: CAD_CAM_EDM_DRO@yahoogroups.com
>From: Jon Elson <elson@...>
>Date sent: Wed, 14 Jan 2004 23:23:59 -0600
>Subject: Re: [CAD_CAM_EDM_DRO] Is a reduction drive necessary
>for servo motors?
>Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com
>
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>Jon
>
>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.

That's more like the way a stepper works.

Servos are DC motors, apply a voltage, and they move and keep on
moving. If you short the rotor out (just two wires here) they will
brake and try to hold position because of the magnetic drag, although
that's not, iirc, the way that they maintain a position.

They have encoders which give a known number of steps/transitions per
revolution. One thing the drive can do is to command the motor to
turn in a particular direction and go X number of steps.

Steppers have multiple coils, and changing the current through these
coils will typically command the motor to align with the new magnetic
field. However, this will move (generally) only one "step". To
simulate motion, the coils are energized in a specific sequence that
forces the motor rotor to "track" those changes.

So the controller is doing different things for each type of motor.

For the servo, it applies a voltage, then counts encoder pulses until
the desired total is met, then stops the motor.

For the stepper, it applies a sequence of voltage changes to the
coils, for the specified number of steps.

Hope that this helps a bit.

Harvey


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