Re: [CAD_CAM_EDM_DRO] Re:Stepper Motor vs Servo Motor

no falloff wrote:

> I was hoping someone could clarify a few points for me.
>
> What is the practical difference between AC and DC, brush and brushless servos?

These terms are really misnomers. There are 3 types of motors.
The oldest, DC brush motors, are pretty familiar. They usually have
permenent magnet fields, but some may have wound fields. These would
be used in shunt connection for a servo motor. These motors have a
wound armature, a commutator, and usually 2 or 4 brushes to connect
the outside world to the rotating windings. The number of brushes will
be equal to the number of field poles.

Second are permanent magnet brushless motors. These are truly AC
synchronous motors, with permanent magnet rotors, and 3 phase windings
in the stators. (Some other motors with only 2 phases are used for fans,
etc.) These are commonly called DC brushless motors, but that is really
an incorrect designation. It comes from when they were first used in
DC fans with internal electronics to drive them.

Finally, there are AC induction motors, which are quite similar to the common
3-phase AC motors. For servo use, there is some difference in the windings
and rotors, but the motor construction and principles of operation are quite
similar. The rotor magnetization is induced from currents in the stator windings.

> Is there a difference between an encoder and a tach? Or is RPM just a calculated by the controller from the encoders feedback?

Yes, a tachometer only indicates speed. An encoder only indicates poition.
Speed can be derived from an encoder, but as the encoder is a quantized
device, it does not produce a useful reading until it has moved at least one
quadrature count. A good DC tach can deliver useful signals down to the
level of a degree per 10 seconds or slower. This can allow a velocity servo
system to continue to provide smooth motion down where the encoder is
delivering only a few counts per second. If you depended only on the encoder
for velocity, the system would start moving in a stair-step motion at some
low velocity.

> I've seen servos that don't look like they have encoders on them. Are servos often sold with the feedback hardware?

Some do, some don't. You can have high-quality servo motors with no encoder or tach,
and you can find them with both integral to the motor.

> In one of the classrooms at school sits an old (plaque says 1967) NC Bridgeport. The brain for this machine was "accidentally thrown away at least 10 years ago. The Chairman of the dept says I can have it if I do all the paper work. It has no manual control, but I thought an upgrade might render me a good machine. It has "slo-syn" steppers ( I think they are steppers as they don't seem to have any type of encoder/feedback provision) and acme screws, however for the type of work I want to do, I think (from what I?ve read here mostly) that ballscrews and servos are what I would want to use.

If the label says "Slo-Syn", they are steppers, as that is the trademark of the
Superior Electric Co. for their line of steppers. I think all Bridgeport-label CNC
machines from 1967 were stepper driven, although other companies made
servo-driven machines based on the Bridgeport iron back then. If the motors
are big blue cans with heat sinks, those are steppers.

> The steppers that are on the machine are 400oz/in so I figured that I should find servos that are at least that powerful does this sound right? Likewise can the gecko 320s handle motors this big? If not does it require a driver with a higher current rating?

Well, you could find servos that capacity pretty easily, or you could just use the steppers.
I think there is a special version of the Gecko drive for these large (size 42) stepper motors.

Jon