Re:Stepper Motor vs Servo Motor

--- In CAD_CAM_EDM_DRO@y..., no falloff <nofalloff@y...> 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?

>

The difference between servo motors? Yoiks long topic there but the
short form any system with adequate specs will run your mill just
fine.

Servo types:
Stepper : Usually a permanent magnet motor key features the magnets
are on the rotor and the coils the outside. No brushes, the motor
steps between poles as alternate coils are energized. The driver
controls the coil sequencing to deturmine motor speed and direction.
They have full torque at 0 speed but tend to drop as speed increases.
Modern drivers have made these motors perform quite well. They are
easy to drive and maintain and are open loop capable. All the
positioning is done by the motor design you just tell it how many
steps to take by driving the coil sequence.

DC brush servo: This is a DC motor usually permanent magnet type. A
typical motor has the magnets on the outside and a wound rotor
commuted with brushes. To make it a position servo feedback is
required to close the control loop. These motors are easy to control
and quite strong. They require brush maintenance and dissipate heat
less effectively than other types so high usage applications tend to
other types these days.

DC brushless servo: Still a DC motor typically permanent magnet as
well but brushes are not used for commutation. You can now put the
magnets on the rotor and the coils on the shell for better heat
disipation and power handling. The drive is more complex as all the
work done by the brushes switching coils must now be done externaly.
The motor must have a controller to operate at all, as you need
sensor feedback from the motors rotor position to drive the coils.
Without the inductance and mechanical limitations of brushed
commutation these motors can run with less EMI and at much faster
speeds.

AC servo: Much like a brushless servo now you remove the rotor
magnets and use induced flux to drive the motor . These controls are
very complex but as the process gets better silicone is cheap magnets
are not. In theory you can get an even better control over the motor
since you do not have the fixed flux component and cogging that the
permanent magnet motor has. Ac motors are cheap.

> Is there a difference between an encoder and a tach? Or is RPM just

a calculated by the controller from the encoders feedback?

A Tach only provides speed and direction usually they are a voltage
proportional to speed. The encoder provides position and can be used
by the controller to differentiate speed and acceleration.

>
> I've seen servos that don't look like they have encoders on them.

Are servos often sold with the feedback hardware?

Steppers don't need encoders, all the other servos are just motors
without feedback and a controller.

>
> 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.

>

Well the servo/ ballscrew path is top notch. Myself I would snag the
machine scrounge the guts and make it go, then see.I'm cheap/broke.

> 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?

You need to double check the motor model but 400oz should be well in
the range of the Gecko, count the wires 4,6 or 8 good 5 bad. Servo
Size I bet there are a few folks with better ideas but you shuld need
no bigger than 400oz if the steppers can keep up. Better to measure
and do some math though you may want to extend the capabilities past
where the old control was able.

Brian
BSP