Re: [CAD_CAM_EDM_DRO] servo motors

george_barr wrote:

>Can someone explain to me the various parts of a servo motor for use
>in a CNC machine?
>

There are many types of motors, and many types of position, etc. feedback.

> What is the resolver, encoder, tachometer, servo
>amp, and anything else I missed and its purpose? I know what the
>encoder is since it reads upto 2000 lines/revolution. It essentially
>reads the position. The tachometer is the speed at which the motor
>is running. What is the resolver and servo amp?
>

Not all motors have a resolver. Generally, you would never see a motor
with BOTH a resolver and an encoder. Back before LEDs, resolvers were
thought
to be more reliable. Still, a resolver can work when filled with water or
oil, an optical encoder would generally fail when much of either fluid got
into it. Resolvers are becoming more rare as the circuits to read them are
more complex, and the reliability gap is at least narrowing.

A servo amp drives the motor. There are DC brush, "DC Brushless"
(really a permanent-magnet AC synchronous motor), AC induction,
and some more exotic types such as switched reluctance motors. So, the
servo amp at least needs to know what type of motor it is driving, and most
servo amps will only drive one type of motor. DC brushless amps need
to get commutation information from the encoder to know when to drive
current to which windings.

Most servo amps need some form of velocity info from the motor to force
the motor to turn at the rate requested by the CNC control's velocity
command.
Some use a tachometer, some use velocity info derived from the encoder or
resolver.

> Is there anything
>else I am missing to build a complete closed loop cnc using servos?
>Also, I want to use a servo motor as a spindle to both drill and tap
>with. What type of spindle servo will allow various rpm drilling
>speeds and allow the cnc machine to do tapping?
>
>

You generally do not need closed-loop servo motors for the spindle to do
tapping. What you do need is a spindle encoder to deliver precise spindle
position info to the CNC control, so the Z axis can be accurately slaved
to the spindle movement. Any servo drive could do this, but delivering
spindle
power can be done much more cheaply with an AC induction motor and
a VFD.

Jon