Re: [CAD_CAM_EDM_DRO] Motor vs. Servo

jtfrimenko wrote:

> Simple question: when does a brush-D.C. motor become a "servo" motor?

In the purest, technical sense, it is when either the speed or position of
the motor (or object controlled by the motor) is sensed by some device, and
this is used to control the speed or position in a closed loop control
mechanism.

> Is it possible to use a non-servo brushed motor in a servo
> application?

Yes, as long as it is a permanent magnet field or shunt wound field.
Series motors will only run one way without reversing connections
inside the motor.

> As soon as the name "servo" is attached, the price goes
> up. Is low speed controll the limiting factor?

Maybe. Also, many servo motors have tachs or encoders built in.

> Are number of
> commutator segments the critical factor?

Not really. Many servo motors have no more segments (or armature
slots) than ordinary industrial-grade DC motors. (Hobby, toy-car
motors often have 3 slots and 3 commutator segments, the theoretical
minimum. They don't count.) There are a number of tricks employed
in servo motors to get rid of magnetic ripple, torque ripple and speed
ripple, all of which are undesirable features of some motors not optimized
for servo use. One is to 'twist' the slots, by assembling the armature
laminations with a slight skew, one to another, so that the slot is actually

slightly helical. This is usually done so the end of the slot is in line
with
the beginning of the next slot. This reduces magnetic ripple, because
the amount of armature iron under the field poles is constant as the
motor rotates. This magnetic ripple can cause a lot of whining noise
in motors, too, as the field structure actually vibrates as the motor
rotates. The skewing of the windings caused by the skewed slots
also means that the number of coil turns under each field pole is much
more constant as the motor rotates, also reducing torque ripple and
speed ripple.

There are other concerns with servo motors, such as cooling of a motor
that may be required to hold a load stationary under external force.
A cooling fan directly rotated by the motor shaft would be useless in
such a condition.

There are special servo motors designed with low, or extremely low
moment of inertia. Some of these are "printed motors", where the armature
is ironless copper traces on a printed circuit board material. Some are
ironless bobbin or thimble types, where a composite hollow rotor that
consists mostly of the copper windings held together with an epoxy
rotates over a stationary solid iron core. These motors ARE, indeed,
very expensive to make, and are totally impractical for machine tool
servo control. They are used in high-end computer tape drives where
the motor is directly coupled to a magnesium or beryllium capstan
that moves a section of vacuum buffered tape that weighs .01 g.
Here, the motor, itself, is the biggest inertia in the system, by a factor
of a thousand or so. Higher speed and smoother control can be achieved
by lowering motor inertia. Where you have a 100 Lb+ machine table, such
exotica is totally overridden by the table's inertia. But, a motor with low

torque and velocity ripple makes the servo system more stable, as the
control loop doesn't have to correct motor defects as well as all other
external forces.

Jon