Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

Bob Engelhardt wrote:

Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

That is an interesting question. Most servo motors come as a packaged unit with motor,
encoder, and sometimes tachometer (old days) all together.

I built a few machines to carry a SRIM injection head into presses years ago, I used AB
1326 DC servo motors back then. The constant cycle of starts stops tended to burn the
commutator in a couple spots. So, I took a few apart and cleaned up the commutator to put
them back in order.

There didn't seem to be all that much remarkable about the things other than provision for
a smaller output shaft on the non drive side to stick the encoder and tach on. I'm
speculating that they may be built for short term overloads (starting torque) but that is
a guess.

Someone likely knows more, should be a good thread Bob!

Wes

"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

A DC servo is just a DC motor with a feedback device. You can make a DC
motor into a servo by adding an encoder. Richard Kinch has a web site
showing his effort.

Now a good DC servo is designed for high torque at low speed without burning
out. A DC motor you pull off the shelf won't be optimized for this.

Karl

On Thu, 11 Jun 2009 21:55:15 -0500, "Karl Townsend"
wrote:


"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

A DC servo is just a DC motor with a feedback device. You can make a DC
motor into a servo by adding an encoder. Richard Kinch has a web site
showing his effort.

Now a good DC servo is designed for high torque at low speed without burning
out. A DC motor you pull off the shelf won't be optimized for this.

Motors that are marketed as servos also usually have lower inertia
than typical motors and magnet materials that are more resistant to
demagnetization under high peak currents. Both important features for
high acceleration, but often overkill in modest applications.

--
Ned Simmons

Bob Engelhardt writes:

Is a servo motor simply a DC motor with an encoder or tach?

That's what is often meant but even that is too restrictive. Any bare
motor with feedback and control over position or velocity would be "servo",
regardless of whether the sensing device is part of the motor. For
example, a CNC drive motor without an encoder is servo operated when the
controls read the DRO table encoders instead of an encoder on the motor
shaft.

The motor principle itself (PMDC, brushless AC, etc) is not a servo
characteristic. Low-inertia motor designs are sometimes thought of as more
properly servo types, but this has more to do with the application
requiring rapid acceleration than with the servo principle itself. There
are servo applications where you want to avoid acceleration and thus the
motor is designed for high inertia.

"Ned Simmons" wrote in message
...
On Thu, 11 Jun 2009 21:55:15 -0500, "Karl Townsend"
wrote:


"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

not all servos are DC - there are AC servo motors in several flavors,
including with a synchro or a resolver, a pot, or a tach - what makes a
servo motor a servo motor is its employment within a servo loop.

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia
than typical motors and magnet materials that are more resistant to
demagnetization under high peak currents. Both important features for
high acceleration, but often overkill in modest applications.

Very high currents from overloads can kill those magnets. I've had to replace motors for
that reason.

Wes

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia

Forgot to add, that is why they tend to be long and slender.

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

On Thu, 11 Jun 2009 23:47:04 -0400, Wes wrote:

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia
than typical motors and magnet materials that are more resistant to
demagnetization under high peak currents. Both important features for
high acceleration, but often overkill in modest applications.

Very high currents from overloads can kill those magnets. I've had to replace motors for
that reason.

Yup, there's a reason for the current limit on amps and drives. A
run-of-the-mill brushed DC motor might tolerate a 2:1 or 3:1 peak over
nameplate current before demag. Some high performance motors with
exotic magnet materials are OK with 7:1 peaks.

--
Ned Simmons

On Thu, 11 Jun 2009 22:15:27 -0400, Bob Engelhardt
wrote:

Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

Servo motors are intended for use in closed-loop control systems. This
makes the motor's dynamic response important: how quickly its speed
can track applied voltage. Time constants of all elements, including
motors, determine how responsive a control system can be with
stability, acceptable damping and freedom from overshoot. Servo motors
are therefore characterized and specified in considerably more detail
than motors that need merely function as prime movers. The system
designer will need to know torque constant as fn of current, speed as
fn of voltage, inductance, moment of inertia, etc etc.

It was once part of the education of young EE candidates to see how
the abstractions of control theory play out in the lab. Seeing red
pencil marks on a paper when one has offended Mr. Nyquist and wandered
into the right half plane is one thing, but seeing a 50 HP DC motor
go building-shakingly berserk does get yer attention. It did mine,
anyway.

On 2009-06-12, Bob Engelhardt wrote:
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

The DC servo motors which I have have several features distinct
from the average DC motor:

1) The number of poles and commutator segments is much higher to
minimize torque "cogging".

2) The rotor is wound on air, and potted in epoxy to hold its
shape, so the inertia is much less than a rotor wound on
laminated steel poles. There are likely to be stationary poles
inside the rotor to help concentrate the flux lines for maximum
torque for a given current.

This lightweight rotor construction allows much quicker
reactions to changes in command current. (However, it probably
does a lot to set the maximum RPM to be allowed. :-)

3) The motor and the tach generator share a common shaft, instead
of being coupled between housings. The tach generator has
a similarly large number of commutator segments, and is also
wound on air.

4) If there is an encoder as well (sometimes the encoder is
a liner one on the ways instead of on the motor), it also shares
a common shaft, and lives in a housing on the back of the motor.

All in all -- a true DC servo motor is designed for the task,
and will give much better performance at the high end than a motor,
generator, and encoder cobbled together.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

Bob,
The answer you are looking for is difficult to formulate, because people
misuse the term. A true servo motor provides drive torque, radial
positioning and position hold torque. These are analogue, multi phase motors
and they are expensive to make, inefficient and consume lots of energy. In
use, there is a transmitter that provides all the power for all the
receivers. They are effectively selsyns and they phase lock together in use,
sometimes used with a differential selsyn for small phase angle adjustment.
Today, this technology is seldom used because although accurate, they are
much more expensive and can do very limited work. They were primarily used
for compass drive and radar screen/antenna synchronization. Today that role
is being performed with stepper motors, with and without position encoders.
However, these are also inefficient and are low torque for the same reason,
using a large portion of the applied power for hold torque, but they are
less expensive. The high torque applications are usually satisfied with
permanent magnet (PM) motors, pulse width modulation (PWM), encoders,
tachometers and brakes because electronics are less expensive than copper
and they are very efficient. Unfortunately, they are all called servo
motors.
Steve

"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob

On Fri, 12 Jun 2009 15:44:11 +0200, "Steve Lusardi"
wrote:

Bob,
The answer you are looking for is difficult to formulate, because people
misuse the term. A true servo motor provides drive torque, radial
positioning and position hold torque. These are analogue, multi phase motors
and they are expensive to make, inefficient and consume lots of energy. In
use, there is a transmitter that provides all the power for all the
receivers. They are effectively selsyns and they phase lock together in use,
sometimes used with a differential selsyn for small phase angle adjustment.
Today, this technology is seldom used because although accurate, they are
much more expensive and can do very limited work. They were primarily used
for compass drive and radar screen/antenna synchronization. Today that role
is being performed with stepper motors, with and without position encoders.
However, these are also inefficient and are low torque for the same reason,
using a large portion of the applied power for hold torque, but they are
less expensive. The high torque applications are usually satisfied with
permanent magnet (PM) motors, pulse width modulation (PWM), encoders,
tachometers and brakes because electronics are less expensive than copper
and they are very efficient. Unfortunately, they are all called servo
motors.

It sounds to me that you're confusing synchros/selsyns with servos. A
synchro, or one of its cousins, may be used for position sensing in
the feedback loop of a servo system. But a pair of synchros does not
constitute a servo, in the classical sense of the term.

--
Ned Simmons

Ned Simmons wrote:
On Thu, 11 Jun 2009 21:55:15 -0500, "Karl Townsend"
wrote:

"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob
A DC servo is just a DC motor with a feedback device. You can make a DC
motor into a servo by adding an encoder. Richard Kinch has a web site
showing his effort.

Now a good DC servo is designed for high torque at low speed without burning
out. A DC motor you pull off the shelf won't be optimized for this.

Motors that are marketed as servos also usually have lower inertia
than typical motors and magnet materials that are more resistant to
demagnetization under high peak currents. Both important features for
high acceleration, but often overkill in modest applications.

In addition to that, servomotors, at least the
ones I've disassembled, appear to be built to
higher standards than plain motors. Better bearings,
closer rotor/armature-stator fit, more rigid frames,
better brush holders and lead terminations.

Jim Stewart fired this volley in news:h0u6qq$jc8
:

A DC servo is just a DC motor with a feedback device.

Cybernetics theory (servo theory) says any "effector" with a feedback
element that the "driver" can read to control the effect is a "servo
system". It doesn't have to be a motor; could be a heater. Your air
conditioner is a cybernetic system.

That's one reason they call what we're discussing "servo motors", and
not (at least not correctly) just "servos".

LLoyd

Ned,
That's the whole point of my reply. There is no classical sense of the term.
You may have different or favorite definitions, but others do as well. That
is the dilemma. Every one of my described devices provide a servo function
and I guess, can be legitimately called a servo. Even Fanuc calls any device
that drives a machine axis a servo, regardless of the technology used.
Steve

"Ned Simmons" wrote in message
...
On Fri, 12 Jun 2009 15:44:11 +0200, "Steve Lusardi"
wrote:

Bob,
The answer you are looking for is difficult to formulate, because people
misuse the term. A true servo motor provides drive torque, radial
positioning and position hold torque. These are analogue, multi phase
motors
and they are expensive to make, inefficient and consume lots of energy. In
use, there is a transmitter that provides all the power for all the
receivers. They are effectively selsyns and they phase lock together in
use,
sometimes used with a differential selsyn for small phase angle
adjustment.
Today, this technology is seldom used because although accurate, they are
much more expensive and can do very limited work. They were primarily used
for compass drive and radar screen/antenna synchronization. Today that
role
is being performed with stepper motors, with and without position
encoders.
However, these are also inefficient and are low torque for the same
reason,
using a large portion of the applied power for hold torque, but they are
less expensive. The high torque applications are usually satisfied with
permanent magnet (PM) motors, pulse width modulation (PWM), encoders,
tachometers and brakes because electronics are less expensive than copper
and they are very efficient. Unfortunately, they are all called servo
motors.

It sounds to me that you're confusing synchros/selsyns with servos. A
synchro, or one of its cousins, may be used for position sensing in
the feedback loop of a servo system. But a pair of synchros does not
constitute a servo, in the classical sense of the term.

--
Ned Simmons

A related question, has anyone here dealth with Allied Motion company,
they make servos and other specialty motors.

i

Wrong attribution, I didn't say that.

Lloyd E. Sponenburgh wrote:
Jim Stewart fired this volley in news:h0u6qq$jc8
:

A DC servo is just a DC motor with a feedback device.

Cybernetics theory (servo theory) says any "effector" with a feedback
element that the "driver" can read to control the effect is a "servo
system". It doesn't have to be a motor; could be a heater. Your air
conditioner is a cybernetic system.

That's one reason they call what we're discussing "servo motors", and
not (at least not correctly) just "servos".

LLoyd

Jim Stewart fired this volley in news:h0un30$7h8
:

Wrong attribution, I didn't say that.

Sorry... got the chain of wrong.

(I don't like it when folks do that to me, either)

LLoyd

On Thu, 11 Jun 2009 23:48:49 -0400, Wes wrote:

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia

Forgot to add, that is why they tend to be long and slender.

Wes

Definitely not true of a "printed circuit" servo motor. The armature
is about 1/8 inch long (thick)and 4-8 inches in diameter and runs
BETWEEN the pemanent magnets.

Thanks, everyone. Very informative - I learned a lot. Bob

wrote:

On Thu, 11 Jun 2009 23:48:49 -0400, Wes wrote:

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia

Forgot to add, that is why they tend to be long and slender.

Wes

Definitely not true of a "printed circuit" servo motor. The armature
is about 1/8 inch long (thick)and 4-8 inches in diameter and runs
BETWEEN the pemanent magnets.


That sounds interesting, what are they used in?

Wes

On Mon, 15 Jun 2009 17:38:06 -0400, Wes wrote:

wrote:

On Thu, 11 Jun 2009 23:48:49 -0400, Wes wrote:

Ned Simmons wrote:

Motors that are marketed as servos also usually have lower inertia

Forgot to add, that is why they tend to be long and slender.

Wes

Definitely not true of a "printed circuit" servo motor. The armature
is about 1/8 inch long (thick)and 4-8 inches in diameter and runs
BETWEEN the pemanent magnets.


That sounds interesting, what are they used in?

Wes
Got mine out of a scrap welding robot. Some are also used in
automotive apps - fan motors and power window winders apparently