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"ordinary" DC motor vs DC "servo" motor?

G

Guest

Jan 1, 1970
0
What is the difference between a brush type "ordinary" DC motor and DC
"servo" motor?
Does adding a quadrature encoder to the ordinary DC motor convert it to a DC
sevo motor?
 
J

John Popelish

Jan 1, 1970
0
DCServoMotor said:
What is the difference between a brush type "ordinary" DC motor and DC
"servo" motor?
Does adding a quadrature encoder to the ordinary DC motor convert it to a DC
sevo motor?

Ordinary brush type DC motors come in several flavors. Series wound
motors (also called universal wound because they can run on AC) have
the field windings wired in series with the armature, so that the
torque produced is proportional to the square of the current (the
armature current reacts against the equal field current). Since these
produce torque in the same direction, regardless of the direction of
current (hence the usability on AC). So these are not usable as servo
motors.

Shunt wound motors have the field winding either wired in parallel
with the armature, or excited by a separate current, entirely. The
separately excited shunt wound motors can be used as servo motors,
since their torque is essentially proportional to armature current and
their speed is approximately proportional to armature voltage.

Permanent magnet field motors are very similar in character to
separately excited wound field motors, since their field's magnetic
strength is not related to armature current. They are commonly used
as servo motors.

Any motor that can produce torque in either direction can, in theory,
be made into a servo motor if you can measure its speed to be used by
the servo loop controller. Being able to measure the torque is very
handy, also, so motors that have torque proportional to armature
current make this easy.

Strictly speaking, a servo is a motor application, not a type of
motor.
 
P

peterken

Jan 1, 1970
0
Ordinary brush motors are just motors starting to turn when a voltage is
applied

Stepper motors can be indicated to be servo motors
http://www.seattlerobotics.org/guide/servos.html

On the other hand, also ordinary motors may be indicated to be servo motors,
depending on their application
http://www.aptronix.com/fuzzynet/applnote/servo.htm

Also more generic info at:
http://users.telenet.be/educypedia/electronics/motorservo.htm

As stated above, depending on their application adding an encoder *might*
turn an ordinary motor into a servo motor


<DCServoMotor> wrote in message
What is the difference between a brush type "ordinary" DC motor and DC
"servo" motor?
Does adding a quadrature encoder to the ordinary DC motor convert it to a DC
servo motor?
 
E

Eric R Snow

Jan 1, 1970
0
Ordinary brush motors are just motors starting to turn when a voltage is
applied

Stepper motors can be indicated to be servo motors
http://www.seattlerobotics.org/guide/servos.html

On the other hand, also ordinary motors may be indicated to be servo motors,
depending on their application
http://www.aptronix.com/fuzzynet/applnote/servo.htm

Also more generic info at:
http://users.telenet.be/educypedia/electronics/motorservo.htm

As stated above, depending on their application adding an encoder *might*
turn an ordinary motor into a servo motor


<DCServoMotor> wrote in message
What is the difference between a brush type "ordinary" DC motor and DC
"servo" motor?
Does adding a quadrature encoder to the ordinary DC motor convert it to a DC
servo motor?
Another requirement of a servo motor used for precise postioning is
it's predictable and consistent response throughout it's operating
range. So if it is poorly built it may react to the same voltage and
current differently as it spins. This means that the motor may pulse
as it spins. It is very hard to control the motor when it acts this
way. Modern motors are often built very well because the methods used
in production require repeatability. So even cheap motors can often be
used as servos if feedback is used. Especially if the application is
not very demanding. The servos used for RC models use a potentiometer
for position feedback and these devices are inexpensive and robust
considering what they cost.
ERS
 
M

Mikal Hodvik

Jan 1, 1970
0
I'm no expert on this, but I'm under the impression that it's primarily LOW
INERTIA that distinguishes a servomotor from the ordinary.

Cheers,
Mikal Hodvik
 
J

John Popelish

Jan 1, 1970
0
Mikal said:
I'm no expert on this, but I'm under the impression that it's primarily LOW
INERTIA that distinguishes a servomotor from the ordinary.

Short mechanical and inductive time constants and high peak torques
(demagnetization resistance) are what justify big bucks for high
performance servo motors. But there are lots of servo applications
that have little use for such prima donnas.
 
J

John Duffus

Jan 1, 1970
0
Mikal Hodvik said:
I'm no expert on this, but I'm under the impression that it's primarily LOW
INERTIA that distinguishes a servomotor from the ordinary.

Cheers,
Mikal Hodvik

Like you, my experience is that the name servomotor was reserved for one
specifically designed for the purpose, not just any motor that was used in a
servomechism. Aside from the inertia requirement I personally always
associated the name with a motor that was supplied with a constant armature
current and push-pull field controlled, but I don't know if that's general.
I think that the term servomotor came into use in the RC field because the
motor is often part of a servo package that contains the motor, transducer
and even electronic controller. Depending on how it is packaged the term
seems to apply variously to the motor, the motor-transducer combination, or
the motor-gearhead-transducer. I doubt whether the motors themselves have
any performance characteristics that particularly suit them to servo
applications.
Regards,
John Duffus
 
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