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

Discussion in 'Electronic Basics' started by Guest, Oct 13, 2004.

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  1. Guest

    Guest Guest

    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?
     
  2. 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.
     
  3. peterken

    peterken Guest

    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?
     
  4. Eric R Snow

    Eric R Snow Guest

    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
     
  5. Mikal Hodvik

    Mikal Hodvik Guest

    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
     
  6. 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.
     
  7. John Duffus

    John Duffus Guest

    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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