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What is the diff between Stepper & Servo motors

Discussion in 'Electronic Basics' started by zalzon, Nov 15, 2003.

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  1. zalzon

    zalzon Guest

    couple o questions.


    Whats the different between stepper and servo motors? My
    understanding was that stepper motors are used for motion while servo
    motors are used for steering the front wheels.


    How many kgs/pounds can two DC 4volt, 0.9amp motors move?
    Could I move say 20 pounds with that on rc car wheels?

    thank you
  2. Stepper motors move a fixed angle per step (unless they are
    overloaded, and then they just vibrate). Servos move any amount based
    on a feedback device that measures their position and reports back to
    the control circuit. They can also be overloaded, but at least the
    control circuit knows that they are not going to where they are
    supposed to be.
    With a smooth enough floor and round enough wheels and bearings, you
    can move the space shuttle with two 4 volt .9 amp motors. With an
    ordinary floor and Rc car wheels, you can move 20 pounds if you gear
    the motors low enough. The motors have to overcome rolling friction,
    air drag, and also have some force to spare to produce acceleration.
    This is a very poorly defined problem as stated. For some actual load
    and wheel situation, you probably need to do some experimenting to
    better estimate the motive power required.
  3. JR North

    JR North Guest

    I assume you know how steppers work.
    Servo motors differ in that they are basically standard
    armature type, either brushed PM or brushless motors. They
    are necessarily feedback devices, using an encoder to
    provide closed loop operation and position ref. Steppers can
    be open loop, with no position ref from the motor itself.
    Steppers have great low rpm torque, low max rpm. Good for
    direct drive positioning. Geared servos (think R/C servos)
    are good for your steering application.
  4. Actually, a servo motor is not a kind of motor, but a feedback control
    system that contain any sort of motor. I have seen servo systems made
    with stepper, synchronous and induction motors, as well as with shunt
    wound and permanent magnet motors. Somebody has probably made servos
    out of series wound motors also, but stabilizing the control loop is a
    bit harder because of the torque proportional to current squared
    property of this type.
  6. Fred Abse

    Fred Abse Guest

    Shouldn't that be "serves"?
  7. -----------
    What? Lunch? It servo-es, damnit! ;->

    Servo-ing is tracking to a signalled value as quickly it is able by
    driving an actuator that moves a sensor.

  8. Yes, in the general sense you're right.

    "Servo" referrs to a feedback technique for controlling position.

    "Servo Mechanism" referrs to any mechanism using this technique.

    "Servo Motor" referrs to any motor using it.

    However, in hobby robotics, "servo" is generally used to mean
    an R/C Servo Motor, rather than the more general concepts.
  9. Joe Pfeiffer

    Joe Pfeiffer Guest

    The shortest answer is that they are completely different and have
    nothing to do with each other, and shouldn't be confused.

    A servo is more than a motor: it's both a motor and a feedback
    mechanism. You tell a servo "I want you to move to the following
    position" and the servo just plain does it. Commercial servoes
    typically have a fixed range of motion; they can turn their output
    shaft something like 270 degrees lock to lock. This makes them good
    for things like steering; you just say "go to straight ahead" and
    quit worrying about it.

    A stepper motor is, in some respects, the exact opposite of a servo.
    You tell a stepper motor you want it to move a step, and it does so.
    Keeping track of how many steps it's gone, and details like not giving
    it too many steps too quickly, are up to you. A stepper can keep
    going round and round, which is why it's good for drive wheels.

    Notice that you can do a servo in software by using a stepper, and
    counting how many steps you've given it.
    Nowhere near enough information. How much torque do the motors
    provide? How are they geared? What sort of acceleration do you want?
  10. Fred Abse

    Fred Abse Guest

    "Serves" as in: "We also serve who only stand and wait"
    I've never come across "Servo" used as a verb before. Interesting
    possibility. Might be Latin, as in:

    Servo - I serve
    Servas - You serve
    Servat - He, she, it serves
    Servamus - We serve
    Servatis - You (plural) serve
    Servant - They serve

    So, your concept would become: "Servo servat" - "The servo

    God! I hated Latin grammar!

  11. Me too. I told my latin teacher "Its all greek to me" one day. the
    old battle ax had absolutely no sense of humor.

    I say, the boy is so stupid that he tried to make a back up copy of his
    hard drive on the Xerox machine!

    Michael A. Terrell
    Central Florida
  12. -----------
    No, servo's, as he "it servoes back and forth".

    As I said:
    Modern English offers the "verbing" of nouns and the nouning of verbs
    as a legitimate word formation device.

    I loved it and won State contest prizes at it. I still read Marcus
    Tullius Cicero, Gaius Julius Caesar, and Publius Vergilius Maro.

    But the use of the word "Servo" in english is not related to Latin
    or the present tense of 1st Conjugation Latin words.

  13. zalzon

    zalzon Guest

    Thanks guys for clearing that up. Its much clearer now.

    One more question :

    What determines torque or turning power of a stepper motor - voltage
    or current?

    i.e. All other things being equal, will a 3 volt, 0.35 A stepper motor
    deliver more torque than say a 5.3 volt, 0.17A motor?
  14. zalzon

    zalzon Guest

    Say u were to tell the servo motor (mechanism) to spin half way around
    and i were to lightly grip the wheel to give it resistance. Would the
    circuitry detect that and transfer more current to the motor to make
    it overcome the resistance and spin to the desired location?
  15. Yes. That is what the feedback signal accomplishes. The motor
    receives just the current it needs to make the desired move.
  16. All torque comes from the interaction of stator and rotor magnetic
    fields. And magnetic fields are proportional to the current that
    causes them (if they come from a coil and not a permanent magnet). So
    torque is proportional to current (if the current energizes one
    electromagnet) or current squared (in the case of a series wound
    motor, where the same current magnetizes both the rotating part and
    the stationary part, in series).

    But it takes voltage to cause current to happen. And the faster a
    motor is turning, the more voltage it generates. The applied voltage
    must overcome the generated voltage before any current can be moved
    through the motor. So voltage must be available (and more for higher
    speed) but that voltage supply must have the ability to deliver
    current to produce torque. Total power is proportional to volts times
    amps (and to torque times speed).
  17. Rich Grise

    Rich Grise Guest

    Wouldn't that be a "wording" device?

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