DC Servos VS Steppers: Identification & Applications.

C.MoherC.Moher Posts: 35
I have scrounged 2 motors out of a printer, one ID'd as a stepper with 5 leads & works fine. I'm told the other motor, with 4 leads, is a DC Servo. Can someone recommend a way to know for sure that this is a servo and a method for lead identification. I don't quite understand the difference between these 2 motor types and when and where you would use one type Vs another. My only book on the stamp has a good section on steppers but little on servos. A·circuit diagram and elementary code for servos would be ideal. Thanks in advance for your help.

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  • 9 Comments sorted by Date Added Votes
  • PJ AllenPJ Allen Banned Posts: 5,065
    edited January 2006 Vote Up0Vote Down
    Here's a link to a current/recent Subject (surprised you missed it) --

    http://forums.parallax.com/showthread.php?p=566652
  • Chris SavageChris Savage Parallax Engineering Posts: 13,778
    edited January 2006 Vote Up0Vote Down
    Depending on the case the motor is in 4 wires could be a bipolar stepper motor, whereas the 5-lead is most likely unipolar.

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    Chris Savage
    Parallax Tech Support
    [url=mailto:csavage@parallax.com]csavage@parallax.com[/url]
    Chris Savage
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  • C.MoherC.Moher Posts: 35
    edited January 2006 Vote Up0Vote Down
    Thanks for the replies. That was a nice intro to servos. I often thought and suggested that a permanent, moderator controlled forum for FAQs would be nice to save repeat answers to repeat questions... anyhow, this motor used to control the ink jet printer's platen moving left & right. There doesn't seem to be·any electronics to it, just four wires into a can. Movement feels a little lumpy, just like the 5 wire stepper. Chris you said "depending on the case the motor is in" ....Is there something about the case that will give it away ?
  • Chris SavageChris Savage Parallax Engineering Posts: 13,778
    edited January 2006 Vote Up0Vote Down
    Well, DC motors tend to have a single molded piece of metal like a small can for their cases.· Stepper motors on the other hand are often made from individual slices of metal, or a much beefier case due to the number of coils and the heat they generate.· Detent feeling on the shaft when spinning is another indicator it is a stepper motor.· I would bet it is a bipolar stepper.· Any numbers or markings?

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    Chris Savage
    Parallax Tech Support
    [url=mailto:csavage@parallax.com]csavage@parallax.com[/url]
    Chris Savage
    Engineering Tech, Parallax Inc.
    (916) 624-8333 x3005
    Website / Twitter / Google+
  • C.MoherC.Moher Posts: 35
    edited January 2006 Vote Up0Vote Down
    Malaysia
    42SIN-15K8NA
    QH4-4039 28 OHMS
    462 OMC
    Thanks....no big deal. I'll just treat it like a bipolar stepper and see what happens. It definately has that detent, as you call it.
  • C.MoherC.Moher Posts: 35
    edited January 2006 Vote Up0Vote Down
    Actually, I just read about the 4 wire stepper in Al Williams book and he says best to avoid them. I don't feel up to building an H bridge which he says is required to drive them so this baby will be going in the garbage. I'll order one of Parallax servos when I am at the point of wanting to use one. Thanks.
  • Chris SavageChris Savage Parallax Engineering Posts: 13,778
    edited January 2006 Vote Up0Vote Down
    Before you scrap that Stepper, bear in mind that the Parallax Professional Development Board uses the L293D, which is a push-pull driver.· It can driver a Bipolar Stepper Motor, and it's cheap.· Don't give up too easily.

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    Chris Savage
    Parallax Tech Support
    [url=mailto:csavage@parallax.com]csavage@parallax.com[/url]
    Chris Savage
    Engineering Tech, Parallax Inc.
    (916) 624-8333 x3005
    Website / Twitter / Google+
  • Steve TobinSteve Tobin Posts: 1
    edited November 2010 Vote Up0Vote Down
    Hi,
    The 4-leaded motor is a bipolar stepper, not a DC Servo. The DC servo is fundamentally a 2-leaded DC motor. It has a separate feedback mechanism usually mounted to the rear with separate leads, for instance an encoder.
    You may be interested in my recent book on DC Servos. It has code, but not for the Stamp because it is too slow in general for servo update rates. There are Analog and digital servo projects in the book:
    www.crcpress.com/product/isbn/9781420080032
    Best,
    Steve
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