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Servo vs. Stepper — Parallax Forums

Servo vs. Stepper

addaadda Posts: 13
edited 2006-10-21 00:55 in Robotics
I need advice about how to control a servo in an analog "clock application", ie the servo turns a hand to one o'clock,·two etc.· The·system must know the starting position of the servo in every instance the system is powered on/off, .· Also, in the event of jam, etc. the system position sensor must not become misaligned.· I want to avoid adding a separate potentiometer (other than·already in the servo)·or other sensor to check position.

I'm currently using the basic stamp 2 with the a GWS continuous·rotation servo.· My basic issue·is that I cannot get the servo to turn a consistent small number of degrees (eg. 10 deg.)·with Pulsout.· Pulsout 500 and Pulsout 1000 will turn the servo nearly 340 degs.· Pulsout between 500 and 1000·or other values does not seem to get me the small increments.· Is there a way to do this with a servo. ·I need about 359 degs of total·rotation in 10 deg or so increments.· I do NOT need to turn a full 360 degs (or more).

Can this type of incrementing (10 degs each step) be done with a continuous rotation servo?· I have the standard parallax servo as well·- this will provide·10 deg increment steps·but this servo·provides only about 150 degs·of total rotation.· Should I consider using a stepper motor?· Is so, I don't believe that steppers have a potentiometer so I will need to use some type of position sensor in the event of a jam - is this correct?

Thanks

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-10-20 21:04
    adda -

    Switch your thinking to a stepper motor, as you somewhat suggest in your subject line, and your problems will all but disappear.

    Regards,

    Bruce Bates

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  • addaadda Posts: 13
    edited 2006-10-20 21:30
    Bruce,

    Thanks for your suggestion. Does a stepper have a means for identifying the axle's position? Is this position relative or absolute?

    Best regards.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-10-20 21:56
    adda,
    ·
    ·· Stepper motors are rated to move a certain number of degrees per step.· However if something throws that off you have no way of knowing if you don’t have some sort of encoder to detect position.· Most Stepper Motors don’t come with an encoder by default, you have to add one.

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    Chris Savage
    Parallax Tech Support
  • Ken GraceyKen Gracey Posts: 7,392
    edited 2006-10-20 22:37
    Yeah, Chris is right. If your CNC mill uses stepper motors and you miss a beat for some reason you can botch up a two-hour piece of aluminum. Just like that! Faster than you can snap your fingers.

    So you say "get bigger steppers". Good idea. And a bigger power supply. Some of the missed step stuff can be avoided with ramping. Take a look at the Little Step U, a great helper for controlling steppers. For bigger ones, try the Gecko G201 drives.

    My advice about motors is to go to a surplus store and get a bunch of each kind you're interested in trying.

    Ken Gracey
  • addaadda Posts: 13
    edited 2006-10-21 00:55
    Thanks.

    Ok - steppers it is. The next question is how best to drive them. Cost is an issue, the parallax solution is a bit pricey. I need to drive to motors independently. One is about .5amps the other about 1.5amps. The leading choice so far is http://www.stepperboard.com/. Any other suggestions would be appreciated.
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