Servo Motors or Step Motors for steering?

Discussion in 'The Projects Forum' started by Panamax, Apr 1, 2009.

  1. Panamax

    Thread Starter New Member

    Apr 1, 2009
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    0
    Hey guys,
    I'm kind of in a rut here. I need to build a reverse car parking system on a small remote control car. The problems is steering... I'm not sure if servo motors or stepper motors are the way to go.
    ill need to control both tires at the front, so any suggestions on how I should start this would be great!

    Cheers :)
     
  2. Metalic soulja

    Member

    Mar 29, 2009
    10
    0
    lol that's funny because i was just about to ask the same think... im using a rack and pinion setup in my rc car and so far I'm going to use a servo, is this the way to go?
     
  3. Feign

    Active Member

    Mar 30, 2009
    50
    2
    RC Servo would be the faster solution. If my memory serves me correctly, an RC servo motor is a stepper motor with a position encode in one package.
     
  4. Metalic soulja

    Member

    Mar 29, 2009
    10
    0
    mmmk i thought it was something like that... how about the rack and pinion? would this be the best way to go for a reasonably heavy duty rc car?
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    In this specific application, a servo on rack and pinion. In any other robotic examples, freewheeling casters with direction determined by speed differential of drive motors.

    Servos have a position sensing element in them so they will deflect at the commanded angle. A Stepper motor without an encoder is an open loop system, so it can only be assumed that the wheels actually did turn when commanded.

    On servo control, from center, they can only move 90 degrees clockwise or counter clockwise. The amount of movement depends on the width of the pulse it receives. It needs to be sent a 1-2ms pulse every 20ms. A 1ms pulse will have the servo turn to max deflection in one direction (usually about 0.3 seconds from center), a 2ms pulse will cause the servo to turn completely to the other limit. A 1.5ms pulse keeps the servo centered.

    For turning an exact angle, the type of servo needs to be determined, 1.25/1.75 turn the servo to the 45 degree marks. It isn't sinusoidal, so the angle is 90/5 (90 degrees/5 milliseconds). Sort of Pulse Width Modulation, but with relatively long delays between pulses.

    The servo and rack and pinion can be gotten from a hobby store local, or as replacement parts for "monster truck" scale R/C car from a site like Tower Hobbies. They are available in metal and plastic (both rack/pinion, and the servo internal gears). Be aware though, metal geared ball bearing servos cost around $90 each, compared to $15 for their plastic counterparts.

    Some of the $5 "Mini Lego" vehicles have a rack & pinion, suspension, wheels and tires as well.
     
    Last edited: Apr 1, 2009
  6. Feign

    Active Member

    Mar 30, 2009
    50
    2
    Could you clairify "reverse car parking system", is that like the auto parallel parking?

    An older scrap printer would provide sufficent parts, cheap relativly heavy duty steppers with rotory encoder and lots and lots of misc components, depending on the vehicle size. The tricky and expensive bits would be avoiding and detecting objects.
     
  7. Metalic soulja

    Member

    Mar 29, 2009
    10
    0
    thanks heaps, great help
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,685
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    Model servo motors are not stepper motors. They are simple, brushed DC motors. The buzzing you sometimes feel is the motor's response to the controller. They can be converted to continuous running motors by removing the mechanical stop and replacing the pot with fixed resistors. That cannot be done with some servos that do not have complete output gears.

    John
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    I'd hate to parallel park with only directional control, speed control would be nice, maybe turn signals; all of that 's available in RC multi channel equipment; or do you have to buld all from scratch?
     
  10. Panamax

    Thread Starter New Member

    Apr 1, 2009
    3
    0
    Thank you all for the responses.

    That is right.
     
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