how can i reduce speed of motor with controlling its direction

Discussion in 'The Projects Forum' started by utilityhacker, May 1, 2014.

  1. utilityhacker

    Thread Starter Member

    May 1, 2014
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    i want to make a two wheeled robot.
    i m using two 3v dc motors.the problem is i want to reduce the speed of the motor without using gears and at the same time control directions of both motors.
    i am using arduino uno as brain of my robot.
    i used l293d to control the directions but couldn't control the speed.
    pls help.......
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Although PWM could provide speed control, I don't see how you can use a (presumably normal) 3V model motor without reduction gearing. Without gearing it would probably mean controlling the motor rotation to an accuracy of part of one revolution. Difficult if not impossible.
     
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  3. utilityhacker

    Thread Starter Member

    May 1, 2014
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    i actualy got them from an old rc car.....
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Fine for car use, but a 2-wheel robot .....? Will it be travelling as fast as the car? Can you post a description or sketch of the intended robot?
     
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  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Similar situation: Elactric tractor with 2 center tapped slide pots for speed & direction. Pots forward of CT, moves slowly forward, R pot advanced, starts L turn; back on L pot to CT & beyond, fast L turn. Discrete components controlling PWM & H bridges. Gave it all away.
     
    Last edited: May 1, 2014
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  6. utilityhacker

    Thread Starter Member

    May 1, 2014
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    Not as fast as car but like an average two wheeled robot
    this is my first robot.
    i just posted what i'm going to do.i haven't done it yet.
    i'm stuck with the hardware and controlling part.
     
  7. ErnieHorning

    Member

    Apr 17, 2014
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    the minimum speed of a DC motor depends on how many commutator sections there are. 6 can go slower then 3 and 12 can go slower than 6. The more there are the more the motor will cost too. Try just attaching a variable power supply to the motor and see how slow it will go with no load. You never be able to go slower than that.

    DC motors have less torque the slower they go, so there will be a bottom end with the weight of your robot.

    [​IMG]
     
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The DC brushed motors I use have maximum torque at zero rpm and is fairly flat up to the rated RPM, this is often typical of medium to high grade motors.
    Max.
     
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  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    True, true.

    DC motors approach zero torque at max speed and no load. Inverse is true, that is why stall torque is such an important datasheet parameter.
     
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  10. ErnieHorning

    Member

    Apr 17, 2014
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    I guess I was looking at this purely analogously. With an input of a DC supply alone, you can only control the speed by varying the voltage. In this case, the lower the voltage, the lower the torque will be.

    When using PWM (includes brushless), the full rated voltage is always applied, therefor full torque is available at any speed. It’s kind of cool to get arm twisting torque from a motor turning at the speed of a second hand. (Do kids these days even know what a second hand is?)
     
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  11. utilityhacker

    Thread Starter Member

    May 1, 2014
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    I think i figured it out...
    there was a tutorial on learn.adafruit about what I was searching.........
    Thankyou all for your help ........
    Have a nice day..
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    When using DC brushed, BLDC or AC P.M. motors of the servo variety, the curves that are important are the continuous torque and peak torque curves,
    The continuous rated torque curve is usually fairly flat from stall up to the rated RPM.
    In designing a drive, it is necessary to limit the current to ensure that the max continuous current/torque is never exceeded or if so, not more than a very limited period, to avoid reaching the max torque where destruction can occur.
    Motor control can be done by a series of methods, PWM being one of them, and now the most common.
    Max.
     
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  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    They will surely tell you it is the one in between the first and the third hand.

    Or, they may tell you it is stuff they bought off of craigslist.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What about clock-wise, anti(counter)- clock wise ?
    Max.
     
  15. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Yes, you should try to show what you intend to do.

    Maybe the OP is thinking of controlling the heading by altering the speed/rotation sense of the rear wheels similar to what is done to control vehicles on tracks or even vessels with two propellers.

    In many robots, the "forward" wheel is just there to provide stability (no traction at all).

    Once I had the chance to drive, using remote control, one heavy Extec machine (mining industry) making her to turn, full speed by turning the tracks in oposite direction. Amazing...and very dangerous! :eek:
     
  16. embpic

    Member

    May 29, 2013
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    i think you have implement PID algo in this project to jet stability.
     
  17. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    hah..... there needs to be a push at the Fed level (US) to get kids more involved with silicon in that k-4 grade range. mind boggling that kids today are more interested in posting their location to face-crook and they show no interest in knowing what actually runs face-crook, etc. most kids dont even know what silicon is, most will say "errr, hmmmm, is that a glue?".

    as to the DC motor Q. i would think PWM with + and - rails will allow a single DC motor to do the job of providing the torq at any speed and provide reversing.
     
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