[ASK] About Servo

Discussion in 'The Projects Forum' started by hokageleads, Oct 2, 2008.

  1. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    Hello Everyone,

    I'm working on a project, it's about to control several dc servo motor with different torque (max 25kgcm @4,8v = DS8711). What i'm asking is ..
    1. What driver that would sufficient to control the motor, considering the heavy current needed by the motor (currently using ULN2003)
    2. Can I have examples of the C codes to control the motor, I use ATMEGA8535 Microcontroller & CVAVR Programmer (just simple one will do)

    And i like to discuss further about dc servo motor.
    Thanks.

    Shi
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Do you have a schematic that you can post?

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Your servo already has control electronics built in. You just ensure that the supply to the servo is sufficient, then give it the usual servo control pulses. ie. 1.5mS null, ~2mS full clockwise, 1mS full counterclockwise..

    Steve

    this is assuming you are using the following servo motor

    http://www.horizonhobby.com/Products/Default.aspx?ProdID=JRPS8711
     
  5. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    here u go .....
     
  6. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    thx, but the motor is a little bit different with the one i used ..
     
  7. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    can u explain what's the difference of the pulses that u mentioned ..

    i still don't get it.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    The electronics for a hobby-type servo are built in, no need to build bridges and etc for the motor. You just need to apply the control pulses and power.

    http://www.seattlerobotics.org/guide/servos.html

    explains the control signal

    The problem is the ambiguity of what a servo is. Technically, it is an actuator that has position feedback, so that qualifies a lot of things. I searched the part number you gave and found a hobby servo

    Steve
     
  9. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    thx .. that's helpful
     
    Last edited: Oct 3, 2008
  10. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    the next problem is the heavy current produced by the motor because it's torque. I still use 1 servo motor .. if i add 3 or 4 more i don't think the ULN2003 will be able to handle the motor, the current needed by the motor by using my driver is almost 1A, any solution??
     
  11. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I am still confused to why you need the ULN2003. You just simply supply the voltage to the servo, then vary the control signal... It has the electronics built in to do this. It isn't like powering a motor directly, there are electronics integrated.

    Steve

    besides, there probably isn't much more than 1-2A going into this thing. You may only get power from: voltage X current or torque X speed, we have voltage, torque, but do not have speed or current. I saw a specified speed, but it didn't have an associated torque.
     
    Last edited: Oct 3, 2008
  12. hokageleads

    Thread Starter New Member

    Oct 2, 2008
    8
    0
    I use ULN2003 because the servo won't give the required torque when a mechanical load applied. Using only the current supplied by the uC won't be sufficient. I conclude this because I know that DC motor require more current to gain more torque. Power is voltage x current, the power required become larger as the mechanical load applied, and so does the current needed by the motor.

    That's why I separate the driver circuit from the uC board (using optoiso) and use ULN2003 (to amplify the current). If it's not separated (have the same common ground) then
    1. The heavy current may damage other component like 7805 or the uC
    2. The uC board can't supply sufficient current so the motor won't move when load applied
    Beside using ULN2003 can save some batteries (proven) ..

    I still can't find a better way (which is also reliable and safe) ..
     
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