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Servo motor

Discussion in 'Robotics & Mechatronics' started by uaefame, Oct 27, 2008.

  1. uaefame

    uaefame New Member

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    Hello everyone,

    I bought a servo motor from a hobby shop JR DS8911.
    I am trying to test it using function generator.

    The problem is I can't get it moving?

    Any ideas How to turn a servo motor.

    It comes with 3 wires 1power 2ground 3control.

    The control wire is hooked up into the function generator.

    Q> What is the required setting to turn a servo motor using function generator.

    Thanks in advance
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Look here for some data.

    Servo Basics
     
  3. Pommie

    Pommie Well-Known Member Most Helpful Member

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    You really should try to use google before asking such basic questions.

    Mike.
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Hesham,
    I have realised you uses the word 'turn' for the digital servo motor.

    A servo will only 'turn' , that is rotate, less than an angle of 300 degress or so, its not a stepper motor.

    Build the 555 circuit shown in the link and test/learn from the servo control before you consider using the 16F818 PIC.
     
  6. uaefame

    uaefame New Member

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    Thanks again,

    BTW, the 555 timer is it important to be build?

    Because I am interested to control the servo angle using microcontroller PIC16f818.

    pic 16f818 contain internal timer. So, I think no need of Timer 555?

    Just asking?

    BTW, the concept of 20ms? and the range 1ms-2ms with 1.5ms to be 90degree.

    The on signal for 1ms-2ms (The on signal to control the angle) and off signal for 10-30ms (Just to make sure the signal servo is on).

    Is this how it is?

    Thanks in advance
     
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I have not seen the datasheet for your servo, but the timings of 1 to 2mSec at 20mSec intervals sound about right.

    I suggested the 555 so that you could just test and check out the servo.

    If you feel confident to write a program for the 16F818 give it a try.:)
     
  8. uaefame

    uaefame New Member

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    I am interested in know the period?
    The on time range?
    The off time range?
    the duty cycle time range?

    If you know how to find these using experiment please provide it!
    Thanks in advance
     
  9. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Issue 1 of the JPUG newsletter has a servo test program for the 18f. A 16f version was posted in the forums ages ago.
     
  10. uaefame

    uaefame New Member

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    hmm, Thanks blueroomelectronics

    BTW its ISSUE 2 of the JPUG newsletter.

    Thanks again, I will start reading it.
     
  11. uaefame

    uaefame New Member

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    I tried seeting the On time to 1.5ms and the off time to 20ms and repeated it 70 times.

    It doesn't go to 90 degree? and it keep going on and on hitting the far right limit and jerking at that point.

    Please help :)

    If you have an idea about servo motor. I tried using the tutorial mentioned above but no use. If you know how to find the information about servo motor please let me know. Information concerning On time and Off time.

    Thanks in advance
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Hesham,
    This is the reason I suggested the 555 servo driver circuit, as you dont have details of the servo, it would enable you to determine experimentally the control required.:)
     
  13. blueroomelectronics

    blueroomelectronics Well-Known Member

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    The 555 is good advice but you better skip it and work on the PIC version or you'll never meet your Dec 8 deadline and you're hoping to control 18 servos? At the rate you're progressing you will not make it.
     
  14. uaefame

    uaefame New Member

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    Hmm, I will build this timer. For the time being, if anyone know the value of frequency for a digital servo let me know. I am working with 70Hz and it was bad!

    Any ideas

    Thanks in advance
     
  15. blueroomelectronics

    blueroomelectronics Well-Known Member

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    It's a pulse between 1ms to 2ms that arrives at 20ms intervals.
     
  16. uaefame

    uaefame New Member

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    I think its even lower because I am using digital servo.

    I could control it using function generator, somehow using 280Hz and duty cycle of 22%-61% (angle 0 till 120degree). I am interested in going up to 180degree. why the servo won't rotate that much. When I try doing it by hand it move the remaining 60degree?

    Strange

    Anyone knows how can i rotate a servo till 180degree its 120degree right now??

    Thanks in advance
     
    Last edited: Oct 28, 2008
  17. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Digital generally means more accurate, do you have the datasheet?
     
  18. uaefame

    uaefame New Member

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    Nope, I am trying to find it NO LUCk YET.

    My motor number is ds8911 from JR company. I wish I know people who can find for me the manual.

    If I have the manual I think it will be easy to program it and even it will be easy to test it.

    Thanks in advance if you can find the manual
     
  19. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Model servos are pretty robust. Although the usual frame rate is 50 Hz (20 mS), they will work over a wide range. I have used higher frequencies, such as 140 Hz, without problems. That fact can be a help when using hardware PWM from a PIC to run them.

    555 servo drivers have generally work OK, but many people (including myself) have found that the simpler circuits may not work well with high-end and digital servos. One of my servos just jitters; another moves but does not stay steady. Older analog servos work fine with the controller. So, I would second the advice to stick with the PIC.

    John
     
  20. uaefame

    uaefame New Member

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    As you said, the analogy servo work fine ( Most of the tutorial in the INTERNET
    uses analogy servo). That confuses a lot of people including me
    I am using 280Hz right now and I only can control between 0 and 120?
    I want to go till 180.

    BTW, If you can find data sheet about my servo I will be very grateful to
    you or anyone. DS8911 servo mtor ??

    THanks in advance
     
  21. Leftyretro

    Leftyretro New Member

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    Most R/C servos only rotate +/- 60 degrees (120 degrees total) by design. The fact that you can manual move it 180 degrees doesn't change the electrical design travel. As far as timing, it's designed for a 1-2msec pulse width but may continue to track an additional small +/- % of that.

    The pulse period is not critical, but 20-25msec is nominally standard. A little experimentation can help you better characterize your specific servo. Start with a "50% travel" timing of 1.5msec. Then track how much travel both above and below Vs your pulse width. Again 1-2msec = 0-120 degrees is industry standard, although most will travel a small additional amount on both ends.

    Lefty
     

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