dither frequency with pwm to drive proportional valve

Discussion in 'Embedded Systems and Microcontrollers' started by mah, Apr 28, 2015.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    i am planning to drive proportional valve using pwm by atmega. i will do functions of ramp up and down in c code . dither frequency is needed to avoid stiction of the valve and to smooth the pwm signal . this dither frequency is ranging from 70–350 Hz and amplitude 0–10% of maximum current . the question here is how to do the dither in software programming?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Proportional valves generally use a closed loop feedback of some kind, does yours include this?
    Max.
     
  3. mah

    Thread Starter Active Member

    Mar 15, 2010
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    for now i will do it without feedback for simplification

    any help?
     
    Last edited by a moderator: Apr 30, 2015
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am still not sure how you are going to position without some kind of feedback indication?
    Especially if the Hydraulic load varies?
    Max.
     
  5. mah

    Thread Starter Active Member

    Mar 15, 2010
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    as i told you i will do it later but let's find out now how to do the dither .
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    :confused:Surely adding dither will introduce additional modulation components, which is the opposite of a smoothing effect?
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    As I understand it, dither is used to counteract the effects of catch and release ("stiction"). Saying it smooths the PWM was a small misstatement. It smooths the movement produced by the PWM control signal (http://www.qualityhydraulics.com/blog/what-dither-versus-pwm/). I know nothing about how to generate that dither, but my current project -- if it gets that far -- will face the same problem.

    Here's an ST Micro reference on generating dither: http://www.st.com/st-web-ui/static/...ical/document/application_note/DM00119042.pdf

    In a practical sense, I wondered whether bit banging the control variables in PWM might work:
    upload_2015-4-30_6-52-35.png
    Thus, alternating between different PR2 or CCPxCON<5:4> values might produce cycle-by-cycle dither. One problem is that dither produced like that would be at the same amplitude as the main PWM.

    Again, I can't add much, but am certainly interested in the answer.

    John
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Is dither also a way of reducing EMI, by using a spread spectrum?
     
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