infinite phase margin

Discussion in 'Homework Help' started by sadiq_bokhari, May 8, 2015.

  1. sadiq_bokhari

    Thread Starter New Member

    May 8, 2015
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    this is the bode plot i'm getting for my system, what type of a lead/lag compensator should i use to bring it to a stable state ?
    and what would be good phase and gain margins for the controller design of such a system
    it is a high order half car suspension system
     
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  2. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    What values of mass, spring, and stick do you have in your system. I suspect you have a problem with units.
     
  3. sadiq_bokhari

    Thread Starter New Member

    May 8, 2015
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    (M1) car body mass 2500 kg
    (M2) suspension mass 320 kg
    (K1) spring constant of suspension system 80,000 N/m
    (K2) spring constant of wheel and tire 500,000 N/m
    (b1) damping constant of suspension system 350 N.s/m
    (b2) damping constant of wheel and tire 15,020 N.s/m
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    The natural frequency, ignoring the wheel and tire, comes out to 5.65 radians/sec, The system is underdamped and can be expected to to oscillate near the natural frequency. In order to reduce the amplitude and duration of the oscillations you need to either:
    1. Reduce the vehicle weight(M1), and/or spring constant of the suspension (K1)
    2. Increase the damping constant (b1) of the suspension
    Keep in mind that most drivers would be highly critical of a critically damped suspension.
     
  5. sadiq_bokhari

    Thread Starter New Member

    May 8, 2015
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    0
    thanks, plus i had made a slight mistake in calculating my TF, now i've corrected it and the system's bode plot is something like this
     
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