infinite phase margin

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sadiq_bokhari

Joined May 8, 2015
11
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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Thread Starter

sadiq_bokhari

Joined May 8, 2015
11
(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
 

Papabravo

Joined Feb 24, 2006
12,514
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.
 

Thread Starter

sadiq_bokhari

Joined May 8, 2015
11
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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