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Relation between suspension switching frequency and body eigefrequency

  1. Jun 21, 2014 #1
    'Slow-active suspensions realize small switching frequencies to control low-frequency body movements,such as roll pitch and lifting motions.Fully-active suspensions reach switching frequencies,like semi-active suspensions,greater than the natural eigen-frequencies of the vehicle.'- an excerpt from a book.

    I have 2 questions:

    1.If body roll,body pitch and lifting make-up for the low frequency body motions,what would be classified as high frequency vibration?

    2.Looks like active suspensions just provide a counter force. How does having a switching frequency greater than the natural eigen-frequency of the vehicle help? Does the author mean to imply that the suspension has to think faster than the rate at which bumps come-up on the road?
  2. jcsd
  3. Jun 21, 2014 #2


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    I'm slightly guessing here. In aircraft design the crossover between "high" and "low" is fairly arbitrary but it is often taken as 20 Hz, since the frequencies of the main structural vibrations like bending and torsion of the wings in flight, or the plane on its undercarriage when landing, are below that frequency.

    I suppose for auto design the same principle applies, i.e. "low" means below the natural frequencies in roll, pitch, yaw of the vehicle and its suspension - in other words the frequency you would see if you push down on a car body and "bounce" the suspension.

    The frequency of the applied forces from bumps in the road, engine vibration, unbalanced wheels, etc would be higher than the largest frequency components of the response. An active suspension could either try to control the response (low frequencies) and more or less ignore what caused it, or counterbalance the forces (high frequencies) to prevent the response developing at all.
  4. Jun 23, 2014 #3
    The use of the word 'response' complicated things for me.Response is what here exactly? You classified response under low frequencies in your text.I'm thinking in terms of the body of the car 'responding' to vibrations.
  5. Jun 24, 2014 #4
    I am thinking in terms of principle of functioning of a damper. When damping is very small,the resonant frequency is almost equal to the natural frequency of the system. Here, the system is split into 2 parts:high natural frequency of vibration,low natural frequency of vibration.

    So,in the odd case that speed bumps equal the natural frequency of any of my system components,I am in trouble. What happens when this is not the case? my active suspension is actually wasting energy. Am I right?
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