| The topic of this dissertation is vehicle suspension design for vibration isolation. Specifically, a nonlinear suspension element --called a variable compliance element--is investigated. Feedback control laws are designed, and the vibration isolation performance is evaluated. This involves computer simulation, because analytical results are difficult or impossible to obtain for nonlinear feedback systems. In addition, Bilinear System Theory is studied because of its close relation to variable parameter systems.; Two example problems are studied. The first is a Switched-Damper- Controlled compliance. One segment of the main spring is in parallel with a damper. By switching the damper between high and low damping values, the effective spring constant is varied. Optimal open loop control is computed numerically for initial condition (IC) response and for a bump input. Feedback control laws are designed and evaluated for the IC case, bump input, random roadway input and random external force input. A quadratic Performance Index weighting acceleration, suspension travel(rattlespace), and road contact force is used. The greatest improvement is in rattlespace and the least in road contact force.; The second example is based on a linear passive tuned isolator. The objective of the variable compliance is to extend the isolation to a wider frequency range. First a variable air compliance is shown to yield very good results if the time-varying input frequency is measurable(feedforward control). A device for varying the effective compliance is also suggested. Then the model is changed by incorporating a lever which has a movable fulcrum (called the Variable Fulcrum Isolator (VFI)). The VFI shows almost the same isolation improvement as the idealized, variable air compliance. Feedback control is then studied with the VFI model. Results--rms acceleration versus rms control power--are compared to linear passive and active systems. The VFI yields better performance than the active system for low control power systems. However, if high power is available, then the active system is eventually a superior isolator. An important characteristic of the VFI with acceleration feedback is the response to inputs of 1 or 2 distinct frequencies. The fulcrum will move to the optimum position, and then remain virtually stationary. Thus, it will use very little control power under these conditions, and achieve very good performance. |
