This dissertation provides a clear path for model-based real-time estimation of transfer case clutch parameters and wheel traction torque with improved accuracy for a 4-Wheel-Drive (4WD) vehicle.Transfer case clutch, distributing the traction torque between front and rear tires, is a key component for 4WD vehicle propulsion system. When the clutch is disengaged, driving torque from transmission is distributed to rear tires only through the solid connection between rear differential and transmission, resulting in 2-Wheel-Drive operating mode for improved fuel economy. When the clutch is engaged, traction torque is distributed to both front and rear tires through transfer case clutch, leading to 4WD operating mode for better traction performance. The amount of traction torque for front or rear tires is determined by the clutch status, given the total traction torque from transmission is known. However, the actual torque distribution ratio, determined by real-time clutch operating status, is typically unknown due to the unavailability of multiple clutch parameters and/or direct measurement of traction torque. Therefore, for accurate traction torque control, real-time estimation of transfer case clutch parameters (such as touchpoint displacement, friction coefficient, output torque, etc.) and wheel traction torque is imperative. One significant step is to estimate the clutch touchpoint displacement based on the clutch actuation system since it is proportional to the clutch normal force (or clutch output torque). The touchpoint displacement is initially designed as a constant. However, there are also many factors that may render it to change. Therefore, this dissertation lumps all the factors that causes variation of touchpoint displacement to a variation displacement parameter and proposes to estimate this variation displacement in real-time with adaptive parameter estimation algorithm. Although the aforementioned approach achieves estimation quite well, the estimation is separated from the desired clutch displacement control. Therefore, an integrated approach is proposed to estimate the touchpoint displacement and track the desired clutch displacement simultaneously. This is realized by using the deadbeat adaptive backstepping control technique based on the clutch actuation system. This approach not only is more concise for implementation but also may reduce production cost. Another significant step to estimate clutch surface friction coefficient is to estimate the clutch output torque. This can be achieved neck-by-neck with the estimation of traction torque on the tires. In this dissertation, the tire traction forces are estimated under different clutch operation conditions: open, slip and overtaken. A integrated model incorporating time-varying effective tire radius, vehicle speed estimation and clutch-slip speed compensation is proposed, which shows good accordance with the measured torque under different clutch conditions. Although the aforementioned modeling approach for clutch output torque calculation shows promising results. It is obvious that this modeling approach is sensitive to measurement noise, especially the clutch output torque calculation is involved with the difference between vehicle speed and tire linear speed. Therefore, an Extended Kalman Filter based estimation algorithm is proposed to deal with measurement noise. This estimation algorithm is developed based on the integrated clutch output torque model suitable for both clutch slip and overtaken condition. Finally, the clutch surface friction coefficient is estimated based on the estimated clutch touchpoint displacement and clutch output torque, note that this estimation approach will be clutch-parameter-dependent. In order to obtain an estimated clutch-parameter-independent friction coefficient, this dissertation further proposes an adaptive lookup table scheme, table nodes of which are updated by the well-known Recursive Least-Squares algorithm. The effectiveness of clutch-parameter-independent friction coefficient is confirmed by comparison and can be used for future clutch output torque control without knowing clutch parameters/operating statuses.
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