Slip-Clutch Torque Estimation via Real-Time Adaptive Lookup Table

Transfer case clutch is crucial in determining traction torque distribution between front and rear tires for four-wheel-drive (4WD) vehicles. Estimating time-varying clutch surface friction coefficient is critical for traction torque control since it is proportional to the clutch output torque. As a result, this paper proposes a real-time adaptive lookup table strategy to provide the time-varying clutch surface friction coefficient. Specifically, the clutch-parameter-dependent (such as clutch output torque and clutch touchpoint distance) friction coefficient is first estimated with available low-cost vehicle sensors (such as wheel speed and vehicle acceleration); and then a clutch-parameter-independent approach is developed for clutch friction coefficient through a one-dimensional lookup table. The table nodes are adaptively updated based on a fast recursive least-squares (RLS) algorithm. Furthermore, the effectiveness of adaptive lookup table is demonstrated by comparing the estimated clutch torque from adaptive lookup table with that estimated from vehicle dynamics, which achieves 14.8 Nm absolute mean squared error (AMSE) and 2.66% relative mean squared error (RMSE).

Transfer Case Clutch Torque Modeling and Validation Under Slip and Overtaken Conditions

The vehicle transfer case clutch plays an important role for four-wheel drive (4WD) vehicles since the torque transmitted through the clutch determines the amount of traction torque on tires, which is important for vehicle performance. However, the clutch torque measurement is usually unavailable on production vehicles and needs to be estimated accurately to improve vehicle performance. This paper proposes a unified scheme to model clutch output torque under all three conditions: open (no torque output), slipping, and overtaken. Specifically, the clutch torque model under clutch overtaken condition is first investigated using the vehicle longitudinal and tire dynamics. It was found that effective radius of front tires, powered by the transfer case clutch torque, cannot be assumed as constant and should be compensated by vehicle acceleration, while the effective radius of rear tires connected directly to the propulsion system does not need to be compensated. In addition, it was found that torque model under clutch overtaken condition cannot be used under slip condition. As a result, a general clutch torque model is developed for both slip and overtaken conditions with a clutch slip speed compensation, resulting a root-mean-square error percentage (RMSE%) of 6.8% comparing with the experimental measurement data. Note that overtaken torque model is a special case of the general torque model by setting slip speed equal to zero. The general clutch torque model is able to calculate clutch output torque accurately under both slip and overtaken conditions.