Parametric model development and quantitative feedback design for automotive torque converter bypass clutch control

Abstract

Customer demands for large high-performance vehicles in the face of increasingly stringent fuel economy targets have led automobile makers to seek innovative ways of meeting these requirements, especially in the North American market. One of the most critical elements in automotive powertrain systems is the torque converter bypass clutch which is now in almost universal use in automatic transmission-equipped vehicles. The clutch is used to bypass the fluid coupling of the torque converter during steady state conditions (i.e. highway cruise), thus eliminating a 15–20 percent loss of efficiency. Generally, the clutch is disengaged during dynamic events (e.g. vehicle launch, braking and gear changes) when the torque converter functionality is required. Further gains in fuel economy can now be realized through modulation of the bypass clutch in some situations where it had been considered necessary that the clutch be disengaged (i.e. during gear changes). In this case, the clutch is operated in a state of continuous slip; the transmitted torque is controlled through modulation of the clutch apply pressure. The dynamics of this system, however, exhibit considerable complexity. Characterization of this behaviour has been of considerable interest among automobile makers in recent years. However, investigation of the non-linear dynamics of this system is beyond the scope of this paper; a more in-depth treatment generally requires bifurcation analysis and/or exhaustive simulation studies. The focus in this paper is upon the detailed development of a linear parametric differential equation model and the design of a linear robust feedback control system. Parametric uncertainty is included to capture the effects of variations in system damping, bulk modulus, coefficient of friction and constants of linearization. Based upon a specific operating point, a linear robust controller is developed using the quantitative feedback theory (QFT) technique. The QFT methodology is aimed at designing feedback controllers so that pointwise frequency response specifications on closed-loop tracking and disturbance rejection are met in spite of large parametric plant uncertainty. The local stability and performance of the non-linear feedback system are subsequently verified by simulation. Since the feedback design is based upon a linear parametric model, no specific guarantees can be made as to the performance of the non-linear closed-loop system, although the results are found to be satisfactory in this case.