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

Customer demands for large high-performance vehicles in the face of increas ingly stringent fuel economy targets have led automobile makers to seek inn ovative ways of meeting these requirements, especially in the North America n market. One of the most critical elements in automotive powertrain system s is the torque converter bypass clutch which is now in almost universal us e 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 per cent loss of efficiency . Generally, the clutch is disengaged during dynamic events (e.g. vehicle l aunch, braking and gear changes) when the torque converter functionality is required. Further gains in fuel economy can now be realized through modula tion of the bypass clutch in some situations where it had been considered n ecessary 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. Characte rization of this behaviour has been of considerable interest among automobi le makers in recent years. However, investigation of the non-linear dynamic s of this system is beyond the scope of this paper; a more in-depth treatme nt generally requires bifurcation analysis and/or exhaustive simulation stu dies. The focus in this paper is upon the detailed development of a linear parametric differential equation model and the design of a linear robust fe edback control system. Parametric uncertainty is included to capture the ef fects of variations in system damping, bulk modulus, coefficient of frictio n and constants of linearization. Based upon a specific operating point, a linear robust controller is developed using the quantitative feedback theor y (QFT) technique. The QFT methodology is aimed at designing feedback contr ollers so that pointwise frequency response specifications on closed-loop t racking and disturbance rejection are met in spite of large parametric plan t uncertainty. The local stability and performance of the non-linear feedba ck system are subsequently verified by simulation. Since the feedback desig n is based upon a linear parametric model, no specific guarantees can be ma de as to the performance of the non-linear closed-loop system, although the results are found to be satisfactory in this case.