6Automatic Transmissions: Control and Calibration

6.1 Introduction

As discussed in the previous chapter, automatic transmissions (AT) that use planetary gear trains (PGT) realize multiple gear ratios by alternately engaging and disengaging hydraulically actuated clutches according to the transmission clutch table. A gear shift in the transmission involves the engagement of an oncoming clutch and the release of an off‐going clutch. Two technical issues naturally arise: (a) when should transmission shifts be initiated, (b) how should the clutches involved in a shift be controlled so that the shift is made quickly and smoothly? The primary functions of transmission control systems are therefore to address these two issues.

Clutches in automatic transmissions of early types were both hydraulically actuated and controlled [1]. A gear shift was initiated by a governor that was driven by the transmission output shaft [1,2]. The governor assembly is actually a centrifugal mechanism that has weights rotating with the governor and swinging about a pivot attached to the governor. As the governor rotates, the weights will swing outward due to the centrifugal effect. The position of the swinging weights depends on the governor rotational speed which is in turn linearly related to the transmission output speed. As the weight swings outward, it switches on a pilot hydraulic valve to trigger a shift corresponding to a specific vehicle speed. Once a shift is initiated, the responsiveness and ...

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