AppendixChallenges and Opportunities in Automotive Transmission Control Zongxuan Sun and Kumar Hebbale Research and Development Center General Motors Corporation Warren, MI 48090Abstract: Automotive transmission is a key element in the powertrain that connects the power source to the wheels of a vehicle. To improve fuel economy, reduce emission and enhance driving performance, many new technologies have been introduced in the transmission area in recent years. This paper first reviews different types of automotive transmissions and explains their unique control characteristics. We then address the challenges facing automotive transmission control from three aspects: calibration, shift scheduling, and sensing, actuation and electronics. Along the way, research opportunists to further improve system performance are discussed.1. Introduction to the Latest Automotive Transmission TechnologiesTo improve fuel economy, reduce emission and enhance performance, automotive manufacturers have been developing new technologies for powertrain systems. In the transmission area, emerging technologies 1 such as continuously variable transmission (CVT), dual clutch transmission (DCT), automated manual transmission (AMT) and electrically variable transmission (EVT) have appeared in the market, which is traditionally dominated by step gear automatic transmission (AT) and manual transmission (MT). Among many different technical challenges for developing these new transmissions, system dynamics and control are crucial to realizing the fuel economy and emission benefits while providing superior performance.The basic function of any type of automotive transmission is to transfer the engine torque to the vehicle with the desired ratio smoothly and efficiently. The most common control devices inside the transmission are clutches and hydraulic pistons. Such clutches could be hydraulic actuated, motor driven or actuated using other means (see Figure 1). Therefore clutch/piston control is essential for transmission operation. In both DCT and AMT, clutch control is the key to ensure smooth torque transfer. In CVT, hydraulic piston control is crucial for not only system performance but also device durability. In many of the new automatic transmissions (AT), clutch-to-clutch shift is adopted to reduce the cost and improve packaging. This involves electronic control of both the oncoming and off going clutches and the timing and coordination between them. In addition to eliminating the shift valves, accumulators, etc., clutch-to-clutch control eliminates the coasting clutches and freewheelers, greatly simplifying the transmission mechanical content. The absence of these devices makes the robust control of clutch-to-clutch shifts a challenge.Figure 1. Schematic Diagram of a ClutchWith the traditional control system in an automatic transmission with clutch-to-clutch shifts, the oncoming clutch fill process is a major source of uncertainty and it makes the clutch coordination during the shift a difficult task. The fill time of the oncoming clutch varies due to many factors, such as, fluid temperature, solenoid valve characteristics, line pressure variations, and time elapsed between shifts. The commanded fill pressure and the commanded fill time are critical to achieving a good fill and a smooth start to the shift process. Even small errors in calculating these two parameters could lead to an overfill or an under fill, as shown schematically in Figure 2. Some algorithms have been developed to detect the end of fill using speed signals, but none of them has proven reliable and fast enough to prevent overfill spikes. An example of an oncoming clutch overfill during an upshift is shown in Figure 3. The oncoming clutch pressure shows a slight overfill, which results in shift tie-up, causing engine pull down and a big drop in the output torque. A more robust off going clutch control was necessary in this example to avoid a tie-up. While the overfill can be corrected using adaptive schemes for future shifts, the real challenge is in preventing it from happening in the first place.Figure 2. Variations in Clutch Fill ProcessFigure 3. Effect of Clutch Overfill on an UpshiftCurrently in most clutch-to-clutch shift production transmissions, the clutch coordination is accomplished by a combination of open-loop, event-driven, and feedback control schemes. Transmission input and output speeds are the primary measured variables used in this control. An adaptive system is used to compensate for shift-to-shift variations and build-to-build variations 2. Recently, an integrated to