1. Introduction The basic form of a hydraulic automatic transmission is a hydraulic torque converter connected in series with a rotary shaft mechanical transmission for power shifting. Because it has excellent automatic adjustment and adaptability to external loads, it ensures smooth vehicle start-up and uniform acceleration. Its vibration damping effect reduces dynamic load and torsional vibration in the transmission system, extends the service life of the transmission system, and improves ride comfort, driving safety, handling, and average vehicle speed. The lower efficiency of automatic transmissions compared to mechanical transmissions has always been a major reason hindering their development. To solve this problem, hydraulic automatic transmissions have undergone various methods, including the use of multi-element working wheel hydraulic torque converters, lock-up clutches, adding planetary gear transmission gears, and electronic control, resulting in improved overall economic performance. Among these, the most effective approach in the last decade has been the extensive application of electronic technology in control, which has significantly improved the performance of hydraulic automatic transmissions. The main work in this area includes shift point control, torque converter lock-up clutch control, and shift quality control. A typical hydraulic automatic transmission control system is shown in Figure 1, mainly including input signal sensors and switches, an electronic control unit, and output actuators. The system includes sensors shared with the electronic fuel injection engine control system for engine speed, load, and temperature; sensors measuring transmission output speed; and sensors for gear selector position, shift mode, and forced downshift switch. The electronic control unit (ECU) consists of an I/O interface, CPU, and memory (RAM/ROM). Its function is to determine the optimal gear and whether the torque converter is locked. These actions are achieved using electromagnetic hydraulic valves. 2. Shift Point Control The shift point is a function of transmission output speed, engine load, gear selector position switch, and shift mode switch. For example, if the driver places the gear selector in the "D" position and selects "E (Economy)" mode, the shift point is determined by the shift diagram shown in Figure 2. The horizontal axis represents the transmission output shaft speed (equivalent to vehicle speed), and the vertical axis represents the engine throttle opening (equivalent to load). Curves 1-2, 2-3, and 3-4 correspond to the shift points when upshifting, while curves 4-3, 3-2, and 2-1 correspond to the shift points when downshifting. When vehicle speed increases, the transmission automatically upshifts one gear as long as the operating point determined by engine load and transmission output speed crosses the corresponding upshift curve to the right. Conversely, when vehicle speed decreases, the transmission automatically downshifts one gear as long as the operating point determined by engine load and transmission output speed crosses the corresponding downshift curve to the left. The curve shape shows that a higher engine load corresponds to a higher vehicle speed during upshifts or downshifts, ensuring sufficient power for low-gear driving under heavy load. Due to limitations in the control unit's memory space, the upshift/downshift curves in the shift diagram are not as continuous and smooth as in older hydraulic control methods, but often exhibit a stepped shape. 3. Lock-up Clutch Control To improve the efficiency of the automatic transmission, the torque converter can be locked. The locking operation is performed by the lock-up clutch, which is jointly controlled by engine load, output shaft speed, gear position, and shift mode. All lock-up clutches employ electronic control. An electromagnetic induction sensor provides the transmission output shaft speed signal, the throttle position switch provides the engine load signal, and signals from the shift lever position and mode switch are transmitted to the control unit for processing and control. Actuators include on/off solenoid valves and pulse solenoid valves; the latter primarily controls the hydraulic pressure of the lock-up clutch piston to improve the smoothness of the transition from hydraulic to mechanical transmission. While lock-up clutches can be engaged in all gears, it is practically limited to third and fourth gear. The two sets of dashed and dotted lines in Figure 2 represent the lock-up clutch engagement and disengagement curves. When the operating point determined by engine load and transmission output speed crosses the dashed line to the right, the lock-up clutch engages, switching from hydraulic to mechanical transmission. Conversely, due to a decrease in vehicle speed or an increase in engine load, when the operating point determined by engine load and transmission output speed crosses the dotted line to the left, the lock-up clutch disengages, switching from mechanical to hydraulic transmission. However, during the transition between the two transmission operating conditions, the change in speed ratio causes a step increase in the transmitted torque, resulting in dynamic load on the transmission system and affecting ride comfort. Therefore, the selection of the locking point becomes a critical issue. A pulse-type solenoid valve can effectively solve this problem. A pulse-type solenoid valve consists of an electromagnetic coil, armature, and valve core, and its function is to control the oil pressure in the oil circuit. Unlike ordinary on/off solenoid valves, the electrical signal controlling the operation of a pulse-type solenoid valve is not a constant voltage signal, but a pulse signal with a fixed frequency. Under the action of the pulse signal, the solenoid valve repeatedly opens and closes the drain hole. The computer changes the pulse width, or the ratio of the current on and off time within each pulse cycle, i.e., the duty cycle (defined as A/(A+B)×100% within one pulse cycle, thus its range is 0-100%), to change the opening and closing time ratio of the solenoid valve, thereby achieving the purpose of controlling the oil circuit pressure. The higher the duty cycle, the more hydraulic oil is discharged through the solenoid valve, and the lower the oil circuit pressure; conversely, the lower the duty cycle, the higher the oil circuit pressure. To utilize the smooth power transmission characteristic of the torque converter, the lock-up clutch can be briefly disengaged during gear shifting, allowing the torque converter to engage. 4. Shift Quality Control Shift quality mainly depends on the smoothness of planetary gear meshing. As shown in the simplified diagram of the planetary gear transmission in Figure 3, second gear is obtained by combining the high-speed gear of the front planetary gear set with the first gear of the rear planetary gear set. The operating elements required are C2, B0, and F2. Third gear is obtained by combining the low-speed gear of the front planetary gear set with the second gear of the rear planetary gear set. The operating elements required are C0, C1, B2, F0, and F1. When shifting from second to third gear, the two planetary gear sets must be synchronized in opposite directions. That is, when shifting from second to third gear, the front planetary gear set must upshift, and the rear planetary gear set must downshift; both must occur simultaneously. The key here is that the release of brake B0 and the engagement of brake B2 must occur simultaneously; otherwise, shifting interference or a brief interruption of power will occur. Three common measures are used to improve shifting quality: first, implementing torque reduction control of the engine during shifting; second, main hydraulic pressure adjustment control; and third, briefly disengaging the lock-up clutch during shifting to allow the torque converter to work, thus smoothly transmitting power. The first two measures are mainly introduced below. Engine torque reduction control reduces the torque generated by the engine. The purpose of controlling engine torque during upshifting under load is to reduce the energy dissipated by friction elements during shifting. This is achieved by reducing the torque generated by the engine during shift synchronization without interrupting power. The purpose of controlling engine torque during downshifting under load is to suppress the vibration caused by the one-way clutch and friction elements during shifting. A common method to reduce engine torque is to delay the ignition timing. This section describes the changes in vehicle longitudinal acceleration during upshifts under conditions with and without engine torque control. Without engine torque control, to keep the clutch slip time within 500ms, torque needs to be increased during slippage. This causes a large torque jump at the end of the shift, resulting in changes in longitudinal acceleration and affecting comfort. If the slip time remains the same but the engine torque is reduced by 50%, and the clutch oil pressure is significantly reduced, the acceleration during the shift is roughly the same as before the shift, and the torque jump at the end of the shift is smaller. This improves shift quality and reduces the load on friction components within the same time frame. The purpose of main hydraulic pressure regulation control is to accurately match the working oil pressure of the relevant shift friction components with the transmission input torque. Specifically, the working oil pressure is divided into two parts: a fixed base pressure and a pressure adjustable by an electro-hydraulic control valve. The transmission input torque can be accurately calculated using parameters such as engine air intake, engine speed, and torque converter speed ratio. Therefore, the pressure parameters can be adjusted according to the operating conditions at different gears. Another approach involves closed-loop control of the shift clutch hydraulic pressure to keep the input shaft angular acceleration within a specified range. To ensure a smooth shift from second to third gear in a planetary gear transmission, the shifting process of both planetary gear sets can be monitored, allowing for precise real-time control of the hydraulic pressure in brakes B0 and B2. For example, in Toyota's A350 automatic transmission, the speed of the sun gear on the front planetary gear set (i.e., the speed of brake B0) is used as the input shaft speed signal, collected by an input speed sensor. Additionally, a sensor is added to measure the speed of the sun gear on the rear planetary gear set (i.e., the speed of brake B2). When shifting from second to third gear, pressure must be applied to the cylinder of brake B0, while pressure must be released from the cylinder of brake B2. In traditional hydraulic control systems, both brake cylinders B0 and B2 are connected to an accumulator, whose hydraulic pressure is regulated by a computer-controlled pulse solenoid valve. Therefore, any change in the accumulator back pressure will cause a change in the hydraulic pressure of brakes B0 and B2, making independent control impossible. With appropriate modifications to the hydraulic circuit, when the transmission shifts from second to third gear, the accumulator is disconnected from the return oil circuit of brake B0. Simultaneously, the oil pressure of brake B0 is controlled by the pulse-type solenoid valve of the torque converter lock-up clutch (at this time, the lock-up clutch lock-up valve is closed, and the lock-up clutch is not engaged), thus allowing independent control of brakes B0 and B2. The oil pressure of B2 begins to rise, and torque transmission gradually shifts from the one-way clutch F2 to brake B2. When the torque transmission effect of F2 is completely lost, the rear planetary gear set begins to shift, the oil pressure of brake B0 begins to adjust, and the front and rear planetary gear sets shift simultaneously. The front planetary gear set has completed its shift, while the rear planetary gear set continues to shift. Thus, due to precise control of the oil pressure, the shift time is short, and the dynamic load on the output shaft is small, achieving smooth shifting. 5. Application of Other Control Technologies Modern automatic transmissions also utilize other control technologies, mainly: a) Adaptive Control. Adaptive control generally has two types: dynamic control and steady-state control. Dynamic control is primarily used to monitor the gear ratio during gear shifts, as mentioned above, for precise control of the shift oil pressure. Steady-state control mainly considers the impact of changes in the friction coefficient of the friction plate material of the actuator. The first is the effect of temperature, which can be addressed by installing a hydraulic oil temperature sensor in the automatic transmission oil tank, allowing the computer control program to correct for the change in the friction coefficient as the oil temperature rises. The second is the impact of a decrease in the friction coefficient. A decrease in the friction coefficient has two causes: firstly, potential leakage, such as clutch leakage, which reduces capacity, causing slippage and overheating of the friction plates, further reducing the friction coefficient; secondly, natural wear. The method used in the GM 4T-80E automatic transmission is as follows: once a gear is determined, the gear ratio is monitored. If it is not the required ratio, the oil pressure is further increased via a microcomputer and regulator until the correct ratio is obtained. This adjustment program is highly sensitive to gear ratio deviations, protecting the automatic transmission and allowing the oil pump to operate at the minimum required pressure, thus improving fuel economy. b) Fuzzy Control Fuzzy control does not rely on a precise mathematical model of the system. Its exploratory control rules are essentially linear, and it is insensitive to changes in process parameters, which is crucial for automatic transmissions that frequently operate under varying conditions. The key to applying fuzzy theory for control is the design of the fuzzy controller. A fuzzy controller is a language-based controller that simulates human control characteristics. Its design mainly includes selecting the controller's structure, choosing fuzzy rules, determining the fuzzification and defuzzification methods, and defining the universes of discourse for the input and output variables. The core is the selection of fuzzy rules. In recent years, some major international automakers have applied fuzzy control theory to automatic transmissions. They use vehicle speed and engine load as the basic input parameters and suppressing the dynamic load generated during gear shifts as the output parameter, while also considering factors such as engine fuel consumption, emissions, and road conditions to control the shifting timing of the automatic transmission. Mitsubishi's new automatic transmission vehicle, the "Fuzzy Shift 4AT," incorporates the concept of "fuzzy control" into its automatic transmission system. The microcomputer within the transmission automatically selects the most appropriate shifting method based on collected driving information and fuzzy logic judgment.