Transmission Control Lecture delivered by: Prof. Ashok C.Meti MSRSAS-Bangalore 1 Session Objectives On completion of this session, the delegate will understand and be able to appriciate: Rl Role of electronic transmission i control system Transmission control System components and configuration Shift control strategies 2 1
Topics Introduction AT Configuration with TCU Basic functions of TCU Hardware and software components Operation of TCU 3 Introduction Automatic transmissions(ats)were popular in America and Japan(80-90%) and in Europe (about 15-20%). In early 1980s, the electronic transmission control units (TCU) were introduced by Renault and BMW. The market for ATs is divided into- Stepped transmissions Continuously variable transmission (CVTs) In stepped transmission, smooth shifts can be optimized by reduction of engine torque during gear shift, combined with the correct matched oil pressure for the friction elements (clutches, brake bands). The reduction of shift shocks to a very low or even to an unnoticeable level has allowed to design of 5 speed gear boxes. 4 2
In today s standard systems, the driver can choose between sport and economic drive programme by means of a selector switch. In sophisticated systems, the self-adoption of shift strategies is common. This leads to Better driveability Significant reduction in the fuel consumption Different shift characteristics are easy to implement in software, and much adoption can be achieved by data change, leaving the transmission hardware and TCU intact. 5 In CVTs, power loss from the transmission s oil pump is a potential problem leading to increased fuel consumption. ECU can achieve optimum yield by matching the oil mass-stream and oil pressure for pulleys to the actual working conditions. 6 3
AT Configuration System Components: Transmission which is adapted for electronic control requirements Electronic control unit with corresponding inputs and outputs and attached sensor elements. Transmission: A four speed or a five speed gear box with a torque converter lockup clutch controlled by the control unit. Mechanical and hydraulic components in the system are being replaced with electro hydraulic or electromechanical components. An electronic shift point control unit detects and controls the relevant components. 7 In this type of systems, the shift points are determined by TCU, resulting in a wide range of freely selectable driving behaviors regarding the shift points. It is also possible to use different shift maps according to switch or sensor signals. The shift smoothness is controlled through an interface to the EMS. The hydraulic pressure is maintained by an electronically controlled hydraulic solenoid. This may use PWM solenoid or a pressure regulator. Present day designs use analog pressure control solenoid that provides proper shift comfort during the entire life of transmission at all temperatures and varying oil quality. 8 4
In some cases, the shift quality is further increased using electronically controllable brake elements(brake bands) for some specific gearings. Present electronically controlled ATs usually use electronically controlled torque converter clutch which can lock-up the torque converter between the engine output and the transmission input. This clutch can be fitted with a solenoid which is activated by the signal from the TCU. This locking up the torque converter eliminates the slip of the converter and efficiency of the transmission system is increased resulting in reduction of the fuel consumption. 9 Electronic Control Unit: The ECU for transmission control can be divide into hardware and related software. The hardware consists of the electronic devices arranged in a suitable enclosure which can be located inside the car or in side the engine compartment. It also consists of specially designed devices in output stages for actuators of pressure regulation and torque converter clutch control. On the functional side, the hardware configuration an be divided into - Power supply Input signal circuits Output stages Microcontroller Peripherals components, monitoring and safety circuits RF interference, EMC 10 5
The microcontrollers are 8 bit or 16bit with real-time and directly addressable capabilities CAN is commonly used in microcontrollers to communicate with systems such as EMS. The EPROMs stores the data and programmes (128 256 Kb). The NVRAMs are increasingly used to store the diagnostic and adaptive data. Watch-dog circuits i are used dfor safety and monitoring. i Special output stages are used for driving the solenoids which are inductive loads. 11 Overview of hardware parts Ignition V bat Internal Supply Voltage Digital inputs EMS Interface Speed Signal Digital signal interface Pulse signal interface Micro controller Actuator output stage Actuators T ATF Analog signal interface EPROM Low signals output stage ECU Interface Diagnostics Serial interface for diagnostics RAM Ext Watchdog (Automatic transmission Fluid) 12 6
Software: The software component of the Electronic Transmission unit is the most important one as its functionalities are increasing. The software consists of two parts- Application programme Data The programme structure is defined by the functions The data are specific for the relevant programme parts and have to be fixed during calibration stage. The most difficult software requirement results from the real-time conditions This is also a criterion for microcontroller selection. 13 Software Structure overview Initialization of microcontroller and control unit Input signal preparation Safety Functions Diagnosticsi Function calculation Output signal translation Data 14 7
Support for standardized communication protocol such as ISO 9141.[The ISO-9141 specification is one of several protocols used in the OBDII system implemented in 1996 and newer cars and light trucks.] Communication with other control unit through protocols such as CAN or J1850. The Data can be divided into two types: Fixed data related to fixed attributes of the system e.g the number of actuators Calibration data data adapted to changing parameters of the system such as engine, vehicle and transmission characteristics. The rising software complexity with simultaneously increasing quality requirements causes higher demands for software quality control. 15 Actuators: Electro hydraulic actuators are important components of electronic transmission control systems. Continuously operating actuators are used to modulate pressure. Switching actuators are used as supply or discharge valves for shift point control. 16 8
Qualities of actuators for ATs: Low hydraulic resistance to achieve high flow rates Operation temperature range 40 to 150 degree centigrade. Small power loss Minimized heat dissipation at output stage of the ECU Small size and low weight Highest reliability in a harsh environment Maximum accuracy and repeatability over lifetime Short reaction times Pressure range up to 2000kPa Maximum vibration and acceleration of 300m/s 2 High number of switch operations The hardware and software of the ECU must be developed taking into account the electrical specification of the solenoid to obtain an optimized complete system. 17 Example of a Gear-Solenoid combination for a four Speed Transmission Solenoid -1 Solenoid-2 Solenoid-3 1 st Gear On On On 2 nd Gear On On Off 3 rd Fear On Off Off 4 th Gear Off Off Off 18 9
Basic functions of TCU Shift point control Lockup control Engine torque control during shifting Safety function and diagnostic functions 19 Structure of basic transmission electronic control unit Ignition V bat Internal Supply Voltage Power Supply Solenoid Position switch KD EMS Interface Speed Signal Turbine Speed T ATF K-Line L-Line Digital signal interface Pulse signal interface Analog signal interface Serial interface for diagnostics Micro controller EPROM RAM Ext Watchdog Solenoid driver stages Digital signals output stage Shifting solenoids Limp home Engine torque reduction 20 10
Shift point control: Basic shift point control uses shift maps. The data maps are selectable over a wide range Shift point limitations based on Highest admissible speed of the engine for each applications Lowest speed of the engine that is practical for driving comfort and noise emission The inputs of the shift points determination- Throttle position Accelerator pedal position Vehicle speed 21 To prevent too frequent shift between the two gears, a hysteresis between the upshift and the downshift characteristic ti is incorporated. The hysteresis is determined by the desired shifting habit of transmission and, alternatively, the car behavior. 22 11
23 Engine torque control during shifting: It requires an interface to an electronic engine management system [EMS]. The purpose is to - Reduce the torque during the shifting Support synchronization of the transmission Prevent shift shocks. In conventional applications, the engine torque reduction is through an ignition angle control. The timing and absolute value of the ignition control depends on the operating conditions concerning actual engine torque and shifting type. 24 12
25 Upshift: Occurs without an interruption of the tractive power. The engine torque reduction may be activated t if the clutch of the target gear stays with the translation of torque. The beginning of the engine torque reduction is determined by the course of engine or transmission input speed. There it is important to detect a decreasing speed. The start of the torque is characterized by a specific difference. The end of the torque reduction is activated at an applicable speed lead before reaching the synchronous speed of the new gear. 26 13
Downshift: Downshift under driving conditions results in a short interruption of the tractive power. At the synchronous point, the tractive power is in operation. The higher revolving energy, on the other hand, results in undesired vibrations of the power train. To prevent such vibrations, it is necessary to reduce the engine output torque before reaching the synchronous point of the new gear. 27 When the transmission input speed reaches the synchronous speed of the new gear, the engine torque has to increase to the nominal value. The increase is usually applied as a torque ramp. The values and timing of the engine torque reduction are generally part of the special calibration data for each combination of vehicle, engine, and transmission. i 28 14
Lockup Control/Torque Converter Clutch: The torque converter clutch connects both functional components of the hydraulic converter, the pump and the turbine. The lockup of the clutch reduces the power losses due to torque converter slip. The shift points of lockup are determined in the same way as the determination of the shift point in the gear shift point control. Normally there is one separate characteristic curve for the lockup for each gear. 29 To prevent power train vibrations, it is advisable to open the lockup during coasting to use the damping effect of the torque converter. In some applications, the lockup is opened during shifting for improved shift comfort. After shifting, the lockup can be closed again. When driving in first gear, the lock is usually open, because the time spent in the first gear is small [High frequency of lockup] Improved acceleration of the car in 1 st gear when using the converter gain for wheel torque. 30 15
Pressure Control The shift comfort is also dependent on the timing and absolute values of the pressure, which is responsible for the torque translation of the friction elements. The TCU offers a large number of possibilities for better function than a conventional hydraulic system. Different algorithms are used to calculate the pressure values during and outside shifting. 31 The inputs for a pressure calculations are Engine torque Transmission input Turbine torque Throttle position and so on the inputs depend on the special signal availability as well as requirement concerning shift control. Variable pressure components along with a constant pressure value are used to determine required pressure 32 16
P mod P const k n P n k tor P tor k s P s Where P mod pressure P k adaptation factor for input speed P pressure component dependent on the revolution signal K P const n n tor constant pressure value adaption factor for engine torque pressure component dependent on torque k adaption factor for vehicle speed s P pressure component dependent on vehicle speed s tor During applications, the factors must be defined d in the calibration phase. In general, to determine these factors, many vehicle tests are required. Another possibility is use of characteristic maps which have to be defined during calibration phase. 33 Safety and diagnostic Functions: Diagnostic functions of TCU can be divided into- Real safety functions to prevent critical driving conditions (uncontrollable shifting, especially unintended down shifting) Diagnostic functions which affect an an increasing availability of the car A better failure detection for servicing. Generally, the monitoring includes the monitoring of the controller. Watchdog (Internal or external) circuits are used monitor the controllers. Incase of actuator malfunction, the limp-home mode is selected. The diagnostic system consists of a failure storage facility (non-volatile) and communication to the service tester. 34 17
Improvement of Shift control: In a second development stage, the basic functions can be revised by a modification of the software functions and by adding new parts to the basic functions. This results in significant enhancement of the driving and shifting comfort. Shift Point Control: The basic function can be improved significantly by adding a software function, the so-called adaptive shift control. This is able to prevent an often-criticized ii i attribute, the tendency for shift hunting especially when hill climbing and under heavy load conditions. In addition, different shift maps can be implemented into the data field of the TCU. (E.g. Economy / Power mode) 35 Lockup Control: There are additional functions which can improve the shift comfort of the lockup. Replacing the on/off control of the lockup actuator by a pulse control during opening and closing The on/off solenoid is replaced by a pressure regulator or a PWM solenoid.it is possible to achieve lockup at low engine speed and low engine load with good shift comfort (resulting in low fuel consumption) by implementing intelligent control strategies and corresponding output stages in within TCU. 36 18
Engine torque reduction during gear shifting: Using PWM signal with related fixed values or a bus interface to the EMS, the engine torque reduction can be directly controlled by TCU. Advantages: Independent calibration of the TCU data over a wide range without changing the EMS data. Possibility of coordination of the engine torque reduction and pressure control within the TCU. 37 Pressure Control: Adaptive software strategy can be implemented. The required inputs for the adaptive pressure control are calculated from available signals in the transmission control. The main reasons for the implementation of the adaptive pressure control are the variations of the attributes of the transmission components like- clutch surfaces and oil quality as well as the changing engine output torque over the lifetime of the car. 38 19
The principle of adaptive pressure control: A comparison of a set value for the shift time with an actual value, measured by the transmission input speed course. At a specific difference of the set value to the actual value, the pressure value is corrected by a certain increment in +ve or ve direction. The original adaptation time and the pressure value increment would be fixed during the calibration phase. For safety reasons, the total deviation of the pressure value from a given value is limited, de[pending upon the particular application. NVRAM is used to store the correction values to have correct values available after power-on of the TCU. 39 40 20
Power train optimization: Adaptive functions in cooperation with carefully designed torque converter clutch control, which allows the clutch to be closed even at low gears, have improved fuel consumption significantly. Based on the driver s behavior, together with an adaptive shift strategy, part of the TCU s adaptive programme software may select an economy or even super-economy by optimization of the drive train. 41 Summary Need for electronic control for transmission has been established. The system configuration and its working of a typical automatic transmission control has been discussed. 42 21