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Because electronic controllers were incorporated, transmissions have become more efficient, smaller, and lighter; use fewer parts; and have longer service life.
Electronic control of automatic transmission performance is an integral part of all new transmission designs. Electronic transmission controllers at various levels of sophistication have been in use since the early 1970s.
Controller technology can be categorized in three functional levels:
Simple reproduction of mechanical and hydraulic functions Complex algorithms involving many parameters Adaptive learning with interaction between systems and self-learning
The original systems and most current production transmission controllers use 8-bit microprocessors to effect program cycle update times of 20 μsec. However, with the expansion in functions, new, more complex, controllers are pushing the limits of
memory space even in 16-bit single-chip computers. A typical controller can have from 30–40 I/Os and may dedicate up to 50% of memory to diagnostic functions.
Integrated powertrain controllers are also becoming common with combined I/O pin counts of more than 100, and they also bring
benefits of higher transmission and engine control integration.
Characteristics common to all electronic controllers include:
Shift point control Converter lockup clutch operation Pressure control of friction elements Engine torque control during shifting Safety functions and diagnostics Hardware protection from driver abuse
Electronic controllers allow automatic transmissions to deliver high levels of performance to satisfy the following performance enhancements:
Provide optimum shift schedule that matches all types of driving conditions and patterns Provide smooth shifting with excellent response Transmit engine torque to power the vehicle and deliver excellent acceleration Improve high reliability
Current controllers are responsible for shift schedules, lockup schedules, and integration with components in other vehicle systems. This requires control over engine torque during shifting and adaptive learning for application of hydraulic pressures.
Sensors and Actuators
A representative listing of the specific types of signals, sensors, and actuators used in conjunction with an electronic controller are as follows:
Spark advance Injector parameters Input sensors Shift selector Engine speed Throttle position Accelerator position Clutch control Turbine speed Transmission output speed Kickdown switch ATF (automatic transmission fluid) temperature Engine coolant temperature Brake light Wheel slip Actuators
2 on-off shift solenoid valves Lockup clutch linear solenoid Clutch pressure linear solenoid
In some transmissions used in luxury cars, as many as four clutch pack pressure solenoids are used.
A foundation block in transmission control has been elimination of the mechanical or vacuum connection between the throttle position and throttle valve and its replacement with an electrical signal produced from a potentiometer mounted on the throttle body. Such throttle
sensors produce full-closed signals and throttle opening signals. Typically, these are 8- or 10-bit A/D converters that identify these signals between the full opening and the full closing of the throttle. TV pressure is generated by a pulse-width modulated
signal that actuates a regulator valve that creates line pressure.
Shift Quality
Basic functions provided by the electronic control unit are:
Smooth shift quality under all conditions, which is effected by controlling clutch hydraulic pressure and engine torque during shifting Consistent shifting over the life of the transmission, which requires feedback control
Torque reduction by modifying spark advance angle during shifting with adaptive pressure control ensures constant shift times through the life of the transmission. Drive-by-wire throttle control, which is being introduced especially in luxury cars, can
also improve shift quality.
During the shift event, feedback allows the electronic control unit to detect any deviation of the actual rotational speed of the input shaft from the target value in the inertia phase. Shift quality is closely correlated to the duration of the inertia phase. Feedback to
the input shaft angular acceleration is performed. Thereafter, the controller is able to maintain accumulator back pressure according to a deviation in the fine adjustment of clutch hydraulic pressure so the variation of the rate of input speed becomes optimal.
Quick learning of clutch fill volumes is necessary to achieve optimum transmission shift quality. Prior to the car assembly plant final test, a transmission controller applies and releases the friction elements in a prescribed sequence. Clutch fill volumes are determined
by monitoring the time it takes for turbine speed to complete the inertia phase. This speed change provides a measure of clutch capacity.
Another factor is the engine run-up speed from the start of the shift, which depends on the time required to fill the oncoming clutch volume. This is a function of fluid flow rate and clutch clearance.
To compensate for elevated fluid temperature and lower engine speeds and the associated lower flow characteristics, an estimate of fluid flow as a function of temperature and engine speed is a learned value adjustment necessary for proper shift quality.
Through the use of feedback control, it is also possible to vary the line pressure according to the shift duration, which makes it possible to eliminate the effects of aging and component variability. Other factors affecting shift quality, such as atmospheric pressure,
road gradient, and additional engine load from the air conditioner and alternator, can be eliminated through feedback control.
Fluctuating factors that affect shift quality include: Coefficient of friction Clutch hydraulic pressure Hydraulic response time Engine torque Spring constant of accumulator and orifice diameter
The most critical factors include friction materials, clutch hydraulic
pressure, and hydraulic response time.
An important function provided by the electronic controller is to create smooth shifting by operating the lockup clutch. For example, upshifting to second gear would be harsh if the lockup clutch was continuously engaged, because this system would contain no dampening. To
overcome this, the converter is momentarily unlocked with precise control on the timing to prevent engine speed fluctuations under idle conditions. An approximate lag between the transmission feedback control initiation and engine torque control response is 30 μsec.
Faster response times occur at higher engine speeds.
Control Parameters
Control parameters include:
An electronic controller currently in use in a luxury car controls eight solenoids and actuators. Four control line pressure, two control clutch pack selection, one controls the torque converter lockup clutch, and one controls timing.
Engine speed and road speed or output speed inputs are digital, while the mode selector and gear position sensor are discrete digital. Sump temperature and throttle position are analog signals that are processed by an 8- and 10-bit precision A/D converter in the ECU.
Typical solenoid outputs are discrete digital, while the pressure regulator is driven at a DC level to provide an inversely proportional pressure output. Most ECM proportional type outputs are achieved through PWM type control of proportional solenoid actuators such as
line pressure regulators.
Heat Balance Calculation
Because a transmission controller has data on torque converter efficiency, pump speed, engine torque, coolant temperature, ambient temperature, car speed, and operating condition, a calculation of continuous heat balance can be estimated for
transmission oil temperature, eliminating the need/cost of an ATF sensor.
Wheel Slip
By communicating with the ABS or traction control systems, it is possible to eliminate upshifting on surfaces with low friction coefficients by relying on continuous wheel slip detection.
Fuzzy Logic
Transmission performance requirements that have become more important in recent years include smooth shift quality and shift schedule that matches the driver's intentions. Even with an electronically controlled automatic transmission, shift hunting
sometime occurs when driving uphill or towing.
The concept of human-friendly vehicles is one direction for the future evolution of the automobile. To detect the variable conditions that a car must operate under would require a large number of sensors. Various attempts have been made by different researchers to devise
a method for estimating the road gradient on the basis of the drive torque generated by a vehicle and the resulting acceleration. If the driver can be thought of as a visual sensor, then it is possible that the driving environment could be detected from the driver's
actions.
One technique is to apply fuzzy logic to derive a quantity corresponding to the road gradient, using the throttle valve opening and vehicle speed as the information inputs. Using statistical analyses makes it possible to develop histograms of throttle valve position for
expressway and winding uphill roads.
For more information about electronic transmission control capabilities at SwRI
or how you can contract with SwRI, please contact
Douglas Fussner at
dfussner@swri.org or (210) 522-3972.
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