AutoShifter Software for AMT Control

The automatic transmission used in the United States uses planetary gear trains and variable-force solenoids with pressure and clutch control to achieve smooth shifting. Because it is so dissimilar in design from a manual transmission, it is an expensive option for car manufacturers whose factories and markets are tooled for manual transmissions. The AMT, on the other hand, provides a logical, moderately priced approach to automated shifting in those markets. More importantly, it also can deliver up to an 8 percent improvement in fuel economy over a traditional automatic transmission and up to a 4 percent improvement over a manual transmission, which is especially significant in high fuel-price markets such as much of Asia and Europe.

The core of AMT technology lies in the control design and development to simulate and even improve on human judgment and shift actions. AutoShifter is a full-authority vehicle controller developed at Southwest Research Institute. The main focus of the AMT controller is in minimizing the interruption of the flow of power to the wheels, providing a human-like clutch feathering/engagement to achieve an anti-stall vehicle launch and provide appropriate safety during uphill or downhill driving. The SwRI AutoShifter software controls four actuators: the engine throttle, clutch, shift fork and select fork. An appropriate software driver controls each actuator while a supervisory controller makes shift decisions and coordinates activation/de-activation and reference signals among the various low-level controllers. AutoShifter provides a software structure within which these goals can be achieved through calibration and some customization. It has the capability of communicating with RS-232 or CAN protocols.

Manual transmissions are intended for human interaction and hence are comparatively loose in tolerance and rely on detent mechanisms to provide engagement feel to the driver. The loose tolerances within the mechanism create unique control problems when gearshift occurs from one leg of the H-bridge to the other (such as from second gear to third gear). The AutoShifter software has a unique set of self-centering algorithms that, when calibrated to a transmission, will simulate human actions of finding the neutral gate so that the selector mechanism will not jam. While this causes an increase in shift time, it also assures a reliable shift every time.

The SwRI AMT research team has developed a list of mechanical modifications and sensor addition strategies that can aid this algorithm even further. The software uses engine speed, vehicle speed, vehicle attitude, throttle position sensor, and encoder position for feedback of shift and select motors.

The friction and nonlinear stiffness of the diaphragm spring in the clutch mechanism poses another unique problem in clutch control during engagement. Rapid slip due to friction change can cause the engine to stall, while excessively slow engagement can cause friction-related sticking or stalling, as well as excessive shift times.

AutoShifter software can control the rate of clutch engagement to coordinate with the control of the ETV (electronic throttle valve) to ensure that the engine does not stall. It also minimizes the overall clutch disengagement and engagement time. The software provides a structure to calibrate and tune control system gains and parameters to match the transmission non-linearities by using clutch encoder position and current feedback.

The ETV is controlled with SwRI proprietary PWM driver circuits and software. The ETV works in harmony with the idle air control function to ensure smooth transition to and from idle conditions. The software also uses throttle position sensor feedback.

Sensors on the brake and accelerator pedals help in understanding driver intentions and are a part of the supervisory control strategy. Memory of required throttle values for driving on level ground is stored and compared against required throttle during vehicle operation, and an estimate of grade is made. Shift performance is stressed during high grade operation to ensure that the vehicle does not roll backward when it is stopped and idling.

The AutoShifter software had been developed using C language. SwRI has packaged it as an S-function with input-output connectivity in Matlab®/Real Time Workshop®. The code can be integrated into a DSP/microprocessor using the integrated development environment of that target processor. At present, the loop time is 5 milliseconds, and it can be tuned according to the needs of target transmission hardware.

Back to Smooth in the Clutch Article