Closed Loop Transaxle Synchronization Control Design 2010-01-0817

This paper covers the development of a closed loop transaxle synchronization algorithm which was a key deliverable in the control system design for the L3 Enigma, a Battery Dominant Hybrid Electric Vehicle. Background information is provided to help the reader understand the history that lead to this unique solution of the input and output shaft synchronizing that typically takes place in a manual vehicle transmission or transaxle when shifting into a gear from another or into a gear from neutral when at speed. The algorithm stability is discussed as it applies to system stability and how stability impacts the speed at which a shift can take place. Results are simulated in The MathWorks Simulink programming environment and show how traction motor technology can be used to efficiently solve what is often a machine design issue.

The vehicle test bed to which this research is applied is a parallel biodiesel hybrid electric vehicle called the Enigma. This vehicle is believed to be the world's first diesel hybrid electric sports car designed in the late 1990's and fabricated in 2001 at San Diego State University. The powertrain couples an AC-Propulsion AC-150 induction motor and a Volkswagen 1.2 direct injection turbo diesel together with a significant energy storage system. This powertrain can propel the vehicle 20 miles on electric power alone with lead acid battery technology and much farther with modern lithium ion. To couple these two power sources together, a custom transaxle was designed. Due to vehicle geometry and powertrain robustness constraints the transaxle design utilizes helical gears for power transmission that are larger than in a standard passenger vehicle. The inertia is further increased by that of the motor rotor. The engine has a dedicated friction clutch and thus can be removed from the synchronization operation. Normal mechanical synchronization methods are not desirable due to the required large size of a conventional synchronizer in this application and amount of rework required to integrate such assembly. Instead, the control system does this job by monitoring input and output shaft speeds as it modulates motor torque to bring the difference between these two shaft speeds within acceptable limits in less than 1 second. Results are presented in units of time to synch vs. RPM delta between the two transaxle shafts.

Application of control system development in this area has the potential to mechanically simplify and reduce the mass of a transaxle or transmission in a hybrid powertrain while maintaining robust operation and shift related drive quality. Here, the transaxle is easily synchronized in less than 1 second allowing for smooth engagement of the transaxle's gears to their respective shaft.