Field of the Invention
The present invention generally relates to a variable magnetization machine controller.  More particularly, the present invention relates to a controller that includes a hysteresis component for changing the magnetization level of a variable magnetization machine, such as an electric motor or other type of variable flux machine, that is employed in an electric or hybrid electric vehicle. 
Background Information
Electric vehicles and hybrid electric vehicles (HEV) include an electric motor that operates as a drive source for the vehicle.  In a purely electric vehicle, the electric motor operates as the sole drive source.  On the other hand, an HEV includes an electric motor and a conventional combustion engine that operate as the drive sources for the vehicle based on conditions as understood in the art. 
Electric vehicles and HEVs can employ an electric motor having variable magnetization characteristics as understood in the art.  For example, the magnetization level of the motor can be increased to increase the torque generated by the motor.  Accordingly, when the driver attempts to accelerate the vehicle to, for example, pass another vehicle, the motor controller can change the magnetization level to increase the torque output of the motor and thus increase the vehicle speed. 
Certain techniques exist for increasing the magnetization level of such an electric motor.  In one conventional technique, a number of magnetization levels (e.g., 3 levels) can be predetermined based on the stator flux linkage.  In another conventional technique, a number of magnetization levels (e.g., 5 levels) can be predetermined based on the magnetization state of the variable magnetization machine (e.g., the motor).  However, if the ideal magnetization state (M/S) of the magnetization machine includes a high frequency component, the number of changes in the M/S will be large (e.g., 400 changes over a certain driving cycle) and the loss due to the number of changes will be significant.  Alternatively, if the ideal M/S settles at a value between 2 nearest values, there will be some steady state error.  Thus, the efficiency of the magnetization machine will not be maximized.