The present invention relates to systems and methods for monitoring the stability of a 
hybrid powertrain.  A 
system and method for monitoring stability of a 
hybrid powertrain system includes monitoring output response signals of the 
system and its subsystems to their inputs to identify system operating conditions.  For example, for a 
hybrid powertrain system equipped with an electrically variable transmission (EVT), three signals are monitored.  These are the two motor speeds representing the immediate subsystem response to its 
feedback control input and the EVT output speed representing the overall system response.  A stability 
monitoring system includes: an average value determination module, which determines an average value 
signal of the 
signal; and an oscillation determination module, which determines the 
signal oscillation signal according to the instantaneous signal and the average value signal.  The 
monitoring system also includes: a signal mean value crossing determination module for determining a signal crossing with its mean value signal; and an oscillation 
peak detection, storage and comparison module for determining a system 
instability indicator for causing the 
control system to take corrective action.