US Patent 10220833 Hybrid powertrain speed control

A vehicle powertrain includes a controller, a torque converter, and an engine and electric machine coupled by a clutch. The torque converter may be configured to couple the electric machine to an output shaft. The controller may be programed to generate a command for the electric machine to output torque to drive the torque converter toward a desired rotational speed, and to modify the command according to a difference between the desired rotational speed and an actual rotational speed to reduce the difference, wherein values of the difference are limited by thresholds that change with powertrain operation.