An agricultural combine includes a rotor drive system to which sensors are coupled to signal the rotational speed of a hydraulic motor and an engine. An electronic controller that contains a mathematical model of the system reads the sensors and using the model estimates additional parameters of the system. The estimated parameters include a gearbox's gear ratio and a planetary ring gear's speed. These estimated parameters and the sensor readings are used in a PID feedback control loop to control the speed of a planetary ring gear by controlling the speed of a hydraulic motor driving the sun gear. In this manner, the controller can drive the ring gear to a predetermined speed that reduces the difference in the input and output speeds of a hydraulic clutch and thereby engage that clutch while reducing shock to the clutch and other mechanical components of the combine.