Method for controlling a rotational speed of an output shaft of a propulsion unit

A control unit for controlling a rotational speed of an output shaft of a propulsion unit, said control unit being adapted to:

• receive information relating to a speed setpoint value, indicative of a rotational speed setpoint for said output shaft, and an actual speed value, indicative of an actual rotational speed of said output shaft,
• receive information relating to an acceleration setting member,
• determine an acceleration value using an acceleration value determination procedure comprising employing an acceleration conversion function that uses said speed setpoint value, said actual speed value and said acceleration setting member as inputs and which produces a resulting value to be used for determining said acceleration value, said acceleration conversion function being such that different resulting values can be obtained for a same set of said speed setpoint value and said actual speed value but for different choices of said acceleration setting member,
• determine a torque request value using a torque conversion operation that uses said acceleration value, wherein said torque conversion operation comprises forming a torque request sum by multiplying said acceleration value by a moment of inertia value, indicative of a moment of inertia associated with said propulsion unit, and adding thereto a resistive torque value, indicative of a resistive torque associated with said propulsion unit and wherein said torque conversion operation further comprises adding a torque disturbance value, indicative of torque disturbance associated with said propulsion unit, to said torque request sum, wherein said torque disturbance value is determined using values determined for at least one previous time instant for each one of an actual torque value, indicative of an actual torque of said output shaft, said moment of inertia value, said resistive torque value, said torque disturbance value and an actual acceleration value, indicative of an actual acceleration of said output shaft and
• issue a signal indicative of said torque request value to said propulsion unit.