Surgical instrument with motorized articulation drive in shaft rotation knob

An apparatus includes a body assembly, a shaft defining a longitudinal axis extending from the body assembly, an acoustic waveguide having a flexible portion, and an articulation section coupled with the shaft. A portion of the articulation section encompasses the flexible portion of the waveguide. The articulation section has a first member and a second member translatable relative to the first member. An end effector has an ultrasonic blade that is in communication with the waveguide. An articulating control assembly has a motor. The motor is operable to translate the first and second members to thereby deflect the end effector away from the longitudinal axis at the articulation section.