In a control system and method for operating an ultrasonic liquid delivery device a first drive signal is delivered according to a first drive signal control schedule to an ultrasonic waveguide assembly to excite the ultrasonic waveguide for a predefined time. At each of a plurality of instances of the first drive signal control schedule a phase angle associated with the current and voltage in the waveguide assembly during ultrasonic vibration thereof is determined. A second drive signal control schedule is determined, including a second ultrasonic drive signal frequency for each of the instances at which the phase angle of the first drive signal was determined. The second drive signal frequency at each of the instances is based at least in part on the phase angle determined at each instance during ultrasonic vibration of the waveguide assembly in accordance with the first drive signal control schedule.