A feedback control system for use with ultrasonic welding. A weld energy set point and/or a reference power curve are used by a controller to provide a power supply voltage set point and a power supply frequency set point for the power that is to be drawn by an ultrasonic weld generator. Information regarding the actual voltage and/or frequency of the drawn electrical power, and/or the frequency and magnitude of an ultrasonic wave transmitted from the generator to an applicator tool used to weld an object can be utilized by an estimator to estimate the weld energy dissipated at the weld site and/or generate a power curve for the welding operation. Using information from the estimator, the controller can determine adjustments for variables for the current and/or future welding operations. Further, welding may not commence until a clamping force exerted on the object satisfies a clamping force set point.