An ultrasonic welding system includes a movable ultrasonic welding stack, an electrically powered linear actuator with a servo motor and a movable element, a controller, and at least two sensors. The controller causes the ultrasonic welding stack to apply a predetermined positive initial force to at least one workpiece prior to initiation of welding. The controller further causes the ultrasonic welding stack to initiate subsequent movement of the ultrasonic welding stack, following initiation of welding, only after the signal outputs from the at least two sensors indicate that a combination of control variables satisfies a predetermined condition.