An ultrasonic welding system and method for making welds on a strip made of polymeric material. The system includes a supporting structure, N welding equipment mounted thereon, and a processor. Each N welding equipment includes a pair of welding heads made of a sonotrode, including a sonotrode welding surface, and an anvil, including an anvil welding surface. One of the two welding heads is movable with respect to the other between a non-operating position and a welding position. A control member is operatively associated with the sonotrode and configured to vibrate the sonotrode at a nominal ultrasonic vibration frequency and to detect a current value of the vibration frequency. A vibration sensor is mounted on the movable welding head. The processor monitors the alignment condition between the sonotrode welding surface and the anvil welding surface by cooperating with the control member and with the vibration sensor.