Abstract: An energy controller is described for a vibration welder wherein the energy imparted into work-pieces being vibration welded is derived and compared with an energy set point to control the vibration welding operation. After a vibration welding operation has encountered the energy set point the welding operation is stopped and the quality of a weld derived from a comparison with another parameter associated with the vibration welding. The other parameter can be the amount of movement of one of the work-pieces during vibration welding or the time involved to inject the energy into the work-pieces to achieve the vibration weld.