Beginning in about 1985, ultrasonic welders began to be equipped with linear encoders measuring weld distance. It quickly became apparent that the two main methods would be absolute distance, giving the same functionality as an end-of-weld limit switch but with much greater accuracy and repeatability, and collapse distance, measuring the amount of carriage travel during the application of ultrasound. Of the two, it is weld by collapse distance that truly revolutionized welding of injection molded parts using an energy director or shear joint. The computerized controller would simply remember the encoder position when ultrasound was turned on via force trigger, and run ultrasound until a certain amount of distance had been traveled. This method made up for a significant amount of variation in part fit-up, plastic density or resin quality variations, and other things that had given users great difficulty before this method was introduced. In a matter of less than four years, all major manufacturers worldwide had introduced a machine with a linear encoder and weld by collapse distance welding mode. Such machines could now be equipped to monitor all major energy, power, time, and distance process inputs for a significant improvement in process stability and visibility.