There are only three basic process variables: amplitude of vibration, pressure (force) and time. The desired final result is the provision of sufficient energy to disrupt oxides and bring opposing surfaces within atomic distance of one another. These variables are combined to provide that energy. Power is a function of horn amplitude and force applied. Energy, in turn, is a function of power used and time as defined by the simplified formulas P = FA and E = PT.
P = Power (Watts)
F = Force (PSIG)*
A = Amplitude (Microns)
T = Time (Seconds)
E = Energy (Watt/Seconds or Joules)
*A simplified pneumatic and mechanical system with all linkage, lever and cylinder characteristics considered as a constant resulting in a direct relationship between PSIG used and force applied.
These process variables are roughly established by prior experience and adjusted to meet the needs of the specific application. In actual production, these variables are easily and accurately controlled.
Amplitude is initially determined by the selection of the booster and the horn design. Amplitude is automatically and accurately controlled and regulated by the electronic power supply. A modern power supply is capable of electronically making small, incremental changes in output amplitude to fine tune the setup. Pressure (force) is usually generated by a pneumatic press and is easily adjusted and regulated. Time is precisely controlled electronically. Within certain limits, the ultrasonic variables can be changed relative to one another in order to achieve the same result (See Figure 4.)
Changes in nonultrasonic variables including part surface condition, size or orientation will usually require changes in the ultrasonic variables to ensure the precise amount of energy is delivered to the weld site. Once the amount of energy required to make a satisfactory weld is determined, consistent quality results can be obtained by ensuring the same amount of energy is applied to each weld. An energy control circuit continuously monitors the power in watts used and integrates this value over time to determine energy delivered. The time variable is automatically adjusted to ensure the optimum energy level is reached thereby assuring consistent quality. (See Figure 5.)
This graph shows two typical curves for two weld conditions. Dirty parts will more freely vibrate against one another until the contamination and oxides are dispersed from the interface. As the parts are cleaned the base metals achieve atomic distance and begin to weld. This requires more time and, therefore, the power curve will rise more slowly until the desired energy level is reached. Welding begins sooner in the absence of contamination and the power curve rises faster reaching the required energy level in a shorter amount of time. The energy control circuit, by monitoring power and changing time, therefore ensures that the same amount of energy is applied to the actual weld regardless of the surface condition of the parts to be welded.
Values for time (t1 - t2) and power (p1 - p2) can be set as limits. Welds exceeding these values will trigger an alarm indicating a suspect weld. Acceptable windows of operation for time and power give flexibility to the process to meet changes introduced by dirty or out of specification parts. Welds causing changes outside these windows trigger alarms to signal a possible defective part.
