Linear vibration welders are used in the industry to weld two plastic parts, by creating linear oscillatory motion of one part relative to another part. As the parts are pressed together by force, the oscillatory motion generates heat, which melts the adjacent surfaces of the plastic parts and creates a weld after the parts cool.
The vibratory movement of one part relative to another part is generated by two electromagnets positioned between movable and stationary parts of the welder. Both electromagnets apply force along the same coordinate line, but in opposite directions. The electromagnets are energized with a 180° phase shift so that when the first electromagnet is energized, the second electromagnet is de-energized. Conversely, when the second electromagnet is energized, the first electromagnet is de-energized.
It is desirable to maintain the frequency of the energizing cycles at the resonant frequency of the movable mechanical part of the welder; to allow for maximum energy transfer to the parts being welded. It is also desirable to control the energy applied to the electromagnets, to maintain a desired level of the plastic melting during welding.
Previous methods to control the electromagnets achieve 180° phase shift between energizing/de-energizing cycles (see, e.g., U.S. Pat. No. 7,520,308), but they still have disadvantages. For example, when a three-phase output drive is used to control two electromagnets, two of the phases are used to drive the two electromagnets, and both electromagnets have a common wire connected to the third phase. The third phase is therefore loaded twice as much as either the first or second phase, which stresses the third phase control element (typically an IGBT transistor). Also, the overall timing of energizing and de-energizing is fixed, while PWM is used to control the amount of energy delivered to each electromagnet because a PWM controller is a standard drive solution in a three-phase motor control. But this has the disadvantage of having a slow response time, limited by the frequency of the PWM controller. In addition, use of the PWM controller for this application causes excessive switching of the output power elements (IGBT transistors), which in turn leads to unwanted power losses, excessive electrical noise and lower system reliability.
Previous methods of measuring the resonance frequency of the movable mechanical system involved a frequency sweep. In the sweep mode a fairly low voltage (typically 10%-25% of the maximum) was applied to the electromagnets and the frequency was stepped in small increments (typically 0.1 Hz) from the lowest to the highest frequency of the machine's operating range (typically from 200 Hz to 240 Hz). As the frequency was stepped, the amplitude feedback and/or the drive current output were monitored. The resonant frequency was determined as the one with highest amplitude feedback and/or lowest current output drive. Once the value of the resonant frequency was determined, it was stored in the memory of the control module (typically a Programmable Logic Controller or PLC) and passed on to the drive as its fixed operating frequency. This method of defining the resonant frequency was fairly accurate, but has several inherent shortcomings. Firstly, it required an operator to remember to go to a “Tuning” mode to sweep the frequency, which was frequently forgotten in the manufacturing environment. Secondly, the procedure itself was fairly time consuming and could take up to 3-5 minutes, which was also undesirable in the high volume production environment. Thirdly, the sweep routing did not address the issues of the machine and tooling warm up in high-volume and high-load types of applications. As the machine and its components get warmer, the resonant frequency goes down. If the new resonant frequency was not found, the machine would be running off its optimum mechanical resonance and therefore draw more current, producing more heat and inducing more stress on its critical components. An avalanche effect (or run away condition) could develop. To remedy this, the operator had to run frequency sweep every hour or so, which again compromised the manufacturing efficiency.
Previous methods to control the welding process were based on the use of a PLC. The linear position of the welded parts and the pressure between welded parts during welding were monitored and controlled by the PLC. Based on the information obtained from the sensors, the hydraulic cylinder lifting the table and engaging welded parts was controlled by the PLC. While the PLC had all the necessary input/output channels to provide such control, its response time was fairly slow (typically from 5 ms to 20 ms), which could affect repeatability and accuracy of the welding process.