Composite materials, which are made from high-tech fibers and resins deliver unique mechanical performances. These materials distinguish themselves by their high stiffness and strength with excellent thermal stability and low weight in comparison with metals.
Kok en Van Engelen Composites Group (KvE) uses composites to design efficient structures used in the aerospace, medical and civil industries. The company makes use of the most efficient assembly method for carbon fiber reinforced thermoplastic laminates, which is induction welding. This technique is being called KVE Induct Welding.
A weld is made with the use of a induction generator and a custom made coil. During this process, the coil is being moved over the composite material. The induction coil generates Foucault currents in the highly electrically conductive carbon fibers of the material. These Foucault currents will heat up the laminate from the inside.
With enough heat and pressure the thermoplastic matrix will melt. Resulting in the two composite parts welding together.
The quality of the weld depends on the characteristics of the materials used. Process parameters containing the amount of energy applied and the movement speed of the induction coil affect the interconnection of the two materials. The goal of this project is to realize a system capable of testing multiple power and speed settings for particular materials. With these settings the optimal weld will be determined.
Starting the project, the requirements and specifications of the induction welding system are determined. These requirements are defined in consultation with KvE. During the design phase of the project, the different requirements are translated to a technical design.
The welding system is divided in subsystems with the use of a Process Breakdown Structure. This structure describes the tasks needed to complete the system. With the use of the MoSCoW method, the tasks are divided according to their importance and dependencies.
During the realization phase of the project, the different tasks will be completed. This will result in the finalization of the induction welding system. Thoroughly testing the individual components will make sure this system is robust.
The system is primarily used to gather the optimal speed and power settings belonging to the characteristics of different types of materials. Among these differences is the thickness of the material, the way the carbon fibers are weaved and the type of thermoplastic. The system has to perform multiple test runs with various power and speed profiles. Therefore, the system is divided in multiple subsystems.
In order to control the output power of the coil, a spare induction generator and coil from KvE is used. With the use of an analog signal, generated by a Siemens S7 1200 PLC, the output power of the induction generator is controlled. This signal provides a full range output to the generator. A local cooling unit makes sure the heat is dissipated from the coil.
The linear movement of the coil is realized with the use of a linear actuator which is supported within an aluminum frame. The carrier of the actuator is powered with a Nema 23 stepper motor and a lead spindle. The position of the carrier is controlled with the use of an end stop switch and position control software. The PLC controls the stepper driver according to the position software.
As a result of the Foucault current flowing through the material, the thermoplastics in the composite will heat up. In order to read these temperatures, multiple thermocouples can be used. These thermocouples can be placed on the material before a test run. With the use of a Siemens thermocouple card, up to eight thermocouples can be read. These temperatures should be used as a guideline for the power and speed settings.
One of the biggest challenges in this project was the human machine interface (HMI). The systems HMI controls the machine and also logs the data aquired from the test run in a SQL database. The HMI runs from a Windows executable and is operable when the computer is connected to the PLC with an Ethernet cable.
Within the HMI, the user is able to generate up to ten power and speed profiles. These settings determine the output power in Amps and the speed of the actuator in millimeters per second. For every step in the profile, the power and speed are adjustable to a self-determinable distance. Besides the profiles, the user is able to enter certain test run data like an ID number, material type and thickness.
By pressing the start button on the HMI, the system will perform the test run according to the imported profiles. With the use of a live feed the actual temperature readings can be monitored within the HMI. When the test run is complete, the user is able to export the gathered data to an Excel file.
The HMI is able to read and write the data directly to the PLC. Subsequently, the PLC will control the complete induction welding system.
The whole induction welding system is capable of performing multiple test runs with multiple speed and power settings for the welding of carbon reinforced thermoplastic laminates. According to the settings, the PLC will control the induction generator and linear actuator with the mounted coil.
During the test run, the HMI provides the user with actual temperature readouts from the thermocouples attached to the composite materials. When the test run is finished, the user is able to export the gathered data for further analysis. These analysis should give the user an indication for the optimal settings of the weld.
Thomas van Mil, Stef Nibbelink, Bert Tuijl, Niels Westerduin