The project reported in this thesis investigated the use of a non-destructive method for monitoring a Resin Transfer Moulding (RTM) process. The measurement of ultrasound energy was proposed for monitoring the RTM flow front and the curing phases. Furthermore the project examined technologies for integrating the ultrasound transducers used in the project.
RTM is a high efficiency low cost manufacturing method. The process allows the production of large size composite components with fibre-reinforcement. This gives the products a light-weight while maintaining a high structural intensity. The RTM process involves inserting fibre-glass mat into the mould’s cavity and infiltrating it with polyester resin. The application of automated control for the resin filling and the curing process helps to produce consistent quality products; this requires constant monitoring of the resin filling and curing processes in the mould. Currently, most monitoring methods require an alteration in the mould or embedding the transducer in the moulding product, for example, the dielectric method requires the electrodes to be built into the walls of the mould. These methods are complicated and significantly increase the cost of the mould.
Ultrasound methods offer non-destructive monitoring and an opportunity for integrating the transducer into a small unit with autonomous measurement and the ability for communication. There have been reports on ultrasound techniques that demonstrated ultrasound velocity as a parameter for interpreting the filling and curing process. Measuring the ultrasound velocity requires advanced electronics with a high initial capital cost. Furthermore, in practice, the fibre reinforcement in the RTM process reduces the ability of the ultrasound to penetrate; consequently, it is difficult to extract a neat echo signal for measuring the ultrasound velocity. In addition, the foam-fill structured material in thick moulded products renders ultrasound waves ineffective, with the foam being unable to transmit the ultrasound. These factors prevent the use of ultrasound velocity measurement in monitoring the filling and curing process.
This project proposes a new ultrasound method for monitoring the RTM process by measuring the energy of the ultrasound echo waves. The measured ultrasound energy is closely correlated to the ultrasound velocity and can be used for detecting the flow front and interpreting the curing stages of the resin.
In a large size moulding, a large number of transducers are distributed on the mould’s surface to sense the flow front and form the flow front contour. A simplification of the system and the reduction of the costs associated with the system can be made by reducing the size and increasing the functionality of the ultrasound transducer. This improvement is feasible through integrating a piezoelectric ceramic with advanced electronics, which can function as an autonomous monitoring unit and send data to a supervisory computer. The reduction of the ultrasound transducer size is achieved by using a bare piezoelectric ceramic that can avoid the need for a backing layer and the associated electrical matching components, and by circuit integration.
The electronic circuitry used for measuring the ultrasound energy involves analogue, digital and mixed signal circuits; these pose a significant challenge for the circuit integration. At present, there is a new technology called System in Package (SiP) that provides flexibility in integration and can integrate complex electronic circuits with various integration processes on a substrate.
In this project, a number of experiments were performed to verify and compare the ultrasound energy measurement method using commercial ultrasound transducers and instruments, and using a prototype transducer built using piezoelectric ceramics and printed circuit board (PCB) circuits. This built transducer, as a prototype, also established the grounds for further investigation of the SiP integration of the ultrasound transducer.
Based on the experimental results and the parameters of the prototype, analogue CMOS (complementary metal–oxide–semiconductor) integrated circuits for the energy measurements were designed. A topology of analogue integrated circuits for CMOS implementation was proposed to compensate for temperature variations in the ultrasound energy measurement circuits. Simulations of the circuits were conducted with both sinusoid signal inputs and an ultrasound wave created by simulation results using piezoelectric ceramic models.
Layout of the SiP transducer was designed with chips of the designed analogue integrated circuit layouts (pulser, ultrasound energy measurement circuits), acquired open source packaging for the microprocessor and IC die for the communication chip, SMD (surface mount device) components for power supply ICs and SMD passive components including capacitors, resistors, diodes and inductors.