Ultrasound-based wireless pressure sensing for injection molding process monitoring
A new sensing methodology utilizing ultrasound as the information carrier for injection mold cavity pressure measurement has been developed in this research. The technique enables wireless data transmission through an enclosed metallic environment, and can be applied to the on-line monitoring of injection molding process.
Ultrasonic transmitters with non-overlapping frequency bands have been designed and analyzed to enable a sensor array configuration for multi-point pressure sensing inside the mold cavity. A modified Mason's equivalent circuit model has been developed to investigate the frequency response behavior of the multiple layer (front, bonding, and coupling) structure of the transmitter. Optimal thickness for each layer was determined that enables high energy efficiency, narrow bandwidth, and unique dominant resonant frequency of the transmitter.
To detect and differentiate multiple ultrasonic pulses that carry timely overlapped pressure information at different spatial locations across the mold cavity, a two-dimensional time-frequency signal representation technique has been developed, based on a variable relative bandwidth wavelet transform. Compared to conventional approaches characterized by a constant ratio between the wavelet bandwidth and center frequency for all the scales, the harmonic wavelet and complex Morlet wavelet investigated in this research enable a variable window size of the wavelet to simultaneously match the frequency characteristics of multiple ultrasonic pulses. This has provided a solution to accurately identify the time and frequency information within the signals. Theoretical framework for the variable relative bandwidth analysis was established, and the performance of the harmonic and complex Morlet wavelet transforms were comparatively studied and experimentally verified.
While this research has been aimed at the injection molding process monitoring, the transmitter design guidelines and signal processing techniques developed are applicable to a wide range of industrial applications where wireless data transmission through an enclosed metallic environment is critical to process state monitoring and controls.