Extraction and digitization of a process signal for self -powering a wireless pressure sensor

• Main Author: Theurer, Charles B
• Language: ENG
• Published: ScholarWorks@UMass Amherst 2004
• Subjects: Mechanical engineering|Electrical engineering
• Online Access: https://scholarworks.umass.edu/dissertations/AAI3136783

The use of sensors for manufacturing process monitoring has been limited by the size, installation, and maintenance requirements of traditional cable-based sensors. To overcome these limitations, a wireless pressure sensor for the injection molding process is analyzed, designed, developed, and validated. The developed sensor extracts energy from the pressure dynamics of the polymer melt during the injection molding process, and then conditions and modulates the stored energy for powering an acoustic transmitter for sending a signal through an injection mold. Energy extraction was accomplished through the development of a stack of polarized piezoelectric rings. Analytical and numerical models were developed to quantify the relationship between the polymer melt pressure and the potential extracted energy. The device was constructed and tested in situ. Temperature and pressure were monitored during the molding process and the performance of the piezoelectric element was evaluated against a conventional strain-gauged based cavity pressure sensor. In addition to corroboration of the developed models, the working temperatures in the prototype sensor were measured, which validated the adequacy of the underlying assumptions and package design. Energy conditioning and modulation was accomplished through the development of a threshold modulator. The charge, proportional to pressure, was stored via a capacitive element. Standard NPN and PNP transistors were used to passively control the flow of charge between the piezoelectric stack and an ultrasonic transmitter. Numerical simulations and bench top prototypes were developed and compared. The results of this comparison indicate the appropriateness of the assumptions and the limiting conditions under which the modulator will effectively measure pressure. Finally the prototype device was optimized with respect to sensitivity, gain, and operating range for use in real time process monitoring and control. The use of multiple transistor pairs was investigated to increase the output voltage to the transmission device. A bi-directional threshold modulator was also developed and validated which was capable of measuring positive and negative pressure changes in the polymer melt of an injection molding process. This research was applied, along with parallel research concerning the design of the ultrasonic telemetry system, in a bench-top experiment that included the application of a force profile, un-powered digitization, ultrasonic transmission through 10 cm of steel, reception, and finally reconstruction of that profile.