Sensor Measurements For Diagnostic Equipment

Piezoelectric sensors are widely used in accelerometers, for detecting failures in structures, and for vibration suppression systems among others. Such sensors can be manufactured in many different sizes, and packages to work in a variety of environments. The main characteristic of the piezoelectric material that allows for this effect is the conversion of mechanical to electrical energy. That is, when a piezoelectric material is subjected to a mechanical stimulus, a response is produced and a conversion of mechanical to electrical energy occurs. A new pre-stressed piezoelectric bender has exhibited an improved response under the same magnitude stimulus when compared to a traditional piezoelectric bender as an actuator. Measurements performed on these new devices demonstrated that energy produced under a single mechanical strike is significant compared to a single piezoelectric slab. The objective of this study is to investigate the feasibility of using this new pre-stressed piezoelectric bender as a self-powered sensor that could be used for monitoring the health of an individual. Introduction Piezoelectric materials have been studied for several years to be applied as effective sensors [1-4]. There are many sensors today that incorporate a multitude of piezoelectric technologies including single crystal, piezoceramic and piezopolymer films. The application requirements typically dictate the type of piezoelectric technology that can be applied. This study has taken advantage of a newly developed pre-stressed device that has demonstrated significant electrical output when compared to conventional piezoelectric devices. By harvesting the energy output of these new devices and redirecting the energy to generate a signal through a known medium, it may be possible to achieve a signal-sensor relationship that could be applied as an aid in monitoring and individual’s health [5-9]. The primary objective of this study was to determine the feasibility of using a pre-stressed device as a self-powered sensor. Results have been obtained for the impedance, capacitance, phase angle and electromechanical coupling of several composite laminates to be considered as potential sensors. Experimental Set-up In order to test the capabilities of a pre-stressed piezoelectric wafer, several tests were performed using high-density piezoelectric ceramic, PZT 5A equivalent, manufactured by CTS Wireless. The ceramic had a surface area of 3 in by 0.5 in and varying thickness. Fifty-six devices manufactured by Dominion Resources consisted of 1 top layer of aluminum 0.001 in thick, a 0.015 in thick PZT layer, and a 0.010 in thick stainless steel type 304 bottom using the polyimide SI as an adhesive. Impedance, phase angle, capacitance, and electromechanical coupling factor Q were measured at a range of frequencies between 100 Hz and 10 kHz using an impedance analyzer (model HP4194A) with a voltage source of 1Vrms. Each device was x-rayed immediately following fabrication using a Philips x-ray machine (model MGC 30/MCN 167). To ensure uniformity, the total thickness of each device was measured using calipers along the length of the device and the result averaged. Dome height, defined as the arc height obtained as a result of pre-stressing, was determined using a non contact laser NAIS, (model ANR5232). The elements were tested for displacement performance under loading at a quasi-static frequency (5 Hz) at voltages up to 800 Volts peak to peak using the same non-contact laser described above, a signal generator (HP 33120A), a TREK power amplifier (model PDZ70-2-L-CE), a LeCroy 500 MHz Oscilloscope (model 9350L), and a data acquisition system (National Instruments) controlled with a PC using LabView software. The layer configuration for the devices utilized was chosen based on previous studies [10]. The results of the previous study states that a “steel thickness ratios of 0.280 to 0.375 seem to give the largest, most reliable displacements for any given voltage,” at 1 Hz and no load. The ratio for this set of tests was 0.31, which falls within that range as seen in Figure 1.