Abstract
Research and /or Engineering Questions/Objective: The objective of this study was to demonstrate a first-generation static/dynamic pressure transducer with the ultimate capability of: (1) frequency response of 0 Hz to up to 35 kHz, (2) maximum sensor head temperature rating of 450oC, (3) sensing head diameter as small as 2.2mm, and (4) the pressure ranges of 0 bar to 350 bar, 0 bar to 2 bar, and 0 Bar to 3000 bar. Such a transducer targets three closed-loop engine control applications based on real-time pressure measurement inside: the engine combustion cylinder, turbocharger, and diesel fuel injector. Methodology: The operation principle of the transducer is based on light intensity changes transmitted by two optical fibers upon reflection from a metal diaphragm deflecting under the effect of pressure. The sensor head is connected to the signal conditioner via a high temperature fiber optic cable ranging in length from a few centimeters to a couple of meters. The transducer uses one SMD LED and two SMD photodiodes, where one photodiode detects light intensity modulation associated with pressure changes while the second photodiode is used for closed loop LED intensity control. A thermocouple welded to the sensor head provides its temperature while a PCB-mounted thermistor measures signal conditioner’s temperature. A microcontroller digitizes the pressure and temperature inputs, performs high bandwidth calculations to minimize temperature errors, and outputs pressure and sensor head temperature voltages. Results: A static/dynamic transducer intended for measuring pressure inside a combustion cylinder over the pressure range of 0 bar to 200 bar and frequency range of 0 Hz to 20 kHz was evaluated both in a laboratory test as well as a single cylinder gasoline engine. All the tests were performed while the signal conditioner was at room temperature. The sensor linearity was better than 0.15%. Over the sensor head temperature change from 20oC to 250oC the offset voltage changed from 0.5V to 0.51 V while the sensitivity changed from 3mV/psi to 3.05 mV/ psi. During an engine test the sensor offset changed from 0.5V at idle, at the sensor head temperature of ~120oC, to 0.52 V at full load, at the sensor head temperature of ~240oC. Limitations of this study: This study was focused on short term transducer performance under the effect of changing pressure and varying sensor head temperature from 20oC to 250oC. No effect of varying signal conditioner temperature on the device sensitivity and offset and in particular the device drift was considered. What does the paper offer that is new in the field in comparison to other works of the author: Unlike the current high temperature pressure sensors offered by Optrand which measure dynamic pressure only the new transducer reported here measures both static and dynamic pressure over the frequency range of 0 Hz to 20 kHz. The device can measure pressures over wide ranges, from 0 bar to 2 bar all the way to 0 bar to 3000 bar. Unlike the previous Optrand sensors which have their tip temperatures limited to 380oC the maximum tip temperature of the new transducer can be as high as 450oC. Finally, the device comes with either a single diaphragm or with a novel dual diaphragm enabling flush mounting of the diaphragm. Conclusion: The feasibility of a fibre optic-based static/dynamic pressure transducer was demonstrated in both laboratory and combustion engine tests. A sensor with 5 mm diameter sensing head rated for maximum temperature of 380oC with the frequency response of 0 Hz to 20 had its offset voltage change from 0.5V to 0.51 V while the sensitivity changed from 3mV/psi to 3.05 mV/ psi when engine load changed from idle to full load. No effect of varying signal conditioner on sensor offset, sensitivity and drift was evaluated yet, to be performed in the follow up studies.
KEYWORDS – pressure, transducer, fibre optic, high temperature, miniature