Presented by Dr. Marek Wlodarczyk, President and CEO of Optrand, Inc. at NASA Fiber Optic Sensors Technical Interchange Meeting, Kennedy Space Center, November 19-20, 1997.

Fiber-Optic Pressure Sensors for Harsh Environment Applications

Marek T. Wlodarczyk, Ph.D.
President & CEO
Optrand, Inc.
Plymouth, Michigan
president@optrand.com

ABSTRACT

Optrand has developed and offers commercially a family of long-life, miniature fiber-optic pressure sensors for use in harsh environments characterized by extreme temperatures (from 40°C to 360°C), high levels of Electromagnetic and Radio Interference, and in the presence of violate chemical materials. In a durable and low-cost design the sensor detects changing light reflection from a specially shaped metal diaphragm under the effect of pressure. Sensors as small as 2.8mm in diameter are available covering pressure ranges from 0-100 psi to 0-30,000 psi.

Two types of sensors are available, FiberPSI and AutoPSI. The FiberPSI sensor uses a single fiber while multiple fibers are used in the AutoPSI sensor. The opto-electronic components of the sensors' signal conditioners include one surface emitting LED and one or two SI PIN photodiodes. To combat LED intensity and photodiode sensitivity changes due to ambient temperature fluctuations, a proprietary auto-referencing technique has been developed that does not require temperature regulation of the optoelectronic package. The referencing technique compensates for other phenomena as well, that can alter baseline light intensity, such as fiber bending, connector mechanical and thermal instabilities, or fiber transmission change due to radiation or temperature.

The FiberPSI sensor opto-electronic and electronic components are l°Cated inside a separate signal conditioner mounted in a rugged metal enclosure (5'x7'x2'). The sensor connects to the conditioner via an optical connector. The AutoPSI conditioner is packaged inside a small, "smart" connector permanently attached to the sensor fiber optic cable. In both FiberPSI and AutoPSI cases, the signal conditioners output pressure and sensor-health signals. When optimized for high linearity and reduced thermal effects, over the sensor tip temperature from 40°C to 350°C the AutoPSI sensor demonstrates ±0.25% FSO accuracy while typical accuracy of the FiberPSI system is at ±0.5% FSO.

The FiberPSI and AutoPSI sensors offer dynamic pressure detection capability over the frequency range of 0.01 Hz to 15kHz, with a possibility to 100kHz. Present sensor applications include R&D and testing of internal combustion engines as well as long-term monitoring of diesel and natural gas engines. Over almost two-year continuous use, the sensors have demonstrated over 800-Million pressure-cycle lifetime in stationary natural gas-burning engines. Currently the sensors are guaranteed for 500 Million cycles of lifetime. Over hundred of sensors are already installed to continuously monitor and balance large-bore stationary engines used in natural gas pumping stations. In it first OEM application, in 1998 the AutoPSI-S sensor will be used to control and monitor natural gas-fueled electricity-generating engines.

A recently developed AutoPSI-TC sensor offers temperature-compensated dynamic pressure capability. Using Optrand's proprietary technique, changes in the sensor diaphragm's deflection with temperature are compensated by changing the optical transmission of the fiber. As a result the AutoPSI-TC matches the performance of water-cooled piezoelectric sensors without water or air-cooling and at fraction of cost. While the signal conditioner of the AutoPSI-TC sensor is currently rated for 40°C to 65°C, a version for 40°C to 125°C will be offered in early 1998.

Finally, the AutoPSI-DC sensor is the newest addition to the AutoPSI family. The sensor offers world's only capability of continuous static pressure detection at 360°C. In addition to pressure the sensor measures temperature by an electric sensor embedded within the sensor package. Over the sensor tip temperature range of 40°C to 350°C the device currently offers ±1% FSO pressure and temperature reading accuracy.