Development of High Temperature Smart Sensor Systems

Smart Sensor Systems operable at high temperatures can meet the needs of a range of aerospace application needs. For example, in order for future aerospace propulsion systems to meet the increasing requirements for decreased maintenance, improved capability, and increased safety, the inclusion of intelligence into the propulsion system design and operation becomes necessary. These propulsion systems will have to incorporate technology that will monitor propulsion component conditions, analyze the incoming data, and modify operating parameters to optimize propulsion system operations. This implies the development of sensor systems that will be able to operate under the harsh environments present in an engine. Likewise, operation in such environments as Venus exploration missions require systems which can operate in the harsh environments present on the Venus planetary surface. Overall, the drive is to make intelligent vehicle by developing Smart Sensor Systems operational in harsh environments. A Smart Sensor System as described here implies the use of sensors combined with electronic processing capability. Silicon carbide (SiC) electronics a leading candidate for meeting a range of harsh environment application needs [1]. Operation of SiC electronic circuit elements at 500°C has been demonstrated for thousands of hours. These circuit elements include transistors, amplifiers, and logic gates. These time frames are now viable for implementation in, for example, engine operations for extended periods. This would allow high temperature signal processing at temperatures far beyond that capable in silicon electronics [2]. However, significant work remains in order to develop and demonstrate complete high temperature Smart Sensor Systems. Previous work has demonstrated a wireless sensor system operational at 300°C. In particular, a wireless pressure sensor has been demonstrated at 300°C, with signal transmission over 1 meter distance and power derived in part from scavenged energy [3]. This work is intended as a stepping stone for Smart Sensor Systems operational at even higher temperatures, such as 500°C. In order to achieve such a high temperature Smart Sensor System, further development and integration of high temperature technologies including sensors, electronics, and packaging is necessary, combined with an extensive testing program. This paper discusses the development of three components of a High Temperature Smart Sensors System: 1) Development and packaging of a SiC-based gas sensor operable at high temperatures; 2) Integration of SiC electronic components and communication technology showing basic wireless operation at 500°C; and 3) Engine testing of system components in relevant environments. Each component has its own technical challenges associated with its maturation and development into a high temperature Smart Sensor System. For example, the implementation of a SiC-based gas sensor into test engine environments highlights some of the challenges involved in transitioning sensor technology into an operational environment. Although a SiC-based gas sensor based on a palladium/palladium oxide/silicon carbide (Pd/PdOx/SiC) Scottky diode sensor design has shown excellent response and stability, packaging and integration of the sensor design into an operational system has posed challenges. The packaging and integration issues involved in moving from a sensor element into a package sensor array operating in an engine test stand environment will be discussed. Further, although circuit elements exist that can operate for extended periods at 500°C, integration challenges exist as well in converting these basic circuit elements into an wireless system operational at 500°C. In particular, the operating frequency and available transmitted power of existing SiC circuit elements drive the design of possible high temperature wireless circuits and limit wireless operation. The use of these circuit elements combined with tailored antenna designs in order to demonstrate a basic 500°C wireless circuit is discussed. Finally, in order for high temperature Smart Sensor Systems to be accepted in an application, extensive operational environment testing is necessary. Examples of multicomponent high temperature sensor system tests will be described. In particular, the use of high temperature sensor systems in engine emission monitoring applications will be discussed. The present approach is to combine sensor elements operating at high temperatures with Smart Sensors System electronics operating in more benign environments. While this approach provides the capabilities of Smart Sensor System for high temperature environment applications, the long-term objective is to integrate the electronics with the sensors in the same package and operating within the harsh environment. It is concluded that the development and application of high temperature Smart Sensor Systems is both a multidisciplinary and multistage project involving different technological capabilities and requirements for each stage of the process. The technologies necessary in order to enable a high temperature Smart Sensor System are still in development, but the realization of such a system can significantly change vehicle system operations.