A Microwave Blade Tip Clearance Sensor for Active Clearance Control Applications

Abstract

Active clearance control has been used to improve efficiency and performance in commercial aircraft engines. Technologies implemented to date rely on compressor bleed air to control clearances based on open loop control and the flight regime (takeoff, cruise, etc.). Open loop systems necessitate a wide safety margin to accommodate the uncertainty in system models that are used to drive such systems. Implementing a feedback control system to optimize clearance has been difficult due to the lack of survivability of clearance measurement technology. Current technologies such as eddy current, capacitive, and laser sensors have been effectively used in laboratory environments but lack the robustness and reliability necessary for long-term use at high engine temperatures. This paper describes a microwave-based sensor designed to operate in temperatures up to 2500°F with a resolution of 0.2 mils and bandwidth up to 25 MHz. The sensor can effectively operate in dirty environments and has the ability to see through oil, combustion products, and other common contaminants. Performance data on the sensor from spin pit testing at 1100°F will be presented to show the viability of the sensor for use in active clearance control.