A novel eddy current (EC) system was developed for the measurement of torque in a rotating shaft, resulting in the following new contributions beyond the current "state of the art" in the field.

1. Outline of a general methodology for shaft torque measurements and the development of an adaptable EC test system that can be applied to a variety of torque measuring applications with diverse constraints.

2. EC characterization of torque-stressed 18% Ni maraging steel at various excitation directions and test frequencies using a specially designed U-core EC sensor and fundamental and harmonic data analysis.

3. Design, development, and evaluation of a novel probe-type EC sensor with dual excitation windings, dual focused U-core elements at 90° to each other, and dual differentially connected sensing windings for static and dynamic shaft torque measurements.

4. System analysis for optimum shaft torque measurement accuracy taking into consideration the EC response behavior of torque-stressed 18% Ni maraging steel, the EC sensor characteristics, the effects of excitation field strength and test frequency, and the applied shaft torque and sensor air gap variations.

5. Design, development, and evaluation of a constant air-gap EC sensor assembly for shaft torque measurements.

6. The developed EC system can be applied in a wide variety of torque measurement or control systems to improve system performance and reliability.