A magnetic field response sensor is designed to measure displacement or rotation rate without a physical connection to a power source, microprocessor, data acquisition equipment, or electrical circuitry. The sensor works with the magnetic field response recorder, which was described in "Magnetic- Field- Response Measurement- Acquisition System," NASA Tech Briefs, Vol. 30, No. 6 (June 2006), page 28. These sensors are wirelessly powered and interrogated, and the measurement acquisition system and sensors are extremely lightweight.
The response recorder uses oscillating magnetic fields to power the sensors. Once powered, the sensors respond with their own magnetic field. For displacement/ rotation measurements, the response recorder uses the sensor's response amplitude, which is dependent on the distance from the antenna. The recorder's antenna orientation and position are kept fixed, and the sampling period is constant.
A sensor with fixed frequency and fixed orientation with respect to the response recorder antenna can be used for position and displacement measurements. If the sensor's orientation is not fixed, but its trajectory is known, it may be possible to calibrate the response amplitude variation with trajectory. For rotational motion such as wheel speed, identifying the number of times the response amplitude exceeds threshold amplitude in a fixed time duration can be used to determine rotation rate. A wheel speed sensor is shown in the figure. The sensor is a thin film circuit placed inside the wall of a tire. As the sensor approaches the antenna, the amplitude increases. The amplitude peaks at the closest point to the antenna then decreases producing changes in amplitude that are cyclical. When two sensors with different respond frequencies are used inside the wheel, rotation direction can be determined by identifying which sensor's amplitude increases first. In addition, there is no mechanical wear because no gears are used in the design.
This work was done by Stanley E. Woodard of Langley Research Center and Bryant D. Taylor of Swales Aerospace.